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Transversely polarized target for Transversely polarized target for CLAS and CLAS12CLAS and CLAS12

Introduction Structure of nucleon and 3D parton distributions Semi-Inclusive processes and TMD distributions Hard exclusive processes and GPDs Projections Summary

Harut AvakianHarut Avakian

CLAS Collaboration Meeting March 1

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Describe the complex nucleon structure in terms of partonic degrees of freedom of QCD

Physics Motivation

QCD evolution tells us how parton distributions evolve, but not original distributions

RHIC Spin & SIDIS

Understanding of the orbital motion of quarks and spin-orbit correlations is crucial!!!

EMC at CERN (85):

J q

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PDFs fpu(x),…

Form Factors F1p

u(t),F2pu(t )..

TMD PDFs fpu(x,kT), d 2k

T

dx (F

T)

GPDs/IPDs Wpu(x,rT),…

d2 r

Wpu(k,rT) “Mother” Wigner distributions

d2 r

d 2kT

Quantum Phase-Space Distributions of Quarks

Measure momentum transfer to targetDirect info about spatial distributionsNo information about underlying dynamics

Measure momentum transfer to quarkDirect info about momentum distributionsNo information about spatial location of partons

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

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Hard Processes

Partonic scattering amplitude

Fragmentation amplitude

Distribution amplitude

proton

SIDIS/DER

lepton lepton

pion

electron

positron

pion

pion

e–e+ to pions

proton

proton lepton

antilepton

Drell-Yan

BNLJPARC

FNAL

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h

Single particle production in hard scattering

Target fragmentationCurrent 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)

xF<0 (target fragmentation)

h

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Semi-Inclusive DIS

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 qf=f1

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

Hard scattering

Favored FragmentationUnfavored

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SIDIS kinematical plane and observables

Trento Conventions

Beam polarization Target polarization

U unpolarized

L long.polarized

T trans.polarized

sinmoment of the cross section for unpolarized beam and transverse target

Phys.Rev. D70, 117504 (2004).

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s=20 GeV, pT=0.5-2.0 GeV/c�0 – E704, PLB261 (1991) 201.�+/- - E704, PLB264 (1991) 462.

Xpp Single Spin Asymmetries in

• Recently, large transverse single-spin effects were observed also in p+p collisions (RHIC), at much higher CM energies.

• In collinear picture, the QCD predict small SSAs with transversely polarized protons colliding at high energies.

Kane, Pumplin, Repko ‘78

FermiLab E-704

FNAL

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The Elephant and the village of the blindThe Elephant and the village of the blind

SSAAsymmetry comes from modulation of the initial distribution function (D.Sivers 1990)

Asymmetry comes from the final state interactions in fragmentation (J.Collins 1993)

Non-PQCD “surface effects (Ma,Boros et al)

correlation between quark fields and the gluonic fields (“twist-3” Qiu&Sterman)

Sivers effect forbidden by time reversal invariance (Collins 1993)

kT – crucial for spin structure studies.

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Mechanisms for SSA

L=1

Collins Fragmentation

• L/R SSA generated in fragmentation•Unfavored SSA with opposite sign•No effect in target fragmenation

Orbital momentum generated in string breaking and pair creation produces left-right asymmetry from transversely polarized quark fragmentation (Artru-93)

fragmentation of transversely polarized quarks into unpolarized hadronsu dd du (favored)

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Mechanisms for SSA

Sivers Distribution f1T┴ : unpolarized quarks in

transversely polarized nucleon

•L/R SSA generated in distribution•Hadrons from struck quark have the same sign SSA•Opposite effect in target fragmentation

T-odd f1T┴, requires final state interactions + interference between different helicity states (Brodsky et al., Collins, Ji et al. 2002)

PP

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Transverse momentum of quarks

•kT – required to describe azimuthal distributions of hadrons and in particular SSAs.

•kT - important for cross section description (also for exclusive production)

•kT – leads to 3D description with 8PDFs

Transverse target provides access to spin-orbit correlations with all possible polarizations of quarks.

Mulders & Tangerman (1995, the TMD “bible”)

Transversity

Off diagonal PDFs related to interference between states with different orbital momentum

Sivers

Boer-Mulders

prot

on p

olar

izat

ion

quark polarization

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Sivers function: First measurement

AUT ~Sivers

• significantly positive asymmetry

requires non-zero orbital angular momentumfirst hint of naïve T-odd DF from DIS

Phys.Rev.Lett.94:012002,2005. (100+ in hep SPIRES citation)

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Collins effect: First measurements

• significantly positive and negative asymmetries

• unexpected large

role of unfavoured (u→ fragmentation

function?.1.1 )()( favunfav zHzH .1.1 )()( favunfav zHzH

First extraction of transversity by Anselmino et al. (hep-ph/0701006)

Soffer bound

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BELLE: Collins function measurements

BELLE: Asymmetries in e+e-→h1h2X (H┴1 H┴

1)_

Efremov et al. Phys.Rev.D. 73,094025 (2006)

positivity limit1

Belle detectorKEKB

Asymmetric collider8GeV e- + 3.5GeV e+

First direct indication of non-0 Collins fragmentation !

