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Harut AvakianHarut AvakianJefferson LabJefferson Lab

Studies of transverse spin effects at JLabStudies of transverse spin effects at JLab

Transversity 2005, Como, September 7-10, 2005

* In collaboration with P.Bosted, V.Burkert and L.Elouadrhiri

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Outline

Physics motivationSIDIS studies at 6 GeV

Factorization testsSpin azimuthal asymmetries

Future plansSummary

3

h

Single pion production in hard scattering

Target fragmentation Current fragmentation

Fracture FunctionsxF

M

0-1 1

h

h

PDF GPD

kT-dependent PDFs Generalized PDFs

Wide kinematic coverage of large acceptance detectors allows studies of hadronization both in the target and current fragmentation regions

xF - momentum

in the CM frame

xF>0 (current fragmentation)

PDF

h

xF<0 (target fragmentation)

h

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Polarized Semi-Inclusive DISCross section is a function of scale variables x,y,z

Parton-Hadron transition: by

fragmentation function D+( (z):

probability for a u-quark to produce a +(-) with momentum

fraction z

Hadron-Parton transition: by distribution function f1

u(x): probability to find a u-quark with

a momentum fraction x

1u

= E-E’y = /Ex = Q2 /2M z = Eh /

z

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Transverse momentum of quarks

To study orbital motion of quarks in semi-inclusive DIS measurements in a wide range of x,z,PT, are required.

•kT – led to introduction of kT dependent PDFs (TMDs)•kT – crucial for orbital momentum and spin structure studies

–led to SSA in hard scattering processes•kT - important for cross section description

- PT distributions of hadrons in DIS- exclusive photon production (DVCS)- hard exclusive vector meson cross section- pp → 0X (E704,RHIC) cross sections

Spin-Azimuthal Asymmetries: sensitive to kT

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SIDIS (*p→X) cross section at leading twist (Ji et al.)

structure functions = pdf × fragm × hard × soft (all universal)

eUnpolarized target

Longitudinally pol. target

Transversely pol. target e

e

p

p

Off diagonal PDFs related to interference between L=0 and L=1 light-cone wave functions.

Boer-Mulders1998

Kotzinian-Mulders1996

Collins-1993

To observe the transverse polarization of quarks in SIDIS spin dependent fragmentation is required!

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Collins Effect: azimuthal modulation of the fragmentation function

D(z,PT)=D1(z,PT)+H1┴(z,PT) sin(hS’)

spin of quark flips wrt y-axisS’ = -S sin(hS)

C

S

STy

x

h

PT sT

S’

C

FUT∞h1H1┴

S’ = -S = -h

S

y

x

h

PTsT

S’

C

sT(p×kT)↔ h1┴

FUU∞h1 ┴ H1┴

S = +h

sT(q×PT)↔ H1┴

S’ = -S = -h

xsin(2h)

sTPT

h

C

S=h

y

FUL∞h1L H1┴┴

(sTkT)(pSL)↔ h1L┴

sinC=sin(hS’)

cos(2h)

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Schafer-Teryaev sum rule

Precise measurement of Collins asymmetries for different hadrons (0 ,+ +-) would allow to test the Schafer-Teryaev sum rule

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High efficiency reconstruction of 0 + opens a new avenue in SIDIS (HMP)

1) SIDIS0 production is not contaminated by diffractive 0 SSA sensitive to the unfavored polarized fragmentation

3) HT effects and exclusive 0 suppressed

4) Simple PID by 0-mass (no kaon contamination)

5) Provides information complementary to +/- information on PDFs

SIDIS with neutral pions

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HT and Semi-Exclusive Pion Production

E. Berger, S. Brodsky 1979 (DY), E.Berger 1980,A.Brandenburg, V. Khoze, D. Muller 1995

A.Afanasev, C.Carlson, C. Wahlquist Phys.Lett.B398:393-399,1997

+

Fragmentation +

0

HT effects and exclusive 0 suppressed

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Experimental Setup (CLAS+IC)

Inner Calorimeter (424 PbWO4 crystals) for the detection of high energy photons at forward lab angles (increases 0 acceptance ~3 times at z~0.5).

