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Exclusive SSA with a transverse polarized target

Exclusive SSA with a transverse polarized target. Cynthia Hadjidakis on behalf of the HERMES collaboration. Transversity Workshop, Athens October 6 th -7 th , 2003. Generalized Parton Distributions Exclusive reaction at HERMES ( p + , r 0 ) SSA on a longitudinally polarized target

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Exclusive SSA with a transverse polarized target

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  1. Exclusive SSA with a transverse polarized target Cynthia Hadjidakis on behalf of the HERMES collaboration Transversity Workshop, Athens October 6th-7th, 2003 • Generalized Parton Distributions • Exclusive reaction at HERMES (p+, r0) • SSA on a longitudinally polarized target • Analysis with a transverse polarized target

  2. Generalized Parton Distributions (GPDs) - Műller(1994) - - Ji & Radyushkin(1996) - Q2>> g*L t p0, r0L, g ... 4 GPDs defined for each quark flavour: HqHqconserve nucleon helicity EqEqflip nucleon helicity ~ -2 x ~ x+x x-x unpolarized polarized N N’ 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. 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 ~ ~ ~

  4. Factorization theorem for hard meson production t M g*L - Collins, Frankfurt & Strikman(1997) - Fas Q2>> t<< ~ ~ H, E, H, E p n handbag diagram Factorisation for longitudinalphotons only sT suppressed by 1/Q2 → at large Q2, sL dominates asymptotically |MM|2 for fixed xB and t

  5. Limiting cases and sum rules ~ Hq(x, x=0,t=0) = Dq(x) DVCS DIS Forward limit (t →0, x→0) Hq(x,x=0,t=0) = q(x) Sum rules dx Eq(x,x,t) = Fq2(t) dx Hq(x,x,t) = Fq1(t) ~ ~ dx Hq(x,x,t) = gqA(t) dx Eq(x,x,t) = hqA(t) Ji sum rules 30%(DIS) 1 ( H(x,x,t=0) + E(x,x,t=0) ) x dx =Jquark =1/2 DS+ D Lz -1

  6. Transverse parton localisation via GPDs GPDs = fourier transform of p.d.f. in position space • Burkard (2000) – • Diehl (2002) – Transverse polarized target: E ~ distortion of →TSA in hadron production reaction • Burkard (2002) – Jakob

  7. GPD predictions for e p → e p+n transverse asymmetry - Frankfurt, Pobylitsa, Polyakov & Strikman(1999) - ~ ~ interference between E and H - Frankfurt, Polyakov, Strikman & Vanderhaeghen(2000) - - Belitsky & Müller(2001) - ~ ~ sS=|ST| sinf E H ~ ~ ~ ~ • SSA linear dependenceE.H/cross section quadratic combination(E+H)2 • SSA higher order corrections cancel scaling region reached at lower Q2 ~ ~ • Constrainpole E and non-pole Hwould help the p FF extraction

  8. e p → e r0p transverse asymmetry interference between E and H - Goeke, Polyakov & Vanderhaeghen(2001) - Erelated to Jqthrough Ji sum rule Ju, Jd: total angular momentumcontribution for the u, d-quarks to the proton spin In s, Eis kinematically suppressed compared to H: AUTmost promising observable which allows an access to E xB

  9. Longitudinal target spin asymmetry → target spin anti-parallel to the beam momentum F Fit: AUL(F)=AULsin F Polarized cross section sS= [ST sL +SLsLT] AULsinF sLT suppressed by 1/Q but SL > ST=|S|sin qg Hermes kinematics: ST/|S|~0.17

  10. Exclusivity for e p → e p+(n) Detection: e, p++only 2 charged tracks (recoil neutron) Missing Mass technique: Missing Mass2 = (e+p-e’-p+)2 = Mn2 Kinematics cuts: Q2 > 1 GeV2, W2 > 4 GeV2 e p→e p+X e p→e p+n p+ enhancement Position and width in agreement with MC based on GPD model - Mankiewicz, Piller & Radyushkin (1999) -

  11. Exclusive p+ asymmetry Background correctedasymmetry: Fit: AUL(F)=AULsin F MX=1.05 GeV AUL=-0.18± 0.05 ± 0.02 ‹xB›=0.15 ‹Q2›=2.2 GeV2 ‹-t›=0.46 GeV2

  12. Exclusive p+ asymmetry x Q2 (GeV2) -t(GeV2) AUL= ST/SAUT =sin qgAUT limits for any asymmetry arising from transverse target component ST asymmetry at low x arises from longitudinal target component SL SL related to sLT (NLO): no theoretical interpretation yet

  13. Transverse target spin asymmetry 2002: run with a transverse polarized target f ↑↓ proton target spin Fit: AUT(F-FS)=AUTsin (F-FS)

  14. Exclusive p+: e p → e p+ (n) e p→e p+X e p→e p+n Fit to data -t<0.5 GeV2 counts Np+-Np- Missing Mass (GeV) Missing Mass (GeV) Missing Mass (GeV) 2002: Nexcl p+ ~ 200 Background subtraction method for different beam energy: Ee=27 GeV Ee=12 GeV

  15. Analysis status Hermes Kinematics (MX<1.3 GeV- no bg subtraction) <Q2> ~ 2.3 GeV2 <xB> ~ 0.15 <t> ~ -0.4 GeV2 Q2 (GeV2) xB -t (GeV2) →For these kinematics, AUT is predicted to be large 50-60% Nexcl p+ ~ 200 Up to now 700 k DIS 4 M DIS expected with the next run period →Nexcl p+ ~ 1200 extraction of exclusive TSA soon

  16. Exclusive reaction at Hermes: ongoing analysis MM→Mn z →1 Inclusive→exclusive region excl. p+ Pseudoscalar ratios: e p → e p+ n /e p → e p0 p e p → e hp /e p → e p0 p e p → e p0 p /e n → e p0 n Detection of the recoiling proton (2 years with recoil detector)

  17. Exclusivity for e d → e r0 (d) → e p+p- (d) Detection: e,p+, p- DE = (M2X-M2p)/2Mp Fit with skewed Breit-Wigner DE < 0.6 GeV -t’=-t+tmin<0.4 GeV2 0.6 < M2p< 1.0 GeV • Monte Carlo simulation of non-exclusive (DIS) background • Subtraction of non-resonant background • sL/ sT separation possible from decomposition of decay angular distributions

  18. Longitudinal SSA for exclusive r0 Background corrected asymmetry:

  19. Exclusive r0 asymmetry Asymmetry compatible with zero sL/sT separation from decay angular separation under way

  20. Summary and outlook • GPDs can be probed by hard exclusive meson production • Large transverse target asymmetry predicted for p+ and r0 at Hermes kinematics • With a longitudinally polarized target for exclusive processes at HERMES • p+ Large AUL At low x, the asymmetry arises from the longitudinal target component • r0 Asymmetry compatible with zero • 2004: end of transverse target runs →transverse spin asymmetry accessible both for p+ and r0→test of the GPDs predictions

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