Overview on hard exclusive production at HERMES

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Overview on hard exclusive production at HERMES
Cynthia Hadjidakis on behalf of the HERMES
collaboration
Conference on Elastic and Diffractive Scattering
Blois, 15-20 May, 2005

• Generalized Parton Distributions
• Compton scattering (DVCS)
• Exclusive mesons production
• Summary and perspectives

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Generalized Parton Distributions (GPDs)
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
- Muller (1994) -
- Ji Radyushkin (1996) -
Q2
Q2gtgt, tltlt
t
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How to access GPDs?
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
DVCS vector mesons pseudo-scalar
mesons pion pairs
? H
? H, E
? H, E
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HERMES at DESY
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
e-beam e/e-, Ee27.5 GeV, PB 55 spin
rotators _at_ HERMES for longitudinal beam
polarization
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HERMES spectrometer
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
Tracking system dP/P 2 , dq lt 1
mrad (charged) Particle Identification RICH,
TRD, preshower, calorimeter Photon calorimeter
dP/P 5 for high energy photon
no recoil detection e p ? e g (p) only e and
g detected Exclusive reaction signed via the
missing mass technique MX ( e p e g
) Exclusive reaction selected with a cut on MX
Background contamination estimated with
non-exclusive MC
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Deep Virtual Compton Scattering e p ? e p g
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
for HERMES kinematics DVCS ltlt Bethe-Heitler
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Beam spin and charge asymmetry
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
? H
PRL87,2001
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Beam charge asymmetry t-dependence
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
? H
GPD calculation different parameterization for
H

• Vanderhaeghen (2002)

AC sensitive to GPD-models
tiny e-p sample (L10 pb-1) HERA 2004-2005 e-
beam (x5)
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Longitudinal target spin asymmetry measurement
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
? H,H

sin f in agreement with GPD models
Coming soon transverse TSA sensitive to GPD E
(Jq)
unexpected large sin 2f from qgq correlations
twist-3 GPDs?
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r0 transverse target spin asymmetry
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
? H,E
xB
x
Large negative asymmetry at low x and large t
Same x-dependence behaviour as GPDs calculations
(no direct theoretical comparison
yet) transverse polarized target still running
(end 2005)
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p cross section measurement
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
? E, H

Comparison with GPD based model

• Vanderhaeghen, Guichon Guidal (1999) -

Q2 dependence is in general agreement with the
theoretical expectation Power corrections (k- and
soft overlap) calculations overestimate the data
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Factorization theorem prediction
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
? E, H

fit 1/Qp p1.90.5 p1.70.6 p1.51.0

interference between E and H leads to a large
transverse TSA asymmetry longitudinal TSA
published PLB535,2002 Coming soon transverse
TSA
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Pion pairs production e p (d)? e p (d) p p-
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
? H, E
Legendre moment
ltP1gt sensitive to the interference between
different pp- isospin states
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Legendre Moment Mpp dependence
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
? H, E
PLB599,2004
GPD model calculations ? quark exchange ? quark
2-gluon exchange
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Future analysis recoil detector
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
clean reaction identification improve statistical
precision (unpolarised data with high density
target) half time e- beam/e beam e/-p?
epg improve detector resolution it will allow
multi-dimensional binning in x and t
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Summary and outlook
Introduction DVCS Vector mesons
Pseudoscalar mesons Pion pairs Perspectives
GPDs probed by hard exclusive photon and meson
production Measurements of exclusive
processes at HERMES DVCS, vector mesons, p,
pp- winter 2005 recoil detector
installation HERMES dedicated to exclusive
processes!
Combining all the measurements will allow to
constrain GPD models