Delia Hasch review of DVCS measurements a brief

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: ≈30% ≈zero

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

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 u-quark d-quark from lattice

the ideal experiment for measuring hard exclusive processes Q 2>>, t<< t

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)…

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

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

small high x 10 -3 HERA-collider 10 -4 < x. B < 0. 02 sea quarks & gluons 0. 2 -0. 5 10 -1 x. Bj HERMES / JLab 0. 02 < x. B < 0. 4 0. 1 < x. B < 0. 6 (valence) quarks

small high x 10 -3 HERA-collider 10 -4 < x. B < 0. 02 sea quarks & gluons 0. 2 -0. 5 10 -1 x. Bj HERMES / JLab 0. 02 < x. B < 0. 4 0. 1 < x. B < 0. 6 (valence) quarks [C. Weiss] see talk by L. Schoeffel

deeply virtual compton scattering DVCS Bethe-Heitler

deeply virtual compton scattering DVCS Bethe-Heitler @H 1/Zeus: DVCS ≈ Bethe-Heitler bilinear in GPDs

deeply virtual compton scattering DVCS Bethe-Heitler @H 1/Zeus: DVCS ≈ Bethe-Heitler @HERMES, JLab: DVCS << Bethe-Heitler bilinear in GPDs

deeply virtual compton scattering DVCS Bethe-Heitler @H 1/Zeus: DVCS ≈ Bethe-Heitler @HERMES, JLab: DVCS << Bethe-Heitler H 1/Zeus: LO sea quark + NLO gluon contribution

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

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 )

selecting exclusive events

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

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

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

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

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

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

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

DVCS cross sections

DVCS cross section [PLB 659(2008)] Zeus 99/00 not included [JHEP 05(2009)] Q 2 range : 1 – 102 Ge. V 2 steep rise with W hard process

DVCS cross section [PLB 659(2008)] measurement of t –slope: provides absolute normalisation

DVCS cross section [PLB 659(2008)] measurement of t –slope: Zeus (p’ tagged events) [JHEP 05(2009)] <Q 2>=8. 0 Ge. V 2 Zeus (p’ tagged events) <Q 2>=3. 3 Ge. V 2 provides absolute normalisation universality of slope parameter: point-like configurations dominate

DVCS cross section [PLB 659(2008)] measurement of t –slope: Zeus (p’ tagged events) [JHEP 05(2009)] <Q 2>=8. 0 Ge. V 2 Zeus (p’ tagged events) <Q 2>=3. 3 Ge. V 2 provides absolute normalisation universality of slope parameter: point-like configurations dominate b average impact parameter: @ x. B=10 -3 x. B~10 -3

DVCS cross section quantifying the skewing LO: R = GPD / PDF zero skewing due to Q 2 evolution of GPDs

DVCS @small x - outlook • more HERA II data to come: H 1: 2007 Zeus: so far HERA I only • combined H 1 & ZEUS analysis (? ) • beam-charge asymmetry @small x : access to interference term first result from HERA II … very limited by statistics

Hall-A DVCS interference term

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

DVCS interference term different charges: e+ e- (only @HERA!): ~ Ds. C ~ cosf ∙Re{ H + x. H +… } I~Ds H polarisation observables: ~ Ds. LU ~ sinf∙Im{H + x. H + k. E} ~ Ds. UL ~ sinf∙Im{H + x. H + …} H Ds. UT ~ sin(f-f. S)cosf ∙Im{k(H - E)+ … } H, E x = x. B/(2 -x. B ), k = t/4 M 2 kinematically suppressed @HERMES and JLab energies ~ H

first DVCS signals: ALU -- from interference term -[PRL 87(2001)] ALU CLAS @E= 4. 2 Ge. V sinf dependence indicates dominance of handbag contribution

call for high statistics JLab: E 1 -DVCS beam-spin asymmetry [PRL 100(2008)] fits of ALU integrated over t (with d=0)

