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Deeply Virtual Meson Production

Deeply Virtual Meson Production. Cynthia Hadjidakis. 6th research conference on Electromagnetic Interactions with Nucleon and Nuclei Milos, 19-24 September, 2005. Generalized Parton Distributions Experimental results Future measurements.

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Deeply Virtual Meson Production

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  1. Deeply Virtual Meson Production Cynthia Hadjidakis 6th research conference on Electromagnetic Interactions with Nucleon and Nuclei Milos, 19-24 September, 2005 • Generalized Parton Distributions • Experimental results • Future measurements

  2. 4 Generalized Parton Distributions (GPDs) HH conserve nucleon helicity EEflip nucleon helicity ~ ~ Pseudoscalar mesons (p, h) Vector mesons (r, w, f) GPDs depend on 3 variables: x, x, t for each quark flavour Hq, for gluon Hg quark flavour decomposition possible from meson production GPDs Hard exclusive meson production -2 x x+x x-x t Ji sum rules 1 30%(DIS) ( H(x,x,t=0) + E(x,x,t=0) ) x dx =Jquark =1/2 DS+ D Lz -1

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

  4. DESY: H1, ZEUS, HERMES: 27.5 GeV e+ CERN SPS: Compass 160 GeV m+ H1, ZEUS: p (920 GeV) HERMES: p , d missing energy ep→er0X Jlab: CLAS up to 5.7 GeV e- Compass: d CLAS: p Experimental facilities H1, ZEUS collider  10-4<xB<0.021: gluons in the proton  distinct signature • Exclusive processes measurement requirements: • high luminosity  s~1/Q4, 1/Q6 • high Q2  hard regime • high resolution  exclusivity • fixed target experiments • COMPASS, HERMES •  0.006/0.02<xB<0.3: gluons/valence and sea quarks • CLAS • 0.15<xB<0.75: valence quarks • signature of exclusive process with missing mass/energy technique kinematics DE = (M2X-M2p)/2Mp

  5. 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

  6. VECTOR MESONS s r, f, J/Y r J/Y s ~ Wd(Q2) with d=0.7 no dependence on Q2 hard scale provided by MJ/Y general transition to hard behaviour at high Q2+M2  Hg, Eg Total cross section vs W at HERA

  7. VECTOR MESONS s  Hg, Eg GPD based model comparison GPD combined with Modified Perturbative Approach corrections to LO: quark transverse momenta, Sudakov suppression NLO calculations for sL [Ivanov et.al. (2004)] [Goloskokov et.al. (2005)] r0 r0 M=MRST2001 C=CTEQ6M different renormalization scale mR calculations of sL, sT, sTL, sTT

  8. VECTOR MESONS s  H, E r0→ p+p-angular distributions: extraction of sL if SCHC holds (VM retains g* helicity): from polarized lepton beam: →weak violation of SCHC COMPASS 02/03 6 times more statistics expected (sL , f, w) HERMES 00-05 4 times more statistics expected

  9. VECTOR MESONS s 2002  H, E r0→ p+p-angular distributions: extraction of sL f p’ if SCHC holds (VM retains g* helicity): Definitions : in g*-p CM frame m’ p g* m r° p+ F inr° rest frame q p- → at Q2 = 2 GeV2, sL=sT

  10. VECTOR MESONS s GPD model calculations for sL: [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  Hq, Eq Cross sections at HERMES [EPJC17,2000] [Frankfurt et.al. (1996)] → talk by Markus Diehl

  11. VECTOR MESONS s Analysis in progress [Vanderhaegen et.al. (1999)] Two-pion invariant mass spectra frozen coupling constant at 0.56 to average out non perturbative effect r+  Hq, Eq r0cross sections at CLAS CLAS (4.2 GeV) • CLAS: analysis of sL for r0 (r+, f) from 5.7 GeV data (higher W, higher Q2) n

  12. VECTOR MESONS s Analysis of ω decay angular distributions: SCHC does not seem to hold → not possible to extract σL handbag diagram estimated to contribute only about 1/5 of measured cross sections [Vanderhaegen et.al. (1999)] Cross sections described by REGGE model [Laget (2005)]  Hq, Eq w production at CLAS CLAS (5.7 GeV) Evidence for unnatural parity exchange  0 exchange very probable even at high Q2 (4 GeV2)

