Nucleon'05, 14/10/05 M. Guidal, IPN Orsay Deep Virtual Compton Scattering : experimental status and perspectives Nucleon'05, 14/10/05 M. Guidal, IPN Orsay

1/ Generalities about GPDs 2/ Review of experimental data 3/ Perspectives

DVCS : Golden process to access GPDs -2x x+x x-x H,E(x,x,t) ~ p’(=p+D) p Leading order/twist (handbag diagram) accessible at moderate Q2 (precocious scaling), DVCS: e g* p e’ p’ g Bethe-Heitler GPDs Interference with Bethe-Heitler process,

x = xB ! Deconvolution needed ! Hq(x,x,t) but only x and t accessible experimentally g* t g,M,... x~xB x= x B 1-x /2 t=(p-p ’) 2 x = xB ! /2 x H,E,H,E ~ p p’ ds dQ d x dt 2 B ~ A H (x,x,t,Q ) q x-x+ie dx +B E (x,x,t,Q ) dx +…. 1 -1 Deconvolution needed ! x : mute variable

GPD and DVCS Beam or target spin asymmetry Cross-section measurement (at leading order:) Cross-section measurement and beam charge asymmetry (ReT) integrate GPDs over x Beam or target spin asymmetry contain only ImT, therefore GPDs at x = x and -x Intro (M. Vanderhaeghen)

ep epg ~ ~ ~ ~ H(x,x,t), H(x,x,t), E(x,x,t) (BSA) H, H (l)TSA H, E Global analysis of polarized and unpolarized data needed for GPD separation ep epg x = xB/(2-xB) k = -t/4M2 A = Ds 2s s+ - s- s+ + s- = Polarized beam, unpolarized target: ~ ~ DsLU ~ sinf{F1H + x(F1+F2)H +kF2E}df H(x,x,t), H(x,x,t), E(x,x,t) (BSA) Kinematically suppressed Unpolarized beam, longitudinal target: ~ ~ H, H DsUL ~ sinf{F1H+x(F1+F2)(H + … }df (l)TSA Unpolarized beam, transverse target: H, E DsUT ~ sinf{k(F2H – F1E) + ….. }df (t)TSA

The actors + theory DESY CERN JLab HERMES ZEUS/H1 COMPASS Hall A Vector & PS mesons DVCS CERN COMPASS Vector mesons DVCS JLab Hall A Hall B Hall C p-DVCS n-DVCS Vector mesons d-DVCS Pseudoscalar mesons + theory (almost) everywhere

DVCS timeline The past : (evidence for handbag mechanism in non-dedicated experiments) HERA s HERMES BSA HERMES BCA, (l,t)TSA (preliminary) CLAS(4.2 GeV) BSA CLAS(4.8 GeV) BSA, (l)TSA (preliminary) The present : (dedicated experiments) JLab Hall A Ds, BSA CLAS(5.75 GeV) Ds, BSA The future : HERMES (recoil detector) COMPASS (recoil detector) JLab(12 GeV)

The past (non-dedicated experiments)

Deep Exclusive reactions: an experimental challenge Missing mass MX2 ep  epX MAMI 850 MeV ep  epX Hall A 4 GeV Resolution Exclusivity Luminosity γ π0 ep  epX CLAS 4.2 GeV are the key issues for this physics! Intro N N+π ep  eγX HERMES 28 GeV

Beam Spin Asymmetry (BSA) (ep epg) (ep epg) S

First experimental signatures First observations of DVCS beam asymmetries in 2000 CLAS HERMES Q2 = 2.6 GeV2 , xB = 0.11, -t = 0.27 GeV2 Q2 = 1.25 GeV2 , xB = 0.19, -t = 0.19 GeV2 Phys.Rev.Lett.87:182002,2001 Phys.Rev.Lett.87:182001,2001 Vdh, Guichon, Guidal (1999) twist-2 + twist-3 : Kivel, Polyakov, Vdh (2000)

