Deeply Virtual Compton Scattering at HERMES

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Deeply Virtual Compton Scattering at HERMES
Weilin Yu II.Physikalisches Institut,
Justus-Liebig-Universität Gießen, Germany
for the HERMES Collaboration
The 5th International Conference on Quarks and
Nuclear Physics

• Spin of nucleon, GPDs and DVCS
• DVCS Results from HERMES
• The HERMES Recoil Detector

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Nucleon Spin and GPDs
HERMES Phys. Rev. D75(2007)
Poor information
Accessible through
Generalized Parton Distributions (GPDs)
four twist-2 GPDs when the
partons do not flip helicity
Ji Relation
t momentum transfer squared to target
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Nucleon Spin and GPDs
HERMES Phys. Rev. D75(2007)
Poor information
Accessible through
Generalized Parton Distributions (GPDs)
3D picture of nucleon
Parton Distribution Function
Form Factors
GPDs
Longi. Momentum
Trans. Position
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Deeply Virtual Compton Scattering (DVCS)
DVCS simplest/cleanest Hard Exclusive Process
to access GPDs
DVCS
Interference term I
Same final state
Difficult to measure the DVCS cross-section
directly at HERMES!!!
Bethe-Heitler
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Deeply Virtual Compton Scattering (DVCS)
DVCS simplest/cleanest Hard Exclusive Process
to access GPDs
DVCS
Interference term I
Unpolarized Proton Target
el beam charge, l beam helicity
Same final state
Bethe-Heitler
Azimuthal Asymmetries possible
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Azimuthal Asymmetries _at_ HERMES

• Beam-charge asymmetry BCA

F1, F2 Dirac and Pauli elastic nucleon form
factors

• Beam-Helicity asymmetry BSA

Compton form factor
convolutions of GPDs
If the beams have both charges
Asymmetries simultaneously extracted from data
via the combined fitting
BCA
BSA(interference)
BSA(DVCS)
Interference and DVCS terms separated!!!
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DVCS _at_ HERMES Without Recoil Detector

• Recoil protons identified by missing mass

different for e- and e
-1.52,1.72
15 overall background
MC studies used to estimate uncertainties from
alignment, acceptance, bin width
12 from associated BH production
Impossible to clean without the Recoil Detector!!!
Semi-inclusive p0 background corrected for the
results
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Beam Charge Asymmetries
GPD model Vanderhaeghen, Guichon, Guidal
(VGG) based on double distribution ansatz
Leading asymmetry
higher twist
fractions of associated BH production
The VGG model with a D-term is dis-favored by
the BCA measurement
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Beam Helicity Asymmetries

• BSA(DVCS term) from proton tagets data

const. term
higher twist
fractions of associated BH production
Pure DVCS squared asymmetries are compatible with
zero, in agreement with model assumptions.
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Beam Helicity Asymmetries

• BSA(interference term) from proton tagets data

const. term
Leading asymmetry
higher twist
fractions of associated BH production
VGG bands obtained by varying input parameters.
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Azimuthal Asymmetries _at_ HERMES

• Transverse target-spin asymmetry
  TTSA

U unpolarized beam, T proton target with
transversal polarization
AC and AUT can be simultaneously extracted if
both beam charges are available
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HERMES First Measurement of TTSA
sensitive to Ju
Only asymmetry where GPD E is not suppressed
Model VGG with variation of Ju while Jd 0
JHEP 06 (2008) 066.
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Azimuthal Asymmetries _at_ HERMES

• Transverse target-spin asymmetry
  TTSA

U unpolarized beam, T proton target with
transversal polarization
AC and AUT can be simultaneously extracted if
both beam charges are available

• Long. target-spin asymmetry LTSA

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DVCS from Deuterium Target
Low t coherent
High t incoherent
Proton and Deuterium targets results are
compatible.
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DVCS from Nuclear Targets
No nuclear mass dependence observed within
uncertainties
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The Recoil Detector

• A Recoil Detector installed to identify the
  recoiling protons
• Dedicated high luminosity run in 2006 - 2007
  with unpolarized hydrogen and deuterium targets

• PID detectors

• Tracking detectors

g
e'
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The Recoil Detector

• Consists of silicon detector, scintillating
  fiber tracker and photon detector
• Detection of recoiling proton
• p-measurement 150-1400 MeV/c
• 76 ? acceptance
• p/P PID via dE/dx

Photon Detector.
Silicon Detector

• Improvement of momenta resolution

Scintillating Fiber Tracker
1 tesla magnetic field
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The Recoil Detector

• Consists of silicon detector, scintillating
  fiber tracker and photon detector
• Detection of recoiling proton
• p-measurement 150-1400 MeV/c
• 76 ? acceptance
• p/P PID via dE/dx

Photon Detector.
Silicon Detector

• Improvement of momenta resolution

Scintillating Fiber Tracker
1 tesla magnetic field

• Backgroud suppression and study
• semi-incl. from 3 to ltlt1
• associated from 12 to 1
• asymmetries from the associated production?

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Conclusions

• Hermes provides a very useful framework to study
  DVCS and GPDs.
• Several DVCS azimuthal asymmetries were measured
  with respect to beam helicity and charge, and to
  target spin in the HERMES experiment.
• The recoil detector took data in 2006 and 2007,
  with which DVCS can be directly measured and
  background can be rejected.

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HERMES Experiment
HERA longi. polar. e beams, 27.5 GeV

• Tracking detectors

• PID detectors

• Pure gas target polarized H, D unpolarized H,
  D, N, Ne, Kr, He,Xe
• Forward spectrometer 40 mrad lt Q lt 220 mrad
• dp/p 2, dQ lt 1 mrad
• PID for e TRD, Preshower, Calorimeter

one lepton and one photon detected in the
calorimeter for DVCS!!!