Measurements of Transverse Spin Effects in SIDIS

COMPASS

Andrea Bressan

Torino 01/04/08

Measurements of Transverse Spin Effects in SIDIS

COMPASS

04/22/23 Andrea Bressan 2

Outlook Introduction Asymmetries

Transversity DF from Single Hadron from Two Hadrons from Hyperons

Sivers DF Other TMD

Near/Far Future Conclusions

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What you will w/not find Review of all Deuteron Results

w/not 2007 proton results They are on their way… ½ PB of data processed Data quality checks ongoing Late Spring

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Spectrometer

Beam:• Luminosity 5 . 1032 cm-2 s-1

• intensity 2 108 µ+/spill (4.8s/16.8s) • momentum 160 GeV/c

• longitudinally polarised muon beam• longitudinally or transversely polarised target• momentum and calorimetry measurements• particle identification

SM1

SM2

beam

Muon Wall

MuonWall

E/HCAL E/HCAL

RICH

Polarised Target

SciFiSiliconMicromegasGEMs

StrawsSDCMWPCW45

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Targetsolid state target operated in frozen spin mode

2002-2004: 6LiD dilution factor f = 0.38

polarization PT = 50%~20% of the time transversely polarised

during data taking with transverse polarization• dipole field always • polarization reversal in the

2 cells after ~ 5 days

dN/dz

2000

zvtx (mm)00 -1000 1000

4000

two 60 cm long cellswith opposite polarisation (systematics)

superconductive Solenoid (2.5 T)

3He – 4He Dilution refrigerator (T~50mK)

Dipole (0.5 T)

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Event SelectionDIS cuts: Q2 > 1 (GeV/c)2

0.1 < y < 0.9 W > 5 GeV/c2

All hadron selection: z > 0.20 pt > 0.1 GeV/c

Plus for leading hadron: zl > 0.25 No signals in the CALOs

from neutral particles with z > zl

Statistics 2002 - 2004:8.5 * 106 positive hadrons7.0 * 106 negative hadrons

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Hadron Identification Hadron identification is based on RICH response: several studies

performed on the stability in time of the detector. Cherenkov thresholds: π ~ 2 GeV/c K ~ 9 GeV/c p ~ 17 GeV/c 2 σ π/K separation at 43 GeV/c In the leading hadron sample: ~76% pions ~12% kaons

positive negative

leading 3.4M 2.8M

leading K 0.7M 0.4M

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Kinematics – inclusive

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Kinematics - SI

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K0 Reconstruction

K0 target pointing Armenteros S/B ~ 15

Z(V0) – Z(prim. vtx.) > 10 cm● angle of target pointing < 0.01 rad● |M– MK0| < 20 MeV/c²● pt, arm > 0.025 GeV/c● pt, K0 > 0.1 GeV/c

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TRANSVERSITY

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Transversity DFTq(x) = q↑↑(x) - q↑↓(x)h1

q(x), q(x), Tq(x)

q=uv, dv, qsea

quark with spin parallel to the nucleon spin in a transversely polarised nucleon

Properties:• probes the relativistic nature of quark dynamics • no contribution from the gluons simple Q2 evolution• Positivity: Soffer bound……………..• first moments: tensor charge……….• sum rule for transverse spin

in Parton Model framework…………• it is related to GPD’s• is chiral-odd: decouples from inclusive DIS

Bakker, Leader, Trueman, PRD 70 (04)

Soffer, PRL 74 (1995)T2 | Δ q | q+ Δq

1 12 2 T q gq L L

T TΔ q dx Δ q(x)

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How Tothe Transversity DF is chiral-odd:

survives only by the product with another chiral-odd function

can be measured in SIDIS on a transversely polarised target via “quark polarimetry”

L Nl’ h X Collins Asymmetry (Collins FF)

L Nl’ X polarization (FF of q

L Nl’ hh X Two hadrons asymmetry (Interference FF)

