of nucleons and polarized fragmentation at high energy. OCR Outputpolarized LEP beam. This is a unique opportunity to study the transverse spin distributionsparasitic measurements of polarized and unpolarized hadronic electroproduction with an unWe propose to install a jet target in LEP. During the normal collider run, it will enable

Abstract

Lyon, Milano, Padova, Praha, 'lkieste, Yerevan, ZiljnaBmo, CERN, Ferrara, Geneva, Genova, Iowa, JINR., Lausanne,

The HELP Collaboration

Experiment at LEPA Proposal for an Internal J et-Target

in Deep Inelastic Scattering3 3 .. 1 H Transverse Polarization

CE R xv LE PCSc P

it /; Y`{`<;September 29, 1993

LEPC / P7CERN/LEPC 93-14/7(;—.·*°* N

ABPatricia

$(:00000617EAMEAEL IOFQEQ

IIIIMIIIIIIHIIMMMMHIIMIIIIMCERN LIBRARIES, GENEVA

W. Kubischta. (CERN) OCR Output

Technical Coordinator

L. Dick (CERN, Milano), B. Vuaridel (Geneva)Spokesmen

B Polarized Cross Section 45 OCR Output

A Analysis of spin observables 43

Cost estimate 42

Moller Energy Monitor 42

Installation and Compatibility with the machine 42

’·.eam pe ..................................55 BPi 40

N ·5.4.4 Momentum Resolution and Particle Identification ......... 39

5.4.3 Calorimet 39

n ’er ................................5.4.2 Track 38

..nmanerenkov .......................541 Rig Igig Ch 33

...................................5.4 Detectors 33

............................. ··53 Spectrometer magnets 31

................................. _ ·52 Cluster Target 31

.............................. . •51 Polarized Jet Target 24

Experimental Setup 24

Experimental Program 23

..areeoarzaon ..............372 Tgt Dpliti 22

on ..............3.7.1 Momentum resoluti 22

.semac nceranes .................37 SyttiUtiti 22

..ncroron raaon ..............365 Syhtditi 22

rons .................3.6.4 Moller Elect 22

..................363 Multiple scattering 21

3.6.2 Electron identification .............. 21

3.6.1 Duty factor and event rates ........... 21

3.6 Particle Identification .................. 21

213.5 Inclusive hadronic production with quasi-real photons

203.4 'Pransverse polarization and target fragments ....

203.3 Polarization transfer mechanism . .

193.2 Electroproduction of Baryons ....

173.1 Electroproduction of light masons . .

17Objectives

142.4 Framentation in DIS ........

122.3.2 Masonic Polarimetcr .....

112.3.1 Baryonic Polarimeter ................

1023 Quark Polarimetry

2.2 Hadronic Electroproduction in DIS .

.n sruons ..........21 SpiDitibti

Polarization in DIS

Introduction

Contents

the dynamics of confinement and make new tests of factorization in semi-inclusive DIS. OCR Outputinitial and final momenta, and it is an ideal process to study these phenomena to enlightenspin structure. In DIS the kinematics of the fragmentation is well defined by the electronphysics which remains to be discovered, and they are the necessary tool to further probe the

These polarized fragmentation functions may represent an important piece of hadronicabout transverse spin.

of the electrons are not correlated because the virtual photon does not carry any informationrequired for such an experiment. Furthermore, the transverse polarizations of the quarks andbe measured from the angular dependence of their decay products. A polarized beam is notrescattering or, with more ease, the polarization of ”self-`analyzing" baryons, like the A, couldof a leading baryon. For example, the polarization of the protons could be analyzed byon the spin of the quark. Another possibility consists in the measurement of the polarizationare detected, then the azimuthal angle of the normal to the hadron plane can also dependazimuthal dependence in the production of the leading hadrons. When two leading hadronsmeans. The fragmentation processes may be sensitive to the quark polarization if there is antransverse polarization in DIS only if the ”struck” quark polarization is measured by some

With a target polarized transversally, the unpolarized electron beam will probe the quarkmost promising process and is therefore one of the aims of this proposal.transverse asymmetries arise at the leading twist level. Semi·inclusive DIS is probably thepressed in this case. In contrast, in polarized Drell-Yan or in polarized semi-inclusive DIS,cannot be measured in purely inclusive DIS because transverse spin asymmetries are supare also fundamental structure functions and are independent of the longitudinal ones. TheyHowever, the transverse spin distributions of the quarks in a transversally polarized nucleon

At present, only the longitudinal spin of the quarks in the nucleons has been probed.Hopefully new results will soon give a more definitive interpretation.large uncertainties imply that they are not necessarily inconsistent with the SLAC results.standard deviations. The CERN data are consistent with the Bjorken sum rule, although theproton data the new SLAC results suggest a violation of the Bjorken sum rule by about twobutanol target, and at SLAC with a new polarized 3He target. Together with the previous

Recently two DIS experiments were performed: at CERN with a polarized deuteratedspin structure function.

an orbital momentum. This has triggered interest in a similar measurement of the neutroninterpret the data, by introducing large strange quark polarization, gluon polarization andEllis and Jaffe, is significantly violated, and many theoretical papers have been written tomeasurements indicate that the quarks carry little spin on average. The sum rule derived bystructure of the quark distribution in the proton. In the framework of the parton model,With both a proton target and a beam longitudinally polarized, DIS can probe the helicity

In contrast very few experiments have exploited the polarization of beams and targets.

out.

of these experiments the targets were not polarized and the spin dependence was averagedCERN and Fermi Lab, were scattered from hydrogen, deuterium or heavier nuclei. In mostmeasurements in which lepton beams of electrons at SLAC and of muons and neutrinos athalf the nucleon momentum. These findings come from an enormous number of cross sectionthe nucleons and that other constituents, identified to be the gluons, are responsible for aboutnuclear matter. For example, it has given confidence that quarks are the building blocks ofDeep inelastic scattering of leptons (DIS) on nuclei has revealed many interesting features of

1 Introduction

range. OCR Output

metries in the light meson electroproduction and A polarizations over a broad kinematicalfragmentation at high energy. We aim for a statistical accuracy of a few % for spin asymThis is an opportunity to study the transverse spin distributions of the nucleons and polarizedwe would like to perform with the unique LEP200 unpolarized beams, in a parasitic manner.spin. Since polarized beams may not be available, we stress here in more detail, the physicsto express this opportunity. Until now, most emphasis was put on DIS with longitudinalintent, memoranda and progress reports [1, 2, 3] were already presented by the collaborationpolarized jet target in the LEP tunnel and to perform a DIS experiment. Various letters of

The HELP (for Hadronic Electroproduction at LeP) collaboration proposes to install aand the only one which has not been measured yet.structure function is one of three structure functions which arise at the leading twist levelopportunity to study, for the first time, the structure function of transverse spin. Thisindication of the limit of the perturbative approach to QCD. Furthermore they offer thesections, the more sensitive spin dependent observables can challenge PQCD and give an

While the validity of the PQCD approach is essentially tested on spin averaged cross

helicity is fixed. OCR OutputFigure 2: The DIS helicity asymmetry in term of unitarity diagrams, when the lepton

+/ : \++/ : Yr +. . .+ · +

and | - >):when the transverse polarization is expressed by a linear combination of helicity states | + >conserves chirality or helicity in a zero mass limit This suppression becomes apparentfunctions. However, the asymmetries are expected to be small since the gauge interaction

With a. transversely polarized target one can probe another combination of structureopposite helicities as shown in fig. 2.The longitudinal two spin asymmetry A1 arises from the difference between diagrams ofparallel and antiparallel target and beam polarization probes the quark helicity distribution.

With longitudinally polarized beam and target, the cross section asymmetry between

Figure 1: DIS cross section represented in term of unitarity diagrams.

+ / i \+ O I O +L l l

—/ ¤ \

.. I ..

neglected.and ”-” denote the helicity of the quarks. Here, for simplicity, higher order diagrams will beScattering (DIS), can be represented by unitarity diagrams as shown in fig. 1, where "+"The amplitude products which contribute essentially to the cross section in Deep Inelastic

2 Polarization in DIS

the final hadron. OCR Output

the transverse spin distribution can be measured in DIS if one measures the polarization ofDrell-Yan process leads to semi-inclusive DIS, as can be seen in fig. 5. With a polarized target,

In semi-inclusive DIS, the situation is similar to the Drell-Yan case. After crossing, thetransverse spin asymmetry in Drell-Yan reactions

with polarized colliding protons in the Relativistic Heavy Ion Collider RHIC to measure thefig. 4 and can produce large asymmetries. An experiment has been proposed at Brookhaven,

In the Drell-Yan process, an interference term can be present as shown by the diagram ofthere will be little sensitivity to the spin structure.mass limit, only small transverse spin asymmetries are expected in fully inclusive DIS andhelicities (chiralities) as shown in fig. 3. Since the helicity is conserved in the zero quarkspin asymmetry arises from interference diagrams where quarks and protons have different

where | +5 > and | -55 > are positive and negative transverse spin states. The transverse

|—=?>=-$z(I+>-|->)

(1)+5='>=-%(l+>+l—>)

Figure 4: Dre11—Yan diagram.

+•..

