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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

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Heidelberg, 05.03.-09.03.2007

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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


  • Spin of the nucleon


  • 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


ΔΣ =0.330±0.011(theo.)±0.025(exp.)±0.028(evol.)

A.Airapetian et al, Phys. Rev. D75(2007)012007


HERMES kinematics:

BH c.s. >> DVCS c.s.

two experimentally undistinguishable processes:

Same initial and final state




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 γin the forward spectrometer

Proton momentum:

P < 1400 MeV/c

Polar angle:

Θ < 1.35 rad

HERMES + Recoil Detector



Recoil proton

Recoil Detector Kinematic

detection of the recoiling proton

P ∈[135 – 1200] MeV/c



76%  acceptance


π/p PID via dE/dx


background suppression

SSD Frame

semi-incl.: 5 %<<1%

associated: 11%~1%

SSD Hybrid



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) modules1318 (1320) fibers in inner parallel (stereo) sub-barrel2198 (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 inScintillating fiber tracker

Ep elastic scattering

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±0.016Sigma 2.436±0.014

Difference between z-vertex reconstructed from silicon and the forward spectrometer


  • 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

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