1. 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,

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

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

4. 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

5. 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

6. 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

7. 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

8. HERMES + Recoil Detector
Hadrons
Recoil proton γ e´

9. 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

10. 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: MeV/c

11. Scintillating Fiber Tracker (SFT)
p-measurement 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

12. 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

13. 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.

14. 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.

15. 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 ±0.016 Sigma 2.436±0.014
Difference between z-vertex reconstructed from silicon and the forward spectrometer

16. 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