-1 ¤ x+

—\ i I +

interfere. These diagrams are suppressed in fully inclusive DIS.Figure 3: Diagrams where two difereut helicity states for the protons and the quarks

+/ 1 \··—/ I \+

· ' ·L};

allel to the transverse spin of the nucleon. ATq(x) is sometimes called hl (X) by analogy to OCR Outputwhere q f£.(z) and qf..;,(z) are distributions of quarks with transverse spin parallel or antipar

AN:(=¤) = 4rs(¢) · 41—s(==) (4)

The transverse spin distribution is then:polarized in the 12 direction, as shown in fig. 7.

Similarly one can define distributions for the transverse spin of the quark in a protonlongitudinally polarized with the parton interpretation schematically represented in fig. 6.the proton of helicity +. These distributions can be measured with a target and a beamwhere qf+(_)(x) are the densities of quarks with flavor ”P’, charge ef and helicity +(-) in

(3)y1(¤=) = $2 ¢}A¤<1:(=¤) = r;X;€il*1J+(")· ax-(=)lcharge:the ilavors of the quarks and antiquarks, and weighted by the square of the quark electricalton and a quark distribution with the helicity antiparallel to the proton one, summed overgl is the difference between a quark distribution with the helicity parallel to that of the promentum carried by the quark, has a very simple interpretation in terms of the parton model.The longitudinal spin distribution A Lq,·(:c) or gl (x), where x is the fraction of nucleon mo

2.1 Spin Distributions

Figure 6: Partonic interpretation of the helicity distribution in the nucleon.

Figure 5: Semi-inclusive DIS with both the initial and final hadrons polarized.

Pl

(ll

7\f

1The polarized cross section are given in appendix B.

[=v”H{‘ + (1 — y)H§‘ + $(2 — y)x/1 - y¢¤¤¢H§‘ + ;€,T;(1 - y)¤¤¤2¢H£`l

d2:dydzdp§.d¢— Q4(6) OCR Output

der 41m22M E

be decomposed into four independent structure fimctions H!‘(a:, Q2, z, p§·) [10]:The unpolarizedl cross section for semi-inclusive electroproduction of hadrons, eN—>e’hX can

2.2 Hadronic Electroproduction in DIS

However, chiral selection rules suppress its effect in inclusive DIS.is as fundamental as ALq(x) and can be measured in semi-inclusive DIS or in Drell-Yan.a classical parton interpretation, unlike gz which has significant twist-3 parts [8, 9]. Aq·q(x)

- The spin structure fnmctions A;,q(x) (or gz) and ATq(x) (or hz) are considered to havemq/M.is not really sensitive because the contribution of Aq·q(x) to gz is suppressed by a factorwhere the quark charge squared and flavor indices have been omitted. However, this method

5 ( )1 d 1 d d m 2 2 = .2 _ / .2- .1 A y1(==.Q )+ az(2=.Q ) A y yi(y) z y dy[M r<1(y) + £(y)]principle, gl and gz could be used to determine the transverse spin distribution A;·q(x) [7]:beam, the other structure function, gz, can be deduced, using already existing data on gl. In

In fully inclusive DIS, with a transversely polarized target and a longitudinally polarizedproton and may help in understanding this highly debated topic.the leading twist level, are therefore not redundant informations on the spin stucture of thecan be shown that Aq·q(z) = q.,.(z) The transverse spin distributions, whic.h arise atcovariant parton model where the baryon is composed of a quark and a scalar diquark, itmotion and orbital momentum, Az-·q(z) is expected to be different &OII1 ALq(a:). In a naive

N aively one would expect that Aq·q(z) = ALq(z). However, because of relativistic internalis necessary and complementary to Drell·Yan experiments.from the decay products of A. Therefore, the experiment is very difhcult. Semi-inclusive DISnot highly polarized and, at present, it is only possible to obtain polarized antiproton beamsquarks are polarized or polarized antiprotons are used. However, the sea quarks are probablyprotons polarized transversely. To be sensitive to Ag·q(z:), it is necessary that either the seagl (x). This transverse spin distribution can be studied in the Drell·Yan processes with both

Figure 7: Partonic interpretation of the transverse spin distribution in the nucleon.

10 OCR Output

polarization can be used to probe the quark polarization.Therefore, one can expect that the same behavior holds for the spin. Then the hadronthe electric charge of the leading hadron are correlated to that of the fragmenting quark.measures the polarization of the struck quark. It is well known that the quark flavor andTransverse spin asymmetries in semi·inclusive deep inelastic scattering can be observed if one

2.3 Quark Polarimetry

carries a fraction z of the quark energy.where fq!.,;,(z,Q2) is the fragmentation function from a quark gf into a hadron which

(9)1?£‘(=,Q'.¤)= E ¢}=q;(=» Q°)fq,-·r»(=» Q2)

Finally, the factorization of the structure functions gives:

model.

where F2 is the usual nucleon structure fnmctionz F;(2:, Q2) = E, e}zqj(z, Q2) in the parton

(8)- Fz(==,Q’) = Ed¤·H£‘(==,Q’.z)A A1

and the fully inclusive structure function F,(z, Q') is given by:

7 ( ),

—-v—-·•-- 1 ——·1—;; · _- • Z [1 + (1 )] EW Q, ) dxdydz Q4 y 2 I lzda 41m22ME

in the approximation R.= ${,5 ~ 0, H; ~ 22H;:one can express the cross section in term of only two independent functions E,;(x,Q2,z), andget information about the four partial structure functions. After integrating over ¢ and p§·

By measuring the 46 and E dependence at fixed x,Q2 of the inclusive cross section one canplane, the z-axis being given by the photon momentum, as shown in fig. 8.where ¢ is the azimuthal angle of the produced hadron with respect to the electron scattering

mass, neglecting the intrinsic transverse momentum of the quarks in the proton.Figure 8: Kinematic variables of the electroproduction of hadrons in the p-1* center of

{ I /D

A 'd”

11 OCR Output

or the electron-Nucleon c.m. frame.Neglecting mass corrections, we can replace this frame by the laboratory frame, the Breit frame

possibilities to probe the quark polarization.The measurement of the polarization of the final proton with a polarimeter offers further

of the s-quark polarization of the nucleon.naively that the spin of the A is carried by the s-quark. In this case, it will be the ideal probe

The A may not be a very good polarimeter for the u and d quarks since one expectsviewed as a " quark polarimeter"Dm, = $5,7-E [13], and the sums are over the flavors. In this picture the baryon can bedistribution as defined previously. Dm, is the depolarization factor of e' quark scattering,baryon B, Aq•qj(::) and Aq·fq!..B(z) are the transverse spin asymmetries of the correspondingof the quark in the target nuclei, fq’...B(z) is the fragmentation function for quark qf into aalong PN in the c.m. frame of the hard processz, q_f(2:) is the transverse spin distribution

where R is the rotation about the normal to the e-quark scattering plane which brings FB

2 Ze; ·q:(=) · fq,~¤(=)-12,,,.Pier-P?-i

ci ‘ Ar9f(’l ‘ A¤·fq;—•B(z)

the transverse spin distribution of the nucleon (N) by:In DIS with a polarized nuclear target (N), the polarization of the baryon(B) is related to

mechanisms, of the highly polarized s·quark in the e"'e' annihilation at the Z° pole.spin of the A. The Lund string model was then used to estimate the spin dilution, by other30% at the Z° pole [16]. For this estimate, it was assumed that the strange quark carries thefragmentation mechanism [15]. Recently, this polarization was estimated to be as high asin the inclusive A production in e+e' —> A + X could give information on the polarizedZ—peak, looking for transverse spin correlations. More simply, the longitudinal polarizationexperimentally demonstrated. It could be done, in principle, at LEP in e`l”e’—+ AA+X at theQCD and there is very little experimental information in this field. This idea should first beof the polarized quark. Unfortunately, this process cannot be calculated in perturbative

It is expected that the leading baryon in quark fragmentation carries part of the spinlimit our attention on this baryon but similar considerations apply to other hadrons.analyzing” particles. They decay into particles which are easy to detect. We will thereforecan be used to determine the baryon polarization. For example, the A’s are well known "selfLarge spin dependent anisotropies arise in the weak decay of some baryons. This phenomenon

2.3.1 Baryonic Polarimeter

In the following sections, we review various quark polarimeters in DIS.production plane of the two highest z hadrons may also probe the quark spin [14].the transverse polarization of the quark [13]. Furthermore, the azimuthal dependence of thethat the azimuthal dependence of the leading hadrons in semi-inclusive DIS is sensitive topolarization of self-analyzing baryons &om fragmentation [4, 6]. Recently Collins suggesteddently, Artru suggested to measure the transverse polarization of the quark by measuring the”handedness" and it was shown how it can be measured in e+e` annihilation [12]. Indepenlations within a jet could be used [11]. Recently this idea was rediscovered and called jet

In the case of longitudinal polarization, it has been suggested that three particles corre

12 OCR Output

shown in fig. 8. However, in first order, if one considers asymmetries between both targetEven if the target is not polarized there is a dependence on the azirnutha.l angle qi, as

quark.