Polarized target

13o

50o

solid NH3 polarized targetproton polarization >75%high lumi ~ 1.51034 s-1cm-2

IC

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Factorization studies with pions

•Double spin asymmetries consistent with simple partonic picture •A1

p inclusive and (~30 times more data expected) an serve as an important check of HT effects and applicability of the simple partonic description.

q

qqq

qq

zDxf

zDxg

A)()(

)()(

11

11

1

LUND-MC

CLAS PRELIMINARY

A1

60 days of CLAS+IC (L=1.5.1034cm-2s-1)

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Unpolarized Semi-inclusive electroproduction of + measured.Complete 5-dimensional cross sections were extracted. Direct separation of different structure functions.

x=0.28-0.32z=0.16-0.19

pT=0.41-0.53 GeV

Q2=2.23-2.66 GeV2

Prelim

inary

Prelim

inary

CLAS

Azimuthal asymmetries at CLAS

M.Osipenko

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• Significant SSA measured for pions with longitudinally polarized target• Complete azimuthal coverage crucial for separation of sinsin2moments

SSA measurements at CLAS

p1sin+p2sin2

0.12<x<0.48

Q2>1.1 GeV2

PT<1 GeV

ep→e’XW2>4 GeV2

0.4<z<0.7MX>1.4 GeV

y<0.85

CLAS PRELIMINARY

p1= 0.059±0.010p2=-0.041±0.010

p1=-0.042±0.015p2=-0.052±0.016

p1=0.082±0.018p2=0.012±0.019

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Flavor decomposition of T-odd f┴

)z(D)x(fe/yy qqq

q,qUU 11

22 21

)()(1 12

,

sin zDxxfeyyQ

MS qq

Lqqq

LUL

4/4)( ,,154 udAduAxxf nULpUL

uL

4/4)( ,,154 duAudAxxf pULnUL

dL

With SSA measurements for and on neutron and proton

() assuming Hfav=Hu→+ ≈ -Hu→-=-Hunfav

)()(1 12

,

sin zHxxheyyQ

MS qq

Lqqq

LUL

In jet SIDIS with massless quarks contributions from H1┴ vanish

gauge link contribution

L

With H1┴ (0)≈0 (or measured) target and beam HT SSAs can be a valuable source of info on HT T-odd distribution functions

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Longitudinally polarized target SSA using CLAS+IC

•Provide measurement of SSA for all 3 pions, extract the Mulders TMD and study Collins fragmentation with longitudinally polarized target•Allows also measurements of 2-pion asymmetries

Hunf=-1.2Hfav

Hunf=-5Hfav

Hunf=0

curves, QSM from Efremov et al

q

qqq

qqL

ULUL zDxf

zHxh

DA)()(

)()(

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112sin 60 days of CLAS+IC

(L=1.5.1034cm-2s-1)

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CLAS12 High luminosity polarized

(~80%) CW beam

Wide physics acceptance(exclusive, semi-inclusive current

and target fragmentation)

Wide geometric acceptance

12GeV significantly increase the kinematic acceptance (x10 lumi)

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Collins Effect

UT ~Collins

Study the Collins fragmentation for all 3 pions with a transversely polarized target and measure the transversity distribution function. JLAB12 cover the valence region.

•SSA in fragmentation•Subleading SSA has opposite sign •No effect in TFR

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Sivers effect

UT ~Sivers

•Asymmetry in distribution•Subleading SSA has same sign•Opposite sign effect in TFR

Measure the Sivers effect for all 3 pions with a transversely polarized target in a wide kinematic range (TFR & CFR).

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Sivers function extraction from AUT (0) does not require information on fragmentation function. It is free of HT and diffractive contributions.

F1T=∑qeq2f1T

┴q

AUT (0) on proton and neutron will allow flavor decomposition w/o info on FF.

In large Nc limit:

f1Tu = -f1T

d

Efremov et al(large xB behavior of

f1T from GPD E)

CLAS12projected

CLAS12projected

CLAS12: Sivers effect projections

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Summary

Current data are consistent with a partonic picture, and can be described by a variety of theoretical models.

Significantly higher statistics of JLab, in a wide kinematical range will provide a full set of data needed to constrain relevant distribution (transversity,Sivers,Collins,…) functions.

Experimental investigation of properties of 3D PDFs at JLab, complementary to planed studies at HERMES, COMPASS, RHIC, BELLE, GSI, would serve as an important check of our understanding of nucleon structure in terms of quark and gluon properties.

