DVCS, Nuclear DVCS & HERMES Recoil Detector

1. DVCS, Nuclear DVCS &HERMES Recoil Detector Roberto Francisco Pérez Benito On behalf the HERMES Collaboration Internetional Workshop Dense and Cold Nuclear Matter and Hard Exclusive Processes Aug 20-24, 2007Het Pand, Ghent University, Ghent, Belgium

2. Outline • Motivation • The Spin Structure of the Nucleon • General Parton Distribution (GPD) • DVCS & BH • Recents HERMES results • Beam Spin Asymmetry (BSA) • Beam Charge Asymmetry (BCA) • Transverse Target-Spin Asymmetry (TTSA) • DVCS on Nuclei • The Hermes Recoil Detector • Design Requirement • Silicon Strip Detector (SSD) • Scintillating Fiber Tracker (SFT) • Photon Detector (PD) • Performance • Outlook

3. Parameterization of the Nulceon Structure • Form Factors → Transverse position ← Elastic Scattering • FFscharacterize charge and magnetization distrubutions in the impact parameter space (transverse plane in the LMF). Form Factors (FFs)

4. Parameterization of the Nulceon Structure Form Factors Parton Distribution (FFs) Functions (PDFs) • Form Factors → Transverse position ← Elastic Scattering • PDFs → Longitudinal Momentum Distribution ← DIS • PDFscharacterize longitudinal momentum distributions (LMF) x– Longitudinal momentum fraction

5. Parameterization of the Nulceon Structure Form Factors PDF GPD • Form Factors → Transverse position ← Elastic Scattering • PDFs → Longitudinal Momentum Distribution ← DIS • GPDs → Access to Transverse Position and Longitudinal Momentum Distribution at the SAME time, 3-D picture ← Exclusive Reactions

6. The Spin Structure of the Nulceon Nulceon Spin: • Spin of quarks measured in DIS HERMES: • Spin of gluons, first Measurements • expected to be small • Orbital angular momentum of quarks • Orbital angular momentum of gluons unknown

7. Generalized Parton Distributions (GPDs) Ji Sum Rule – Ji, PRL 78(1997)610 GPDs parton longitudinal momentum fractionsfraction of the momentum transferinvariant momentum transfer to the nucleon

8. Generalized Parton Distributions (GPDs) Nucleon Structure: GPDs GPDs → PDFs (GPDs in the limite t → 0) GPDs → FFs (First moments of GPDs) conserve nucleon helicityflip nucleon helicity

9. Generalized Parton Distributions (GPDs) Simplest/Cleanest Hard Exclusive Process Deeply-Virtual Compton Scattering (DVCS): • Longitudinal momentum fractions: (not accessible) ( Momentum Transfer) • Measurements as function of DVCS: Access to all four

10. GPDs and the DVCS process DVCS final state is indistinguishable from theBethe-Heitler Process (BH) → Amplitudes add Coherently • Photon-Production cross section: Interference Term

11. DVCS Measurements • calculable in QED with the knowledge of the Form Factors • is parameterized in terms of Compton Form Factors (convolutions of GPDs ) • At HERMES kinematics: GPDs accessible through cross-section differences and azimuthal asymmetries via interference term I → Magnitude + Phase(GPDs enter in linear combinations)

12. Azimuthal Asymmetries • Beam-Charge Asymmetry (BCA) • Beam-Spin Asymmetry (BSA) Longitudinal Target Spin Asymmetry (LTSA) • . • Transverse Target Spin Asymmetry (TTSA) TTSA is only asymmetry where GPD E is not suppressed Models for depend on TTSA is sensitive to

13. The HERMES Spectrometer • HERA Beam: 27.6GeV, or , • Polarize gas targets: H,D,He • Unpolarize gas targets: H,D,N,He,Ne,Xe,Kr Events with exactly one positron or electron and exactly one photon in the calorimeter

14. Exclusivity for DVCS via Missing Mass • Exactly one DIS lepton and one photon detected in the Calorimeter. • Recoiling proton undetected. • Exclusivity via Missing Mass exclusive region Overall background contribution in exclusive region

15. Beam-Spin Asymmetry (BSA) Expected sindependence in exclusive regionsinamplitudes small and positive above exclusive region

16. Beam-Charge Asymmetry (BCA) • Calculate „Symmetrized“ BCA ( → || ) to get rid of all sin - dependences due to polarized beam. cos- amplitudes zero for higher missing masses Symmetrizised BCA in exclusive bin  → || → Cancel sin - dependences

17. Transverse Target-Spin Asymmetry Only asymmetry with GPD E is not suppressed Access total angular momentum largely independent on all model parameters but (F. Ellinghaus, Nowak, Vinnikov, Ye, EPJ C46(2006), HEP-PH/0506264) sensitive to (calculations for ) First model dependent extraction of possible!

