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Hard exclusive production at HERMES

Hard exclusive production at HERMES. Cynthia Hadjidakis. 2 nd workshop on the QCD structure of the Nucleon Rome, 12-16 June, 2006. Generalized Parton Distributions Compton scattering (DVCS) Exclusive mesons production Summary and perspectives. p 0 , r 0 L , g.

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Hard exclusive production at HERMES

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  1. Hard exclusive production at HERMES Cynthia Hadjidakis 2nd workshop on the QCD structure of the Nucleon Rome, 12-16 June, 2006 • Generalized Parton Distributions • Compton scattering (DVCS) • Exclusive mesons production • Summary and perspectives

  2. p0, r0L, g GPDs depend on 3 variables: x, x, t for each quark flavour Hq, for gluon Hg 0.2-0.3 (DIS) quark flavour decomposition possible from meson production Ji’s sum rule: 1 = D S + J L q q 2 Hard exclusive production of photons and mesons Q2 4 Generalized Parton Distributions (GPDs) HH conserve nucleon helicity EEflip nucleon helicity Q2>>, t<< ~ -2 x ~ x+x x-x Pseudoscalar mesons (p, h) Vector mesons (r, w, f) DVCS (g) depends on 4 GPDs t t 30%(DIS) 1 ( H(x,x,t=0) + E(x,x,t=0) ) x dx =Jquark =1/2 DS+ D Lz -1

  3. HERMES kinematics coverage GPDs formalism: Q2>>, t<< HERMES: <Q2>=2.4 (1-10) GeV2, -t < 0.5 GeV2 • collider experiments • H1, ZEUS • 10-4<xB<0.021 :gluons in the proton • fixed target experiments • COMPASS, HERMES •  0.006/0.02<xB<0.3 : gluons/valence and sea quarks • CLAS •  0.15<xB<0.6: valence quarks

  4. → → e +/ e - 27.5 GeV PB= 55% Target: polarized H, D / unpolarized H, D, N, Ne, Kr, Xe 1H→<|Pt|> ~ 85 % 2H→<|Pt|> ~ 85 % 1H↑ <|Pt|> ~ 75 % HERMES spectrometer Tracking system: dP/P = 2 %, dq < 1 mrad (charged) Particle Identification: RICH,TRD,preshower, calorimeter Photon: calorimeter: dP/P = 5 % for high energy photon no recoil detection e+ p → e+g (p) only e+ and gdetected Exclusive reaction signed via the missing mass technique MX = ( e + p – e’ – g ) Exclusive reaction selected with a cut on MX Background contamination estimated with non-exclusive MC

  5. DVCS ~ ~  H, H, E, E Bethe-Heitler DVCS DVCS-BH interference leads to non-zero azimuthal asymmetry Deep Virtual Compton Scattering: e p → e’ p’ g for HERMES kinematics: DVCS <<Bethe-Heitler

  6. DVCS ~ ~  H, H, E, E ~ DsC~cosfRe{ H+ xH + k E} DsLU~sinfIm{H+ xH+ k E} DsUT  Different polarisations : ~ DsUL ~sinfIm{H+ x(H+ …} beam target DVCS asymmetries I~Ds  Different charge : e+ e-(only at HERA!) : H ~ H ~ H, H DsUT ~sinfIm{H- E + … } H, E Suppressed by kinematical factor x = xB/(2-xB ),k = -t/4M2

  7. DVCS ~ ~  H, H, E, E Beam spin and charge asymmetry Beam Spin Asymmetry Beam Charge Asymmetry [PRL87,2001] symmetrizationf → |f| (cancel sin f terms from polarized beam) [hep-ex/0605108, subm. to PRL] e+/- p→ e+/- p g (MX<1.7 GeV) ─ P1 + P2 cos f + P3 cos 2f + P4 cos 3f L=140 pb-1 L=10 pb-1 P1 = -0.01±0.02 P2 = 0.06±0.03 P3 = 0.02±0.03 P4 = 0.03±0.03 <-t> = 0.12 GeV2,<xB> = 0.1, <Q2> = 2.5 GeV2

  8. DVCS e+/- p→ e+/- p g (MX<1.7 GeV) (in HERMES acceptance) Regge, D-term Regge, no D-term fac., D-term fac., no D-term ~ ~  H, H, E, E Beam charge asymmetry: t-dependence GPD calculation: different parameterization for H [Vanderhaegen et.al. (1999)] H = double distribution~ q(x) with skewing effect D-term or not t dependence: Regge-inspired t-dependence factorized t-dependence (ebt) →ACsensitive to GPD-models tiny e-p sample (L=10 pb-1) HERA: 2004-2005e- beam (x10) P1 = -0.01±0.02 P2 = 0.06±0.03 P3 = 0.02±0.03 P4 = 0.03±0.03 <-t> = 0.12 GeV2,<xB> = 0.1, <Q2> = 2.5 GeV2 symmetrizationf → |f| (cancel sin f terms from polarized beam)

  9. DVCS ~ ~  H, H, E, E Longitudinal target spin asymmetry Lp = 50 pb-1 Ld = 170 pb-1 sin f in agreement with GPD models unexpected large sin 2f (NLO contributions): from qGq correlations twist-3 GPDs?