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support slides…

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Higher Twist SSAs

Target sin SSA (Bacchetta et al. 0405154)

Beam sin SSA

In jet SIDIS only contributions ~ D1 survive

Discussed as main sources of SSA due to the Collins fragmentation

With H1┴ (0)≈0 (or measured) Target and Beam SSA can be a valuable source of info on HT T-odd distribution functions

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SIDIS: factorization studies

JLab data at 6GeV are consistent with factorization and partonic description for variety of ratio observables

P.Bosted

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Collinear Fragmentation

quarkThe only fragmentation function at leading twist for pions in eN→e’X is D1(z)

Ee =5.7 GeV

No significant variation observed in z distributions of + for different x ranges (0.4<z<0.7, MX>1.5) and for A1p as a function of PT

q

qqq

qq

LLTLL zDxf

zDxg

DPA)()(

)()(

11

11

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CLAS12: Transversity projections

AUT ~Collins

Simultaneous measurement of, exclusive with a transversely polarized target

10-3

27•Study the Collins fragmentation mechanism with long. polarized target• For - and 0 SSA is sensitive to unfavored fragmentation

)()( 12

,

sin zHxxheQ

MS q

Lqqq

LUL

SSA: x-dependence

HT–SSA significant for + and 0 (non-Collins?) AUL ( 0) ~ H1

favore+H1unfavored

PRELIMINARY 5.7 GeV

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• Indicate a negative sin2 moment measured for +. • Some indication of negative SSA (more data required for - and 0)• More data required to correct for exclusive 2 contribution.

SSA: kinematical dependence

q

qqq

qqL

ULUL zDxf

zHxh

DA)()(

)()(

11

112sin

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Non-perturbative TMD Perturbative region

PT-dependence of beam SSA

sinLU(UL) ~FLU(UL)~ 1/Q (Twist-3)

In the perturbative limit 1/PT

behavior expected (F.Yuan SIR-2005)

Asymmetries from kT-odd and kT-even (g1) distribution functions are expected to have a very different behavior (flat A1

p(PT) observed at 5.7 GeV).

2.0

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Exclusive production background from PYTHIA

Pions from string (direct) present the lower limit for current fragmentation events

Filled (open) symbols represent pions from exclusive (all) vector mesons.

electron

0 sample “clean” at large z (non-string pions are mainly from semi-inclusive +, )

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For Collins fragmentation use chirally invariant Manohar-Georgi model (Bacchetta et al)

Systematic error only from unknown ratio of favored and unfavored Collins functions (R= H1

d→/H1u→), band

correspond to -2.5<R<0

- and 0 SSA will also give access to h1Ld

CLAS-5.7GeV

First glimpse of Twist-2 TMD h1L┴

PRELIMINARY

•More data required with - & 0 •Exclusive 2 pion background may be important

Distribution functions fromQSM from Efremov et al

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CLAS+Inner Calorimeter (IC)

CLAS

CLAS+IC

Reconstruction efficiency of high energy 0 with IC increases ~ 3 times at large z due to small angle coverage (target in ~60cm from IC)

IC E/E=0.0034/E+0.038/√E+0.022

0

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TransversitySimple string fragmentation

(Artru model)

Sub-leading pion opposite to leading

(into page)

L=1

production may produce an opposite sign AUT

Leading opposite to leading (into page)

Understanding of 2 pion asymmetries will help to understand transversity measurements

+

0

SIDIS @11 GeV: 2 pions

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SIDIS: factorization studies

• A1 inclusive, from sum and are consistent (in range 0.4<z<0.7 )

GRVSHERMES

•There is an indication that A1p of +is lower than inclusive at large z.

•More data required for 2 pion () final state studies

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GEANT simulation

Angular acceptance for charged tracks for eg1+IC configuration (polarized target at -67 cm from IC)

Maximum angle ~50o

Minimum angle ~14o

13o

50o

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exclusive production backgroundPions from string present the lower limit for current fragmentation events

Fraction of pions from non-diffractive vector mesons adds up to SIDIS sample

Fraction of pions from exclusive rho-0(black squares) should have a special treatment

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exclusive production background

Fraction of charged pions from rho-0 especially high for neutron target

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production background from exclusive events

Non string pions are mainly from semi-inclusive rho+

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SSA: PT-dependence of sin moment

sinLU(UL) ~FLU(UL)~ 1/Q (Twist-3)

ALU CLAS @4.3 GeV

Beam and target SSA for + are consistent with increase with PT

In the perturbative limit is expected to behave as 1/PT

AUL (CLAS @5.7 GeV) AUT HERMES @27.5 GeV

PRELIMINARY

TMD pQCD