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Harut AvakianHarut AvakianJefferson LabJefferson Lab

Hard scattering studies at JLabHard scattering studies at JLab

XI Workshop on High Energy Spin Physics , Dubna, September 2005

* Talk presented by A.Kotzinian

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Outline

Physics motivationSIDIS studies at 6 GeVHard exclusive processesFuture plansSummary

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Physics Motivation

Parton Distribution Functions generalized to contain information not

only on longitudinal, but also on the transverse distribution of partons:

Generalized Parton Distributions (GPD) H, E ...Transverse-momentum dependent (TMD) parton distributions

Orbital Angular Momentum (OAM) in the focus.

Complementary sets of non-perturbative functions sensitive to different aspects of transverse distributions

½ = ½ (u+d+s) + Lq + Jg Proton’s spin

~20-30%

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PDFs fpu(x,kT), g1, h1 FFs F1p

u(t),F2pu(t)..

d2k

T

=0,

t=0 dx

Wpu(x,k,r) “Parent” Wigner distributions

d3 r

d 2kT(FT) GPD

Measure momentum transfer to quark

Measure momentum transfer to target

Probability to find a quark u in a nucleon P with a certain polarization in a position r and momentum k

kT-integrated PDFs same in exclusive and semi-inclusive analysis

Analysis of SIDIS and DVMP are complementary

TMD

TMD PDFs fpu(x,kT),

GPDs Hpu(x,,t)..

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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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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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Semi-Inclusive Pion Electroproduction with a Polarized Semi-Inclusive Pion Electroproduction with a Polarized Beam and Longitudinally Polarized Target at 6 GeVBeam and Longitudinally Polarized Target at 6 GeV

and the CLAS collaboration

JLab

exp

erim

ent

P-0

5-11

3

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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)()(

)()(

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11

1

LUND-MC

CLAS PRELIMINARY

A1

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

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

1) SIDIS0 production is not contaminated by diffractive2) 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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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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• Significant SSA measured for pions with longitudinally polarized target• Complete azimuthal coverage crucial for separation of sinsin2moments

Target 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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SSA: x-dependence

HT SSA from Collins mechanism AUL ( 0) ~ H1favore+H1

unfavored

PRELIMINARY 5.7 GeV

With H1┴ (0)≈0 (or measured) target and beam HT SSAs can be a

valuable source of info on HT T-odd distribution functions f┴ ,g┴L

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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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DVCSDVCS DVMPDVMP

Hard Exclusive Processes and GPDs

hard vertices

hard gluon

DVCS – for different polarizations of beam and target provide access to different combinations of GPDs H, H, E

long. only

DVMP for different mesons is sensitive to flavor contributions ( select H, E, for u/d flavors, , K select H, E)

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Deeply Virtual Compton Scattering ep->e’p’

•GPD combinations accessible as azimuthal moments of the total cross section.

DVCS

BH

LU~ sinIm{F1H + (F1+F2)H +kF2E}~

Polarized beam, unpolarized target:

Unpolarized beam, longitudinal target:

UL~ sinIm{F1H+(F1+F2)(H +.. }~

= xB/(2-xB ),k = t/4M2

Kinematically suppressed

Kinematically suppressed

d4dQ2dxBdtd ~ |TDVCS + TBH|2DVCS BH

TBH : given by elastic form factorsTDVCS: determined by GPDs

GPD

Page 18

Target Spin Asymmetry: t- Dependence

Higher efficiency photon detection will also improve significantly DVCS target SSA measurements.

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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

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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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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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Collins Effect and Kotzinian-Mulders Asymmetry

Study the Collins fragmentation with longitudinally polarized target.Measure the twist-2 Mulders TMD (real part of interference of L=0 and L=1 wave functions)

UL ~KM

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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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Sivers effect in the target fragmentationA.Kotzinian

High statistics of CLAS12 will allow studies of Q2 dependence of the Sivers effect in target fragmentation region

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polarization in the target fragmentation

xF - momentum

in the CM frame

Wide kinematical coverage of CLAS12 allows studies of hadronization in the target fragmentation region

p

e

Λ1 2

e’

polarization in TFR provides information on contribution of

strange sea to proton spin

J.Ellis, D.Kharzeev, A. Kotzinian ‘96 W.Melnitchouk and A.W.Thomas ‘96

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CLAS12 - DVCS/BH Beam Asymmetry

L = 1x1035

T = 2000 hrs

Sensitive to GPD H

LU~sinIm{F1H+.}d

e p ep

E=4.3 GeV

ALU

S. Stepanyan et al. Phys. Rev. Lett. 87 (2001)

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e p ep

CLAS12 - DVCS/BH Target Asymmetry

<Q2> = 2.0GeV2

<x> = 0.2<-t> = 0.25GeV2

CLAS preliminary

E=5.75 GeV

AUL

Longitudinally polarized target

~sinIm{F1H+(F1+F2)H...}d~

E = 11 GeVL = 2x1035 cm-2s-1

T = 1000 hrsQ2 = 1GeV2

x = 0.05

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GPDs H from expected DVCS ALU data

bval=bsea=1

MRST02 NNLOdistribution

Q2=3.5 GeV2

Other kinematics measured concurrently

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Exclusive production on transverse target

2 (Im(AB*))/ T

22)22t/4m2) - Re)22UT

A ~ 2Hu + Hd

B ~ 2Eu + Ed0

K. Goeke, M.V. Polyakov, M. Vanderhaeghen, 2001

Q2=5 GeV2

Eu, Ed needed forangular momentum sum rule. 0

A ~ Hu - Hd

B ~ Eu - Ed+

Asymmetry is a more appropriate observable for GPD studies at 12 GeV as possible corrections to the cross section are expected to cancel

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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 3D distribution functions (TMDs,GPDs)

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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• 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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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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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