  10. DVCS + 2005: 2 times more statistics ~ ~  H, H, E, E Transverse target spin asymmetry ~ AUT~sin(f-fS) cos(f)Im{H- E + … }+ cos(f-fS) sin(f)Im{H + … } GPD calculation: [Goeke et.al. (2001)] , [Ellinghaus et.al. (2005)] H = double distribution Regge-inspired t-dep. D-term E = double distribution ~ sensitive to Jq: Ju(Jd=0) factorized t-dep. (dipole form factor) L= 64 pb-1 →First (model dependent) constraints on Ju and Jd ! talk by Zhenyu Ye

  11. DVCS ~ ~  H, H, E, E DVCS on nuclear target GPDs modification in nuclear matter: spatial distribution of energy, angular momentum and shear forces inside the nuclei • coherent nuclear DVCS (-t<0.05 GeV2) different from proton DVCS • incoherent nuclear DVCS similar to proton DVCS (small BH cross section on neutron at small t) • proton and deuteron data consistent • highest t-bin may be affected by associated production (30%) 2H (720 pb-1), 4He (30 pb-1), 14N (50 pb-1), Ne (86 pb-1), Kr (135 pb-1), Xe (80 pb-1) study of properties of quarks and gluons inside nuclei

  12. DVCS ~ ~  H, H, E, E Beam spin asymmetry on nuclear target L=30 pb-1 L=86 pb-1 • → clear sin f amplitude in the exclusive region for Ne and Kr • → soon: Anucleus/Aproton (He, N, Ne, Kr, Xe) • t-dependence (separation of coherent and incoherent part) • A-dependence for coherent production [Guzey et al. (2003)], [Liuti et al. (2005)]

  13. Meson production: factorization for longitudinal photons only sTsuppressed by 1/Q2→at large Q2, sL dominates Meson production: wave function: additional information/uncertainty « scaling law » asymptotically for fixed xB and t Factorization theorem for meson production Q2 Q2>>, t<< hard scale t

  14. VECTOR MESONS  H, E Vector Mesons cross sections • ~ |∫ dx H(x,x,t) + E(x,x,t) |2 E kinematically suppressed at low t H = double distribution~ q(x)/G(x) with skewing effect factorized t-dependence (ebt with slope from data) transverse target spin asymmetry AUT~ Im(H .E ) E = double distribution~ sensitive to Jq factorized t-dependence (dipole form factor) higher order corrections cancel: scaling region reached at lower Q2

  15. VECTOR MESONS Fit with skewed Breit-Wigner 0.6 < M2p< 1.0 GeV -t’< 0.4 GeV2 • data • non exclusive MC DE < 0.6 GeV -t’< 0.4 GeV2 e p → e r0(p): exclusive r0 selection r0→p+p- : h+h- detected Missing energy DE = (M2X-M2p)/2Mp (MX = e + p – e’ – h+ – h- ) 0.6 < M2h< 1.0 GeV -t’=-t+tmin<0.4 GeV2 DE < 0.6 GeV Monte Carlo simulation of non-exclusive (DIS) background

  16. VECTOR MESONS extraction of sL: r0→ p+p-angular distributions g*-p CMS 23 SDMEs (15 unpolarised, 8 polarised) extracted in 3-D: F, f, cos q r° rest frame f p’ e’ p g* e L=250 pb-1 r° p+ F q p- if SCHC holds (VM retains g* helicity): →violation of SCHC → at Q2 = 2 GeV2, sL=sT

  17. VECTOR MESONS GPD model calculations for sL: H [Vanderhaegen et.al. (1999)] • indication of a larger gluon contribution --- 2-gluon exchange --- quark exchange corrections to LO: quark transverse momenta [Diehl et.al. (2005)] [Vinnikov et.al. (2005)] • quark exchange dominates  H, E r0 longitudinal cross sections [EPJC17,2000] L = 106 pb-1 [Frankfurt et.al. (1996)] • more data to come: r, f, w, r+