distribution of the leading hadron may depend strongly on the polarization of the ’mother’azimuthal angle 45, with respect to the quark polarization, as shown in :fig.9. The azimuthalangle 0. Then the quark fragments into hadrons emitted with transverse momentum pq- andmagnitude of the polarization vector is reduced by a factor ’D,,,,’ depending on the scatteringand the scattering plane will be unchanged after the scattering, as shown in fig. 9. Only thecomponents, D,,,,=D,,. This means that the angle between the transverse polarization vectorthe depolarization parameters, can be calculated in QED and are equal for both transversesomewhat depolarized after scattering by the electron. In the hard subprocess center of mass,initial quark will have fi and § polarization components. The quark polarization will bein the scattering plane or sideways and longitudinal to the particle momentum (lc), thethe standard polarization notation for a vector normal to the electron scattering plane (fi),transversely polarized target, e.g. a proton, the quark will be transversely polarized. Usingschematically represented in fig. 9 in the center of mass of the hard subprocess. With aazimuthal distribution of a single leading hadron. The method proposed by Collins [13] isAn interesting and novel idea for quark polarimeters consists in the measurement of the

2.3.2 Mesonic Polarimeter

therefore interesting to consider also non-baryonic polarimeters.has to create a baryon pair which cost energy [17]. From an experimental point of view, it isbaryonic charge has to migrate from the target fragmentation region in rapidity space, or onein the kinematical region of interest. If one is interested in quark fragmentation, either the

One problem of baryon polarimeters for DIS is the low cross section for baryon production

S,,(S,,·) and the scattering plane is conserved since D,,,, = D,,, i.e. a' = oz.model, in the hard process center of mass. The angle a(a') between the spin vectorFigure 9: Quark polarization in semi-inclusive DIS within the framework of the parton

P r

1 S.

[isl cf , ev. ,5q S

P //

S" EPZEFIEscatle-zlmq gd [ Fl

13 OCR Output

favored in comparison to the right-hand side.momentum of the created quark pair, the fragmentation with pq- on the left-hand side isantiparallel spin and because of the compensation between the total spin and the orbitalbreaks with the creation of a qq pair. If this leading meson is a pion, q;,,,d;,,,, and q haveis transversely polarized, as shown in fig. 11. The leading meson is emitted when the stringprovides a mechanism for the Collins type asymmetry. Let us assume that the leading quarkorder of those shown in tig. 10. On the theoretical side, the string fragmentation modelare due to similar mechanisms, we can expect that the analyzing power is at least of theHowever, if we assume that the single spin asymmetries observed in pTp —» 1r + X reactions

At present, we have no direct experimental information on the magnitude of this effect.observed in the pp —> w+X reactions at large transverse momentum.The asymmetry is expected to reach up to a few times 10 %, similar to the large asymmetriesat qq·=O, the leading twist asymmetry and the higher twist asymmetry when qy >> Mh.where My, is a typical hadronic mass. This expression exhibits the kinematical zero expected

12. A ~ sm ¢·——Mn · qr

[13];The asymmetry A is proportional to sin(¢,) and has a dependence in QT roughly given by

momentum, qq- ·: pq-/z, and Pfv is the transverse target polarization.where qy is the transverse momentum of the virtual photon with respect to the hadron

E; cy · q;(==) · fq,-·».(¢.. qr, Z)(11)E 62 · A1·¢1;(== Q2) · Arf -·¢ q Z A(,,’Q2,¢__qT_z) = Dm . pf .

by [13]:the high Q2 limit this asymmetry is related to the transverse spin distribution of the nucleonpolarization states, i and -2, only the spin dependent part of the cross section will remain. In

Figure 10: Single spin asymmetries in pp -+ vr + X at 200 GeV proton beam energy.

0 0.2 0.4 0.6 0.8

-0.4 I- ¤`=°

1|·°=¤

1|’+=U Q-0.2

Z <1 O ‘

it fl! >l<0.2

O.4

14 OCR Output

direction in the virtual photon-proton center of mass and p,,,,,,, is the maximum possiblethe variable xf = ik ~ 5,lL, where pu is the hadron momentum parallel to the virtual photonl3

It is possible to attribute hadrons to one of the two fragmentation regions by considering

nucleon.

leptons up to contributions from the intrinsic transverse momentum of the quark in thefragmentation, and the kinematics of the quark may be defined by that of the scatteredpartonic picture of the nucleon, the interpretation is most straightforward for the quarkthe target fragmentation and the quark fragmentation as represented in fig. 13. In theIn the deep inelastic scattering, hadrons arise essentially from two types of fragmentations,

2.4 Fragmentation in DIS

our measurements with a method described in section 3 and appendix A.from the target polarization to the final quark. Both these assumptions will be checked inD,,. Furthermore, the polarization of the final state may not come from a pure spin transfercoefhcients determined in the framework of the parton model with QED are equal, i.e. D,,,, =

The spin vector may not behave as simply as shown in fig. 9 where the spin transferpion may be simpler.However, the multiplicity in the forward region is rather limited and the method with a singleinteresting because it can be applied to a broader range of processes, e.g. in pp reactions.between the spin of the quark and the vector 51 x 5;, as shown in fig. I2. This method ismesons. In this case, one expects an azimuthal dependence in cosqbzh, where ¢2;, is the angle

A variant of this method has been proposed [14]. It relies on the measurement of twotheir relative contribution to the nucleon spin.and deuteron targets, it will be possible to distinguish between various quark species andfunctions. Looking at various hadrons, 1+, 1r', 1r° and kaons produced from polarized protonized hadronic production using different reactions to get a deeper insight into the structureand a hagmentation function. Clearly it will be necessary to perform experiments in polar

The experiment will be sensitive to the product of two terms; namely a structure function

preferably produced on the left-hand side.the spin direction of leading quarks denoted by the circular arrows/The leading rr isversely polarized quarks. The left and right directions are defined with respect toFigure 11: Schematic representation, in the string fragmentation model with trans

\ mglwt,,»’/ 6\’<\ - <`\ C5 PT

leadingCl A UQ

F8`/Opecl P T qLeading

15 OCR Output

Figure 13: Target and quark fragmentation in DIS.

torget frogments

quork fragments

of the two leading mesons.azimuthal angle ¢z;, is between this plane and pz >< pz, where pz and pz are the momentapolarization of the initial state quark, Sq, lies on the plane in the righthand side; theFigure 12: Quark polarization and the fragmentation into two leading hadrons. The

P1lw2··/ A.

P2

P1 >< P2.. .. an

16 OCR Output

LEP200 is crucial to study hadronic electroproduction semi-inclusive DIS.However, the hadronic production yields are then very low. Therefore the high energy ofAt higher z the W limit is somewhat relaxed, allowing for measurements at lower energies.

• z > 0.5 and W2 > 9GeV

• z > 0.2 and W2 > 23GeV2 or

• z > 0.1 and W2 > 55GeV° or

somewhat smaller values of W may be adequate. Berger propose the following cuts:inequality together with the standard DIS criteria is about E,,,;,,=35 GeV. For larger z-valuesessentially the full range of z. The minimum lepton beam energy which allows fulfilling thisIf this inequality is satisfied, it should be possible to measure fragmentation fimctions over

(13)2 1 _ W2 = 9-g-J2 > 55G'eV2

x) /x), this criterion is equivalent to:expressed in terms of W, the invariant mass of the system X, and, with Y=lnW2=ln(Q2(1·that the rapidity range be larger than about 4. Since the full range of rapidity, Y, may befragmentation is to be studied with no contamination from the target remnant, it is necessarythat the typical hadronic correlation length in rapidity is about 2. Therefore if the quarkbased on the rapidity yi, of the final state hadron [10]. Experimeutally it has been established

In order to separate these distinct fragmentation regions, Berger has worked out criteriaregion of overlap in between.principle, to the target fragments, the positive ones to the quark fragments and there is amomentum. Note that If ~ z for z or xy 20.2. The negative values of xy correspond, in

17 OCR Output

between 100 and 300 mrad, as described in section 5.spectrometer for the hadrons, called spectrometer H, with an angular acceptance ranging

In addition, we consider a two-step spectrometer, the spectrometer I and a large acceptancedetector and the already available UA6 magnet with angular acceptance of about i 100 mrad,spectrometer I which consists of tracking detectors, an electromagnetic calorimeter, a RICH

In tables 2 through 4 we present DIS electrons and mesons, ·rr and K, detected in thebe varied by a few hundred MeV without significantly affecting the statistical error estimate.equation 12 could be changed into a simple linear dependence in qq- while the mass M h couldthe Berger cuts of section 2.4. The transverse momentum dependence of the asymmetry of

h

2 through 6 for electrbproduction of 1r, kaons and A from the quark fragmentation, satisfyingand G = D · P, - éaifgl,-. The yields and statistical errors of the asymmetry are given in tableswhere the sums are over the all the Nw events for each target polarization direction, T and L,

Nw eu}ev evi e" evi Nw culK : [JT E Simp} - E mai]/[Q E G · sin.2¢I + Q- Z G · sinztpf] (is) NN

efficiency, c.f. appendix A:from systematic uncertainties arising from luminosity variation, spectrometer acceptance and

···. Such an asymmetry between yields of both transverse polarization signs can be analyzed free

