The EMC effect – history and futureK. Rith,

LNF Frascati

26.5.2008

Quark- and gluon-distributions are different for free nucleons and for bound nucleons inside nuclei

Specifically:

Open question: Do quarks and gluons play any role for the understanding of nuclear forces?

Can at least the short-range part be directly described by the exchange of quarks, gluons or multigluon states? (Analogue: Van der Waals force)Can the model of nuclear forces mediated by meson exchange currents be replaced by a fundamental theory based on the strong interaction between quarks and gluons?Is confinement influenced by the nuclear medium?Do nucleons swell due to the neighbourhood of other nucleons?Do they form multiquark clusters or even one big bag?

Deep-inelastic Lepton-Nucleon-Scattering

Q2 = -(k-k‘)2 = 2EE‘(1-cos) = E - E‘, y = /Ex = Q2/(2M) = fraction of nucleon‘s momentum P, carried by struck quark

From angular and momentum distribution of scattered leptons

P

hadrons

nucleonk= (E, k)

k‘= (E‘,k‘)

* xP1/ Q2

Internal structure of the nucleon Structure functions F1(x,Q2), F2(x,Q2), g(x,Q2)

d21/dxdQ2 = 42/Q4 F2(x,Q2)/x [1 –y –Q2/4E2 + (1 -2m2/Q2)(y2 + Q2/E2)/(2[1 + R(x,Q2)])]

F2(x,Q2) = x zq2 [ q(x,Q2) +

q(x,Q2) ] q = u,d,s,..

R(x,Q2) = [ F2(x,Q2) ( 1 + Q2/2 ) – 2xF1(x,Q2) ] / 2xF1(x,Q2)If RA1(x,Q2) = RA2(x,Q2) :(d21/dxdQ2)A1 / (d21/dxdQ2)A2 = F2

A1(x,Q2)/ F2A2(x,Q2)

End of the 1970‘s:Second generation of DIS experiments: CDHS, CHARM, CCFRR, CHIO, EMC, BCDMS majority used nuclear targets (Fe, CaCO3, C,.. ) Main aim: study scale breaking of structure functions predicted by QCD, determine QCD, gluon distribution g(x,Q2) via Altarelli-Parisi equations

Underlying assumption: Quark and gluon distributions obtained from nuclear targets are identical to those from free nucleons

Assumption: Nucleons do not change their internal properties (mass, radius, spin…) when being embedded in nuclei

Apart from Fermi-motion

qN(x) is convolution ofquark momentum distribution in free nucleon andnucleon momentum distribution in nucleus

Bodek, RitchieBerlad et al.

…………….. Frankfurt, Strikman

The EMC experiment at CERN

H2, D2

Fe calorimeter target

Fit to Fe-dataExpectation for D-data including Fermi-motion

EMC data for F2N(Fe) and

F2N(D)

The EMC effectJ.J. Aubert et al., Phys. Lett. 123B (1983) 275

A lot of excitement: up to now 814 citations

statistical errors

Published: March 31, 1983 25th anniversary

Consequence: Quark (and gluon) distributions are modified by the nuclear environmentBig surprise for high-energy physicists, but in principle expected by nuclear physicists and possible effects discussed in the 70th at several conferences about ‚Quarks in nuclei‘First review:

Proceedings of the 18th Rencontre de Moriond, March 13-19, 1983, pp. 207-222

!

Data from SLAC - 1

H

D

ee‘

N1 = NWalls + NH,D

N2 = NWalls

NH,D = N1-N2

H,D

1970-72

Archeology 1983

Fe,Al

empty

Data from SLAC-1, archeology

A. Bodek et al., PRL 50 (1983) 1431; PRL 51 (1983) 543

Data from SLAC-2, dedicated experimentR.G. Arnold et al., PRL 52 (1984) 727 ; J. Gomez et al., PRD 49 (1994)

4348

?

Data from SLAC-2, A-dependence

Data from neutrino experiments

EMC Spectrometer – phase 3

Problem with old H and D data at low x due to correlated inefficiencies of drift chambers W4/5, cured by additional proportional chambers P4/5

Data from EMC – phase 3

No enhancement at very low x,Some enhancement at 0.1 < x < 0.3

J. Ashman et al., PL B202 (1988) 603

Shadowing data from EMC – phase 3 M. Arneodo et al., PL B211 (1988)

493

Large-x behaviour Multiquarkclusters – Short Range

Correlations?

