Experimental overview on exclusive processes Delia Hasch 1st Duality Workshop June 06-08, 2005; LNF-INFN, Frascati (Italy)

hard exclusive processes prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm hard exclusive processes Q2 t Q2>>, t<< high luminosity  s~1/Q4, 1/Q6 high Q2  hard regime high resolution  exclusivity Quantum number of final state selects different GPDs: Vector mesons (r, w, f): H E Pseudoscalar mesons (p, h): H E DVCS (g) depends on H, E, H, E ~

prerequisites H1, ZEUS, HERMES: H1, ZEUS 27.5 GeV e+, e- prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm prerequisites H1, ZEUS, HERMES: 27.5 GeV e+, e- H1, ZEUS  distinct signature in 4p det. Compass: 100-190 GeV m+ Jlab: HallA,B up to 6 GeV e- HERMES, COMPASS, JLab:  missing mass/energy technique: CLAS: DMX ~ 30 MeV HERMES: DMX ~ 400 MeV CLAS HERMES H1, ZEUS: p (820 GeV) HERMES: p , d unpol. H, D, 4He, 14N, 20Ne, Kr, Xe Compass: d , p Hall A: p , n Hall B: p, d

Deeply virtual Compton Scattering prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~ Deeply virtual Compton Scattering Bethe-Heitler HallA,B HERMES H1/ZEUS DVCS BH  NLO analysis of H1, ZEUS cross sections

DVCS cross section first measurement of t –slope: prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  Hg Hq DVCS cross section first measurement of t –slope: ds/dt = ds/dt|t=0∙exp(-b t) b = 6.02±0.35±0.39 GeV-2 absolute normalisation! [A.Freund, M. McDermott EPJC23(2002)] comparison to NLO QCD: band width given by b measurement

DVCS asymmetries DsLU ~ ~ ~ ~ beam target prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~ DVCS asymmetries DVCS-BH interference leads to non-zero azimuthal asymmetry DsLU beam target  separating GPDs through polarisation: DsLU ~ sinf∙Im{F1H + x(F1+F2)H +kF2E} ~ ~ H, H, E DsUL ~ sinf∙Im{F1H + x(F1+F2)(H + …} ~ ~ H, H DsUT ~ sinf∙Im{k(F2H - F1E) + … } H, E kinematically suppressed x = xB/(2-xB ),k = t/4M2

beam spin asymmetry CLAS ~  H H E 0.15 < xB< 0.4 prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~ beam spin asymmetry [PRL87,2001] CLAS [PRL87,2001] PRELIMINARY (not for circulation) 0.15 < xB< 0.4 1.5 < Q2 < 4.5 GeV2 -t < 0.5 GeV2 Vanderhaeghen -

BSA on nuclear targets heavier targets : 4He, 14N, 20Ne, Kr, Xe prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~ BSA on nuclear targets heavier targets : 4He, 14N, 20Ne, Kr, Xe … study dependence of coherent production on nuclei

target spin asymmetry CLAS ~  H H prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H ~ target spin asymmetry CLAS PRELIMINARY (not for circulation)

target spin asymmetry transverse TSA: sensitivity to GPD E (Jq) ~ prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~ target spin asymmetry Vanderhaeghen - transverse TSA: sensitivity to GPD E (Jq)

beam charge asymmetry (only @ HERA) ~  H H E prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~ beam charge asymmetry (only @ HERA)

beam charge asymmetry (only @ HERA) prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~ beam charge asymmetry (only @ HERA) GPD calculations: different parametrisations for H Vanderhaeghen (1999/02) – AC: high sensitivity to GPD models (tiny e-p sample ~10pb-1) HERA: 2004/05 e- beam (5x) … H1/ZEUS ?

prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~  H E ~  H E DVCS  DVMS DVCS theoretically cleanest process to study GPDs - azimuthal asymmetries from interference term relate directly to amplitude hard exclusive meson production: flavour separation ρ0 2u+d, 9g/4 ω 2u-d, 3g/4 f s, g ρ+ u-d ! factorisation theorem for sL only !

s vs W in bins of Q2 r f J/Y s ~ Wd with d=0.7 no dependence on Q2 prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm s vs W in bins of Q2  H E J/Y r f s ~ Wd with d=0.7 no dependence on Q2  general transition to hard behaviour at high Q2+M2

sL separation from angular distribution prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H E sL separation from angular distribution [Vanderhaegen et.al. (1999)] --- 2-gluon exchange --- quark exchange if SCHC holds (VM retains g* helicity):  r0 2002 < 10% CLAS: analysis of sL for r0 (r+) from 6 GeV data (higher W !) going on …

- Goeke, Polyakov & Vanderhaeghen (2001) - prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H E r0 transverse TSA interference of H and E: sS ~ |ST| sin (f-fS) E∙H - Goeke, Polyakov & Vanderhaeghen (2001) - E related to Jq  TSA sensitive to Jq  same x-dependence behaviour as GPD calculations

p+ cross section sT suppressed by 1/Q2 Comparison with prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H E ~ p+ cross section Comparison with GPD model for sL: [Vanderhaegen, Guichon, Guidal (1999)] Q2 dependence: sT suppressed by 1/Q2 Q2 dependence: in general agreement with the theoretical expectation LO calculations too small power corrections (k┴ ,soft overlap) overestimate data

cross section ratios p0 production: no p-pole ~ x measure at prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H E ~ cross section ratios p0 production: no p-pole  H dominates measure at  HERMES  Jlab: ~ [Mankiewicz,Piller & Radyushkin (1999)] x PRELIMINARY (not for circulation)

p+ transverse TSA ~ ~ sS ~ |ST| sin (f-fS) E∙H ~ ~ ~ prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H E ~ p+ transverse TSA - Frankfurt, Pobylitsa, Polyakov & Strikman (1999) - - Frankfurt, Polyakov, Strikman & Vanderhaeghen (2000) - ~ ~ - Belitsky & Müller (2001) - interference of H and E: sS ~ |ST| sin (f-fS) E∙H g*L p → p+ n ~ ~ TARGET SPIN ASYMMETRY  gives access to relative size of H and E ~ soon from HERMES Compass, Jlab …

dedicated experiments prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm dedicated experiments DVCS with recoil: 2006/07 background “free” DVCS: semi-inclusive bg: 5%  <<1% associated bg: 10%  1%

JLab dedicated DVCS experiments in 2004 - 2005 M. Garcon JLab/Hall A JLab/CLAS Inner Calorimetrer and supraconducting magnet within CLAS torus Dedicated, high statistics, DVCS experiments → Detection of 3 particles e, p and γ in final state → Firmly establish scaling laws (up to Q2 ~ 5 GeV2), if observed, or deviations thereof understood, first significant measurement of GPDs. → Large kinematical coverage in xB and t leads to femto-tomography of the nucleon → Opens the way for an ambitious program with JLab@12GeV (CLAS12 and other) Intro p e’ e γ

conclusions first exciting results on DVCS and DVMS available prerequisities perspectives DVCS s asymm | VECTOR MESONS s asymm | PS MESONS s asymm  H H E ~  H E ~  H E conclusions first exciting results on DVCS and DVMS available  polarisation provides new observables sensitive to different (combinations) GPDS dedicated experiments for DVCS measurements going on at JLab or starting soon at HERMES new facilities (high luminosity/energy/resolution) needed to map GPDs in an global analysis of all hard exclusive processes

backup slides t – dependence of BSA for photon and pion production: PRELIMINARY (not for circulation)

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