1. Ignazio Vilardi DUBNA SPIN 07, September 2007 1 Hermes Recoil Detector Status Ignazio Vilardi On behalf of the HERMES Collaboration DUBNA-SPIN-07 September 3 – 7, 2007

2. Ignazio Vilardi DUBNA SPIN 07, September 2007 2 Outline  GPD’s and hard exclusive processes;  Hermes results: Beam Spin Asymmetry (BSA) and Beam Charge Asymmetry (BCA);  The Hermes Recoil Detector: Motivation and Design;  Hermes Recoil Detector Status;  Outlook;

3. Ignazio Vilardi DUBNA SPIN 07, September 2007 3 Study of hard exclusive processes leads to a new class of PDF’s Generalized Parton Distributions possible access to L q exclusive: all reactions are reconstructed missing energy (DE) and missing Mass (M x ) = 0 from incl.-DIS: HERMES ~0.3 The Hunt for L q A. Airapetian et al, Phys. Rev. D 75 (2007) 012007 a theoretical framework that provides the most complete description of the nucleon Ji’s sum rule

4. Ignazio Vilardi DUBNA SPIN 07, September 2007 4 What do GPD’s characterize? unpolarized polarized conserve nucleon helicity flip nucleon helicity not accessible in DIS Data from HERMES on tape DVCSDVCSDVCSDVCS pseudo-scalar mesons vector mesons A C,A LU, A UT, A UL A UT,   + A UT,   Observables: GPD’s Introduction

5. Ignazio Vilardi DUBNA SPIN 07, September 2007 5 two experimentally indistinguishable processes: HERMES kinematics: BH c.s. >> DVCS c.s. isolate BH-DVCS interference term non-zero azimuthal asymmetries beam helicity asymmetry: beam charge asymmetry: HERA (polarised electrons and positrons) only place in the world so far to measure the complete Compton amplitude Deep Virtual Compton Scattering DVCSBethe-Heitler

6. Ignazio Vilardi DUBNA SPIN 07, September 2007 6  Exactly 1 DIS lepton in the spectrometer and 1 photon in the calorimeter;  Recoiling proton undetected  Exclusive reactions via the missing mass technique  ;  exclusive region: DVCS event selection at HERMES Overall background contribution  15% in exclusive region

7. Ignazio Vilardi DUBNA SPIN 07, September 2007 7 Beam Spin Asymmetry (BSA) Expected sin  dependence in exclusive region  Im H sin  amplitudes small and positive above exclusive region A. Airapetian et al., Phys. Rev. Lett. 87 (2001) 182001

8. Ignazio Vilardi DUBNA SPIN 07, September 2007 8 Beam Charge Asymmetry (BCA) Symmetrizised BCA in exclusive bin   |  |  Cancel sin  dependence Solid curve – 4 Parameter fit P 1 + P 2 cos  + P 3 cos2  + P 4 cos3  cos  - amplitudes zero for higher missing masses A. Airapetian et al., Phys. Rev. D75 (2007) 11103 Expected cos(  ) dependence  Re H

9. Ignazio Vilardi DUBNA SPIN 07, September 2007 9 The end of polarized targets at HERMES  Improve Exclusivity Selection  Detect recoiling proton modify target region Detector to measure recoiling proton

10. Ignazio Vilardi DUBNA SPIN 07, September 2007 10 10 cm 3D Model of the Recoil Detector Photon Detector (PD) ScintillatingFibersTracker(SFT) 1T Magnet not shown Silicon Strip Detector(SSD) BEAM

11. Ignazio Vilardi DUBNA SPIN 07, September 2007 11 detection of the recoiling proton detection of the recoiling proton p: 135 -1200 MeV/c; p: 135 -1200 MeV/c; 76%  acceptance (because of 76%  acceptance (because of layout of SSD); layout of SSD);  /p PID via dE/dx;  /p PID via dE/dx; Performance of the Recoil Detector

12. Ignazio Vilardi DUBNA SPIN 07, September 2007 12 background suppression Benefits of the Recoil Detector semi-incl. DIS: 5% -> <<1% associated BH: 11% -> ~1%

13. Ignazio Vilardi DUBNA SPIN 07, September 2007 13 Target cell Target cell inside beam pipe Thickness = 75  m and length = 150 mm

14. Ignazio Vilardi DUBNA SPIN 07, September 2007 14 Silicon Detector: protons < 0.5 GeV/c  16 double sided Silicon sensors, size 99 x 99 mm 2, 300 µm thickness;  128 strips per sensor side, 8192 read out channels in total, 758 µm strip pitch;  High and Low Gain read out;   strip orientation for space point reconst.;  p-measurements from dE/dx 135 - 500 MeV/c;  PID  /p from dE/dx for p < 250 MeV/c;  76% azimuthal acceptance (  );

15. Ignazio Vilardi DUBNA SPIN 07, September 2007 15 SFT Detector: protons > 0.3 GeV/c  2 barrels with each parallel and stereo layer (10°) for space point reconstruction and tracking;  4992 fibers (1 mm diameter) in total;  p - measurements 300-1200 MeV/c;  PID  /p from dE/dx for p < 700 MeV/c;  100% azimuthal acceptance;

16. Ignazio Vilardi DUBNA SPIN 07, September 2007 16 Photon Detector:  from  + (  +  p  0  p  )  3 layers of scintillators after tungsten converter: the first parallel and the last 2 stereo (± 45°);  60 strips in the parallel one and 44 in the other ones;  Detects photons from intermediate  resonance (  +  p  0  p   );  Reconstructs  0 if both  are detected;  Contributes to PID  /p (together with SFT) for p > 600 MeV/c;  can provide cosmic trigger;

