1. Exclusive production at HERMES
Cynthia Hadjidakis
on behalf of the HERMES collaboration
Rencontres de Moriond, La Thuile
March 28th – April 4th, 2003
Exclusive physics and Generalized Parton Distributions
Compton scattering (DVCS)
Vector meson production
Pion production

2. Generalized Parton Distributions (GPDs)
Q2>>, t<<
g*L t
p0, r0L, g ...
- Műller (1994) -
- Ji & Radyushkin (1996) -
-2 x
4 GPDs defined for each quark flavour:
Hq Hq conserve nucleon helicity
Eq Eq flip nucleon helicity ~
x+x
x-x ~ N N’
unpolarized
polarized
3 variables: x, x, t
x+x
Longitudinal momentum fraction of the quark
-2x
Exchanged longitudinal momentum fraction t
Squared momentum transfer
GPDs = probability amplitude for N to emit a parton (x+x) and for N’ to absorb it (x-x)

3. Limiting cases and sum rules
30%(DIS)
Ji sum rules
( H + E ) x dx = Jquark
=1/2 DS + D Lz

4. Generalized Parton Distributions (GPDs)
Quantum number of final state selects different GPDs
Vector mesons (r, w, f)
unpolarised GPDs H E
Pseudoscalar mesons (p, h)
polarised GPDs H E
DVCS (g) depends on both unpolarised and polarised GPDs ~ ~
Different combination of GPDs according to the reaction:
flavour decomposition ~ ~ ~
Hpp0: 2/3 Hu/p + 1/3 Hd/p
Hpp+: Hu/p Hd/p ~ ~ ~

5. HERMES Spectrometer → →
e+, e-
27.5 GeV
Tracking: 57 tracking planes dP/P = 2 %, dq < 0.6 mrad
Particle Identification: RICH, TRD, preshower, calorimeter
lepton identification 99%
p identification % for pp > 2 GeV

6. Deep Virtual Compton Scattering: e p → e p g
BH larger than DVCS for HERMES kinematics:
DVCS
Bethe-Heitler
DVCS-BH interference
leads to non-zero azimuthal asymmetry

7. Azimuthal asymmetry g DVCS-BH interference gives
a non zero azimuthal asymmetry: g
- Beam helicity asymmetry
- Beam charge asymmetry

8. Exclusivity for e p → e p g
Missing Mass2 = ( p + g* – g )2
Missing Mass resolution : 0.8 GeV
Exclusive region: Missing Mass < 1.7 GeV

9. DVCS asymmetry measurement
e p → e p g
e+/- p → e p g
PRL, 87 (2001),
‹xB›=0.11
‹Q2›=2.6 GeV2
‹-t›=0.27 GeV2
ALU = -0.230.04(stat)0.03(syst)
AC=0.110.04(stat)0.03(syst)
Expected cos F dependence
sin F dependence
Kivel, Polyakov & Vanderhaeghen (2001) –
Signal of DVCS process
Can be described by GPD calculation

10. HERMES upgrade: the recoil detector
projection of the statistical accuracy
of ALU for 2 fb-1 (1 year of data)
Around the target: reconstruct the kinematics of the recoil particles

11. Vector mesons g*L r0L,wL,fL p p Missing Mass2 = ( g* - p+ - p- )2
e p → e r0 (p)
p+ p-
Missing Mass2 = ( g* - p+ - p- )2
e p → e w (p)
p+ p- p0
e p → e f (p)
K+ K-
DE = (M2X-M2p)/2Mp
Monte Carlo simulation of non-exclusive background
sL/ sT separation from decomposition of decay angular distributions

12. Cross sections for r0, f W [GeV] GPD calculation:
2 gluon exchange mechanism
quark exchange mechanism
Guichon, Guidal & Vanderhaeghen (2001) –

13. Exclusivity for e p → e p+(n)
Missing Mass2 = ( g* + p - p+ )2
Use of p- yield to subtract the non exclusive background
p+ enhancement
Exclusive neutron peak: position and width in agreement with MC based on GPD model
- Mankiewicz, Piller & Radyushkin (1999) -

14. Longitudinal target spin asymmetry
e p → e p+ n
AUL=-0.18 ± 0.05 ± 0.02
Polarized cross section:
sS= [ST sL +SL sLT] AUL sinF
‹xB›=0.15
‹Q2›=2.2 GeV2
‹-t›=0.46 GeV2
sLT suppressed by 1/Q
but SL > ST=|S|sin qg
Hermes kinematics: ST/|S|~0.17
AUL = ST/S AUT
2005: end of run with a tranverse polarized target

15. Exclusive reaction at Hermes: ongoing analysis
Run with transverse polarized target ( )
- Goeke, Polyakov & Vanderhaeghen (2001) -
- Frankfurt, Pobylitsa, Polyakov, Strikman (1999) -
g*L p → p+ n
TARGET SPIN ASYMMETRY
Target spin asymmetry: most promising observable which allow to access J

16. → transverse spin asymmetry accessible both for p+ and r0
Summary and outlook
GPDs can be probed by hard exclusive meson production
Measurements at HERMES:
DVCS: clear signal of the process in agreement with GPD
Vector mesons: longitudinal cross sections measurement in agreement with GPD calculation
Pion: large longitudinal target spin asymmetry
2005: end of transverse target runs
→ transverse spin asymmetry accessible both for p+ and r0
2005: run with the recoil detector