Roberto Francisco Pérez BenitoOn behalf the HERMES Collaboration

DVCS, Nuclear DVCS &

HERMES Recoil Detector

Internetional WorkshopDense and Cold Nuclear Matter

and Hard Exclusive Processes

Aug 20-24, 2007Het Pand, Ghent University, Ghent, Belgium

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OutlineOutlineOutlineOutline

Motivation The Spin Structure of the Nucleon General Parton Distribution (GPD) DVCS & BH

Recents HERMES results Beam Spin Asymmetry (BSA) Beam Charge Asymmetry (BCA) Transverse Target-Spin Asymmetry (TTSA) DVCS on Nuclei

The Hermes Recoil Detector Design Requirement Silicon Strip Detector (SSD) Scintillating Fiber Tracker (SFT) Photon Detector (PD)

Performance

Outlook

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Parameterization of the Nulceon StructureParameterization of the Nulceon StructureParameterization of the Nulceon StructureParameterization of the Nulceon Structure

Form Factors → Transverse position ← Elastic Scattering

FFs characterize charge and magnetization distrubutions in

the impact parameter space (transverse plane in the LMF).

Form Factors (FFs)

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Parameterization of the Nulceon StructureParameterization of the Nulceon StructureParameterization of the Nulceon StructureParameterization of the Nulceon Structure

Form Factors Parton Distribution (FFs) Functions (PDFs) Form Factors → Transverse position ← Elastic Scattering

PDFs → Longitudinal Momentum Distribution ← DIS PDFs characterize longitudinal momentum distributions (LMF)

x – Longitudinal momentum fraction

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Parameterization of the Nulceon StructureParameterization of the Nulceon StructureParameterization of the Nulceon StructureParameterization of the Nulceon Structure

Form Factors PDF GPD

Form Factors → Transverse position ← Elastic Scattering

PDFs → Longitudinal Momentum Distribution ← DIS

GPDs → Access to Transverse Position and Longitudinal Momentum

Distribution at the SAME time, 3-D picture ← Exclusive Reactions

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The Spin Structure of the NulceonThe Spin Structure of the NulceonThe Spin Structure of the NulceonThe Spin Structure of the NulceonNulceon Spin:

Spin of quarks

measured in DIS

HERMES: Spin of gluons, first Measurements

expected to be small Orbital angular momentum of

quarks Orbital angular momentum of

gluons

unknown

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Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)

Ji Sum Rule – Ji, PRL 78(1997)610

parton longitudinal momentum fractions

fraction of the momentum transfer

invariant momentum transfer to the nucleon

GPDs

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Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)Nucleon Structure: GPDs

GPDs → PDFs (GPDs in the limite t → 0)

GPDs → FFs (First moments of GPDs)

conserve nucleon helicity

flip nucleon helicity

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Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)Generalized Parton Distributions (GPDs)Simplest/Cleanest Hard Exclusive Process

Deeply-Virtual Compton Scattering (DVCS):

Longitudinal momentum fractions:(not accessible)

( Momentum Transfer)

Measurements as function of

DVCS: Access to all four

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GPDs and the DVCS processGPDs and the DVCS processGPDs and the DVCS processGPDs and the DVCS process

DVCS final state is indistinguishable from theBethe-Heitler Process (BH) → Amplitudes add Coherently

Photon-Production cross section:

Interference Term

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DVCS MeasurementsDVCS MeasurementsDVCS MeasurementsDVCS Measurements

calculable in QED with the knowledge of the Form Factors

is parameterized in terms of Compton Form Factors

(convolutions of GPDs )

At HERMES kinematics:

GPDs accessible through cross-section differences and azimuthal asymmetries via interference term I → Magnitude + Phase(GPDs enter in linear combinations)

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Azimuthal AsymmetriesAzimuthal AsymmetriesAzimuthal AsymmetriesAzimuthal AsymmetriesBeam-Charge Asymmetry (BCA)

Beam-Spin Asymmetry (BSA)

.

TTSA is only asymmetry where GPD E is not suppressed

Models for depend on TTSA is sensitive to

Transverse Target Spin Asymmetry (TTSA)

Longitudinal Target Spin Asymmetry (LTSA)

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The HERMES SpectrometerThe HERMES SpectrometerThe HERMES SpectrometerThe HERMES SpectrometerHERA Beam: 27.6GeV, or ,

Events with exactly one positron or electron and exactly one photon in the calorimeter

Polarize gas targets: H,D,HeUnpolarize gas targets: H,D,N,He,Ne,Xe,Kr

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Exclusivity for DVCS via Missing Mass Exclusivity for DVCS via Missing Mass Exclusivity for DVCS via Missing Mass Exclusivity for DVCS via Missing Mass Exactly one DIS lepton and one photon detected in the Calorimeter.

Recoiling proton undetected.

Exclusivity via Missing Mass

exclusive region

Overall background contribution in exclusive region

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Beam-Spin Asymmetry (BSA)Beam-Spin Asymmetry (BSA)Beam-Spin Asymmetry (BSA)Beam-Spin Asymmetry (BSA)

Expected sin dependence in exclusive regionsin amplitudes small and positive above exclusive region

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Beam-Charge Asymmetry (BCA)Beam-Charge Asymmetry (BCA)Beam-Charge Asymmetry (BCA)Beam-Charge Asymmetry (BCA)

Calculate „Symmetrized“ BCA ( → || ) to get rid of all sin - dependences due to polarized beam.