M, , In + 91*(14)2 _ 3 · A(=¤.Q .¢..qz~.=) — A¤(=.Q .=)··¤m(¢.)·D-Pr · MhqT

larization parameter D in the hard process, the asymmetry is:roughly by iyhfg`? Taking into account the target polarization P, and the calculable depox,Qz, ¢•,, qy and z. The 45, dependence should be given by sin(¢,) and the qg· dependenceAs seen in the previous sections, the asymmetries will depend on several kinematical variables:

3.1 Electroproduction of light mesons

2·10"“ cm""s“Luminosity

1·10"° cm`2s"Luminositytarget polarization (H or D) | 0.95target thickness (H or D) I 3-1013atoms/ cm

6 mAbeam intensityTable 1: Beam intensity and target densities for the HELP project.

of the f1·agmentation was implemented.is based on the Lund string model. For systematic uncertainties studies a spin dependencesimulation using the program JETSET implemented into Lepto for DIS [18]. This programwe consider the parameters of table 1. The yields presented here were obtained from acorresponds to 1 or 2 real years. For the beam intensity, target thickness and luminositieskinematical range. For yield estimates we consider a run of 6 fully efficient months. Thisfor spin asymmetries in the light meson electroproduction and A polarizations over a broadtarget and the unpolarized LEP200 beam. We aim for a statistical accuracy of —a few %In this section we present the experimental objectives of the program with a polarized jet

3 Objectives

18 OCR Output

for precise measurements at medium and large x values.be large enough for the low x region. However, a large acceptance spectrometer is requiredthese tables show that the yields for the spectrometer I with the existing UA6 magnet will

If the asymmetries are as large as a few times 10 % as expected from other processes,of a distribution in z for x ranges are given in table 5.

Clearly the z and gg- dependence will be investigated. For example, the statistical errors

• charged kaons: 8 GeV in I and 5 GeV in H

• neutral pions: 2 GeV in I and 2 GeV in H for both 7

• charged pious: 5 GeV in I and 2 GeV in H

• electrons: E' > 5G'eV,W> 4GeV,Q> 1GeVin spectrometer I2 2° 2

electron in spectrometer I and the leading hadron:The following minimum energy cuts were considered in addition to Berger cuts for the

0.130.05 0.80.30-1.00 6 I 0.0212 I 0.02

0.050.03 4s I 0.010.15-0.30 vs I 0.01

0.030.02 51 I 0.010.10-0.15 80 I 0.01

0.030.01 as I 0.010.08-0.10 49 I 0.01

30 0.010.0142 107 I 0.010.04-0.08 141 I 0.01

0.0142 90 I 0.010.020.02-0.04 53 106 I 0.01

66 I 0.030.02450.0252 vs I 0.02.006-0.02

x I [10°events] I ugh I [10°events] I o·A&II[103events] I aA_ I [103events] I a·A__yieldsyieldsyieldsyields

I(i100mrad) I I+H (i300mrctd) I (d:100m1·ad) I I+H (;h300m1·ad)

tical errors for the transverse one-spin asymmetry A0.Table 3: Estimate of semi-inclusive 1r+ and vr" production yields and expected statis

0.040.170.40.30-1.00

44 0.020.040.15-0.30

0.023613 0.030.10-0.15

0.03160.050.08-0.10

0.025225 0.020.04-0.08

0.020.02 43250.02-0.04

0.02390.02.006-0.02 27

x I [10°events] I UA, I [103events] I aA_yieldsyields

1 (xioommd) | 1+11 (iaoommd)spectrometer I and II.rors for the transverse one·spin asymmetry Ao for spectrometer I and the two—stepTable 2: Estimate of semi-inclusive 1r° production yields and expected statistical er

19 OCR Output

*The total A production will be much larger, c.f. {ig. 14 through 17.the trackers up to an angle of 500 mrad

°Tl1e or will be observed iu the RICH detector up to 300 mrad. Additional or will be detected in

estimated errors on the A polarization are given with the yield in table 6.Note that all the A considered here satisfy Berger cuts for quark fragmentation? The

larger than 0.3 GeV in spectrometer I and H

crease further the efliciency of the A detection. For the 1, we required only a pion momentummomentum. Additional tracker planes along the experiment may be necessary in order to in

In the A decay, the proton carries most of the A momentum while the 1k'- has loweris required.

trackers. A minimum momentum for the proton of 2 GeV in spectrometers I or 2 GeV in IIbefore the spectrometer magnets in order to analyze the proton and pion momenta in the

the protons from the 1r* of a K° decay. Furthermore we selected A’s which have decayedters in the two-step spectrometers I and H. The Cherenkov counters are used to distinguish

Here, we consider A with a proton detected by the tracking detectors and Cherenkov coun

vector of the A.

where oz=0.64 and gi', is a unit vector in the direction of the proton and PA is the polarization

(16)= ill + ¤Pix·17p)In the A rest frame the angular distribution of the decay protons can be written in the form:

3.2 Electroproduction of Baryons

0.5-1.0 I 4s I 0.01 I 107 I 0.01

0.3-0.5 I 72 I 0.01 I 93 I 0.01

0.2-0.3 I 69 I 0.01 I 48 I 0.01

0.1-0.2 I 94 I 0.02 I 27 I 0.02z I [10°events] I ch I [103events] I ah

yieldsyields

0.006< x < 0.1 I 0.1< x <1for the transverse one-spin asymmetry Ag for the two-step spectrometer I+II.Table 5: Estimate of semi-inclusive 1r`*' production yields and expected statistical errors

0.050.20-1.00 I 0.2 I 0.26

0.020.16-0.20 I 0.s I 0.09

0.020.10-0.15 I 2 I 0.06

0.030.00-0.10 I 2 I 0.05

0.02170.04-0.08 I 7 I 0.0z

0.03140.02-0.04 I 9 I 0.03

0.0711.006-0.02 | 9 I 0.06

x l [103events] I aA_,I [103events] I a·A_yieldsyields

I (t100mrad) I I+H (;l:300mrad)for the transverse one-spin asymmetry A0.Table 4: Estimate of semi-inclusive K+ production yields and expected statistical errors

20 OCR Output

tracking detectors before and after the plate. The azimuthal dependence of the rescatteredwith a polarimeter [20, 21]. The most efficient polarimeters consist of thick graphite plates and

The transverse polarization of a few 100 MeV up to a few GeV protons may be measuredhope to measure very precisely polarization phenomena in this case.polarized targets. This may be sensitive to the strange quark polarization in the proton. Webaryon polarization. One can measure the A polarization on longitudinally or transversallywill have a polarization correlated to that of the target nucleons and one may also expect large

With a polarized target in DIS and an unpolarized electron beam, the target fragmentsmore precisely in DIS with electrons.polarization of target fragments like the A. This polarization correlation could be studiedus that there is a polarization correlation between the struck quark polarization and thewith valence quarks of left-handed helicity only. The large observed A polarization tellscurrent interaction longitudinally "po1arizes” the struck quark, because the W bosons interactfound to be as large as about 60% in the target fragmentation [19]. In this case the charged

The polarization of A was measured in the semi-inclusive reaction 17Ne-> p+A+X andto 100 times larger than in the quark fragmentation region, as shown in figs. 14 trough 17.In the target fragmentation region, the yields for the production of baryons will be about 10

3.4 'I‘ransverse polarization and target fragments

the quark and target hagmentation regions in the semi-inclusive ep-—>eh+X reaction.asymmetries which are independent of the target polarization. This could be done in bothit would be interesting to study the contributions to the baryon polarization and to thethe A are strongly polarized, independently of the target or beam polarizations. Therefore,are dependent on the target polarization. However, in pp-» A+X, at large 2:; and large py,the baryon or to the mesonic fragments. This means that these polarizations and asymmetriesgiven by equations 10 and 11, are produced by pure spin transfers from the target nucleon to

The polarization of the leading baryons or the asymmetries in the production of mesons,uncertainties, as shown in appendix A.be performed in a model independent way, free from acceptance and luminosity systematicthe event sample. This direction is the same for all the events and the reconstruction canfrom one event to another, we propose to reconstruct the target polarization direction fromthe orientation of the initial quark polarization with respect to the e-p scattering plane differsin fig. 9 of section 2. This will be checked by the azimuthal distribution of the hadrons. SinceThe polarization orientation of the final quark, $4:, may be different by the direction shown

3.3 Polarization transfer mechanism

0.140.30-1.00 I 0.4

0.060.15-0.30 I 1.8

0.050.08-0.15 I 3.0

0.060.05-0.08 I 2.2

0.04.006-0.05 I 5.0

r I [103events] I crpz, ap, or cp,yields

errors for the A polarization measurement.Table 6: Estimate of semi-inclusive A production yields and the expected statistical

21 OCR Output

is necessary to reach the required luminosity.can reach as much as one hadronic interaction length. In th.is case, the large target thicknessstate targets with ii beams, the experiment can be very difficult, since the target thicknesstarget. Therefore the chance for multiple scattering is extremely small. For polarized solidof the beam electrons will pass through a hydrogen atom when a LEP bunch crosses the jetextremely thin and the particles have almost no chance to interact twice. Only about 1%For a semi-inclusive DIS experiment, the target has to be relatively thin. A jet target is

3.6.3 Multiple scattering

is required, which can be obtained with the setup proposed in section 5.

after the spectrometer magnet is less than 10‘2. Therefore a rejection factor of about 10'The ratio of the DIS electrons to the total photoproduction of 1r which reach the countersThe DIS electrons have to be identified in the background of a large number of hadrons.