Origin: superfast nucleons and/or superfast quarks

SLAC

Large-x behaviour Multiquarkclusters – Short Range

Correlations?

To be studied in detail at JLAB12 – Hall C (E12-06-105)

CLAS, K.S. Egiyan et al., P.R.L. 96 (2006)082501

Overall picture of nuclear effects

Interpretation

Several hundred publications with different approachesNo unique model for the whole x-rangeComplications: ‚Any configuration of quarks, antiquarks and gluons coupled to overall color-singlet can be expanded in a basis of mesons, baryons and antibaryons‘ ‚Nobody knows how to boost the wavefunction of a bound system into the infinite momentum frame‘

Reviews: e.g.: M. Arneodo, Phys. Rep. 240 (1994) 301 D.F. Geesaman et al., Ann. Rev. Nucl. Part. Sci. 45 (1995) 337

Some approches

Convolution

F2A(x,Q2) = dy fc

A(y) F2c(x/y, Q2)

c = ‚cluster‘: N, , , 6q, ………fc

A(y): probability of finding ‚cluster‘ of momentum y in nucleus AF2

c(x/y, Q2): quark distribution in c

c x

A

Badly known, a lot of freedom

Change of confinement scale, swelling of nucleons, i.e., Q2 rescalingIdea: relevant quantity is (QR)Data should be identical for (QDRD)2 = (QARA)2

F2

Q2

small x

large x

Require increase of about 15%But: from quasielastic scattering (y-scaling): radius increase is at most ~3% (Sick et al.)

nucleus

Change of nucleon mass, x-rescaling

A

N pi = (M + Ei, pi )

xi‘ = Q2/2piq = Q2/[2(M+Ei) - 2 pi q]x‘ x / (1 + <Ei>/M) > x, <Ei> - 25 MeVContains both ‚binding correction‘ and ‚Fermi-motion‘

Ei = removal energy

Conventional nuclear physics with improved nucleon wavefuctions, removal energies and correlated many body approach (applicable for 0.3 < x < 0.9 ?)

Reasonable agreement for 0.3 < x < 0.7 room for additional contributions

C. Ciofi degli Atti and S. Liuti, PL B225 (1988) 215

Example:

2

Shadowing at high Q2Generalized vector-meson dominance

model in lab frame (property of photon)

mean free path: L = 1/( VN) 2.5 fm

fluctuation length:d = 2/(mv2 +

Q2) = 15 GeV d 10 fm

L

d

Absorption on surface

A/AN ~ A-1/3

d 1/Mx 1/(1 + mv

2/Q2)Effect dies out for x ~ 0.1

d >> L

Parton-parton fusion: ‚overcrowding‘ of low-x partons in infinite momentum frame (property of nucleus) d‘ d M/p

Lorentz contracted nucleon

DA‘

z

D ~ 1/Q2: transv. resolutionz ~ 1/xp: longt. size of gluonz > d‘, i.e., x < 1/Md 0.1

Low x gluons (and seaquarks) of different nucleons overlap and interact

Modified gluon and quark distributions

D

The NMC experiment at CERN

Main aims:Precision measurement of F2

p, F2D, F2

n/F2p, F2

p-F2n

Precision measurement of F2A1/F2

A2 (x,Q2) and (RA1-RA2)(x,Q2) for several nuclei; dependence on nuclear density and radius

Collected statistics: ~2 108 DIS events

Helium-4 4.00Lithium-6 6.05

Relevant publications from NMC:

P. Amaudruz et al., Z. Phys. C 51 (1991) 387 Z. Phys. C 53 (1992) 73 Phys. Lett. B 294 (1992) 120 Nucl. Phys. B 371 (1992) 553M. Arneodo et al., Phys. Lett. B 332 (1994) 3 Nucl. Phys. B 441 (1995) 3 Nucl. Phys. B 441 (1995) 12 Nucl. Phys. B. 481 (1996) 3-22 Nucl. Phys. B 481 (1996) 23-39

Complementary target setup: Minimize systematic errors due to incident flux I and acceptance A

I1

I2

A1

A2H

HD

D

I1

I2

I3

A1

A2 A3 A4 A5 A6(H)/(D) = (N11 N22)/(N12 N21)

/ = ………………..

