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Delia Hasch

Delia Hasch. review of DVCS measurements. a brief introduction. DVCS: from low to high x : cross sections and asymmetries (selected results). DVCS on nuclei. conclusion & perspectives. workshop on GPDs@COMPASS , CERN, March 04, 2010. GPDs and the spin puzzle. nucleon spin:.

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Delia Hasch

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  1. Delia Hasch review of DVCS measurements a brief introduction DVCS: from low to high x : cross sections and asymmetries (selected results) DVCS on nuclei conclusion & perspectives workshop on GPDs@COMPASS,CERN, March 04, 2010

  2. GPDs and the spin puzzle nucleon spin: ≈30% ≈zero

  3. GPDs and the spin puzzle nucleon spin: ≈30% ≈zero [X. Ji, 1997] requires orbital angular momentum proton helicity flipped but quark helicity conserved

  4. nucleon tomography [M. Burkardt, M. Diehl 2002] FT(GPD) : momentum space  impact parameter space: probing partons with specified long. momentum @transverse position b T polarised nucleon: spin-orbit correlations d-quark u-quark from lattice

  5. Q2 t the ideal experiment for measuring hard exclusive processes Q2>>, t<<

  6. the ideal experiment for measuring hard exclusive processes high+variable beam energy • hard regime • wide kinematic range high luminosity • small cross sections • measure in 3 kinematic variables simultanously complete event reconstruction  ensure exclusivity … doesn’t exist (yet)…

  7. experimental prerequisites HERA till 2007 • polarised 27GeV e+/e- • unpolarised 920 GeV p • ≈full event reconstruction • polarised 27GeV e+/e- • long.+transv. polarised p, d targets • unpolarised nuclear targets • missing mass/energy technique • 2006/7: data with recoil detector CEBAF

  8. Hall-A experimental prerequisites HERA till 2007 • polarised 27GeV e+/e- • unpolarised 920 GeV p • ≈full event reconstruction • polarised 27GeV e+/e- • long.+transv. polarised p, d targets • unpolarised nuclear targets • missing mass/energy technique • 2006/7: data with recoil detector CEBAF • highly polarised continuous up to 6GeV e- • long. polarised effective p and n targets • missing mass/energy technique

  9. small  high x 0.2-0.5 10-3 10-1 xBj HERA-collider HERMES / JLab 0.02 < xB < 0.4 0.1 < xB < 0.6 10-4 < xB < 0.02 sea quarks & gluons (valence) quarks

  10. small  high x 0.2-0.5 10-3 10-1 xBj HERA-collider HERMES / JLab 0.02 < xB < 0.4 0.1 < xB < 0.6 10-4 < xB < 0.02 sea quarks & gluons (valence) quarks [C. Weiss] see talk by L. Schoeffel

  11. deeply virtual compton scattering DVCS Bethe-Heitler

  12. deeply virtual compton scattering DVCS Bethe-Heitler @H1/Zeus: DVCS≈ Bethe-Heitler • bilinear in GPDs

  13. deeply virtual compton scattering DVCS Bethe-Heitler @H1/Zeus: DVCS≈ Bethe-Heitler @HERMES, JLab: DVCS<< Bethe-Heitler • bilinear in GPDs • linear in GPDs

  14. deeply virtual compton scattering DVCS Bethe-Heitler @H1/Zeus: DVCS≈ Bethe-Heitler @HERMES, JLab: DVCS<< Bethe-Heitler H1/Zeus: LO sea quark + NLO gluon contribution

  15. DVCS & GPDs: caveats • x is mute variable (integrated over):  apart from cross-over trajectory (x=x) GPDs not directly accessible • extrapolation t  0 is model dependent

  16. DVCS & GPDs: caveats • x is mute variable (integrated over):  apart from cross-over trajectory (x=x) GPDs not directly accessible • extrapolation t  0 is model dependent cross sections & beam-charge asymmetry ~ Re(T DVCS ) beam or target-spin asymmetries ~ Im(T DVCS )

  17. selecting exclusive events

  18. exclusivity ≈ 4p acceptance for e and g  p escapes in beam pipe LPS: p tagged sample

  19. exclusivity LPS: p tagged sample e-sample: BH control sample e+e-, J/Y bg sample g-sample: BH + DVCS

  20. exclusivity LPS: p tagged sample e-sample: BH control sample e+e-, J/Y bg sample g-sample: BH + DVCS ( BH + DVCS ) - BH

  21. exclusivity full data sample e-sample: BH control sample e+e-, J/Y bg sample g-sample: BH + DVCS ( BH + DVCS ) - BH

  22. exclusivity via missing mass / energy part of the signal (ep  e’ g X) subtracted very well understood

  23. exclusivity via missing mass / energy part of the signal  D+ ID: transition GPDs (ep  e’ g X) subtracted very well understood  reference samples 2006/7

  24. Hall-A exclusivity via missing mass / energy part of the signal (ep  e’ g X) subtracted very well understood

  25. DVCS cross sections

  26. DVCS cross section [PLB659(2008)] Zeus 99/00 not included [JHEP05(2009)] Q2 range : 1 – 102 GeV2 steep rise with W  hard process

  27. DVCS cross section [PLB659(2008)] • measurement of t –slope: provides absolute normalisation

  28. DVCS cross section [PLB659(2008)] • measurement of t –slope: Zeus (p’ tagged events) provides absolute normalisation [JHEP05(2009)]  universality of slope parameter: point-like configurations dominate <Q2>=8.0 GeV2 Zeus (p’ tagged events) <Q2>=3.3 GeV2

