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GPD and TMD Studies at HERMES

GPD and TMD Studies at HERMES. Frank Ellinghaus University of Colorado October 2007 DNP 2007, Newport News, USA. scattered electron. real photon. electron. recoiling proton. proton. Generalized Parton Distributions (GPDs). Simplest/cleanest hard exclusive process:

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GPD and TMD Studies at HERMES

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  1. GPD and TMD Studies at HERMES Frank Ellinghaus University of Colorado October 2007 DNP 2007, Newport News, USA

  2. scattered electron real photon electron recoiling proton proton Generalized Parton Distributions (GPDs) Simplest/cleanest hard exclusive process: e p -> e’ p’ g , Deeply-Virtual Compton Scattering (DVCS) • Longitudinal momentum • fractions: • (not accessible) • Experimental challenges: • Measurements as a function of xB, Q2, AND t ! • Exclusive -> detect full final state! Access to the four leading quark GPDs in real photon (DVCS) and Meson production !

  3. DVCS Bethe-Heitler (BH) Indistinguishable processes interfere with each other! Azimuthal Asymm. in BH-DVCS Interference Small at HERMES, JLab kinematics Calculable in QED with knowledge of FFs Access to real part of DVCS amplitude Access to imaginary part of DVCS amplitude Needs polarized (electron) beam (other asymmetries with pol. Target!) Needs both beam charges (e+ and e-) beam (simplest approx.)

  4. The HERA accelerator at DESY (Hamburg) 7/1/07@ 1:09:56 am 27.6 GeV e+ and e- <P> about 35-55%

  5. Event Selection at HERMES pol. and unpol. Gas targets -> H, D, He, N, Ne, Kr, Xe All data in the following taken before installation of Recoil Detector !

  6. 1 Tesla Superconducting Solenoid Photon Detector • 3 layers of Tungsten/Scintillator • PID for higher momentum • detects Δ+pp0 Scintillating Fiber Detector • 2 Barrels • 2 Parallel- and 2 Stereo-Layers in each barrel • 10° Stereo Angle • Momentum reconstruction & PID HERA BEAM Silicon Detector • 16 double-sides sensors • 10×10 cm2 active area each • 2 layers • Inside HERA vacuum • momentum reconstruction & PID Target Cell Recoil Detector Overview

  7. First physics signals Data taking with the recoil detector in 2006 and 2007 • p / p PID • Promising first physics signals…

  8. Exclusivity for DVCS via Missing Mass elastic BH: e p -> e p g assoc. BH: e p -> e D+g (mainly) semi-incl. : e p -> e p0 X (mainly) • Exclusive p0 (e p -> e p p0 ) not shown (small) • DVCS process not simulated (DVCS c.s. unknown, c.s. << BH) • Radiative corrections to BH not simulated (-> Excl. Peak overestimated, BG underestimated) About 15 % BG in exclusive bin (-1.5 < Mx < 1.7 GeV)

  9. BCA versus -t (PRD 75, 2007) ( simplest approx.) HERMES, (PRD 75, 2007) Model by Vanderhaeghen, Guichon, Guidal (VGG), based on double-distributions (Radyushkin) Guzey/Teckentrup, PRD 74, 2006 Analysis based on tiny e- p sample (~ 700 events), Now about 20 times more data on disk! BCA has high sensitivity to GPD models!

  10. Beam-Spin Asymmetry (BSA) • Compare to model calculations at average kinematics per bin • Flat kinematic dependence (kinematics correlated !) described by models • Too large absolute asymmetries Simplest approx. : HERMES preliminary Model by Vanderhaeghen, Guichon, Guidal (VGG), based on double-distributions (Radyushkin)

  11. BSA: Model comparison II Model (Guzey, Teckentrup) based on dual-parametrization (Polyakov, Shuvaev) are in agreement with “all other” DVCS data so far: -> Cross sections from H1/ZEUS (used for normalization) -> BCA at HERMES -> Published (PRL, 2001) AVERAGE BSA values from HERMES and CLAS Size and kinematic dependence of the asymmetry is reproduced More data with improved systematics to come, but BSA not very sensitive to models.

  12. New BSAs can be defined and measured Fourier expansion for unpolarized Hydrogen target: <- Let’s not neglect it… Zero Order approximation: More realistic approximation:

  13. New BSAs “Usual” BSA is not only sensitive to interference term but gets contribution from DVCS term. “Usual” BSA depends on beam charge and size of the BCA. Define: New asymmetries can disentangle (both charges needed) the contributions from the interference and the DVCS term

  14. What about Jq? X. Ji, 1997 So far: Access to GPD H using unpolarized hydrogen targets How to get to GPD E ? GPD E (on a proton target) is always kinematically suppressed except in the transverse target-spin asymmetry TTSA:

  15. DVCS TTSA and model calculations Result from data taken on transversely polarized Hydrogen: Largely independent on all model parameters but Ju (F.E., Nowak, Vinnikov, Ye, EPJ C46 2006, hep-ph/0506264) First model dependent extraction of Ju possible!