6

NMC – Example of target arrangement

E665: M.R. Adams et al., Phys. Rev. Lett. 68 (1992) 3266; Z. Phys. C 67 (1995), 403

Detailed study of shadowing region

Dependence on nuclear mass A and density

Dependence on nuclear radius A1/3

a + b A-1/3

a + b A-1/3 + c A-2/3

Dependence on A1/3 or ?Ultimate experiment: Polarised 67Ho98 (J = 7/2)

4He(=0.089)/3He(=0.051)

4/3 =1.75JLAB-proposal E-03-103,…

But:

precise knowledge of F2

n/F2p at large x required

R4/R3 (4/3)1/3 =1.10 ??

Q2-dependence

Q2-dependence

F2A1/F2

A2 = a + b ln Q2

Sn/C

Gluon ‚overcrowding‘ in infinite momentum frame (property of nucleus)

d‘ d M/pLorentz contracted nucleon

DA‘

z

D ~ 1/Q2: transv. resolutionz ~ 1/xp: longt. size of gluonz > d‘, i.e., x < 1/Md 0.1

Low x gluons (and seaquarks) of different nucleons overlap and interact

Modified gluon and quark distributions

c

cc

J/e+, +

e-, - pt

g

*

(Hard scale: mass of c-quark)

Modification of gluon distributionQCD: If quark distributions are modified by the nuclear environment, then also the gluon distribution must change

Is enhancement at 0.1 < x < 0.3 due to ‚merged‘ gluons?Experimental tool: Inelastic J/-production

P. Amaudruz et al., Nucl. Phys. B 371 (1992) 553

Inelastic J/ production: GSN(x)/GC(x) = 1.13 0.08

Modification of gluon distribution

Modification of gluon distribution

f1(x) = F2Sn(x)/F2

C(x); r(x) = GSn(x)/GC(x) from Q2-dependence

T. Gousset, H.J. Pirner, PLB 375 (1996) 349

xtarget xbeam

proton

proton

}X

}X

-

+

Additional information from Drell-Yan

Additional information from Drell-Yan (E772)

Selection: x1 – x2 > 0.3 Ratio ~ qA1 / qA2

No indication of enhancement of sea-quarks , Valence-only effect?

Additional information from Drell-Yan

Very precise data expected from FNAL Main Injector DY

Additional information from neutrinos - MINERA

Linear combinations of () and ():Separate valence (xF3) and sea (q)

Additional information from neutrinos - MINERA

Also H,D

Overall picture of nuclear effects

Outlook - Polarized EMC effectI.C. Cloet, W. Bentz, A.W. Thomas, PLB 642 (2006) 210

Detailed study of shadowing region - 2

Dependence on nuclear radius A1/3 and scaling parameter n(x,Q2,A)

n number of gluons probed by hadronic fluctuations of photon B. Kopeliovich and B. Povh, PL B367 (1996) 329

a + b A-1/3

a + b A-1/3 + c A-2/3

Nuclear dependence of R = L/T

Dependence of R on A could indicate nuclear effects on g(x) or different higher twist contributions to RA1 and RA2

(d21/dxdQ2)A1 / (d21/dxdQ2)A2 = F2A1(x,Q2)/ F2

A2(x,Q2)requires RA1(x,Q2) = RA2(x,Q2)

Method: use different beam energies Ei

A1/A2(Ei) = (F2A1/F2

A2)[(1+RA2)(1+RA1)] [(1+ziRA1)(1+ziRA2)] (F2

A1/F2A2){1 – R (1-zi)/[(1+R)(1+ziR)]}

withR = RA1 - RA2, R = ½(RA1 + RA2)zi = [1 + ½ (yi

2 + Q2/Ei2)/(1 – yi – Q2/4Ei

2)] -1

Nuclear dependence of R = L/T

3 beam energies Ei: 120 GeV, 200 GeV, 280 GeV

R = RSn – RC = 0.040 0.021 (stat.) 0.026 (syst.)