  29. DVCS cross section [PLB659(2008)] • measurement of t –slope: Zeus (p’ tagged events) [JHEP05(2009)] provides absolute normalisation  universality of slope parameter: point-like configurations dominate <Q2>=8.0 GeV2 Zeus (p’ tagged events) <Q2>=3.3 GeV2 b  average impact parameter: @ xB=10-3 xB~10-3

  30. DVCS cross section quantifying the skewing LO: R = GPD / PDF zero skewing  skewing due to Q2 evolution of GPDs

  31. DVCS @small x - outlook • more HERA II data to come: • H1: 2007 • Zeus: so far HERA I only • combined H1 & ZEUS analysis (?) • beam-charge asymmetry @small x : •  access to interference term first result from HERA II … very limited by statistics

  32. Hall-A DVCS interference term

  33. DVCS interference term ~ DsC~cosf∙Re{ H+ xH +… } DsUT polarisation observables: beam target I~Ds  different charges: e+ e-(only @HERA!): U, L U, L, T Unpolarised Longitudinally Transversely polarised

  34. DVCS interference term ~ DsC~cosf∙Re{ H+ xH +… } ~ DsLU~sinf∙Im{H+ xH+ kE} polarisation observables: ~ DsUL~sinf∙Im{H+ xH+ …} I~Ds  different charges: e+ e-(only @HERA!): H H ~ H DsUT~sin(f-fS)cosf ∙Im{k(H- E)+ … } H, E kinematically suppressed @HERMES and JLab energies x = xB/(2-xB ),k = t/4M2

  35. ALU first DVCS signals: ALU -- from interference term -- [PRL87(2001)] CLAS @E= 4.2 GeV  sinf dependence indicates dominance of handbag contribution

  36. call for high statistics JLab: E1-DVCS beam-spin asymmetry [PRL100(2008)] fits of ALU(with d=0) integrated over t

  37. <-t> = 0.18 GeV2 <-t> = 0.30 GeV2 <-t> = 0.49 GeV2 <-t> = 0.76 GeV2 call for high statistics JLab: E1-DVCS beam-spin asymmetry [PRL100(2008)] 3D binning in x, Q2 and t fits of ALU(with d=0) integrated over t

  38. call for high statistics JLab: E1-DVCS beam-spin asymmetry [PRL100(2008)] a ~ Im[F1H]

  39. call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data  separation of interference term + DVCS2 amplitude

  40. call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data  separation of interference term + DVCS2 amplitude ‘classical’ single-charge asymmetry:

  41. call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data  separation of interference term + DVCS2 amplitude ‘classical’ single-charge asymmetry: • charge difference asymmetry: • charge average asymmetry:

  42. call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data  separation of interference term + DVCS2 amplitude beam spin asymmetry:[JHEP11(2009)] regge-ansatz for t-dependence GPD models: VGG bands obtained by varying input parameters bval & bsea between 1 and 8 regge-ansatz for t-dependence Dual-GT

  43. call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data  separation of interference term + DVCS2 amplitude beam spin asymmetry:[JHEP11(2009)]  contribution from DVCS2 term small regge-ansatz for t-dependence Dual-GT

  44. call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data  beam charge asymmetry [JHEP11(2009)] GPD models: VGG regge-ansatz for t-dependence, no D-term regge-ansatz for t-dependence Dual-GT

  45. Hall-A call for high precision data Hall-A: DVCS cross section from interference term

  46. Hall-A call for high precision data Hall-A: DVCS cross section from interference term [PRL97(2006)] [VGG model]

  47. Hall-A call for high precision data Hall-A: DVCS cross section from interference term  Q2 dependence of DVCS coeff.: dominance of tw-2 contribution (handbag diagram ) (however, very limited Q2 range…) [PRL97(2006)]

  48. a word about ‘user friendly’ GPD models VGG:[Vanderhaegen, Guichon, Guidal 1999] • double distributions ; factorised or regge-inspired t-dependence • D-term to restore full polynomiality • skweness depending on free parameters bval & bsea • includes tw-3 (WW approx) Dual:[Guzey, Teckentrup 2006, 2009] • GPDs based on infinite sum of t channel resonances (minimal: truncated k=[0,2]) • factorised or regge-inspired t-dependence • tw-2 only

  49. describes well AC, AUT,L and ALL data • fails for ALU • Ac favours ‘no D-term’  contradicts cQSM & lattice results • describes well kine dependence of charge and spin asymmetries • after correction in calculation: magnitude off by factor 2-4 see talk by D. Muller a word about ‘user friendly’ GPD models VGG:[Vanderhaegen, Guichon, Guidal 1999] • double distributions ; factorised or regge-inspired t-dependence • D-term to restore full polynomiality • skweness depending on free parameters bval & bsea • includes tw-3 (WW approx) Dual:[Guzey, Teckentrup 2006, 2009] • GPDs based on infinite sum of t channel resonances (minimal: truncated k=[0,2]) • factorised or regge-inspired t-dependence • tw-2 only • call for new, more sophisticated parametrisations of GPDs & new approaches … on the way:- generalisation of Mellin transform technique &dispersion relations - ‘model independent fitter’…

  50. …nevertheless: first attempts to constrain Jq observables sensitive to E: (Jq input parameter in ansatz for E) • DVCS AUT : HERMES • nDVCS ALU : Hall A • r0 AUT : HERMES

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