A recoil detector to identify hard exclusive A recoil detector to identify hard exclusive reactions at the HERMES experimentreactions at the HERMES experiment

Die Deutsche Physikalische Gesellschaft e.V. (DPG)Die Deutsche Physikalische Gesellschaft e.V. (DPG)

Fachverband TeilchenphysikFachverband Teilchenphysik

T 510 Detektorsysteme IT 510 Detektorsysteme I

Heidelberg, 05.03.-09.03.2007Heidelberg, 05.03.-09.03.2007

Roberto Francisco Perez BenitoRoberto Francisco Perez Benito

OutlineOutline

• Spin of the nucleonSpin of the nucleon• DVCS at HERMESDVCS at HERMES• How to improve DVCS measurementsHow to improve DVCS measurements• Recoil DetectorRecoil Detector

– DescriptionDescription– PerformancePerformance

Spin of quarks

Spin of gluons

Orbital angular momentum of quarks

Orbital angular momentum of gluons

~ 30%!

How to access ?

Spin of the nucleon Spin of the nucleon

Generalized Parton DistributionsGeneralized Parton Distributions

Study of hard Study of hard exclusive processesexclusive processes leads to leads toa new class of PDFsa new class of PDFs

GGeneralized eneralized PParton arton DDistributionsistributions

possible access topossible access toorbital angular momentumorbital angular momentum

1

1 0

1

2q

qq

t

HJ Exdx

1

2q qLJ

from DISfrom DISHERMES HERMES

ΔΣΔΣ =0.330±0.011(theo.)0.330±0.011(theo.)±±0.025(exp.)0.025(exp.)±±0.028(evol.)0.028(evol.)A.Airapetian et al, Phys. Rev. D75(2007)012007A.Airapetian et al, Phys. Rev. D75(2007)012007

exclusive:exclusive: all products of all products of the reaction are detectedthe reaction are detected

missing energy (missing energy (E) and missing Mass (ME) and missing Mass (Mxx) = 0) = 0

qqqq EHEH~

,~

,,

DVCS at HERMESDVCS at HERMES

two experimentally undistinguishable processes:two experimentally undistinguishable processes:Same initial and final state Same initial and final state

HERMES kinematics:HERMES kinematics:BH c.s. >> DVCS c.s.BH c.s. >> DVCS c.s.

DVCSDVCS Bethe-Heitler (BH)Bethe-Heitler (BH)

* 2 2*~ | | | |BH DVCS DVC BS HB DVCSHd

DVCS can be accessed via azimuthal asymmetriesDVCS can be accessed via azimuthal asymmetries

DVCSDVCS

BHBH

Events selection without Recoil DetectorEvents selection without Recoil Detector

Exactly one lepton track and one trackless clusterExactly one lepton track and one trackless cluster

No proton detected but reconstructed via missing mass:No proton detected but reconstructed via missing mass: from Monte Carlo from Monte Carlo

n

pp

0

Associated Bethe Heiler:Associated Bethe Heiler:

Can we detect the recoil proton?Can we detect the recoil proton?

DVSC/BH recoil protons with lepton DVSC/BH recoil protons with lepton and and γγ in the forward spectrometerin the forward spectrometer

Proton momentum:Proton momentum:P < 1400 MeV/cP < 1400 MeV/cPolar angle:Polar angle: ΘΘ < 1.35 rad < 1.35 rad

γRecoil proton Hadrons

e´

HERMES + Recoil DetectorHERMES + Recoil Detector

Recoil Detector KinematicRecoil Detector Kinematic detection of the recoiling protondetection of the recoiling proton

P P ∈∈ [135 – 1200] MeV/c[135 – 1200] MeV/c76% 76% acceptance acceptanceππ/p PID via dE/dx/p PID via dE/dx

background suppressionbackground suppressionsemi-incl.: 5 semi-incl.: 5 %<<1%%<<1%associated: 11%~1%associated: 11%~1%

improved exclusivityimproved exclusivity

improved t-resolutionimproved t-resolution

SFT SFT

SFT 1SFT 1

SFT 2SFT 2

SSDSSDConnectorsConnectors

SSD HybridSSD Hybrid

SSD FrameSSD Frame

1 TESLA magnetic field1 TESLA magnetic field

pp

Silicon Strip Detector (SSD)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)Scintillating Fiber Tracker (SFT)

42 (36) inner (outer) modules1318 (1320) fibers in inner parallel (stereo) sub-barrel2198 (2180) fibers in outer parallel (stereo) sub-barrel

• 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

Photon Detector (PD)Photon Detector (PD)• 3 layers tungsten-scintilator3 layers tungsten-scintilator::

A layer parallel to beam line,A layer parallel to beam line,B and C layer stereo under +45°/-45°B and C layer stereo under +45°/-45°

• StripsStrips: 2x1x28cm³: 2x1x28cm³• Use the same PMTs to the SFTUse the same PMTs to the SFT• Main functionMain function

– 11γγ from from ππ° decay ° decay – Reconstruct Reconstruct ππ° if 2 ° if 2 γγ‘s detected‘s detected

E-P elastic scattering inE-P elastic scattering in Photon Detector Photon Detector

Selection of elatic protons by making cuts on the momentum Selection of elatic protons by making cuts on the momentum of the lepton detcted by the forward spectrometer.of the lepton detcted by the forward spectrometer.

E-P elastic scattering inE-P elastic scattering in Scintillating fiber trackerScintillating fiber tracker

Select again elacstic candidate tracks in the forward spectrometer.Look at tracks reconstructed in SFT detector. Resolution meets specifications.

Ep elastic scatte

ring

Ep elastic scatte

ring

E-P elastic scattering in E-P elastic scattering in Silicon Strip DetectorSilicon Strip Detector

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

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 0.066Mean 0.066±0.016±0.016Sigma 2.436±0.014Sigma 2.436±0.014

ConclusionConclusion

• Recoil Detector: successfully installedRecoil Detector: successfully installed

• SSD,SFT & PD:SSD,SFT & PD:

fully commisioned and taking data until the end of fully commisioned and taking data until the end of HERA – June, 2007 HERA – June, 2007

• With the recoil detector, DVCS and other With the recoil detector, DVCS and other hard exclusive reactions can be precisely hard exclusive reactions can be precisely measuredmeasured