Exclusive pion electroproduction at HERMES

1
Exclusive pion electroproduction at HERMES
LNF Spring School May 19th-23rd, 2003
Cynthia Hadjidakis

• Physics motivation GPDs
• Exclusive pion electroproduction at HERMES
• Single Spin Asymmetry measurements
• Cross section studies

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Physics motivations GPDs
Q2 gtgt t ltlt
(Collins, Frankfurt, Strikman)


Hq, Eq
4 GPDs
Hq, Eq
vector meson
pseudoscalar meson
final state select GPDs
handbag diagram
Links between GPDs, form factors and structure
functions
2nd moment of the GPDs give access to the
contribution of the quark orbital angular
momentum D Lz to the spin of the nucleon
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GPD predictions for e p ? e pn asymmetry

• interference between E and H

transverse spin asymmetry
Frankfurt, Pobylitsa, Polyakov, Strikman PRD
60(1999), 014010

• NLO corrections cancel in asymmetry measurements

scaling region reached at lower Q2 than for
absolute cross section
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Exclusivity at Hermes

• Detection e, p
• Missing mass technique Missing Mass2
  (ep-e-p)2 Mn2

excl. events
e p ? e p X
e p ? e p n

• s 230 MeV do not resolve the exclusive peak
• Use of p- yield to subtract the background

excl. events
clear peak at Missing Mass Mn
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Single-spin asymmetry on a longitudinally
polarized target
xB0.15 Q22.2 GeV2 -t0.46 GeV2
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Result on a longitudinal target AUL
Hermes kinematics
Lines indicate limits for any asymmetry arising
from transverse target component
2002 run with a transversally polarized target
Lines indicate limit for any asymetry arising
from transverse target component
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Cross Section - Monte Carlo studies
To correct the data from the Hermes detector
acceptance

• Monte Carlo GPDs model for exclusive p

L. Mankiewicz, G. Piller, A. Radyushkin

• Good agreement between data and Monte Carlo

• Low efficiency 10-3-10-1 systematic studies to
  do using another Monte Carlo

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Conclusion and perspectives

• Exclusive p electroproduction at Hermes
• Results with a longitudinally polarized target
• Actually
• Analysis of the transverse polarized target data
• Cross section studies