Jim Stewart Jefferson Lab HERMES – Selected Highlights for the Hermes collaboration

May 2008 J. Stewart HERA: e + /e - (27GeV) - proton (920 GeV) collider DESY HERA HERA-e self-polarizing due to Sokolov-Ternov effect

May 2008 J. Stewart The HERMES Spectrometer Internal Gas Target: He, H, D, H unpol: H 2,D 2,He,N 2,Ne,Kr,Xe Kinematic Range: GeV 2 and W > 2 GeV Reconstruction:  p/p < 2%,  < 1 mrad Particle ID: TRD, Preshower, Calorimeter a 1997: Cherenkov, 1998 a : RICH + Muon-ID

May 2008 J. Stewart Semi-Inclusive Deep Inelastic Scattering The cross section can be expressed as a convolution of a distribution function and a fragmentation function.

May 2008 J. Stewart Virtual Photon Asymmetry - Path to  q Virtual photon can only couple to quarks of opposite helicity Virtual photon can only couple to quarks of opposite helicity Select quark helicity by changing target polarization direction Select quark helicity by changing target polarization direction Different targets give sensitivity to different quark flavors Different targets give sensitivity to different quark flavors

May 2008 J. Stewart Quark Polarizations Correlation between detected hadron and the struck quark allows flavor separation Linear System in Inclusive DIS →  Semi-inclusive →

May 2008 J. Stewart Polarized Quark Densities  First complete separation of pol. PDFs without assumption on sea polarization   good agreement with NLO-QCD  No indication for < 0  No indication for  s(x) < 0   In measured range (0.023 – 0.6)  u(x) > 0  d(x) < 0 ~ 0   u(x),  d(x) ~ 0

May 2008 J. Stewart Fit x-dependence of multiplicities to get S(x) PDF and Kaon FF dotted: CTEQ-6L & fit dotted: dashed:dashed: dashed-dotted:dashed-dotted: solid:S(x)= Q(x): CTEQ-6L & DSS solid:S(x)= HERMES: polarized and unpolarized s-PDF  Need longitudinal polarized deuterium target → strange quark sea in proton and neutron identical → fragmentation simplifies  All needed information can be extracted from HERMES data alone → inclusive A 1,d (x,Q 2 )and kaon A K 1,d (x,Q 2 ) double spin asym. → Kaon multiplicities  D Q K and D S K  Only assumptions used: → isospin symmetry between proton and neutron → charge-conjugation invariance in fragmentation s(x) + sbar(x)

May 2008 J. Stewart Inclusive Asymmetry Kaon Asymmetry HERMES: polarized and unpolarized s-PDF

May 2008 J. Stewart Distribution Functions HERMES HERMES Leading Twist Leading Twist  3 distribution functions survive the integration over transverse quark momentum unpolarized DF Helicity DF Transversity DF Transversitybasis

May 2008 J. Stewart Properties of the Transversity DFs  For non-relativistic quarks  q(x)=  q(x) →  q(x) probes the relativistic nature of the quarks  Due to Angular Momentum Conservation → Different QCD evolution → No gluon component  → Predominately sensitive to valence quarks  Bounds   Soffer Bound:  T-even  Chiral odd → Not measurable in inclusive DIS

May 2008 J. Stewart Measuring Transversity  Need a chiral odd fragmentation function: ‘ Collins FF ’  Transverse quark polarization affects transverse hadron momentum  Observed asymmetry in azimuthal angle about lepton scattering plane Forbidden Forbidden Need chiral odd fragmentation function Need chiral odd fragmentation function

May 2008 J. Stewart Sivers Function f  1T (x,p T 2 )  Distribution function → Chiral even BUT! → Naïve T-ODD  A remnant of the quark transverse momentum can survive the photo- absorption and the fragmentation process, thus the cross section depends on the angle between the target spin direction and the hadron production plane → Azimuthal distribution  The transverse momentum distribution is nucleon-spin dependent.  Non-vanishing Sivers function requires quark orbital angular momentum → Requires L z

May 2008 J. Stewart How to Measure Transversity Collins azimuthal moment Sivers azimuthal moment Assuming gaussian distributions for initial an final quark momentum

May 2008 J. Stewart Collins Amplitudes  All Pions  + large positive  - large negative u+u+u+u+ u-u-u-u-  0 data lies nicely between the  + and  - data

May 2008 J. Stewart Collins Amplitudes for Kaons  No significant K + ampl.  K+ comparable to  +  K - pos. trend?

May 2008 J. Stewart Sivers Amplitudes  All Pions  + large  - ~zero

May 2008 J. Stewart Large pos. K + K - consistent zero K + larger than  + ? Significant sea quark contribution?

