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

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)

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

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 ~ ~ ~

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 98% for pp > 2 GeV

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

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

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

DVCS asymmetry measurement e p → e p g e+/- p → e p g PRL, 87 (2001), 182001 ‹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

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

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

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

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) -

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

Exclusive reaction at Hermes: ongoing analysis Run with transverse polarized target (2002-2005 ) - 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

→ 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