Roberto Francisco Perez Benito A recoil detector to identify hard exclusive reactions at the HERMES experiment Die Deutsche Physikalische Gesellschaft e.V. (DPG) Fachverband Teilchenphysik T 510 Detektorsysteme I Roberto Francisco Perez Benito Heidelberg, 05.03.-09.03.2007

Outline Spin of the nucleon DVCS at HERMES How to improve DVCS measurements Recoil Detector Description Performance

Spin of the nucleon Spin of quarks Spin of gluons Orbital angular momentum of quarks Orbital angular momentum of gluons ~ 30%! How to access ?

Generalized Parton Distributions 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 (DE) and missing Mass (Mx) = 0 from DIS HERMES ΔΣ =0.330±0.011(theo.)±0.025(exp.)±0.028(evol.) A.Airapetian et al, Phys. Rev. D75(2007)012007

DVCS at HERMES two experimentally undistinguishable processes: Same initial and final state HERMES kinematics: BH c.s. >> DVCS c.s. DVCS DVCS BH Bethe-Heitler (BH) DVCS can be accessed via azimuthal asymmetries

Events selection without Recoil Detector Associated Bethe Heiler: Exactly one lepton track and one trackless cluster No proton detected but reconstructed via missing mass: from Monte Carlo

Can we detect the recoil proton? DVSC/BH recoil protons with lepton and γ in the forward spectrometer Proton momentum: P < 1400 MeV/c Polar angle: Θ < 1.35 rad

HERMES + Recoil Detector Hadrons Recoil proton γ e´

Recoil Detector Kinematic detection of the recoiling proton SFT P ∈ [135 – 1200] MeV/c p 76%  acceptance SFT 1 π/p PID via dE/dx SFT 2 background suppression SSD Frame semi-incl.: 5 %<<1% associated: 11%~1% SSD Hybrid SSD Connectors 1 TESLA magnetic field improved exclusivity improved t-resolution

Silicon Strip Detector (SSD) 2 layers of double sided TRIGER sensors operate in beam vacuum strips , pitch=758μm, thickness=300 μm HELIX chips with high and low gain Helix chips was disagne in Heidelberg Proton momentum: 135-500 MeV/c

Scintillating Fiber Tracker (SFT) p-measurement 250-1300 MeV/c π/p PID from dE/dx for p<700 MeV/c 2 cylinders of 2x2 layers, 10° stereo angle 1mm Kuraray fibers, mirrored ends and double cladding PMT Hamamatsu H7546B 5120 channels total 42 (36) inner (outer) modules 1318 (1320) fibers in inner parallel (stereo) sub-barrel 2198 (2180) fibers in outer parallel (stereo) sub-barrel

Photon Detector (PD) 3 layers tungsten-scintilator: A layer parallel to beam line, B and C layer stereo under +45°/-45° Strips: 2x1x28cm³ Use the same PMTs to the SFT Main function 1γ from π° decay Reconstruct π° if 2 γ‘s detected

E-P elastic scattering in Photon Detector Selection of elatic protons by making cuts on the momentum of the lepton detcted by the forward spectrometer.

E-P elastic scattering in Scintillating fiber tracker Select again elacstic candidate tracks in the forward spectrometer. Look at tracks reconstructed in SFT detector. Resolution meets specifications.

E-P elastic scattering in Silicon Strip Detector Difference between reconstructed azimuthal angle from particule detected in the silicon by the forward spectrometer and by the silicon detector Single lepton in foward spectrometer + single track reconstructed in silicon Mean 0.066±0.016 Sigma 2.436±0.014 Difference between z-vertex reconstructed from silicon and the forward spectrometer

Conclusion Recoil Detector: successfully installed SSD,SFT & PD: fully commisioned and taking data until the end of HERA – June, 2007 With the recoil detector, DVCS and other hard exclusive reactions can be precisely measured