CLAS/DVCS at 4.8 and 5.75 GeV PRELIMINARY PRELIMINARY : 0.15 < xB< 0.4 1.50 < Q2 < 4.5 GeV2 -t < 0.5 GeV2 PRELIMINARY 5.75 GeV data (H. Avakian & L. Elhouadrhiri) PRELIMINARY Vdh, Guichon, Guidal (1999) Kivel, Polyakov, Vdh (2000) calculation twist-2 + twist-3 GPD based predictions (BMK) 4.8 GeV data (G. Gavalian) :

Charge Spin Asymmetry (BCA) (e-p epg) (e+p epg) S

Beam charge asymmetry t-dependence Vdh, Guichon, Guidal (1999) calculation : Guidal, Polyakov, Radyushkin, VdH (2005) tiny e-p sample (L~10 pb-1) HERA: 2004-2005 e- beam (x5)

(longitudinal) Target Spin Asymmetry (l TSA) (ep epg) (ep epg) S

Longitudinal target spin asymmetry Preliminary CLAS data Experimental Studies with CLAS Data were collected as a by-product during the eg1 2000 run: 5.75 GeV with NH3 longitudinally polarized target, <Q2> ~ 1.8 GeV2 HERMES Preliminary target spin asymmetries on p and d but unexpected large sin 2f sin f in agreement with GPD models Preliminary HERMES data

(transverse) Target Spin Asymmetry (t TSA) (ep epg) (ep epg) S

Transverse target spin asymmetry

Guidal, Polyakov, Radyushkin, VdH (2005) x b (fm) y z x

orbital angular momentum carried by quarks evaluated at μ2 = 2.5 GeV2 PROTON 2 Jq (GPRV 05) Δq HERMES (1999) 2 Lq u 0.63 0.57 ± 0.04 0.06 ± 0.04 d -0.06 -0.25 ± 0.08 0.19 ± 0.08 s 0.03 -0.01 ± 0.05 0.04 ± 0.05 u + d + s 0.60 0.30 ± 0.10

The present (dedicated experiments)

JLab dedicated DVCS experiments in 2004 - 2005 High statistics and unambiguous epg final state determination JLab/Hall A JLab/CLAS Calorimeter and supraconducting magnet within CLAS torus Intro p e’ e γ

Add an EM calorimeter at forward angles A typical event in CLAS (Hall B, JLab) p 1/ DVCS (Hall B) epa epg g Add an EM calorimeter at forward angles 420 PbWO4 crystals : ~10x10 mm2, l=160 mm Read-out : APDs +preamps JLab/ITEP/ Orsay/Saclay collaboration Add a “Moller shield” solenoid around the target

Data taking : March to May 2005 Calibration from π0→γγ σ = 7.5 MeV Mγγ (GeV) η CLAS (preliminary analysis of a 2 hours run) All (eγp) events (eγp) events after kinematical cuts

Expected Kinematical Dependencies About 380 bins in f, xB, t

2/DVCS in JLab/Hall A Experiment completed (Nov.-Dec. 2004) High Resolution Hall A spectrometer for electron detection 100-channel scintillator array for proton detection 132-block PbF2 electromagnetic calorimeter for photon detection Detection of all 3 final-state particles ensures exclusivity

Sensitivity to quark angular momentum J DVCS on the neutron DVCS-BH interference generates a beam spin cross section difference Main contribution for the proton Main contribution for the neutron Intro → (within a model) Sensitivity to quark angular momentum J Veto detector added to the p-DVCS set-up

~ New CLAS experiment : Longitudinal Target Spin Asymmetry DsUL ~ sinfIm{F1H+x(F1+F2)(H +.. } CLAS eg1 (preliminary) CLAS (eg1+IC) projected 6 GeV run with NH3 longitudinally polarized target (CLAS + IC) 60 days of beam time Approved at the latest JLab PAC

The future

precision (unpolarised data with high density target) Recoil detector nov. 2005 for 2 years Detection of the recoiling proton clean reaction identification improve statistical precision (unpolarised data with high density target)