May 2008 J. Stewart Theoretical fits to Hermes-COMPASS data M. Anselmino,et al., arXiv: v1

May 2008 J. Stewart The Hunt for L q Study of hard exclusive processes leads to a new class of PDFs Generalized Parton Distributions possible access to orbital angular momentum exclusive: all products of the reaction are detected missing energy (  E) and missing Mass (M x ) = 0 missing energy (  E) and missing Mass (M x ) = 0 from DIS: ~0.3

May 2008 J. Stewart GPDs Introduction What does GPDs charaterize? unpolarized polarized conserve nucleon helicity flip nucleon helicity not accessible in DIS DVCSDVCSDVCSDVCS quantum numbers of final state select different GPD pseudo-scaler mesons vector mesons A C,A LU, A UT, A UL A UT,   + A UT,  

May 2008 J. Stewart DVCS ASYMMETRIES  C ~ cos  ∙Re{ H +  H +… } ~  LU ~ sin  ∙Im{H +  H + kE} ~ polarization observables:  polarization observables: H different charges: e+ e-  different charges: e+ e-  UT ~ sin  ∙Im{k(H - E) + … }  UL ~ sin  ∙Im{H +  H + …} ~ kinematically suppressed  = x B /(2-x B ),k = t/4M 2  UT beam target H H, E H ~

May 2008 J. Stewart Selection of the exclusive event sample Identification by the missing mass technique Not possible to separate associated from elastic production. Possible with Recoil Detector for 2006/2007 data. (~12%)

May 2008 J. Stewart Extraction of Asymmetries Transverse Target and Beam Charge  Full transverse target data set analyzed → Electron 100 pb -1 and positron 70 pb -1 arXiv: submitted to JHEP

May 2008 J. Stewart Beam Charge Asymmetry Goeke et al., Prog. Part. Nucl. Phys. 47 (2001) 401 Code: VGG [Vanderhaegen et al., priv. comm] NLO twist-2 gluon helicity flip The factorized t dependence is disfavoured and the Regge ansatz with no D term best describes these BCA data

May 2008 J. Stewart Hermes: Transverse Target Spin Asymmetry Sensitivity to J q

May 2008 J. Stewart Suppressed Asymmetries

May 2008 J. Stewart Extraction of Asymmetries Longitudinal Beam Spin and Beam Charge Preliminary results from data sets

May 2008 J. Stewart Beam Charge Asymmetry Changes in the new analysis  2.5 times more statistics than previous publication  6-bins in all kinematical variables  Systematical errors include new model-dependent studies Results agree with former publications Results agree with former publications

May 2008 J. Stewart Beam Charge Asymmetry (higher twist) Bin-wise fractions of associated production. The factorized ansatz and the VGG variant with the D-term is dis-favored by the beam charge asymmetry.

May 2008 J. Stewart Beam Spin Asymmetry (higher twist) Pure DVCS squared asymmetries are compatible with zero, in agreement with model assumptions.

May 2008 J. Stewart Beam Spin Asymmetry Result agrees with Dual model predictions, but fractions of associated productions are not corrected for. (higher twist)

May 2008 J. Stewart Hermes: Charge and Beam Spin Asymmetry Heavy Targets Beam Charge Asymmetry Beam Spin Asymmetry Why nuclear DVCS: constrain nuclear GPDs constrain nuclear GPDs constrain models attempting constrain models attempting to describe nuclear matter to describe nuclear matter neutron and proton matter neutron and proton matter distribution in nuclei distribution in nuclei

May 2008 J. Stewart Recoil Detector – Installed Jan 06 Photon Detector → 3 layers of Tungsten/Scintillator → PID for higher momentum → detects Δ +  p  0 Scintillating Fiber Detector (SFT) → 2 Barrels → 2 Parallel- and 2 Stereo-Layers in each barrel Silicon Detector → 16 double-sides sensors → 2 layers → Inside HERA vacuum Target Cell 1 Tesla Superconducting Solenoid

May 2008 J. Stewart Recoil Detector running  Installed January 06  e + running till July 06 → Si not available for e + data  Switched to e - July 06 → Detector fully functional  End of data taking July 07  Total luminosity luminosity 06 and 07 1 fb -1 of data collected ! ++ -- p E-loss in outer SFT PID proton-pion What's to come?  Analysis of full data set using only forward spectrometer  Analysis of recoil data to suppress associated background  Exclusive meson production

May 2008 J. Stewart Summary  Isoscaler extraction of the strange sea polarization → Positive but consistent with zero → Strange quark distribution different than expected  The full data sets for the Sivers and Collins analyses are available as preliminary results. → Impressive fits are available  New major data sets are available for DVCS on the proton both for hydrogen and heavy nuclei. → Deuteron will come soon. → 2006 and 2007 data starting to be analyzed  1 fb -1 of data on tape  Recoil detector to quantify associated production  Many other results not shown … Which Hadron ( ,K,p) is Which