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Single hadron asymms.

h azimuthal angle of the hadron

s azimuthal angle of the transverse spin of the initial quark

s’ azimuthal angle of the transverse spin of the fragmenting quark

C = h - s’

Collins angle

S = h - s Sivers angle

s’ = - s (spin flip)

Collins and Sivers terms in SIDIS cross sections depend on different combination of angles:

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Collins EffectCollins effect:

a quark with an upward (downward) polarization, perpendicular to the motion, prefers to emit the leading meson to the left (right) side with respect to the quark direction

i.e.the fragmentation function of a transversely polarized quark has a spin dependent part

And the resulting measured asymmetry

=h h h h 0 h hq T q T T q T h s'D (z,p ) D (z,p )+Δ D (z,p )×sin(φ -φ )

qhq

2q

qhq

0TT

2q

T

hC

Coll DqeDΔqΔe

PfAA

nnD

h s 0 hh C h C C CN N 1 A sin

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Unidentified Hadrons

• only statistical errors shown (~1%), systematic errors considerably smaller • small asymmetries compatible with 0 for both + and – hadrons

[NP B765 (2007) 31-70]

http://wwwcompass.cern.ch/compass/publications/papers/locked/2007_npb765_031.pdf

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Identified Hadrons

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04/22/23 Andrea Bressan 18F. Bradamante Genova, 27 febbraio 2008

Naïve Interpretation

unfavored Collins FF ~ – favored Collins FF at variance with unpol case u quark dominance (d quark DF ~ unconstrained)

(parton model, valence region)

• deuteron data

some (small) effect expected even if cancellation between Tu (x) and Td (x)

access to Td (x)

10T2

0T DD ΔΔ

10T2

0T DD ΔΔ

• proton data

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Two Hadronsz-axis = virtual photon directionx-z plane = lepton scattering plane

R

21

1221

zzPzPzR

R= angle between lepton scattering plane and two-hadron plane

S= azimuthal angle of initial quark versus lepton scattering plane

RS= R - S´ = R + S - π (A. Bacchetta, M. Radici, hep-ph/0407345)

S´= π - S (fragmenting quark)

(X. Artru, hep-ph/0207309)

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Azimuthal Asymmetries

( ) ( )( ) ( )∑

∑ h∠

q2h

hq

2q

q2hqT

2q2

hRS Mz,D xq e

Mz,H xq Δe=)M,z,x(A

( ) { }RSsinUT0RS

± sinΦ A±1N=ΦN RS

Target single spin asymmetry ARS(x,z,Mh2):

RSsinUT

TRS A

D P f1

=A

f: Dilution factor ≈ 0.38D: Depolarisation factor D=(1-y)/(1-y+y2/2)PT: Target polarisation ≈ 0.5

N±(RS): Number of events for target spin up (+) and down (-)

and

(X. Artru, hep-ph/0207309)

z=z1+z2

Under measurement in e+e- (BELLE)

expected to depend on the hadron pair invariant mass

h∠qq H ,Dh

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SelectionHadron selection:

• z1,2 > 0.1 (current fragmentation)• xF1,2 > 0.1• z1+z2 < 0.9 (exclusive rho)• RICH identification of π, K

DIS cuts:

• Q2 > 1 GeV2/c2

• 0.1 < y < 0.9• W > 5 GeV/c2

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Two hadron asymm.s

(Radici/Bacchetta, PRD74(2006)114007)

Expected Small

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Identified 2 Hadrons

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z-order two identified hadrons Motivations:

Hadrons with higher relative energy carry more information about the fragmenting quark polarization

For leading hadron pairs signal enhancement is predicted

PID with 2003-04 rich

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z-order two identified hadrons

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Polarimetry

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Results

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SIVERS DF

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SIVERS Mechanism The Sivers DF is probably the most famous between TMDs… gives a measure of the correlation between the transverse momentum and

the transverse spin Requires final/initial state interactions of the struck quark with the spectator

system and the interference between different helicity Fock states to survive time-reversal invariance