3.6.2 Electron identification

5 seconds. Therefore, there will be almost no accidental sem.i~inclusive events.bunches. The DIS cross section in our spectrometers is 0.2 pb, corresponding to 1 event everyWith our luminosities this corresponds to about 10 events per second or 1 event per 10’0O0cross section for photo-production with photon energy larger than 0.1 GeV is about 10 pb.dominated by hadronic photo-production. At an electron beam energy of 100 GeV, the totalWe will have no problem with the duty factor in this experiment. The total rates will be

3.6.1 Duty factor and event rates

3.6 Particle Identification

polarization.the assumption of independent fragmentation and will be used to monitor the effective targetpg- dependence of target fragmentation functions. The spin asymmetry will probe more deeplysection is concemed. In general, the comparison of 7p —-> h+X and pp -+ h+X shows similarthe target fragmentation region has been extensively studied as far as the unpolarized cross

A relative to the beam momentum. The projectile independence of the inclusive reactions inphoton direction is essentially the beam direction. Therefore the asymmetry can be measuredor quark axis in the hard process center of mass system. In quasi-real photoproduction theas a function of pg-. In electroproduction the asymmetry is measured with respect to the jetwill be rather straightforward to measure the spin dependence of transverse spin asymmetriesreal photoproduction, where Q2 is close to zero. The yields for this process will be larger and itIn the totally inclusive process, ep —» h+X, the hadronic production is dominated by quasi

3.5 Inclusive hadronic production with quasi-real photons

method can probably be used only for the protons from the target fragments.low energy protons and copious numbers of protons are required. For this experiment, thisreached for polarimeter efficiencies as high as 3 to 5 %. However this method is limited toproton gives the polarization of the initial protons. Analyzing powers of 20 to 30% can be

22 OCR Output

value with no beam.

axis and over time, the nuclear polarization is expected to decrease by less than 5 % from itsa reasonable beam size and averaging over position of the atoms with respect to the bunchturns out that it is an advantage to increase the transverse holding field to about 1.5 kG. Forand therefore on the beta-function at the target, and on the strength of the holding field. Itnegligible because of the high bunch density. Depolarization depends on the size of the beam,has been studied [22]. The effects of the bunch magnetic field on the hydrogen spin are notThe influence of the passage of the LEP-bunches on the polarization of the target atoms

3.7.2 Target Depolarization

negligible.bilities one can reach at LEP with a fixed jet target, these systematic uncertainties will beusing the program Lepto. With the resolutions of the apparatus and the calibration capavariables could cause an uncertainty in the asymmetries measurement. This was investigatedhard process center of mass, as shown in fig. 9. Therefore the uncertainties of the kinematicalmenta to define the final quark polarization direction via parton model assumptions in theFor measurement of asymmetries, we rely on measurements of the electron and hadron mo

3.7.1 Momentum resolution

some sources of systematic uncertainty, which are described in the following sections, remain.true when the polarization is flipped at a fast rate as proposed in our experiment. Howeversince asymmetries between two target polarization states are considered. This is especiallyFor spin observables, the systematic uncertainties are usually lower than for cross sections,

3.7 Systematic Uncertainties

swept away by the UA6 magnet.by synchrotron radiation, have low momenta and will be either confined by the solenoid orshielding for spectrometer I. The charged particles, which are produced in the beam pipe

In the tunnel, the detectors will be shielded against radiation by the solenoid and by extraonly the normal LEP quadrupoles are present.quadrupoles. In the long straight section of LEP, where we propose to install the experiment,Synchrotron radiation is essentially produced in bending magnets, wigglers and low beta

3.6.5 Synchrotron radiation

probability for a DIS event to be contaminated with a Moller electron is very low.calorimeter of spectrometer ”I”, i.e. less than 1 electron every 240 bunches. Therefore thea small fraction of the total, less than 1%, will pass the UA6 magnet and reach the EMthe UA6 magnet. The remaining electrons have higher momenta and smaller angles. Onlythan 100 mrad. They will be confined in the inner part of the solenoid and deflected bybunches. 90 % of these electrons have momenta below 100 MeV with scattering angle largerof the angle. With our luminosity this corresponds to an average of 1 electron every 2.4these recoil electrons in our spectrometer acceptance is about 10'25 cm2 almost independentlyThe M¢ller recoil electrons will be scattered in the spectrometers. The lab cross section 5% for

3.6.4 Moller Electrons

23 OCR Output

the solenoid

95-96 the detectors andWinter | Installation of the polarized jet target,

some detectors

the UA6-magnet and

94-95 installation of the cluster jet,

Winter Civil engineering

Nov. 1993 Approval?

Time Activityfable 7:_Tentative installation program.

with a transversely polarized jet target could start in 1996 or 1997.synchrotron radiations, as well as first measurements with an unpolarized target. Data taking1995 it should be possible to perform detector and background tests, especially concerningA tentative time schedule for the installation of this experiment is summarized in table 7. In

4 Experimental Program

24 OCR Output

and for conceptual studies of large aperture RF—transitions. Measurements of atomic beamas part of the general setup in figs. 18 and 19, for the investigation of atomic beam optics

R&D work on this target has advanced during recent years, for layout studies, as shownthan 90%.

1 >< 4cm2, giving a target thickness of 1013<1t0ms/cmz with an effective polarization of moreexpect a beam flux of about 1018at0ms/s in two hyperfine states into a beam cross section ofaperture superconducting sextupoles, a slit nozzle and an improved pumping geometry. Wea few 1011 to 1012at0ms/cm2 . We intend to increase the performance further by using largeand 1017at0m.s/s in a beam diameter of about 1 cm, corresponding to a target thickness ofin many polarized ion sources. Typical performances of existing beams range between 1016We propose the use of an atomic beam source of the Stern-Gerlach type, similar to those used

5.1 Polarized Jet Target

figs. 18·20.RICH detectors, the tracking detectors and the electromagnetic calorimeters are shown in

The polarized jet target, the UA6 magnet, the large acceptance spectrometer magnet, thewill be required and the setup will be compatible with normal operations.this region and can be available for such an experiment [23]. Only minor civil engineeringin diameter and 22m long. About 30 meters of LEP are &ee &·om any machine elements in4.4m diameter of the txmnel. This section is part of UJ56, an underground building, 13.5m[P5. There is a 22m long section of the txmnel which is much larger and higher than the usualwill be especially comfortable in a part of the LEP tunnel which is close to the access shaft ofeter magnet, the detectors and the polarized jet can fit into the LEP tunnel. The installation

Installation studies have shown that the UA6 magnet, the new large acceptance spectromthe detectors, except for the magnetic field from spectrometer H and a thin window.this angular range, the particles will pass through spectrometer H without being affected by

In spectrometer H, no detectors will be installed between 0 and 100 mrad. Therefore, inadequate to analyze the momentum of hadrons at angles above 100 mrad.will cover a wide angular range and, with a longitudinal field of about 2Tm, the solenoid isthe high electron momenta. For the large angle spectrometer a solenoid will be used. Thismagnet. With a transverse field of more than 2 Tm this magnet is well suited to analyzeand an electromagnetic calorimeter. The magnet of spectrometer I will be the former UA6

The detectors consist of tracking devices, a Ring Imaging Cherenkov counter (RICH)particles in both spectrometers.momentum hadrons. The momentum distributions are shown in fig. 16 and 17 for thesespectrometer H, for the angles between 100 and 300 mrad will be used to detect the lower10 and 100 mrad for the high momentum electrons and hadrons while a second spectrometer,angle of interest. A small angle spectrometer, spectrometer I, will cover the angles between

We are considering two spectrometers to adequately cover the largest part of the solidfig. 15. with and without the criteria for quark fragmentation.A are presented in fig. 14. The longitudinal and radial position of the A decay, are shown indistributions for electrons, 1r, K and protons as well as those of the r' and protons from theangles with lower momenta of up to a few tens of GeV, depending on the angle. The angularand their momenta range between 5 and 85 GeV, while the hadrons are produced at largerIn semi-inclusive DIS at 90 GeV beam energy, the electrons are scattered at small angles

5 Experimental Setup

25 OCR Output

criteria for the quark fragmentation, shaded area, and without, solid line.semi-inclusive production of 7I'+, K+, protons, 1r' and protons of the A, with BergerFigure 14: Angular distribution, dN/d•9 for a 3 weeks run, of the DIS electron, DIS

9 mrad8 mrad

0 200 400 500 -0000 200 400 600 800 1000

250200

500400

750GOO

A decay800 1000A decay

6 mrad0 mud

100 200 300 400100 200 300 400

10001000

20002000

30003000

4-0004000

0 mrad0 mrad 100 200 soc Eno

O 20 40 60 80 100

10002500

20005000

30007500

clcctron4000

10000

x 10

26 OCR Output

area., and without, solid line.weeks run, in semi-inclusive DIS, with the criteria for the quark fragmentation, shadedFigure 15: Longitudinal and radial position of the A decay, dN/ dz and dN/ dr for a 3

r cm

0 2 4 6 8 10

500

1000

1500

2000

z cm

0 20 40 50 80 100

1000

2000

3000

27 OCR Output

line.

of the A with the criteria for the quark fragmentation, shaded area, and without, solidDIS electron, and DIS semi-inclusive production of 1r+, K+, protons, vr" and protonsFigure 16: Momentum distribution in spectrometer I, dN / dp for a 3 weeks run, of the

p Gcvjc P GeV/cQ 2 4 5 5 15 O 10 20 30 40

25

5050

75100

A decayA decay 100

EGcv/P Gcv/C O 10 20 IS? :0

0 10 20 50 40

500500

10001000

15001 500

p ccv/CP G¤V/¤O 10 20 30 40O 20 40 50 80

50020000

1000

400001500

electron60000

28 OCR Output

criteria for the quark fragmentation, shaded area, and without, solid line.DIS semi—inclusive production of vr“", K`*', protons, wr" and protons of the A with theFigure 17: Momentum distribution in spectrometer II, dN/dp for a 3 weeks run, of

P Gev/CP GeV/ c0 2 4 5 8 10O T Z 3 4

100200

200400

100 P A amy600 A d°°"

P GeV/cP Gcv/c

O 5 ,0 ,5 200 5 10 15 20

50005000

10000*0000

P Gcv/¢;O 5 10 15 Eg

5000

10000

29 OCR Output

Figure 18: Axoncmetric view of the HELP experimental Setup.