Symmetrizised BCA in exclusive bin → || → Cancel sin - dependences

cos - amplitudes zero for higher missing masses

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Transverse Target-Spin AsymmetryTransverse Target-Spin AsymmetryTransverse Target-Spin AsymmetryTransverse Target-Spin Asymmetry

Only asymmetry with GPD E is not suppressed

Access total angular momentum

largely independent on all model parameters but

(F. Ellinghaus, Nowak, Vinnikov, Ye, EPJ C46(2006), HEP-PH/0506264)

sensitive to

(calculations for )

First model dependent extraction of possible!

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Investigate the internal structure of Investigate the internal structure of NNUCLEIUCLEI Investigate the internal structure of Investigate the internal structure of NNUCLEIUCLEI

DVCS on NEON (HEP-EX/0212019) triggerred first calculation for DVCS on NUCLEI

Possibility to explore nuclear structure in term of Quarks and gluons, EMC effect, (Anti-)shadowing, Color Transparency,...

Hermes nuclear tragets:

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Contributions from different processes from MC Contributions from different processes from MC Contributions from different processes from MC Contributions from different processes from MC

LEPTO BETHE-HEITLER simulation

Coherent BETHE-HEITLER process dominates at small –t'

Incoherent BETHE-HEITLER process dominates at large –t'

Background contributions: Semi-inclusive Resonances

Chose –t' cut for each enriched sample to provide target-independent 〈 -t' 〉 : Coherent: 〈 -t' 〉 = 0.018 GeV² Incoherent: 〈 -t' 〉 = 0.2 GeV²

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p

e´

HERMES Detector HERMES Detector HERMES Detector HERMES Detector

95 - 05 exclusivty through missing mass cut 06 - 07 Recoil detector installed to identify the recoiling proton

γ

Green PID detectors Red track detectors

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RD - Design RequirementsRD - Design Requirements RD - Design RequirementsRD - Design Requirements

Improve Exclusivity Detect recoiling proton Background suppression

semi-incl. : 5%→<<1% BH with resonace excitation:

11%→~1%

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Recoil DetectorRecoil Detector Recoil DetectorRecoil Detector Photon Detector 3 layers of Tungsten/Scintillator PID for higher momentum detects Δ+p0

1 Tesla Superconducting Solenoid

Scintillating Fiber Detector 2 Barrels 2 Parallel- and 2 Stereo-Layers

in each barrel 10° Stereo Angle Momentum reconstruction & PID

Silicon Detector 16 double-sides sensors

perpendicular with respect to each other

97×97 mm2 active area each 2 layers Inside HERA vacuum Momentum reconstruction & PID

Target Cell

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Target CellTarget CellTarget CellTarget Cell

Target cell inside beam pipe

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Silicon Strip Detector (SSD)Silicon Strip Detector (SSD) Silicon Strip Detector (SSD)Silicon Strip Detector (SSD) 2 layers of double sided TIGRE sensors 16 TIGRE sensors operate in beam vacuum few cm close to the beam Size 97mmΧ 97mm, thickness=300μm 128 strips per side , perpendicular w.r.t. each other, pitch=758μm HELIX chips are ADC and running under same condition The high and low gain yields from charge sharing

8192 channels in total

Proton momentum 135-500 MeV/c

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Scintillating Fiber Tracker Scintillating Fiber Tracker Scintillating Fiber Tracker Scintillating Fiber Tracker 2 cylinders of 2Χ2 layers,

10° stereo angle 1mm Kuraray fibers,

mirrored ends and double cladding

PMT Hamamatsu 64 channels 5120 channels in total

Proton momentum

250-1200 MeV/c

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3 layers of Tungsten/Scintillator A layer parallel to beam line,

B and C layer stereo under +45°/-45°

Strips: 2x1x28cm³ same PMTs as for SFT are used Main purpose

1γ from π° decay Reconstruct π° if 2 γ‘s detected

Photon DetectorPhoton Detector Photon DetectorPhoton Detector

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Recoil DetectorRecoil Detector Recoil DetectorRecoil Detector

electron/positron beam proton beam

HERMES magnet

PMTs SFT

Recoil detector

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Recoil detector AlignmentRecoil detector AlignmentRecoil detector AlignmentRecoil detector Alignment Six parameters (three translations and three rotations) which are common for all

tracks are fitted Residuals and dependence of residuals on coordinates used as a tool to check

alignment procedure

=0.28mm

SFT Paralleltop

Si

Si alignment respect SFT with tracks

Next step is Recoil-Hermes alignment using e-p elastic

=0.26 strip

Recoil detector AlignmentRecoil detector AlignmentRecoil detector AlignmentRecoil detector Alignment

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Momentum ReconstructionMomentum Reconstruction Momentum ReconstructionMomentum Reconstruction

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Momentum ReconstructionMomentum Reconstruction Momentum ReconstructionMomentum Reconstruction

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Momentum ReconstructionMomentum Reconstruction Momentum ReconstructionMomentum Reconstruction

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Full tracking including alignment is in production

Efficiency of the tracking algorithm studied on MC and found to be above 98%

Starting to study the efficiencies, residuals and ghost tracks

p

π-π +

PID with real dataPID with real dataPID with real dataPID with real data

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Collected statistic (preliminary) with recoil detector

Electron beam 2006 (only SFT operational):

H : 5k DVCS (3Mio. DIS), D : 1k DVCS (0.8Mio. DIS)

Positron beam 2006/07 (all subdetectors fully operational)

H : 42k DVCS (28Mio. DIS), D : 10k DVCS (7Mio. DIS)

Data tacking and PerformanceData tacking and PerformanceData tacking and PerformanceData tacking and Performance

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2 2

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Outlook Outlook Outlook Outlook Analysis with Recoil Detector.

DVCS Beam Charge Asymmetry (BCA) Challenging as only Fiber Tracker operational

DVCS Beam Spin Asymmetry (BSA)

Hard exclusive meson production Different flavor combinations of GPD’s