  16. First constraint on quark angular momentum First model dependent constraint on total quark angular momentum! Similar Method used by JLab Hall-A; “neutron” data has higher sensitivity to Jd Value to be taken with care, since VGG does not seem to describe the available BSA data, but it is important to have a (first) method!

  17. What about the other model? Second comparison to model calculations (Guzey/Teckentrup, PRD 2006) suggest small/negative value for Ju if Jd=0. Model uncertainty bigger than uncertainties from measurements! The way to go: Constrain models for GPD H first by BSA/BCA. (some model parameters might be the same for GPD E) Compare remaining models to asymmetries sensitive to GPD E (Ju ,Jd).

  18. Exclusive Vector-Meson Production The (only) other (promising) access to the GPD E (J) on a p target : AUT in excl. r0 production Factorization proof for only (Collins, Frankfurt, Strikman PRD 56, 1997) • Event selection: • r0 -> p+p- • No recoil detection -> Missing energy

  19. Transverse Target Spin Asymmetry (TTSA) (strongly simplified) • L / T separation done via angular dist. in r0 decay, SCHC assumed • Data in agreement with earlier calc. (at higher –t ) based on quark • production only (Goeke, Polyakov, Vanderhaeghen, Prog. Part. Nucl. P. 47, 2001) • Data in agreement with theoretical calc. including gluons • (F.E., Nowak, Vinnikov, Ye, EPJ C46, 2006) • Additional gluon contr. dilutes asymmetry -> decreases sensitivity • Positive Ju suggested by both models • Additional model unc. due to gluons when compared to DVCS

  20. Exclusive Pseudo-Scalar Meson Production • Access to and : • Exclusive p+ production: Factorization proof for only (Collins, Frankfurt, Strikman PRD 56, 1997) Frankfurt, Pobylitsa, Polyakov, Strikman, PRD 60 (1999) • Large amplitudes for AUT predicted • Also sensitive to different pion distribution amplitudes • TTSA not yet available, • but final result for cross section available (hep-ex/0707.0222)….

  21. Exclusivity for e p -> e n p+ N(p+) – N(p-) versus • No excl. p- production on p target • -> Excl. p+ survive • Some BG contributions not (well) • described by MC cancel, e.g., • excl. r0 and resonances (partially) • Excl. peak at neutron mass2 • after BG subtraction by MC • Agreement with excl. MC based on • GPD model (VGG, PRD 60, 1999) • suggests little remaining contr. • from resonances

  22. Cross section: g* p -> p+ n • GPD model (VGG, PRD 60, 1999) including power corrections applicable only at low values of –t’ << Q2 -> agreement with data • Good description by Regge model (Laget, PRD 70, 2004) except maybe at small –t’

  23. The Sivers function “tranverse momentum distribution of unpolarized quarks in a transversely polarized proton • non-zero Sivers function implies non-zero orbital angular momentum Sivers Collins

  24. Sivers amplitudes • Published (PRL 94, 2005) results • for p+ and p- confirmed with • full statistics: • p+ clearly positive • -> Non-zero Sivers fct. • -> Non-zero orbital ang. momentum • Additional p0 measurement • -> Isospin symmetry for Sivers • amplitudes fulfilled • Prelim. kaon results comfirmed with • full statistics: • K+ clearly positive • Unexpected result: • K+ amplitude (still) > p+ amplitude • role of sea-quarks?

  25. DVCS on Nuclei? First measurement of DVCS on Neon (F.E. et al, hep-ex/0212019) triggered first calculations for DVCS on Nuclei. ->Opens possibility to explore nuclear structure in terms of quarks and gluons, EMC effect, (Anti-)Shadowing, CT, ….

  26. Contribution from different processes from MC Task: Find for each target upper (lower) -t cut in order to compare the BSA for the coherent (incoherent) production at similar average kinematics: Coherent BH Incoherent BH Semi-Inclusive BG Resonances 82% coherent for heavier targets at –t =0.018 GeV2 and very similar average x and Q2

  27. A-Dependence of BSA • No obvious A-dependence seen. • sin(2f) moment consistent with zero for all targets

  28. BSA ratios Nuclei/Hydrogen • Coherent enriched: 2 sigma above unity. Prediction of R=5/3 for Spin-0 and Spin ½ tragets (Kirchner, Mueller, EPJ 2003). R=1-1.1 for 4-He (Liuti, Taneja, PRC 2005), but large stat. error, calculations for heavier targets underway/promised • Incoherent enriched Consisten with unity as expected

  29. BSA Ratios Nuclei/Hydrogen Guzey/Siddikov (J. Phys. G, 2006) Promising, more data needed… Guzey/Strikman (Phys.Rev. C, 2003)

  30. Summary • Indications for small DG stress the importance of the investigation of the quark orbital angular momentum via GPDs <- 3 Dim structure • HERA /HERMES shut down, but much more to come for GPDs (e.g., all data taken with the recoil detector !)

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