Deeply Virtual Compton JLab

Transcription

1 Deeply Virtual Compton JLab Franck Sabatié CEA Saclay For the Hall A and Hall B collaborations Exclusive 07 - JLab May 1 st 007 Introduction Non-dedicated measurements E experiment in Hall A E experiment in Hall A [Prelim. deep-π 0 E result] E1-DVCS experiment in Hall B What I didn t cover Summary Exclusive 07

2 Experimental observables linked to GPDs Experimentally, DVCS is undistinguishable with Bethe-Heitler However, we know FF at low t and BH is fully calculable Using a polarized beam on an unpolarized target, observables can be measured: 4 d σ DVCV C S T + T Re ( T ) + T dx dq dtd ϕ dx BdQ dtd 4 4 dx BdQ d Kroll, Guichon, Diehl, Pire, BH BH D DVCS d σ d σ BH DVCSS T Im ( T ) + T T dx dq dd tdt ϕ BH DVCS DVCS At JLab energies, T DVCS supposed small

3 Into the harmonic structure of DVCS d 4 σ dxbdq dtd 1 = Γ + + ϕ Ρ ( ϕ) Ρ ( ϕ) 1 1 BH BH BH { 0 1 ϕ ϕ} 1( xb, Q, t) c c cos c cos 1 + Γ ( x Q t Ρ ( ϕ) Ρ ( ϕ) I I I I { c0 c1 ϕ c ϕ c3 ϕ} B,, ) cos cos cos3 T BH d dx 4 B 4 σ d σ = dq dtdϕ ( xb, Q, t) Γ Ρ ( ϕ) Ρ ( ϕ) 1 I I { s1 sinϕ+ s sin ϕ} Interference term hadronic plane e - γ ϕ Ρ 1 ( ϕ) Ρ ( ϕ) 1 e - γ* leptonic plane p Belitsky, Mueller, Kirchner BH propagators ϕ dependence

4 Tests of scaling d 4 dxbdq dtd d σ 4 4 σ d dxbdq dtd 1 = Γ 1( xb, Q, t) c0 + c1 cos + c cos ϕ Ρ ( ϕ) Ρ ( ϕ) I I { s1 sin ϕ s sin ϕ} BH BH BH { ϕ ϕ} 1 + Γ x Q t Ρ ( ϕ) Ρ ( ϕ) σ Γ( xb, Q, t) = + ϕ Ρ ( ϕ) Ρ ( ϕ) I I I I { c0 c1 ϕ c ϕ c3 ϕ} ( B,, ) cos cos cos3 1. Twist- terms should dominate σ and Δσ. All coefficients have Q dependence which can be tested!

5 Special case of the asymmetry The asymmetry can be written as: d d 4 4 σ d 4 4 σ + d σ =Γ σ s sinϕ+ s sin ϕ + + ( + )cos ϕ +... I I 1 A ( xb, Q, t) I BH I BH c0 c0 c1 c1 Pro: easier experimentally, smaller RC, smaller systematics Con: direct extraction of GPDs is model- (or hypothesis-) dependent (denominator complicated and unknown) It was naturally the first observable extracted from non-dedicated experiments

6 Published non-dedicated JLab/Hall B results on A LU and A UL JLab/Hall B - E1 JLab/Hall B - Eg1 A LU A UL PRL 97, 0700 (006) PRL 87, 1800 (001) Both results show, with a limited statistics, a sin ϕ behavior (necessary condition for handbag dominance) In the A LU result, models (VGG) tend to over-estimate the data

7 E experimental setup and performances 75% polarized.5ua electron beam 15cm LH target Left Hall A HRS with electron package 11x1 block PbF electromagnetic calorimeter 5x0 block plastic scintillator array Δt (ns) for 9-block around predicted «DVCS» block σ Pbeam=75.3% Vertex ± 0.07% resolution σ.5 (stat) 1.mm x y mm σ E =.7% E at 4.GeV

8 E kinematics The calorimeter is centered on the virtual photon direction 50 days of beam time in the fall 004, at.5μa intensity Lu dt = 1394 fb 1

9 Analysis Looking for DVCS events HRS: Cerenkov, vertex, flat-acceptance cut with R-functions Calo: 1 cluster in coincidence in the calorimeter above 1 GeV With both: subtract accidentals, build missing mass of (e,γ) system

10 Analysis π o subtraction effect on missing mass spectrum Using π 0 γ events in the calorimeter, the π 0 contribution is subtracted bin by bin After π 0 subtraction

11 Analysis Exclusivity check using Proton Array and MC Using Proton-Array, we compare the missing mass spectrum of the triple and double-coincidence events. The missing mass spectrum using the Monte-Carlo gives the same position and width. Using the cut shown on the Fig.,the contamination from inelastic channels is estimated to be under 3%. M X cut Normalized Monte-Carlo (e,p,γ) triple coincidence (e,γ)x (e,p,γ) events

12 Difference of cross-sections PRL97, 600 (006) Q x B =.3 GeV = 0.36 Twist- Twist-3 Corrected for real+virtual RC Corrected for efficiency Corrected for acceptance Corrected for resolution effects Checked elastic ~1% Extracted Twist-3 contribution small! New work by P. Guichon!