call for high statistics JLab: E 1 -DVCS beam-spin asymmetry [PRL 100(2008)] 3 D binning in x, Q 2 and t <-t> = 0. 18 Ge. V 2 <-t> = 0. 30 Ge. V 2 <-t> = 0. 49 Ge. V 2 <-t> = 0. 76 Ge. V 2 fits of ALU integrated over t (with d=0)

call for high statistics JLab: E 1 -DVCS beam-spin asymmetry [PRL 100(2008)] a ~ Im[F 1 H]

call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data separation of interference term + DVCS 2 amplitude

call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data separation of interference term + DVCS 2 amplitude ‘classical’ single-charge asymmetry:

call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data separation of interference term + DVCS 2 amplitude ‘classical’ single-charge asymmetry: • charge difference asymmetry: • charge average asymmetry:

call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data separation of interference term + DVCS 2 amplitude beam spin asymmetry: GPD models: VGG [JHEP 11(2009)] regge-ansatz for t-dependence Dual-GT regge-ansatz for t-dependence 8 bands obtained by varying input parameters bval & bsea between 1 and

call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data separation of interference term + DVCS 2 amplitude beam spin asymmetry: [JHEP 11(2009)] contribution from DVCS 2 term small Dual-GT regge-ansatz for t-dependence

call for new analysis methods HERMES: combined analysis of charge & polarisation dependent data beam charge asymmetry [JHEP 11(2009)] GPD models: VGG Dual-GT regge-ansatz for t-dependence, no D-term regge-ansatz for t-dependence

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

Hall-A call for high precision data Hall-A: DVCS cross section from interference term [PRL 97(2006)] [VGG model]

Hall-A call for high precision data Hall-A: DVCS cross section from interference term Q 2 dependence of DVCS coeff. : dominance of tw-2 contribution (handbag diagram ) (however, very limited Q 2 range…) [PRL 97(2006)]

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

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

…nevertheless: first attempts to constrain Jq observables sensitive to E: (Jq input parameter in ansatz for E) • DVCS AUT : HERMES • n. DVCS ALU : Hall A • r 0 AUT : HERMES

…nevertheless: first attempts to constrain Jq Jq input parameter in ansatz for E: [JHEP 06(2008)] [VGG] Jd=0

…nevertheless: first attempts to constrain Jq Jq input parameter in ansatz for E: difference of polarised cross sections on p&D n. DVCS: [PRL 99(2007)] Hall-A [VGG] Hall-A n. DVCS ALU VGG [PRL 99(2007)] fgg n. DVCS: enhanced sensitivity to GPD E to Jq

…nevertheless: first attempts to constrain Jq Jq input parameter in ansatz for E: difference of polarised cross sections on p&D n. DVCS: [PRL 99(2007)] Hall-A [VGG] Hall-A n. DVCS ALU VGG : [PRL 99(2007)] fgg

DVCS @large x - summary Hall-A beam charge asymmetry beam spin asymmetry Hall-A transverse target spin asymm. longitudinal target spin asymm. double spin asymmetry

conclusions 10 -3 10 -1 0. 2 -0. 5 HERA-collider HERMES / JLab sea quarks & gluons (valence) quarks x. Bj increasing amount and precision of experimental data large “flow” of new data expected soon: JLab dedicated experiments HERMES data with recoil multi-D analysis & results

conclusions 10 -3 0. 2 -0. 5 10 -1 HERA-collider x. Bj HERMES / JLab Compass JLab 12 Ge. V EIC increasing amount and precision of experimental data large “flow” of new data expected soon: & bright future JLab dedicated experiments, JLab 12 Ge. V HERMES data with recoil COMPASS with recoil EIC prior to any conclusion about GPDs from data: call for new, more sophisticated parametrisations and approaches

outlook - I H 1/ZEUS: more statistics from HERA II running HERMES: beam charge & spin asymmetries with improved statistics separation of associated prod. with recoil info CLAS: - longitudinal target spin asymmetry - beam spin asymmetry with 4 He - prove of principle for transverse HD target - CLAS 12 Hall. A: - (helicity dependent) cross sections for p&d @diff. beam energy Rosenbluth-like separation of DVCS 2 – I terms, scaling tests - Hall. A @JLab 12