Time-reversal invariance implies:

…to be checked In SIDIS:

T 2 T 20 T 0 TΔ q (x,k ) Δ q (x,k )SIDIS DY

qhq

2q

qhq

T0

2q

T

hS

Siv DqeDqΔe

PfAA S

hS

0hSh sinA1NN

T0Δ q

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Unidentified hadrons

• only statistical errors shown (~1%), systematic errors considerably smaller • small asymmetries compatible with 0 for both + and – hadrons

[NP B765 (2007) 31-70]

http://wwwcompass.cern.ch/compass/publications/papers/locked/2007_npb765_031.pdf

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Identified Hadrons

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Naïve Interpretation

F. Bradamante

the measured asymmetriescompatible with zero suggest

asymmetry for + > 0, asymmetry for - ≈ 0 Sivers DF for d-quark ≈ - 2 Sivers DF for u-quark

• proton data

2

• deuteron data

preliminary 2002-2004

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Small SIVERS on D: hint for small Lg

the measured asymmetry on deuteron compatible with zero has been interpreted as

Evidence for the Absence of Gluon Orbital Angular Momentum in the Nucleon

S.J. Brodsky and S. Gardner, PLB643 (2006) 22

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Other TMD’s

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Sidis XsectionFrom A. Bacchetta et al., JHEP 0702:093,2007. e-Print: hep-ph/0611265

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Results - LO

g1T is the only parton DF which ischiral-even, T-even, leading twist function

in addition to the unpolarised DF and to the helicity DF

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Results - LO

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Transverse Target SSA for exclusive Q Motivations:

Hard exclusive meson production (HEMP) is a way, complementary to DVCS, to access GPDs

Vector mesons Transverse Target Single Spin Asymmetry AUT(h,S) connected to GPD E

E allows flip of proton helicity, while quark helicity is not flipped → overall helicity is not conserved

angular momentum conservation implies transfer of orbital angular momentum

Ji Sum Rule

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Selection Exclusive selection: •only 3 outgoing particles

•0.01 < pT2 < 0.5 [(GeV/c)2]

• missing energy -2.5 GeV< Emiss<2.5 GeV• Inv. Mass -0.3 MeV/c2 < M – M < 0.3 MeV/c2

Besides standard DIS cuts:

• Q2 > 1 GeV2/c2

• 0.1 < y < 0.9• W > 5 GeV/c2

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Results

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2007 RUN

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Target in 2007

2 3 cells

solid state target operated in frozen spin mode

2007: NH3

dilution factor f = 0.14 polarization PT = 90%

Beam line Cavity

1Cavity 2 Cavity 3

Thermal screen

Microwave power

Coupling hole

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Projected errors for full statistics (45x1012 )

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Summaryprecise deuteron data from COMPASS are now available

Collins and Sivers asymmetries h±, ±, K±

+ other 6 TMDs [sin(3h-S), sin(S), sin(2h-S), cos(h-S), cos(2h-S), cos(h)]

Two hadron asymmetries , , KKTransverse Lamba polarization

all the measured deuteron asymmetries are very small, and compatible with zero

COMPASS data on deuteron allows to determine the d-quark contribution

present phenomenological studies can describe at the same time the BELLE (FF), the HERMES (proton) and COMPASS (deuteron)…and have allowed a first extraction of the transversity distribution Tq

At the same way COMPASS and HERMES data have allowed a first extraction of the Sivers DF

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Coming Soon Deuteron

Cahn and Boer-Mulders asymmetries unpol g2

Proton Collins + Sivers… Little further in time… all the rest

Future programs Drell-Yan [see … M. Chiosso talk] Precision measurement … for better flavour separation and

measurements of first moments…

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THAT’S ALL…(FOR NOW!)

Measurements of Transverse Spin Effects in SIDIS

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