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Exclusive p + production at HERMES

Exclusive p + production at HERMES. Cynthia Hadjidakis, Delia Hasch on behalf of the HERMES collaboration. DIS 2004, Strbske Pleso, Slovakia 14 - 18 April, 2004. Generalized Parton Distributions Exclusive p + production at HERMES Target spin asymmetry and cross section measurements.

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Exclusive p + production at HERMES

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  1. Exclusive p+production at HERMES Cynthia Hadjidakis, Delia Hasch on behalf of the HERMES collaboration DIS 2004, Strbske Pleso, Slovakia 14 - 18 April, 2004 • Generalized Parton Distributions • Exclusive p+ production at HERMES • Target spin asymmetry and cross section measurements

  2. Generalized Parton Distributions (GPDs)

  3. Factorization theorem for meson production - Műller(1994) - Q2>>, t<< - Ji & Radyushkin(1996) - t M - Collins, Frankfurt & Strikman(1997) - g*L Fas 4 Generalized Parton Distributions (GPDs) HH conserve nucleon helicity EEflip nucleon helicity ~ ~ ~ ~ H,E,H,E unpolarized polarized N N’ handbag diagram Quantum number of final state selects different GPDs Vector mesons (r, w, f): unpolarized GPDs H E Pseudoscalar mesons (p, h): polarized GPDs H E (pion pole) ~ ~ Factorization for longitudinalphotons only asymptotically for fixed xB and t

  4. Exclusivity for e p → e p+(n) Detection: e, p+ (recoil neutron) Missing Mass technique: Missing Mass2 = (e+p-e’-p+)2 Use of p- yield to subtract the non exclusive background e p→e p+n e p→e p+X p+ enhancement Np+- Np- normalisation region Missing Mass 2 (GeV2) Missing Mass 2 (GeV2) Exclusive peak clearly centered at the nucleon mass

  5. Exclusivity for e p → e p+(n) Ee=27.5 GeV Ee=12 GeV mean=0.880.05 sigma=0.290.07 mean=0.870.05 sigma=0.680.04 Missing Mass 2 (GeV2) Missing Mass 2 (GeV2) For different beam energy, same exclusive peak at the nucleon mass stot = sT + esL L/T separation not possible Hermes kinematics: e > 0.80 sT suppressed by 1/Q2→ at large Q2, sL dominates

  6. Cross section determination L integrated luminosity DxDQ2 range in Q2 and x G(x,Q2) virtual photon flux factor Nexcl number of p+ background subtracted 1996-2000: hydrogen target (unpolarized and polarized) 14.2 M DIS events - 3500 exclusive p+ k(x,Q2): detection probability using 2 exclusive MC (different GPD parameterization) - Mankiewicz, Piller & Radyushkin (1999) - - Vanderhaeghen, Guichon & Guidal (1999) -

  7. Monte Carlo 1 1 Q2 (GeV2) xB -t (GeV2) • Data distributions background subtracted • Monte Carlo - arbitrary normalization - Mankiewicz, Piller & Radyushkin (1999) - - Vanderhaeghen, Guichon & Guidal (1999) -

  8. Detection probability probability to detect e and p+ (generated in 4p) with the Hermes detector - Mankiewicz, Piller & Radyushkin (1999) - - Vanderhaeghen, Guichon & Guidal (1999) - Systematic error ~ 5% (< 10 %)

  9. Cross section: Q2 dependence @ fixed x First measurements for Q2 dependence at fixed x

  10. Cross section: comparison with model No L/T separation but sT suppressed by 1/Q2 and e > 0.8 • Vanderhaeghen, Guichon & Guidal (1999) - p production: H pseudovector contribution E pseudoscalar contribution (pion pole related to Fp) ~ ~ Q2 dependence is in general agreement with the theoretical expectation Power correction calculations overestimate the data

  11. Asymmetry measurement for e p → e p+n - Frankfurt, Pobylitsa, Polyakov & Strikman(1999) - Transverse target spin asymmetry - Frankfurt, Polyakov, Strikman & Vanderhaeghen(2000) - ~ ~ - Belitsky & Müller(2001) - g*L p → p+ n interference between E and H ~ ~ sS: |ST| sinF E H TARGET SPIN ASYMMETRY ~ ~ ~ ~ TSA linear dependenceE.H/cross section quadratic combination(E+H)2 TSA higher order corrections cancel: scaling region reached at lower Q2 ~ ~ Constrain pole E and non pole H would help the p FF extraction

  12. Transverse target spin asymmetry F ↑↓ proton target spin Fit: AUT(f-fS)=AUTsin (f-fS) 2002-2004: run with a transverse polarized target Nexcl ~ 1000 extraction of exclusive TSA soon

  13. Longitudinal target spin asymmetry Fit: A(f)=AULsin f AUL=-0.18± 0.05 ± 0.02 ‹xB›=0.15 ‹Q2›=2.2 GeV2 ‹-t›=0.46 GeV2 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 asymmetry arises from longitudinal target component SL SL related to sLT (NLO): no theoretical interpretation yet

  14. Exclusive reaction at Hermes: future analysis Detection of the recoiling proton Pseudoscalar ratios: e p → e p+ n /e p → e p0 p e p → e hp / e p → e p0p e p → e p0 p /e n → e p0 n 2005: (2 years with recoil detector)

  15. Summary and outlook • GPDs can be probed by hard exclusive meson production • p+ measurements at HERMES: Cross section: first measurements for Q2 dependence at fixed x SSA: large longitudinal target spin asymmetry • 2004: end of transverse target runs: transverse spin asymmetry accessible for p+ • 2005: run with the recoil detector

  16. 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 2 < pp < 15 GeV

  17. GPDs: Q2 and x dependence - Vanderhaeghen, Guichon & Guidal (1999) - t=tmin Q2=2.4 GeV2 dsL/dt(nb/GeV2) kperp+overlap overlap ~ ~ LO LO (H+E) H E ~ kperp ~ x

  18. Binning choice Pp < 14 GeV

  19. Power correction • Vanderhaeghen, Guichon & Guidal (1999) - intrinsic transverse momentum of the active quark (kperp) (Q2>>) →0 soft overlap contribution → no gluon exchange

  20. Piller – VGG - Mankiewicz, Piller & Radyushkin (1999) - g*p  p+n - Vanderhaeghen, Guichon & Guidal (1999) - xB=0.1 Q2=2.4 GeV2 VGG Tuned VGG (E*et ) Piller parametrisation ~ t=tmin xB=0.1 Hermes kinematics arbitrary scale Q2 -t different dependence for VGG and Piller t=tmin Q2= 2.4 GeV2 t=-0.4 GeV2 Q2= 2.4 GeV2 xB xB

  21. p+ production g*L p+ H pseudovector contribution E pseudoscalar contribution(pion pole) ~ Fas ~ ~ ~ H,E p n - Vanderhaeghen, Guichon & Guidal (1999) - At low t, dominance of the pion pole: E related to Fp ~ ~ ~ Constrain pole E and non pole H would help the p FF extraction

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