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Different final states selects different combinations of GPDs

N

*L

t

N’

Q2>>, t<<

H,E,H,E~ ~

Hunting for LHunting for Lq q in hard exclusive processes

Generalised Parton Distributions - - MMűűllerller (1994)(1994) - -

- - Ji & RadyushkinJi & Radyushkin (1996)(1996) - -

10-30%(DIS)10-30%(DIS)

( H + E) x dx = JJqq

= = 1/2 1/2 LLzz

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GPDs and spatial distributions

Transverse position distribution Transverse position shift

Shift in the transverse space of quarks in the transversely polarized proton first predicted in GPD framework, confirmed by Lattice (hep-ph/0612032)

Transversely polarized proton

Unpolarized quark

HU Feb 15 18

→ First (model dependent) constraints on Ju and Jd !

HERMES: DVCS with Transverse target and GPD E

L = 64 pb-1

UT ~ sin(Scos(Im{ H - E + … }+..

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CLAS Transversely Polarized Target

22.5 degree impact

22.5o

Measurements at different beam energies will allow study of Q2 dependence for a fixed x in a wide range.

CLAS12 CLAS6

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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.

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Sivers effect

UT ~Sivers

Requires: non-trivial phase from the FSI + interference between different helicity states

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Sivers effect in the target fragmentation

Significant effect predicted in the target fragmentation region, in particular for baryons (target remnant also asymmetric)

A.Kotzinian

CLAS12 will allow studies of kinematic dependences of the Sivers effect in the target fragmentation region

HU Feb 15 23

CLAS12 - Exclusive Target Asymmetries

Asymmetry for photons and rho highly sensitive to the GPD E and u-quark contributions to proton spin.

Transversely polarized target

e p ep

~ GPD-E ~ Ju

E = 11 GeV ep

ep

CLAS12

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Summary

Studies of exclusive and semi-inclusive hard processes with transverse target at CLAS6 and CLAS12 related to the spin, spin orbit correlations and orbital angular momentum (in particular Sivers function and GPD-E) are crucial for understanding of the transverse structure of the nucleon

•Transverse target design in progress at OXFORD (field maps available)•GSIM studies of scattering of high lumi beam with transverse target in progress.

Few independent proposals for PAC32:Inclusive DISSemi-Inclusive DISExclusive processes……………..(proton & deuteron targets)

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Support slides…..

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PDFs and orbital momentum

Transverse position shift using GPD-E

Good agreement of HERMES (filled) and JLab (open) data with BBS curves for quarks aligned with proton spin, where the orbital motion is not as important.

BBS (NP B441, 197 (1995) using helicity structure of perturbative QCD coupling at x→1

n=minimal number of spectator quarks =2

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Hard Scattering Processes: Kinematics Coverage

Study of high xB domain requires high luminosity

27 G

eV

com

pass

herm

es JLab (upgraded)

JLab@6GeV

Q2

EIC

collider experiments H1, ZEUS (EIC)10-4<xB<0.02 (0.3): gluons (and quarks) in the proton

fixed target experiments COMPASS, HERMES 0.006/0.02<xB<0.3 : gluons/valence and sea quarks JLab/JLab@12GeV 0.1<xB<0.7 : valence quarks

HERA

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Nonperturbative TMDPerturbative region

Boer-Mulders Asymmetry

CLAS12 and EIC studies of transition from non-perturbative to perturbative regime will provide complementary info on spin-orbit correlations.

Transversely polarized quarks in the unpolarized nucleon-

CLAS12

EIC

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CLAS12 a full acceptance, general purpose detector for high luminosity electron scattering experiments, is essential for high precision

measurements of GPDs and TMDs in the valence region. Provide new insight into

- quark orbital angular momentum contributions to the nucleon spin- 3D structure of the nucleon’s interior and correlations- quark flavor polarization

EIC will extend studies of 3D nucleon structure, to low x and high Q2 , important for all processes of interest:

- deeply virtual exclusive processes (DVCS, DVMP) - semi-inclusive meson production with polarized beam and polarized targets

Summary