13 Q dependence and test of scaling <-t>=0.6 GeV, <x B >=0.36 Twist- Twist-3 No Q dependence: strong indication for scaling behavior and handbag dominance Twist 4+ contributions are smaller than 10%

14 Total cross-section PRL97, 600 (006) Q x B =.3 GeV = 0.36 Corrected for real+virtual RC Corrected for efficiency Corrected for acceptance Corrected for resolution effects Extracted Twist-3 contribution small! but impossible to disentangle DVCS from the interference term (more on this in J. Roche talk)

15 DVCS on the neutron in JLab/Hall A: E See talk from M. Mazouz MODEL-DEPENDENT Ju-Jd extraction LD target 4000 fb-1 x B =0.36, Q =1.9 GeV VGG Code GPD model : LO/Regge/D-term=0 Goeke et al., Prog. Part. Nucl. Phys 47 (001), 401.

16 Deep-π 0 electroproduction in JLab/Hall A Same data as E00-110, but: -γ requirement in the calorimeter at π 0 mass, -eπ 0 X at proton mass. counts Fit to data ϕ d σ d σt d σ L d σ LT d σtt = + ε + (1 ε + ε) cosφ + ε cosφ dt dt dt dt dt

17 Cross-sections for deep-π 0 production in JLab/Hall A Q x B =.3 GeV = 0.36 VGG Laget Laget +rediff. -Low-t cross-section largely overshoots GPD & JML models. -TT term is large, suggesting a large transverse component. PRELIMINARY -But π 0 cross-section ratio for proton and deuteron targets is found 0.95 More data needed! (J. Roche talk)

18 E1-DVCS with CLAS : a dedicated DVCS experiment in Hall B Beam energy: ~5.8 GeV Beam Polarization: 75-85% Integ. Luminosity: 45 fb -1 M γγ (GeV ) Inner Calorimeter + Moller shielding solenoid More details in FX Girod s talk ~50 cm

19 E1-DVCS kinematical coverage and binning W > 4 GeV Q > 1 GeV

20 E1-DVCS : Asymmetry as a function of x B and Q <-t> = 0.18 GeV <-t> = 0.30 GeV <-t> = 0.49 GeV <-t> = 0.76 GeV Accurate data in a large kinematical domain Integrated over t

21 E1-DVCS : A LU (90 ) as a function of t + models VGG twist-+3 VGG twist- JM Laget

22 E1-DVCS : Cross-sections over a wide kinematical range PhD Thesis H.S. Jo (IPNO) 0.09<-t<0. 0.<-t< <-t< <-t<1 1<-t< <-t< PRELIMINARY

23 What I did not have time to talk about Unpublished non-dedicated data from CLAS (G. Gavalian, H. Avakian, ) More details about E1-dvcs in Hall B (FX Girod talk) More details about E in Hall A (M. Mazouz talk) Plans for the short and long term studies of DVCS at JLab in both Hall A (J. Roche talk) and B (L. Elouadrhiri talk)

24 Summary DVCS BSA (Hall B/CLAS): Data in a large kinematical range and good statistics. It will give a very hard time to models, to fit the whole range in Q, x B and t. Data seem to favor JM Laget model at low-t and VGG model at high t. DVCS Cross-section difference (Hall A): High statistics test of scaling: Strong support for twist- dominance First model-independent extraction of GPD linear combination from DVCS data in the twist-3 approximation Upper limit set on twist-4+ effects in the cross-section difference: twist>3 contribution is smaller than 10% DVCS Total cross-section (Hall A): Bethe-Heitler is not dominant everywhere DVCS terms might be sizeable (more on this in J. Roche s talk)

25 GPDs from Theory to Experiment x+ξ GPDs x-ξ γ γ t Theory Handbag Diagram T DVCS γ Factorization theorem states:. The GPDs enter Q and the ν DVCS large amplitude as an integral over x: In the suitable asymptotic limit, - GPDs appear in atthe x B and real t fixed part throughthe a PP handbag integral diagram over is x the leading - GPDs appear in the imaginary part but contribution at the line to x=ξ DVCS. 1. Needs to be checked!!! + 1 GPD( x, ξ, t) dx + L γ γ γ = 1 x ξ + iε Experiment = P but it s not so simple GPD( x, ξ, t) dx x ξ Physical process - i πgpd( x = ξξ,,) t + L Collins, Freund

26 Observables and their relationship to GPDs T DVCS = GPD( x, ξ, t) dx x ξ + iε = P L GPD( x, ξ, t) dx x ξ - i πgpd( x = ξξ,,) t + The cross-section difference accesses the imaginary part of DVCS and therefore GPDs at x = ξ L The total cross-section accesses the real part of DVCS and therefore an integral of GPDs over x