17. Ignazio Vilardi DUBNA SPIN 07, September 2007 17  Removes Møller (and Bhabha) electrons by letting them spiral forward and in this way protects the silicon detector from background;  Provides a 1T solenoid longitudinal superconducting magnetic field for tracking. For this reason the homogeneity of the field has to be better than 20% in either direction; Magnet: Møller e - suppression and track bending

18. Ignazio Vilardi DUBNA SPIN 07, September 2007 18 Data Taking and general performance Fiber tracker fully operational  2.5 Mio. DIS from hydrogen target;  0.5 Mio. DIS from deuterium target; Fiber tracker and Photon detector fully operational; Finished commissioning of Silicon Detector in September 06;  28 Mio. DIS events from hydrogen target;  7 Mio. DIS events from deuterium target;  Switch to positron beam in July 06  Data taking started in February 06 with electron beam Recoil Detector ran stable for 10 months

19. Ignazio Vilardi DUBNA SPIN 07, September 2007 19 First Recoil Detector Results (SSD)  Signal from the Silicon detector divided by a coupling capacitor (10 pF) into HG and LG readout channels: its ratio OK;  Correlation between inner and outer Silicon modules OK (different disposition of Silicon strips);  Energy deposition in inner Silicon module vs. outer Silicon module OK; Protons  Deuterium target OK; stopped in 2 nd layer 106 MeV/c 135 MeV/c 500 MeV/c

20. Ignazio Vilardi DUBNA SPIN 07, September 2007 20 Momentum reconstruction with Recoil D. Higher momentum protons  dE/dx (Bethe-Block formula); Low momentum protons (stopped in outer Silicon)  Sum of energy deposits; High momentum particles  Bending in magnetic field;

21. Ignazio Vilardi DUBNA SPIN 07, September 2007 21 Internal Alignment (magnet-off data)  Six parameters (three translations and three rotations) which are common for all tracks are fitted;  Each track is fitted with a straight line taking into account alignment parameters at current iteration;  After iterative procedure converges it is repeated with new initial values of alignment parameters to be sure that alignment procedure does not depend on initial approach;  Residuals and dependence of residuals on coordinates used as a tool to check alignment procedure;

22. Ignazio Vilardi DUBNA SPIN 07, September 2007 22 Residuals for the SFT from cosmic data SFT Parallel top SFT Parallel Bottom  = 0.28mm 1mm fibers  = 0.31mm

23. Ignazio Vilardi DUBNA SPIN 07, September 2007 23 Residuals for SSD (magnet-off data)  = 0.28strip  = 0.26strip SI inner SI outer

24. Ignazio Vilardi DUBNA SPIN 07, September 2007 24 Tracking  Full tracking is in production including alignment;  Efficiency of the tracking algorithm studied on MC and found to be 98.4 %;  Starting to study ghost tracks;

25. Ignazio Vilardi DUBNA SPIN 07, September 2007 25 First recoil-spectrometer correlation e-p elastic scattering Selection of single recoil tracks by making cuts on the momentum of the lepton detected by the forward spectrometer. Clear correlation can be observed;  correlation  correlation Z correlation

26. Ignazio Vilardi DUBNA SPIN 07, September 2007 26 Demonstration of recoil principle p ++ -- Use Recoil Detector to remove background!!!

27. Ignazio Vilardi DUBNA SPIN 07, September 2007 27 Outlook  Analysis with Recoil Detector;  Conventional analysis (without Recoil Detector);  Once background contribution is measured: refine analysis of pre-recoil DVCS data; DVCS Beam Charge Asymmetry (BCA) challenging as only Fiber Tracker operational during e - running DVCS Beam Spin Asymmetry (BSA) Hard exclusive meson production (  ,  0,  0 )

28. Ignazio Vilardi DUBNA SPIN 07, September 2007 28

29. Ignazio Vilardi DUBNA SPIN 07, September 2007 29 HERA: e + /e - (27.6 GeV) - proton (920 GeV) collider HERA @ DESYHERMES @ HERA

30. Ignazio Vilardi DUBNA SPIN 07, September 2007 30 Particle ID: EM-Calorimeter: energy measurement for leptons and  Preshower, TRD, a 1997 : Cherenkov, 1998 a : RICH + Muon-ID Momentum: measured by deflection of particle in magnetic field Particle trajectories: measured by hits in tracking chambers HERMES: DIS e +/- (27.6 GeV) on p Internal Gas Target: He, H, D, H unpol: H 2,D 2,He,N 2,Ne,Kr,Xe

31. Ignazio Vilardi DUBNA SPIN 07, September 2007 31 The spin structure of the nucleon Naïve parton model BUT 1989 EMC measured  = 0.120 0.094 0.138 ± ± Spin Puzzle Gluons are important !! Sea quarks  q s GGGG Don’t forget the orbital angular Momentum!! Quark orb. ang. mom. Gluon spin Gluon orb. ang. mom. Proton spin Quark spin

32. Ignazio Vilardi DUBNA SPIN 07, September 2007 32 BCA – Comparison to Model calculations P 1 = - 0.011 ± 0.019 P 2 = 0.060 ± 0.027 P 3 = 0.016 ± 0.026 P 4 = 0.034 ± 0.027 M. Vanderhaegen et al., Phys. Rev. D 60 (1999) K. Goeke et al., Prog. Part. Nucl. Phys. 47 (2001) Large contribution from associated production in last t-bin (not included in models) GPD’s Model (only protons) Regge-inspired with D-term disfavored