18. Investigate the internal structure of NUCLEI Hermes nuclear tragets: DVCS on NEON (HEP-EX/0212019) triggerred first calculation for DVCS on NUCLEI Possibility to explore nuclear structure in term of Quarks and gluons, EMC effect, (Anti-)shadowing, Color Transparency,...

19. Contributions from different processes from MC LEPTO BETHE-HEITLER simulation • Coherent BETHE-HEITLER process dominates at small –t' • Incoherent BETHE-HEITLER process dominates at large –t' • Background contributions: • Semi-inclusive • Resonances • Chose –t' cut for each enriched sample to provide target-independent 〈-t'〉: • Coherent: 〈-t'〉= 0.018 GeV² • Incoherent: 〈-t'〉= 0.2 GeV²

20. p HERMES Detector γ • Green PID detectors • Red track detectors • 95 - 05 exclusivty through missing mass cut • 06 - 07 Recoil detector installed to identify the recoiling proton e´

21. RD - Design Requirements • Improve Exclusivity • Detect recoiling proton • Background suppression • semi-incl. : 5%→<<1% • BH with resonace excitation: 11%→~1%

22. HERA BEAM Recoil Detector 1 Tesla Superconducting Solenoid Photon Detector • 3 layers of Tungsten/Scintillator • PID for higher momentum • detectsΔ+pp0 Scintillating Fiber Detector • 2 Barrels • 2 Parallel- and 2 Stereo-Layers in each barrel • 10° Stereo Angle • Momentum reconstruction & PID Silicon Detector • 16 double-sides sensors perpendicular with respect to each other • 97×97 mm2 active area each • 2 layers • Inside HERA vacuum • Momentum reconstruction & PID Target Cell

23. Target Cell Target cell inside beam pipe

24. Silicon Strip Detector (SSD) • 2 layers of double sided TIGRE sensors • 16 TIGRE sensors operate in beam vacuum few cm close to the beam • Size 97mmΧ 97mm, thickness=300μm • 128 strips per side , perpendicular w.r.t. each other, pitch=758μm • HELIX chips are ADC and running under same condition • The high and low gain yields from charge sharing • 8192 channels in total • Proton momentum 135-500 MeV/c

25. Scintillating Fiber Tracker • 2 cylinders of 2Χ2 layers, • 10° stereo angle • 1mm Kuraray fibers, mirrored ends anddouble cladding • PMT Hamamatsu64 channels • 5120 channels in total • Proton momentum 250-1200 MeV/c

26. Photon Detector • 3 layers of Tungsten/Scintillator • A layer parallel to beam line,B and C layer stereo under +45°/-45° • Strips: 2x1x28cm³ • same PMTs as for SFT are used • Main purpose • 1γ fromπ° decay • Reconstructπ° if 2 γ‘s detected

27. Recoil Detector HERMES magnet PMTs SFT Recoil detector electron/positron beam proton beam

28. Six parameters (three translations and three rotations) which are common for all tracks are fitted Residuals and dependence of residuals on coordinates used as a tool to check alignment procedure Recoil detector Alignment Recoil detector Alignment SFT Parallel top =0.28mm Si alignment respect SFT with tracks Next step is Recoil-Hermes alignment using e-p elastic =0.26 strip Si

29. Momentum Reconstruction

30. Momentum Reconstruction

31. Momentum Reconstruction

32. Full tracking including alignment is in production Efficiency of the tracking algorithm studied on MC and found to be above 98% Starting to study the efficiencies, residuals and ghost tracks PID with real data SFT DATA SI DATA + p π - π

33. Collected statistic (preliminary) with recoil detector Electron beam 2006 (only SFT operational): H : 5k DVCS (3Mio. DIS), D : 1k DVCS (0.8Mio. DIS) Positron beam 2006/07 (all subdetectors fully operational) H : 42k DVCS (28Mio. DIS), D : 10k DVCS (7Mio. DIS) Data tacking and Performance 2 2 2 2

34. Analysis with Recoil Detector. DVCS Beam Charge Asymmetry (BCA) Challenging as only Fiber Tracker operational DVCS Beam Spin Asymmetry (BSA) Hard exclusive meson production Different flavor combinations of GPD’s Outlook