30 OCR Output

Figure 19: Side view of the HELP experiment.

Z Ld E.

31 OCR Output

magnets. The angular acceptance of the magnet will range from 10 mrad to more thancan be compensated in a similar way as in the UA6 experiment with two small dipolebeam pipe region from f B dl ~ 2 Tm down to less than 7·10"2 Tm. This weak fieldthe beam region. The original intermediate plate of the magnet reduces the field in thespecifically to be installed around an accelerator with only a very weak magnetic field inThe UA6 magnet is available now for this experiment. This magnet was designed

• Spectrometer I:

5.3 Spectrometer magnets

luminosity 10 times larger than that of the polarized target.similar to the UA6-target [25]. With a target thickness of 4 · 101°atoms/cm2, it will give aIn a first phase of the experiment, it is planned to install an unpolarized cluster jet target

5.2 Cluster Target

the target dimension in the direction of the storage ring beam.exploring different focusing systems to reach a target thickness of 3·1013at0ms/ cmz, extending

The performance expected for the beam under design is not an absolute limit. We areby less than 5%, depending on the actual beam size.been foimd that the nuclear polarization averaged over the interaction volume is decreasedLEP bunches. The depolarizing effect of a bunch crossing has been studied [22], and is hasof target atoms by the circulating beam are minimized in spite of the very high density of the11 ps, each bunch meets a ”new” target, therefore effects due to depolarization and ionizationz 1200m/s, the vertical LEP beam dimension of 40*, ~ 1.2mm and the bunch spacing ofthe single-passage atomic beam target at LEP is the fact that with the atomic velocity ofconstruction of the first superconducting sextupole is in progress. A particular feature ofcharacteristics have been performed and a new type of dissociator has been developed. The

Figure 20: The HELP setup viewed from the positron beam direction.

L-_\LA

owwou

/ - -

vm so

nm zm

WHLL

32 OCR Output

field with the same power supply. An additional power supply could be used to reach 1T, i.e. 3Tm.For this particular experiment, the solenoid could have the shape of an ellipsoid to obtain a higher

correspond to the field in the beam pipe.correcting dipoles and the jet target holding field are indicated in fig. 23. These valuesin fig. 23. The magnetic fields of the HELP Setup, the solenoids, the UA6 magnet, the

The existing machine elements around UJ56 and of the HELP magnets are represented

spacers with apertures will allow the passage of cables and fibers.solenoid will be made from modular coils with annular iron yokes. Between some coils,solenoid and is 0.62T with a 875kW standard power supply, 25OOA at 350VS. Theare from a calculation with POISSON. The field is very homogeneous over the wholelong solenoid with normal conducting coils. The field lines in cylindrical representationThis solenoid is shown in fig. 22 with its return yoke. It is a 2m inner diameter, 3m

and, with a compensating solenoid, it can be made almost transparent to the beam.With no transverse field on the axis, a solenoid can be integrated in the LEP latticeWe are proposing to use a solenoid with a longitudinal field of about f Bdl ~ 2Tm.

provide a natural shielding for the detectors.This is especially important to detect the decay products of the A. Furthermore it wouldwe prefer a solenoid design because it has a larger acceptance than the double dipole.the center would vanish and only a small quadrupole component would remain. HoweverWith two regions of opposite magnetic field transverse to the beam, the magnetic field inApart from the UA6 magnet design, a double opposite dipole magnet could be used.

• Spectrometer H:

originating from the target, independently of their scattering angle.field in this magnet is, in the first approximation, orthogonal to the particle trajectoriesabout 100mm, reduces acceptance at the small angles. As! shown in fig. 21, the magneticis put closer to the target, then the shielding plate, which has a minimum thickness of100 mrad. With this type of magnet, it is difficult to cover larger angles. If the magnet

Figure 21: A transverse section of the UA6 magnet.

SHIHDINS PLRTESi

33 OCR Output

Cherenkov ring image as shown in fig. 25. The liquid radiator will be CGFM, with n=l.2808the detector planes, less than 15%X0, and produce a. hit in addition to the hits from theare located about 200mm from the radiator. In this case the charged particles will crosscurvature radius, and a 3mm quartz window. The fast-cathode pad-photon detector planeslarge angles. The 10 mm thick liquid radiator is contained between a focusing mirror, 1.5mRICH. In our case, these requirements are essential for a RICH operating in the solenoid atIt can operate at lower momenta, in a magnetic field and is more compact than a gaseous

The second RICH for spectrometer II, RICH 2 as shown in fig. 29, has a liquid radiator.(68)GeV for 31:4 (5x7)mm2 pixel sizes.resolution is 140 (210)prad and the momentum upper limit for 30* 1r/K separations is 82Cherenkov counter, the RICH constant, k,=0y;/,82 is about 6 (8)·10‘°, the incident anglepressure, refractive index n=1.0017 with a Cherenkov radiation threshold 7,:17. In such awith little material in the electron path [29]. We consider here CSFIZ gas at atmosphericdetectors [26]. This geometry is specially designed for this kind of fixed target experimentplanes. Two thin flat and orthogonal minors will reflect the light onto the top and bottomimaging Cherenkov counter will focus the image onto the fast-cathode pad photon detectorof the RICH for spectrometer I, RICH 1, is given in fig. 28. The spherical mirror of the ringof 1r’s, K’s and protons, and provide a direction measurement. A schematic representation

These detectors will have an excellent hadron rejection factor, allowing an identificationlarger than the expected resolution of 12 mrad from an analytic calculation.angle distribution is shown in fig. 27. It shows a resolution a·;(,_.,_m__,m of 14 mrad, slightlybecause of the finite beam size. A single typical event is shown in fig. 26. The Cherenkovcan be detected, as shown in fig. 25 for 2000 events. The image here is somewhat fuzzyfocused RICH with a LiF solid radiator. Because of the optics, only part of the ring imagebeen very successfully tested at CERN on a 1r test-beam [27]. The prototype is a proximityphoton detectors [26, 28]. In summer 1993 such a RICH prototype, shown in fig. 24, hasthe particles. We propose to use recently developed RICH detectors with fast-cathode padRing imaging Cherenkov (RICH) counters will be used in the spectrometers I and H to identify

5.4.1 Ring Imaging Cherenkov

5.4 Detectors

drical representation.A Figure 22: The solenoid for spectrometer II with the homogeneous field lines in cylin

coils

iron return yoke

34 OCR Output

separator and quadrupoles.field, the compensating dipoles and solenoid magnets between the LEP electrostaticFigure 23: Preliminary layout of the HELP spectrometer magnet, the jet target holding

*1

dwégéQsE¤

$‘

37 OCR Output

Figure 29: Side and aaconometric view of RICH 2. 6 of the 8 segments are shown.

•ELf-ZCTRDNICS

P1-DTD DETECTOR

Us FmLIGUIU

WINDOWumm \ \ mmm

SPHERICFI.

Figure 28: Side and axonometric view of RICH 1.

RFl]lHT[R=C

OCR OutputOCR OutputSPHERHZFI. M!RROR

{QU P5-UTON UETEET¤RS·B_ECTRONxCS

38 OCR Output

slightly these figures. A more detailed study is under way to take all these effects into account.6Because of the limited detector size, part of the 33 photo·electrons will not be detected, degrading

preceding ones.at low multiplicities, especially if the various planes are rotated by 60° with respect to thesplit in 3 parts and fits into the solenoid. The 3-fold segmentation can remove ambiguities

shown in fig. 30. This provides the coordinates in two transverse directions. The detector iswe propose using a design similar to the "diamond°’ tracker of the CHORUS experiment asX0, and allows reaching a FWMH resolution of 32s?um. For the tracking in the solenoidlayers of 500pm diameter scintillating fibers. One plane is about 3.1mm thick, i.e. 0.7%collaboration and recently installed in their experiment [31]. A tracker plane consists of 7We propose to use scintillating fiber planes similar to those developed by the CHORUS

5.4.2 Tracker

devices. For example, threshold Cherenkov and TRD do not match all these advantages.pad structure of the detector will remove ambiguities which arise in one·dimensional trackingcation. Furthermore the RICH have the capability to cover a large angular range and thetions. This is important for the momentum analysis and the hadron rejection and identifi

The RICH detectors allow to identify the particles and to measure accurately their direcprototype now operates with 12’000 pads of 5x7 mmzA total of 6 mz detector planes represent 170’00O pads of 5x7mm2 pads. The fast RICH[30]. With a readout time of 2-3 ps per 4000 pads, this system can be used in our experiment.

