Hard scattering studies at JLab Harut Avakian Jefferson

Hard scattering studies at JLab Harut Avakian Jefferson Lab XI Workshop on High Energy Spin Physics , Dubna, September 2005 * Talk presented by A. Kotzinian 1

Outline ØPhysics motivation ØSIDIS studies at 6 Ge. V ØHard exclusive processes ØFuture plans ØSummary 2

Physics Motivation Orbital Angular Momentum (OAM) in the focus. ~20 -30% Proton’s spin ½ = ½ (Du+Dd+Ds) + Lq + Jg Parton Distribution Functions generalized to contain information not only on longitudinal, but also on the transverse distribution of partons: Complementary sets of non-perturbative functions sensitive to different aspects of transverse distributions ØGeneralized Parton Distributions (GPD) H, E. . . ØTransverse-momentum dependent (TMD) parton distributions 3

Probability to find a quark u in a nucleon P with a certain polarization in a position r and momentum k Wpu(x, k, r) “Parent” Wigner distributions 2 d d 3 r k. T (F GPD T) TMD 0, t =0 T), PDFs fpu(x, k. T), g 1, h 1 dx d 2 k. T Measure momentum transfer to quark GPDs Hpu(x, x, t). . x= TMD PDFs fp u(x, k Measure momentum transfer to target FFs F 1 pu(t), F 2 pu(t). . k. T-integrated PDFs same in exclusive and semi-inclusive analysis Analysis of SIDIS and DVMP are complementary 4

x. F>0 (current fragmentation) Single pion production in hard scattering h x. F<0 (target fragmentation) x. F - momentum in the CM frame Target fragmentation Current fragmentation h M h h h -1 Fracture Functions PDF 0 k. T-dependent PDFs PDF GPD 1 x. F Generalized PDFs Wide kinematic coverage of large acceptance detectors allows studies of hadronization both in the target and current fragmentation regions 5

SIDIS (g*p→p. X) cross section at leading twist (Ji et al. ) e Unpolarized target Longitudinally pol. target Transversely e pol. target e p Boer-Mulders 1998 Kotzinian-Mulders 1996 p Collins-1993 structure functions = pdf × fragm × hard × soft (all universal) Off diagonal PDFs related to interference between L=0 and L=1 light-cone wave functions. To observe the transverse polarization of quarks in SIDIS spin dependent fragmentation is required! 6

JLab experiment P-05 -113 Semi-Inclusive Pion Electroproduction with a Polarized Beam and Longitudinally Polarized Target at 6 Ge. V and the CLAS collaboration 7

Experimental Setup (CLAS+IC) Polarized target 4 solid NH 3 polarized target 4 proton polarization >75% 4 high lumi ~ 1. 5 1034 s-1 cm-2 50 o 13 IC o Inner Calorimeter (424 Pb. WO 4 crystals) for the detection of high energy photons at forward lab angles (increases p 0 acceptance ~3 times at z~0. 5). 8

Factorization studies with pions 60 days of CLAS+IC (L=1. 5. 1034 cm-2 s-1) A 1 LUND-MC CLAS PRELIMINARY • Double spin asymmetries consistent with simple partonic picture • A 1 p inclusive and p 0 (~30 times more data expected) an serve as an important check of HT effects and applicability of the simple partonic description. 9

SIDIS with neutral pions 1) 2) 3) 4) 5) SIDIS p 0 production is not contaminated by diffractive r p 0 SSA sensitive to the unfavored polarized fragmentation HT effects and exclusive p 0 suppressed Simple PID by p 0 -mass (no kaon contamination) Provides information complementary to p+/- information on PDFs High efficiency reconstruction of p 0 r+ , h opens a new avenue in SIDIS (DVMP) 10

HT and Semi-Exclusive Pion Production E. Berger, S. Brodsky 1979 (DY), E. Berger 1980, A. Brandenburg, V. Khoze, D. Muller 1995 Fragmentation p+ A. Afanasev, C. Carlson, C. Wahlquist Phys. Lett. B 398: 393 -399, 1997 p+ p 0 HT effects and exclusive p 0 suppressed 11

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 p 0 sample “clean” at large z (non-string pions are mainly from semi-inclusive r+, 12 w)