Summary EXPERIMENT : The most complete information on the structure of the nucleon : GPDs (f (x), g (x), F (t), G (t), F(z), pion cloud, Jq…) 1 A EXPERIMENT : Up to 2005 : first experimental signatures in different Kinematics, in different observables, are very encouraging We enter a new era with high-statistics, high resolution dedicated experiments: definitely sign the validity of the approach (factorization, scaling,...) and we are on our way to extract/strongly constraint the GPDs and extract the physics

DVCS with a polarized target: New CLAS experiment ~ DsLL ~ CBH+ cosfRe{F1H+x(F1+F2)(H +xE/2)+.. } 5.7 GeV run with NH3 longitudinally polarized target+IC Projections for 60 days The double spin asymmetry in DVCS with longitudinally polarized target will provide access to the real part of Compton form factors

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

Status of GPDs Studies at Jefferson Lab GPD Reaction Obs. Expt Status ep→epγ (DVCS) BSA CLAS 4.2 GeV Published PRL CLAS 4.8- 5.75 GeV Preliminary (+ σ) Hall A 5.75 GeV Fall 04 CLAS 5.75 GeV Spring 05 ep→epγ (DVCS) TSA CLAS 5.65 GeV Preliminary e(n)→enγ (DVCS) BSA Hall A 5.75 GeV Fall 04 ed→edγ (DVCS) BSA CLAS 5.4 GeV under analysis ep→epe+e- (DDVCS) BSA CLAS 5.75 GeV under analysis From ep → epX Dedicated set-up Dedicated set-up Intro

CLAS/DVCS at 4.8 and 5.75 GeV PRELIMINARY PRELIMINARY 0.15 < xB< 0.4 1.50 < Q2 < 4.5 GeV2 -t < 0.5 GeV2 PRELIMINARY 5.75 GeV data (H. Avakian & L. Elhouadrhiri) PRELIMINARY GPD based predictions (BMK) 4.8 GeV data (G. Gavalian)

Beam spin and charge asymmetry Introduction | DVCS | Vector mesons | Pseudoscalar mesons | Pion pairs | Perspectives → H Beam spin and charge asymmetry Beam Spin Asymmetry [PRL87,2001]

GPD Reaction Obs. Expt Status ep→epγ (DVCS) BSA CLAS 4.2 GeV Published PRL CLAS 4.8 GeV Preliminary CLAS 5.75 GeV Preliminary (+ σ) Hall A 5.75 GeV Fall 04 CLAS 5.75 GeV Spring 05 ep→epγ (DVCS) TSA CLAS 5.65 GeV Preliminary e(n)→enγ (DVCS) BSA Hall A 5.75 GeV Fall 04 ed→edγ (DVCS) BSA CLAS 5.4 GeV under analysis ep→epe+e- (DDVCS) BSA CLAS 5.75 GeV under analysis ep→epρ σL CLAS 4.2 GeV Published PLB CLAS 5.75 GeV under analysis ep→epω (σL) CLAS 5.75 GeV Accepted EPJA + other meson production channels π, η, Φ under analyses in the three Halls. From ep → epX Dedicated set-up Intro

Energy dependence BH DVCS Calculation (M.G.&M.Vanderhaeghen)

DDVCS: first observation of ep → epe+e- * Positrons identified among large background of positive pions * ep→epe+e- cleanly selected (mostly) through missing mass ep→epe+X * Φ distribution of outgoing γ* and beam spin asymmetry extracted (integrated over γ* virtuality) but… A problem for both experiment and theory: * 2 electrons in the final state → antisymmetrisation was not included in calculations, → define domain of validity for exchange diagram. * data analysis was performed assuming two different hypotheses either detected electron = scattered electron or detected electron belongs to lepton pair from γ* Hyp. 2 seems the most valid → quasi-real photoproduction of vector mesons Intro Lepton pair squared invariant mass

Compton Scattering “DVCS” (Deep Virtual Compton Scattering)