A fast integrated readout system for the cathode pad photon detector has been developedincident angles have the most extreme values in our spectrometer°[29] shows that the RICH constant k, is less than 9·10'* even if the impact parameters andpoint. This minimizes the impact parameter and incident angle. An analysis of the detectorThe detector will consist of 8 identical segments, with mirrors oriented toward the targetbetween 0.7 and 1.5 mrad and the momentum upper limit for 3cr 1r/K separations is 9 GeV.and 1,:1.6. In such a Cherenkov counter, k,. is about 4·10'*, the incident angle resolution is

orthogonal to each other but tilted by 60°Figure 30: A tracker plane. In one detector segment, the two fiber directions are not

Tccmcggg ' T0 CCUCFFERR

ru cm CFPERB / \ TD cm mm

I l/me {rg gg) gcwgqn { 1 I TD CCD CQMERR

39 OCR Output

protons in addition to the momentum analysis.momentum analysis for the hadron, 0,,/p = (k,. ·p2)/(,81022). This can be used for K’s and

The particle having been identified, the Cherenkov angle measurement provides a goodrequired for a better separation of the electrons and wr.are well separated. However, at the high momenta the electromagnetic calorimeter will bek,.=6-10`6 in I, k,=4·10"° in H and the above resolution, is shown in fig. 31. The hadrons

The results of a simple simulation of the experiment with mass reconstruction, usingFor spectrometer H, a mass resolution of about 10% is good enough.than cr, / p, the momentum resolution dominates the uncertainty of the mass determination.af,/mz = ,82 -1* · kf + ag/pz where k,. is the RICH constant given above. If Byzk,. is smallerparticles can be reconstructed. The masses are given by m = p · x/B-? — 1 with a resolutionvery accurately. With the ,6 measurement provided by the RICH detectors the mass of thecellent resolution in spectrometer I will allow the determination of the electron kinematicsI and 0,,/p g 1 - 10'°·p/0 in spectrometer H, where p is in GeV/c and O in rad. The exlating fibers, the momentum resolution will be 0*,,/ p g 4 · 10"4 · p GeV'1 in spectrometerWith the angular resolution of the RICH detectors and the spatial resolution of the scintil

5.4.4 Momentum Resolution and Particle Identification

and 3500 channels.

sufiicient space resolution. The two EM calorimeters will represent a surface of about 6mzSince there are almost no 1r at larger momenta, the 4x4 cm lead glass blocks should provideminimum distance in the EM-calorimeter 1 for 20 GeV 1r° is 14 cm at 10m from the target.the 1r°, the minimum angular aperture between the two photons is 2/7. For example, theterm, were obtained recently by the Nomad collaboration [32]. For the identification ofreadout should be adequate for this experiment. Resolution of about 4% / x/E + 1% constantthe experiment. Lead-glass scintillator blocks, 25Xq in length, 4cm in width, with photodiodewith a good spatial resolution. Furthermore they will provide a simple and fast trigger forMoreover they will allow for 1r° detection and will help to remove multitrack ambiguitiesElectromagnetic calorimeters, 1 and 2, will provide an additional electron identification.

5.4.3 Calorimeter

preshower layer.and the UA6 magnet will also be prepared to install additional tracking planes as well as a

,.` with little acceptance losses and can be built at a reasonable cost per channel. The SolenoidWe propose to use scintillating fibers because they can fit into the spectrometer magnets

experiment.experiment. The relatively slow readout of the CCD camera, 20 ms, is fast enough for ourplates will be used to detect the fiber light [31]. About 30 chains will be required for the

Chains of CCD cameras with fast clear, image demagnetifiers and gated microchannelexperiment.A total of 8 planes or 600’000 fibers, i.e. 1/2 the number of fibers installed in the CHORUSin the UA6 magnet as well as 2 planes before and after RICH 1, as shown in figs. 18 and 19.

A total of 4 bi-dimensional tracking planes will be installed in the solenoid and 2 planesRICH and of the electromagnetic calorimeter, the ambiguities will be removed.trometer I and 3.6 in H. With the 3-fold segmentation of the trackers, the pad structure ofa mean total multiplicity, of charged hadrons + photons from the 1r° decay, of 3.3 in spec

In our experiment the multiplicities are rather limited. The Lund string model predicts

40 OCR Output

cone, as shown in fig. 32.known [34]. The aluminum beam pipe in the solenoid will have the shape of a trimcatedwill be carried out once the exact location of the experiment and the final field strength arethat there will be no serious interference with LEP operations. More detailed calculations

With a magnetic field of 7·10"2 Tm on the axis and these aperture dimensions it seems

ay = \/by - Ev ~ (/B, · c,/25 = 0.3mm

(19)az = (/B, · es, = 1.7mm

The beam dimensions at the jet target location will be:

could be reduced.

constraint. The horizontal ellipse axis dimension of 120mm and the vertical one of 60mmthe UA6-magnet shielding plate has an elliptical aperture for the beam pipe and fulfills thisbeam optics, B,=100m and B,=23m, i.e. I.m{”:Zb47IDII1 and y,,,;,,=;i:14mm. At presentwhere s is the location of the aperture. For example, in the middle of UJ 56, with the LEP200

153],,1](18)¤y(¤)[ml = 35 · ;t y :1; mm [/

198[rn](17)¤·(~·)[ml = 65.5 · z zh mm 4/

aperture of the beam pipe should be [33]:In order to keep a standard LEP beam pipe aperture in our experiment, the minimum free

5.5 Beam Pipe

for a better identification of the few electrons in spectrometer II at large Q2For this analysis, the information from the EM-calorimeter was neglected. It will allow

41 OCR Output

electrons.

trometers. A two-dimensional scatter plot shows mz versus momentum for 1r andFigure 31: Reconstructed masses using momentum analysis and RICH in both spec

P GeV/c302010

¢ J`·‘ r-Z·'·i\’»:»¢'

\T, V$`€·I·[ TE} `°'· '—····$ . fa. ‘····N !4"Eq”¥

‘·· 1;:.f·.’··,;y ;,t·._-w-" 'é£f electron ` `0.0 2 . . .. A { _

. ·..=··"*Z;Z· i?

0.04

mz GeV2

m GeV

0 0.25 0.5 0.75 1mécv0 025 05 075

500k P5000

100010000

1500Spectrometer H15OOg I,. I T

spcctromctul

an/dmdN/dm

42 OCR Output

construction of the experiment is in progress.

tion system, collaboration members and a tentative list of institution contributions to theA report on cost estimate of the experiment, including implementation and data acquisi

8 Cost estimate

jet target of the HELP experiment.AE/E could be better than 10*. Such a monitor could be installed in the UJ56, using thewhere 7 is the CM 7 factor, 7=(E,+m,)/ The accuracy of the energy determination,

(21)gu = 2/1

90° in the center of mass, the laboratory opening angle is minimal and is given by:of elastically scattered electrons of the beam from electrons of the hydrogen target AtThe energy of the LEP200 beam could be determined very accurately by measuring the angle

7 M¢ller Energy Monitor

been found [35]. A detailed report on the various aspects is in progress.Bayard, B. Bianchi, M. Schmitt, T. Taylor and B. Zotter. So far no incompatibility haveJ .P. Koutchouk, R. Magnin, R. Jung, G. von Holtey, M. Placidi, O. Grobner, R. Veness, O.have been studied in a preliminary way with C. Bovet (coordinator), A. Hofmann, B. Danner,The installation in the LEP tunnel and the compatibility of the HELP experiment with LEP

6 Installation and Compatibility with the machine

Figure 32: Beam pipe from the jet target to the UA6 magnet.