Target SSA measurements at CLAS ep→e’p. X p 1 sinf+p 2 sin 2 f W 2>4 Ge. V 2 Q 2>1. 1 Ge. V 2 y<0. 85 CLAS PRELIMINARY 0. 4<z<0. 7 MX>1. 4 Ge. V p 1= 0. 059± 0. 010 p 2=-0. 041± 0. 010 p 1=-0. 042± 0. 015 p 2=-0. 052± 0. 016 p 1=0. 082± 0. 018 p 2=0. 012± 0. 019 PT<1 Ge. V 0. 12<x<0. 48 • Significant SSA measured for pions with longitudinally polarized target • Complete azimuthal coverage crucial for separation of sin , sin 2 moments 13

SSA: x-dependence PRELIMINARY 5. 7 Ge. V HT SSA from Collins mechanism AUL (p 0) ~ H 1 favore+H 1 unfavored With H 1┴ (p 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 14

Longitudinally polarized target SSA using CLAS+IC 60 days of CLAS+IC (L=1. 5. 1034 cm-2 s-1) curves, c. QSM from Efremov et al Hunf=-5 Hfav Hunf=-1. 2 Hfav Hunf=0 • 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 15

Hard Exclusive Processes and GPDs DVMP DVCS long. only hard gluon hard vertices DVCS – for different polarizations of beam and target provide access to different combinations of GPDs H, H, E DVMP for different mesons is sensitive to flavor contributions (r 0/r+ select H, E, for u/d flavors, p, h, K select H, E) 16

Deeply Virtual Compton Scattering ep->e’p’g DVCS d 4 d. Q 2 dx. Bdtd BH GPD ~ |TDVCS + TBH|2 TBH : given by elastic form factors TDVCS: determined by GPDs Polarized beam, unpolarized target: ~ Ds. LU ~ sinf. Im{F 1 H + x(F 1+F 2)H +k. F 2 E} DVCS Kinematically suppressed BH Unpolarized beam, longitudinal target: ~ Ds. UL ~ sinf. Im{F 1 H+x(F 1+F 2)(H +. . } Kinematically suppressed x = x. B/(2 -x. B ), k = t/4 M 2 • GPD combinations accessible as azimuthal moments of the total cross section. 17

Target Spin Asymmetry: t- Dependence Higher efficiency photon detection will also improve significantly DVCS target SSA measurements. Page 18

CLAS 12 High luminosity polarized (~80%) CW beam Wide physics acceptance (exclusive, semi-inclusive current and target fragmentation) Wide geometric acceptance 12 Ge. V significantly increase the kinematic acceptance 19

PT-dependence of beam SSA ssinf. LU(UL) ~FLU(UL)~ 1/Q (Twist-3) In the perturbative limit 1/PT behavior expected (F. Yuan SIR-2005) 2. 0 Non-perturbative TMD Perturbative region Asymmetries from k. T-odd and k. T-even (g 1) distribution functions are 20 expected to have a very different behavior (flat A 1 p(PT) observed at 5. 7 Ge. V).

Collins Effect Collins UT ~ • SSA in fragmentation • Subleading SSA has opposite sign • No effect in TFR Study the Collins fragmentation for all 3 pions with a transversely polarized target and measure the transversity distribution function. JLAB 12 cover the valence region. 21

Collins Effect and Kotzinian-Mulders Asymmetry KM UL ~ 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) 22

Sivers effect Sivers UT ~ • 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). 23

CLAS 12: Sivers effect projections In large Nc limit: f 1 T u= -f 1 T d CLAS 12 projected F 1 T=∑qeq 2 f 1 T┴q Efremov et al (large x. B behavior of f 1 T from GPD E) CLAS 12 projected Sivers function extraction from AUT (p 0) does not require information on fragmentation function. It is free of HT and diffractive contributions. AUT (p 0) on proton and neutron will allow flavor decomposition w/o info on FF. 24

Sivers effect in the target fragmentation A. Kotzinian High statistics of CLAS 12 will allow studies of Q 2 dependence of the Sivers effect in target fragmentation region 25

L polarization in the target fragmentation e’ e 1Λ 2 x. F - momentum in the CM frame p L 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 Wide kinematical coverage of CLAS 12 allows studies of hadronization in the target fragmentation region 26