  18. VECTOR MESONS - Goeke, Polyakov & Vanderhaeghen(2001) - interference between E and H sS: |ST| sin (f-fS) E H Erelated to Jq TSA sensitive to Jq  H, E r0 transverse target spin asymmetry [Vinnikov et.al. (2005)] L=64 pb-1 xB x GPD model calculations (quarks+gluons GPDs) Erelated to Jq TSA sensitive to Jq 2 times more data with 2005: sL/sTseparation → talk by Armine Rostomyan

  19. PION PAIRS  H, E Pion pairs production: e p (d)→ e’ p (d) p+ p- Legendre moment: <P1> sensitive to the interference between different p+p- isospin states

  20. PION PAIRS interference between S-wave and lower r0 tail mpp < 0.6 GeV indication of r0 –f2 interference mpp ~ 1.3 GeV minimum interference between S-P waves mpp ~ 0.77 GeV GPD model calculations for sL: ■▲ quark exchange ― quark + 2-gluon exchange [Lehmann-Dronke et.al. (2001)]  H, E Legendre Moment: Mpp dependence [PLB599,2004] L=250 pb-1

  21. PS MESONS ~ ~  H, E ~ At low tand large x, Edominated by thepion pole E related to Fp ~ p+ production: Pseudoscalar Mesons cross sections ~ ~ • ~ |∫ dx H(x,x,t) + E(x,x,t) |2 ~ E kinematically suppressed at low t H = double distribution~ Dq(x) with skewing effect factorized t-dependence ~ target spin asymmetry ~ ~ AUT~ Im(H .E )

  22. PS MESONS ~ ~  H, E GPD model calculations for sL: [Vanderhaegen et.al. (1999)] p+cross section measurement L/T separation not possible sT suppressed by 1/Q2 → at large Q2, sL dominates L=250 pb-1 supported by REGGE model [Laget (2005)] Q2 dependence is in general agreement with the theoretical expectation Corrections to LO (k┴ and soft overlap) calculations overestimate the data

  23. PS MESONS ~ ~  H, E interference between E and H sS: |ST| sin (f-fS) E H Transverse target spin asymmetry for exclusive p+ ~ ~ g*L p → p+ n [Frankfurt et al. (1999)] ~ ~ [Belitsky et al. (2001)] L = 145 pb-1

  24. Recoil detector Jan. 06 - Jun. 07 Detection of the recoiling proton associated prod. ~11% semi-incl. ~5% associated prod. ~1% semi-incl. <<1% Future analysis: recoil detector clean reaction identification improve statistical precision (Lp = 750 pb-1,Ld = 200 pb-1) → talk by Ralf Kaiser

  25. ~ ~  H E ~ H H E E ~ ~ H DVCS: LTSA ~ E excl. p+ : s ~ ~  H E  H E  H, H, E, E CONCLUSION GPDs probed by hardexclusivephoton and meson production H DVCS: BSA, BCA excl. r0: sL excl. pions pairs E: TTSA DVCS, excl. r0 Corrections to leading order are needed to describe the cross sections Leading order calculationsdescribe asymmetries Jan. 06: polarized target removed, recoil detector installed and under commissioning → HERMES dedicated to exclusive processes! ~ Asymmetries: powerful tool to constrain GPD models ~ ~ Reaction Observable GPDs ~ ep→epg BCA, BSA, L(T)TSAH (2u+d) ep→epρ0σL H (2u+d) TTSA H.E ep→epfσL H(s) ep→epωσLH(2u-d) ep→epp+p-Legendre MomentH ep→epp+σtot E(u-d) TTSA H.E ep→epp0σtotH(2u+d) ~ ~ ~ Hpp0: 2/3 Hu/p + 1/3 Hd/p Hpp+: Hu/p - Hd/p ~ Polarisation provides observable sensitive to different combinations of GPDS ~ ~ dedicated experiments for exclusive measurements starting soon at HERMES

  26. HERMES at DESY e-beam: e+/e-, Ee=27.5 GeV, PB= 55% spin rotators @ HERMES for longitudinal beam polarization

  27. ~ ~  H, H, E, E Longitudinal target spin asymmetry:sin 2f unexpected large sin 2f: from qGq correlations twist-3 GPDs? upper limits for qGq correlations twist-3 GPDs [D. Mueller]

  28. ~ ~  H, H, E, E Model dependent constraint on Ju and Jd

  29. e p→e p+n 1 • Monte Carlo (arbitrary norm.) • data p+ enhancement #events -t (GeV2) Exclusive peak clearly centered at the nucleon mass Mean and width in agreement with exclusive MC Good description of data by MonteCarlo (acceptance determination) - Vanderhaeghen, Guichon & Guidal (1999) - Exclusive p+ production: e p → e p+(n) Missing Mass2 = (p-g*-p+)2 e p → e p-n : use of p- yield to subtract the non exclusive background e p→e p+X

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