43

(29) OCR Outputc0s(¢») = @1* · g C05(¢;_)TH 1 #*..11

.sin(¢). The values of c0s(qS)and sin(¢)can be obtained by statistical averaging, e.g.:u H Tl

where am : fall · P(z,y, z,qq·,¢;,,¢) · dzdydzdqydefvhdqb. A similar expression holds for

-—-— cv-·=(¢)= - ·¤¤¤(¢) ·¢"(=¤.y. Z. qr.¢».. ¢) · P(¤¤. y. Z. qr. ¢>).. ¢) · dZdydZdqrd¢».d¢ (28)u 1 yftot

P(x.y.Z.<1r.4>¤..¢)=ditional factors like geometrical acceptance or apparatus efficiency for particle detections,and sin(¢). In order to take into account the experimental situation, one has to add ad

Using all as a distribution function over 45, one can calculate the mean values of cos(¢)to cx -+ a + 1r.

where T and L denote the spin orientation of the target, with the spin reversal corresponding

[1 ¥¤.(==.y. Z. ar) - G(qr. y) · ~•i¤(¢) ¥¢..(Z. y. Z. qr) · G(qr. y) ·<=¤Z(¢)]

(27)¢u(==.y.Z.1>r.¢r.. ¢) = ¤¤(Z.y.Z.qz·.¢:.)·

¢;, ·—¢,·, in a form relevant to the target spin orientation:In addition, the cross section can be written in terms of a new independent variable, ¢ =

e, = A · cos(a) (26)

e,, = A · sin(a) (25)related to the asymmetry A by:vector of the target, c.f. 9. It is useful to define new observables e, and 6,,, which are

45,. and a are laboratory azimuthal angles of the scattered electron and of the polarization

= D · P · -———·— (qr.y) (v) » Mz + ,1%Mhqr

where:

¤(Z.y. Z.pr.¢»..¢.·) = ¤¤· [1 - A(Z. y. Z. ar) · G(qr.y) · ~·i¤(¢». - ¢.· + ¤)] (23)

kinematics infers the polarization of the final quark. Thus:polarization vector of the target and of the initial quark is known. Then the DIS electronIn the framework of the parton model, the direction of ¢·‘}' is then defined since the laboratoryprocess simply rotates the polarization vector of the initial quark with some depolarization.in the hard subprocess center of mass, c.f. fig. 9. In this system QED predicts that the hardpolarization vector of the scattered quark defined with respect to the electron scattering planewhere qi, is defined in fig. 9, 46, = 95;, —¢‘}', 45;, is the azimuthal angle of hadron and 45*}, is the

_ . `_ + A(z7y$ zi · . I Pt U `,1'n(¢U)]Mhqrdv dvu

Collins parametrization for the asymmetry in the high Q2 limit [13], becomes:This spin dependent part of the polarized cross section can be factorized and, using thesection for the production of a leading hadron can have a leading order sin(q5,) dependence.In semi-inclusive DIS with a transversely polarized target, the spin dependent part of the cross

A Analysis of spin observables

44 OCR Output

mechanism shown in fig. 9.

data. This provides an interesting test of the partonic interpretation of the spin transferefficiency corrections and the target spin direction is reconstructed using the experimentalmethod for data analysis has the advantages that the results are free from acceptance andWith a similar method it is possible to obtain equation 15 [36]. In leading order, the proposed

tan(a) = EQ/E (33)

(32)X = V G?. + E

azimuthal angle of the target polarization in the laboratory frame can be calculated:of the above relations, one can get the values of EQ and EZ, and the averaged asymmetry andusing the experimental data. Thus, excluding c0s(¢)and sin(¢)from the left-hand side

u uwhole kinematical region. The angles 45; could be simply reconstructed from event to event

In these expressions EI and `éj are the values of the observables 6,, and e, averaged over thewith a similar relation for sin(¢)

u

31T 2 T €l 1 -—L- · G-cos 45- + -2- · G·c0s2¢i? **..1 -- **..1

v/H T I T T E l [-55;; · EQ Gi.szn¢ic0.s¢i + EN; · g: Gisin¢}c0s¢}] ip

lVe't lVe'1

cos(¢) = E; - g c0s(¢;)T + 5-E · E c0s(¢,)1¥Tl 1 IV"] 1 lV¢'1

Again, with statistical averaging, one gets:

(30)[1 qi e, · G · sin(¢) qi en · G · c0s(d>)]dzdydzdqTd¢hd¢

¤¤~·(¢)·(¢+¤) - P(==.y,z.qr.¢»..¢)' *1 5;-/tot[1 QZ e, · G · sin(¢) ZF c,, · G · c0s(¢)]dzdydzdqTd¢hd¢ .·:

¤¤··(¢) = ¢<>¤(¢)·¤¤(¤=.y.=.<1r.¢».¢)·P(=.y.=.qr.¢h.¢)Tl 1 E;/

On the other hand, the integration gives:where Nw, [ denotes the total number of events for the two opposite target polarization states.

45 OCR Output

20% of H1 and S1; respectively.experimental conditions we have estimated that these functions may represent only 10 tothe structure functions, H2, ...H4, Sz, ...58 vanish in the high Q2 limit [40]. For the HELP

We have performed parton model calculations similar to that of Cahn [39] and found that.»·.. Collins type asymmetry.

one has P; 2 -Pg·sin(¢;, —¢,· + oz) and in the cross section H; + P; · S1 corresponds toIn the high Q2 limit, cos¢.,· 2 1, sim9.,. 2 0. Therefore, for transversely polarized nucleon

azimuthal dependence was found by Dombey [38].sion angle in the eN laboratory frame. For exclusive hadron electroproduction, an identicalwhere a is the azimuthal angle of P as shown in fig. 9 and 0., is the virtual photon emis

P; = —PTsin0.,·cos(a ·- ¢,·) + 1}|cos0.,· (35)

P; = PTcos6,•sin¢;,c0s(a - ¢,») + P;•c0s¢;,sin(a —¢,») + 1§]sin0.,·sin¢;,Pf = Pg·c0s0.,· cos¢;,cos(a —¢,») — Pq·sin¢;,sin(a - ¢,») + P[|sin0.,·cos¢;,

laboratory frame by:on Q’,z,z,p§., and P; = (Pf‘,P{,P;) is related to the target polarization P in the eNangle of the final hadron in the 7'N frame, the structure functions H1, ...H4, S1, ...58 dependswhere e is the polarization of the virtual photon. In the above equation, ¢;, is the azimuthal

(34)(Pf S; + P;5'6) · \/e(e + 1)sin¢;, + (P{‘S7 + P; S8) · csin2¢;,]

eH4cos2¢;, + P;(S1 + 65; + \/e(e + 1)S;»,c0s¢;, + eS4c0.s2¢;,) +

........._...:—-.—-——-H+H+ +1H ¢+ .1q=.1»4¢,.4z4p;4¢, 12sw4ME=Q=p,,(1 - 6) [ ‘°’ l/€(° ) °°°" "(Pa cxzv

an explicit way. Using the method developed in ref. [37], we find:In the laboratory ·y*N frame, one can describe the final hadron azimuthal dependence in

dependent part of the cross section.12 structure functions. 4 of them corresponds to the spin independent part and 8 to the spinshow that the cross section for unpolarized electron and polarized nucleons is described byGourdin ref.[37]. Neglecting Z exchange, using Lorentz covariance and P invariance, one canThe differential cross section for polarized semi·inclusive leptoproduction was analyzed by

B Polarized Cross Section

46 OCR Output

[20] J. Antille, et al., CERN 76-05, (1976).

[19] S. Willocq et al., Z. Phys. C53(1992)207.

[ls] G. Ingelman, ISSN 0284-2769, proceedings ‘Physics at HEB.A‘, Hamburg, oct. 1991.

[17] X. Artru and M. Mekhfi, Nucl. Phys. A532(1991)351c.

[16] G. Gustafon and Jari Hakkinen, Phys. Lett. B303(1993)350.

[ls] M. Burkhardt and R.L. Jaffe, Phys. Rev. Lett. 70(1993)2537.

[14] J. Collins et al., PSU/TH/101, (1993).

[13] J. Collins, PSU/TH/102 (1992).

[12] A.V. Efremov et al., Phys. Lett. B284(1992)394.

[11] O. Nachtmann, Nucl. Phys. B127(1977)314.

and R. Minehart, SLAC report 316(1987)82.Electronuclear Physics with Internal Targets, SLAC, January, 1987, eds by R. Arnold

[10] Edmond L. Berger, ANL-HEP-PR.-87-45(1987) and Proceedings of the Workshop on

the SLAC workshop on high energy electroproduction, SLAC-392 (1992);:.80.[9] R.L. Jaffe and Xiangdong Ji, Nucl.Phys. B375(1992)527 and R.L. Jaffe, proceeding of

[8] R.L. Jaffe and Xiangdong Ji, Phys. Rev. D43(1991)724.

[7] J.L. Cortes et al., Z. Phys. C55(1992)409.

[6] X. Artru and M. Mekhfi, Z. Phys C45(1990) 669.

[5] Proposal on Spin Physics using the R.I·HC Polarized Collider, (1992).

Collins, S.F Heppelman, R.W. Robinett, AIP No.223, (1990)p. 176.[4] X. Artru, Proceeding of the Polarized Collider Workshop, University Park, Edts. J.

LEPC/M106.[3] The HELP collaboration, CERN/LEPC 92-9, LEPC/M105 and CERN/LEPC 92-14,

LEPC/M 88, and the ref. therein.Hadronic Electroproduction at LEP, HELP collaboration, CERN / LEPC 89-10,[2]

LEPC/I 9.Polarized Electroproduction at LEP, Letter of intent to the LEPC, CERN / LEPC 88-16,{1]

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Acknowledgements: We wish to express our gratitude to C.B0vet, J .P. Koutchouk, A.

47 OCR Output

[40] A. Kotzinian, to be published.

[39] R. Cahn, Phys. lett. 78B(1978)269, Phys. Rev. D40(1989)3107.

[38] N. Dombey, Rev. of Modern Phys., 41(1969)236.

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in the LEP tunnel, 23 oct., 1992.[35] C. Bovet, memorandum: preliminary study of installation of a gas jet target experiment

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