  13. VECTOR MESONS asym [Goeke et.al. (2001)] 2002-04 sS ~ |ST| sin (f-fS) E∙H Erelated to Jq TSA sensitive to Jq + Hg: smaller AUT [Vinnikov et.al. (2005)]  Hq, Eq r0 transverse target spin asymmetry xB • same xB-dependence behaviour as GPD calculations • no sL/sTseparation yet • 2 times more data for 2005 r0 TSA can be investigating at COMPASS

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

  15. 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)]  Hq, Eq Legendre Moment: Mpp dependence [PLB599,2004]

  16. 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 )

  17. PS MESONS s ~ ~  Hq, Eq - Vanderhaeghen et al. (1999) - - Huang, Wu & Wu (2004) - CEA, Cornell, Jlab GPD model calculations for sL: [Vanderhaegen et.al. (1999)] LO + power corrections Leading Order p+ cross section at HERMES L/T separation not possible sT suppressed by 1/Q2 → at large Q2, sL dominates 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 Pion Electromagnetic Form Factor

  18. PS MESONS s ~ ~  Hq, Eq [Mankiewicz et al. (1999)] xB Cross section ratios p0 production: no p-pole e p→e p+ n / e p→e p0 p measure at  HERMES  Jlab: e p → e p+ X CLAS (5.7 GeV) Q2~2.3 GeV2 x~0.3 Missing mass (GeV) xB

  19. - Belitsky & Müller(2001) - - Frankfurt, Pobylitsa, Polyakov & Strikman(1999) - - Frankfurt, Polyakov, Strikman & Vanderhaeghen(2000) - PS MESONS asym. ~ ~  Hq, Eq p+ transverse TSA ~ ~ g*L p → p+ n sS ~ |ST| sin (f-fS) E∙H [Frankfurt et al. (1999)] [Belitsky et al. (2001)] TARGET SPIN ASYMMETRY • HERMES • COMPASS

  20. 2006/2007 Detection of the recoiling proton PERSPECTIVES HERMES with a recoil detector clean reaction identification improve statistical precision (unpolarised data with high density target) improve detector resolution: multi-dimensional binning in xB and t → poster by Yves Van Haarlem

  21. TARGET SPIN ASYMMETRY xB = 0.3-0.4 -t = 0.2-0.3GeV2 AUT sL r0 sT Other bins measured concurrently xB PERSPECTIVES Jlab upgrade to 12 GeV beam energy Projections for 11 GeV (sample kinematics) sTot COMPASS: 2010 CERN proton luminosity upgrade proposal for a recoil detector  up to Q2= 20 GeV2     up to Q2= 7 GeV2

  22. ~ ~  H E  H E  H E CONCLUSION GPDs can be probed by hardexclusivemeson production Reaction Observable Exploration Experiment ep→epρ0σL H (2u+d) H1, ZEUS, HERMES,CLAS TSA H.EHERMES ep→epρ+σL H (u-d) ep→epfσL H(s)H1, ZEUS, HERMES ep→epωσLH(2u-d) CLAS ep→epp+p-Legendre MomentHHERMES ep→epp+σtot E(u-d)HERMES TSA H.E ep→epp0σtotH(2u+d) ~ ~ ~ ~ Corrections to leading order are needed to describe the cross sections Scaling region expected to be reached at lower Q2for TSA, cross section ratios Combining all the measurements will allow to constrain GPD models Measurements of different exclusive processes: vector mesons, p+p-, pseudoscalar mesons ~ ~ ~ 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

  23. asymptotically for fixed xB and t Factorization theorem prediction fit: 1/Qp p=1.9±0.5 p=1.7±0.6 p=1.5±1.0

  24. SDME from GPD model Modified Perturbative Approach [Goloskokov et.al. (2005)] Good agreement with the data (sL,SDME) for Q2>5 GeV2 t-dependence can be different for stot and sL no separate measurements for the t-slope on the data

  25. ~ ~ Hq(x, x=0,t=0) = xDG(x) Hq(x, x=0,t=0) = Dq(x) Sum Rules and Limited cases Forward limit (t →0, x→0) Hq(x,x=0,t=0) = q(x) Hq(x,x=0,t=0) = xG(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

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