CLAS 12 - DVCS/BH Beam Asymmetry ep epg Ds. LU~sin Im{F 1 H+. . }d Sensitive to GPD H ALU L = 1 x 1035 T = 2000 hrs E=4. 3 Ge. V S. Stepanyan et al. Phys. Rev. Lett. 87 (2001) 27

CLAS 12 - DVCS/BH Target Asymmetry ep epg Longitudinally polarized target E = 11 Ge. V L = 2 x 1035 cm-2 s-1 T = 1000 hrs DQ 2 = 1 Ge. V 2 Dx = 0. 05 ~ Ds~sin Im{F 1 H+x(F 1+F 2)H. . . }d AUL CLAS preliminary E=5. 75 Ge. V <Q 2> = 2. 0 Ge. V 2 <x> = 0. 2 <-t> = 0. 25 Ge. V 2 28

GPDs H from expected DVCS ALU data Q 2=3. 5 Ge. V 2 p bval=bsea=1 MRST 02 NNLO distribution § Other kinematics measured concurrently 29

Exclusive r 0 production on transverse target 2 D (Im(AB*))/p T AUT = |A|2(1 -x 2) - |B|2(x 2+t/4 m 2) - Re(AB*)2 x 2 Q 2=5 Ge. V 2 r 0 A ~ 2 Hu + Hd B ~ 2 Eu + Ed r+ A ~ Hu - Hd B ~ Eu - Ed Eu, Ed needed for angular momentum sum rule. K. Goeke, M. V. Polyakov, M. Vanderhaeghen, 2001 Asymmetry is a more appropriate observable for GPD studies at 12 Ge. V 30 as possible corrections to the cross section are expected to cancel

Summary q Current data are consistent with a partonic picture, and can be described by a variety of theoretical models. q Significantly higher statistics of JLab, in a wide kinematical range will provide a full set of data needed to constrain relevant 3 D distribution functions (TMDs, GPDs) q Experimental investigation of properties of 3 D 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. 31

support slides… 32

Higher Twist SSAs Discussed as main sources of SSA due to the Collins fragmentation Target sinf SSA (Bacchetta et al. 0405154) In jet SIDIS only contributions ~ D 1 survive Beam sinf SSA With H 1┴ (p 0)≈0 (or measured) Target and Beam SSA can be a valuable source of info on HT T-odd distribution functions 33

SIDIS: factorization studies P. Bosted JLab data at 6 Ge. V are consistent with factorization and partonic description for variety of ratio observables 34

Collinear Fragmentation p quark The only fragmentation function at leading twist for pions in e. N→e’p. X is D 1(z) Ee =5. 7 Ge. V No significant variation observed in z distributions of p+ for different x ranges (0. 4<z<0. 7, MX>1. 5) and for A 1 p as a function of PT 35

SSA: kinematical dependence • Indicate a negative sin 2 moment measured for p +. • Some indication of negative p- SSA (more data required for p - and p 0) • More data required to correct for exclusive 2 p contribution. 36

First glimpse of Twist-2 TMD h 1 L┴ For Collins fragmentation use chirally invariant Manohar-Georgi model (Bacchetta et al) PRELIMINARY CLAS-5. 7 Ge. V Distribution functions from c. QSM from Efremov et al Systematic error only from unknown ratio of favored and unfavored Collins functions (R= H 1 d→p+/H 1 u→p+), band correspond to -2. 5<R<0 p- and p 0 SSA will also give access to h 1 Ld • More data required with p- & p 0 • Exclusive 2 pion background may be important 37

exclusive production background Pions 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 38

exclusive production background Fraction of charged pions from rho-0 especially high for neutron target 39

production background from exclusive events Non string pions are mainly from semi-inclusive rho+ 40

SSA: PT-dependence of sin moment sin LU(UL) ~FLU(UL)~ 1/Q (Twist-3) AUL (CLAS @5. 7 Ge. V) ALU CLAS @4. 3 Ge. V AUT HERMES @27. 5 Ge. V PRELIMINARY TMD p. QCD Beam and target SSA for p+ are consistent with increase with PT In the perturbative limit is expected to behave as 1/PT 41