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

2 Outline  Physics motivation  SIDIS studies at 6 GeV  Hard exclusive processes  Future plans  Summary

3 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) + L q + J g Proton’s spin ~20-30%

4 PDFs f p u (x,k T ), g 1, h 1 FFs F 1p u (t),F 2p u (t).. d2kTd2kT  =0,t=0 dx W p u (x,k,r) “Parent” Wigner distributions d3rd3r d2kTd2kT (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 k T- integrated PDFs same in exclusive and semi-inclusive analysis Analysis of SIDIS and DVMP are complementary TMD TMD PDFs f p u (x,k T ), GPDs H p u (x, ,t)..

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

6 SIDIS (  *p→  X) cross section at leading twist (Ji et al.) structure functions = pdf × fragm × hard × soft (all universal) e Unpolarized 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-Mulders 1998 Kotzinian-Mulders 1996 Collins-1993 To observe the transverse polarization of quarks in SIDIS spin dependent fragmentation is required!

7 Semi-Inclusive Pion Electroproduction with a Polarized Beam and Longitudinally Polarized Target at 6 GeV and the CLAS collaboration JLab experiment P

8 Experimental Setup (CLAS+IC) Inner Calorimeter (424 PbWO 4 crystals) for the detection of high energy photons at forward lab angles (increases  0 acceptance ~3 times at z~0.5). Polarized target 13 o 50 o  solid NH 3 polarized target  proton polarization >75%  high lumi ~ 1.5  s -1 cm -2 IC

9 Factorization studies with pions Double spin asymmetries consistent with simple partonic picture A 1 p inclusive and   (~30 times more data expected) an serve as an important check of HT effects and applicability of the simple partonic description. LUND-MC CLAS PRELIMINARY A1A1 60 days of CLAS+IC (L= cm -2 s -1 )

10 High efficiency reconstruction of  0  +  opens a new avenue in SIDIS (DVMP) 1)SIDIS  0 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

11 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: ,1997 ++ Fragmentation  + 00 HT effects and exclusive  0 suppressed

12 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  +,  )

13 Significant SSA measured for pions with longitudinally polarized target Complete azimuthal coverage crucial for separation of sin  sin2  moments Target SSA measurements at CLAS p 1 sin  +p 2 sin2  0.12<x<0.48 Q 2 >1.1 GeV 2 P T <1 GeV ep→e’  X W 2 >4 GeV 2 0.4<z<0.7 M X >1.4 GeV y<0.85 CLAS PRELIMINARY 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

14 SSA: x-dependence HT SSA from Collins mechanism A UL (  0 ) ~ H 1 favore +H 1 unfavored PRELIMINARY 5.7 GeV With H 1 ┴ (  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

15 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 H unf =-1.2H fav H unf =-5H fav H unf =0 curves,  QSM from Efremov et al 60 days of CLAS+IC (L= cm -2 s -1 )

16 DVCS DVMP 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)

17 Deeply Virtual Compton Scattering ep->e’p’  GPD combinations accessible as azimuthal moments of the total cross section. DVCS BH  LU  ~ sin  Im{F 1 H +  (F 1 +F 2 ) H +kF 2 E } ~ Polarized beam, unpolarized target: Unpolarized beam, longitudinal target:  UL  ~ sin  Im{F 1 H +  (F 1 +F 2 )( H +.. } ~  = x B /(2-x B ),k = t/4M2 Kinematically suppressed d4d4 dQ 2 dx B dtd  ~ | T DVCS + T BH | 2 DVCSBH T BH : given by elastic form factors T DVCS : determined by GPDs GPD

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

19 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

20 Non-perturbative TMD Perturbative region P T -dependence of beam SSA  sin  LU(UL) ~F LU(UL) ~ 1/Q (Twist-3) In the perturbative limit 1/P T behavior expected (F.Yuan SIR-2005) Asymmetries from k T -odd and k T -even (g 1 ) distribution functions are expected to have a very different behavior (flat A 1 p (P T ) observed at 5.7 GeV). 2.0

21 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 oppositesign No effect in TFR

22 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

23 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).

24 Sivers function extraction from A UT (  0 ) does not require information on fragmentation function. It is free of HT and diffractive contributions. F 1T =∑ q e q 2 f 1T ┴q A UT (  0 ) on proton and neutron will allow flavor decomposition w/o info on FF. In large Nc limit: f 1T u = -f 1T d Efremov et al (large x B behavior of f 1T from GPD E) CLAS12 projected CLAS12 projected CLAS12: Sivers effect projections

25 Sivers effect in the target fragmentation A.Kotzinian High statistics of CLAS12 will allow studies of Q 2 dependence of the Sivers effect in target fragmentation region

26  polarization in the target fragmentation x F - momentum in the CM frame Wide kinematical coverage of CLAS12 allows studies of hadronization in the target fragmentation region p e Λ 1 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

27 CLAS12 - DVCS/BH Beam Asymmetry L = 1x10 35 T = 2000 hrs Sensitive to GPD H  LU ~sin  Im{F 1 H +.  }d  e p ep  E=4.3 GeV A LU S. Stepanyan et al. Phys. Rev. Lett. 87 (2001)

28 e p ep  CLAS12 - DVCS/BH Target Asymmetry = 2.0GeV 2 = 0.2 = 0.25GeV 2 CLAS preliminary E=5.75 GeV A UL Longitudinally polarized target  ~sin  Im{F 1 H +  (F 1 +F 2 ) H... }d  ~ E = 11 GeV L = 2x10 35 cm -2 s -1 T = 1000 hrs  Q 2 = 1GeV 2  x = 0.05

29 GPDs H from expected DVCS A LU data b val =b sea =1 MRST02 NNLO distribution Q 2 =3.5 GeV 2  Other kinematics measured concurrently 

30 Exclusive   production on transverse target 2  (Im(AB*))/  T          t/4m 2 ) - Re      UT  A ~ 2H u + H d B ~ 2E u + E d 00 K. Goeke, M.V. Polyakov, M. Vanderhaeghen, 2001 Q 2 =5 GeV 2 E u, E d needed for angular momentum sum rule. 00 A ~ H u - H d B ~ E u - E d ++ Asymmetry is a more appropriate observable for GPD studies at 12 GeV as possible corrections to the cross section are expected to cancel

31 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.

32 support slides…

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

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

35 Collinear Fragmentation  quark The only fragmentation function at leading twist for pions in eN→e’  X is D 1 (z) E e =5.7 GeV No significant variation observed in z distributions of  + for different x ranges ( ) and for A1p as a function of P T

36 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

37 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= H 1 d→  /H 1 u→  ), band correspond to -2.5<R<0  - and  0 SSA will also give access to h 1L d CLAS-5.7GeV First glimpse of Twist-2 TMD h 1L ┴ PRELIMINARY More data required with  - &  0 Exclusive 2 pion background may be important Distribution functions from  QSM from Efremov et al

38 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

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

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

41 SSA: P T -dependence of sin  moment  sin  LU(UL) ~F LU(UL) ~ 1/Q (Twist-3) A LU GeV Beam and target SSA for  + are consistent with increase with P T In the perturbative limit is expected to behave as 1/P T A UL GeV)A UT GeV PRELIMINARY TMDpQCD