Probing Generalized Parton Distributions in Exclusive Processes with CLAS

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1 Probing Generalized Parton Distributions in Exclusive Processes with CLAS Volker D. Burkert Jefferson Lab The nucleon: from structure to dynamics First GPD related results in DVCS and DVMP Experimental DVCS program at 6 GeV Prospects for the JLab energy upgrade Conclusions N*2002 Workshop, Pittsburgh, October 8-11, 2002

2 A real 3-dimensional Scotty z y x Scotty reduced to one dimension probablity Water Calcium Carbon Carbon x

3 Snapshot of a Nucleon (taken through a low resolution microscope) meson cloud quark spin valence quarks sea quarks A one-dimensional slice of the nucleon from DIS experiments correlations orbital angular momentum We need to map out the full nucleon wave function to understand its internal dynamics q + q Longitudinal parton momentum and helicity distribution q - q x 1

4 Deeply Virtual Exclusive Processes Inclusive Scattering Forward Compton Scattering θ = 0 Deeply Virtual Compton Scattering (DVCS) t Probes the internal nucleon dynamics at the amplitude level. H(x, ξ, t),.

5 Modeling of the Generalized Parton Distributions Cross section difference Quark distribution q(x) Cross section qq distribution

6 GPDs may be accessed in Deeply Virtual Exclusive Processes DVCS DVMP hard gluon Scaling cross sections for photon and meson production hard vertices 1/Q 6 1/Q 6 1/Q 4 DVCS depends on all 4 GPDs and u, d quarks DVMP allow separation of un(polarized) H, E (H, E) GPDs, and contributions of u, d quarks M = ρ, ω H, E γ M = π, η, Κ H, E In hard scattering, photons dominate at high Q 2 Need to isolate γ * L

7 DVCS in the Interference with the Bethe-Heitler (BH) Measures DVCS in the interference with the known BH process: d 4 σ dq 2 dx B dtdφ ~ T DVCS + T BH 2 E o = 11 GeV E o = 6 GeV E o = 4 GeV BH DVCS Beam helicity cross section difference in twist-2: σ + σ ~ sinφim{(f 1 H+ k 1 (F 1 +F 2 )H+k 2 F 2 E }dφ

8 Measurement of exclusive DVCS with CLAS 2000 data, E=4.8GeV Beam Spin Asymmetry 1999 data, E=4.2GeV p ary n i relim A. Belitsky et al. S. Stepanyan et al. Phys. Rev. Lett. 87 (2001) Α(φ) = αsinφ + βsin2φ g g α = ± 0.028stat ± 0.013sys β = ± 0.021stat ± 0.009sys AJM ansatz for GPDs in L.O. α = 0.20 A. Feund, M. McDermott, M. Strikman, hep-ph/ g g α = ± 0.017stat β = ± 0.013stat fit gives very small twist-3 contributions β/α g

9 Total cross section for γ*p p ρ 0 CLAS E = 4.2 GeV σ (γ*p->pρ 0 ) (µb) Comparison to world data CLAS DESY Cornell

10 Separation of σ L /σ T in γ*p pρ 0 π + π angular distribution d dn 3 = cos 2 ( ϑ) cos( ϑ r r ) 4 (r : probablity that ρ o has helicity 0) With SCHC, Q 2 = 2.4 GeV 2 from the shape of the angular distribution σ L /σ T cos(θ π)

11 Total, longitudinal, transversal cross section CLAS γ*p p ρ data p r e l i m i n a r y GPDs (frozen coupling constant + k perp effects) Regge/VDM (Laget) 1 Q 2 (GeV 2 ) 2001 data at 5.75 GeV more than 10 times statistics

12 Dedicated DVCS Program with CLAS at 6 GeV Measure the beam spin asymmetry and helicity-dependent cross section difference for several Q 2 bins x B -dependence Φ - dependence t dependence o Imaginary part of DVCS amplitude o Kinematical dependence of DVCS observables and GPDs o Relative importance of twist-2 and higher twist contributions, QCD corrections o Improve modeling of GPDs

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14 DVCS - Expected Kinematical Dependencies sensitivity to twist 3 probes ξ dependency t= x B =0.22 φ = 90 ο x B =0.22 ~ 380 bins in φ, x B, t

15 Prospects for GPD at 12 GeV

16 L = cm -2 s -1 The CLAS ++ Detector Forward Cerenkov Forward DC Forward EC Inner Cerenkov Central Detector Preshower EC Torus Cold Ring Inner Calorimeter Forward TOF

17 DVCS with CLAS ++ at 11 GeV Q 2,x B, t, ranges measured simultaneously

18 DVCS with CLAS ++ at 11 GeV 2% of all data points that are measured simultaneously.

19 DVCS with CLAS ++ at 11 GeV sinφ-moment of A LU: t - dependence

20 Deeply Virtual ρ 0 Production at 11 GeV CLAS ++ σ L Separation of σ L /σ T by analysis of angular distribution of the π + π system, and use of SCHC cm -2 s -1

21 Separated Cross section for ep eπ + n Rosenbluth separation for σ L /σ T x B = many other bins at -t = GeV 2 the same time σ L Measure pπ 0, pη, KΛ, KΣ,.. simultaneously σ TOT σ T

22 Summary g First exclusive DVCS measurement with CLAS at 4 GeV g New 6 GeV data in hand for DVCS and DVMP g Some 6 GeV DVCS data with longitudinal polarized target g Dedicated setup under design in Hall B and in Hall A for a DVCS measurement at 6 GeV with complete event reconstruction and large kinematic coverage High luminosity data at 6 GeV and 12 GeV will allow to map the Q 2,x B and t dependences necessary for extracting information about the GPDs in the essentially unexplored kinematical region: W > 2 GeV, Q 2 > 1.5 GeV 2, 0.1 < x B <0.7 Elevate the study of nucleon dynamics to a new level

23 First Measurement with CLAS Data (March 1999) CEBAF Large Acceptance Spectrometer Beam polarization (~70%) E beam = 4.2 GeV Integrated L tot = 1.4x10 39 cm -2 (1.4fb -1 ) Corresponds to 3 days of beam time DVCS: Exclusive measurement: ep e'p(γ)

24 Hall A DVCS at 6 GeV Experimental Setup High Resolution Hall A spectrometer for electron detection 100-channel scintillator array for proton detection 150-block PbF 2 electromagnetic calorimeter for photon detection Detection of all 3 final-state particles ensures exclusivity

25 Hall A DVCS at 6 GeV Methods and goals Unique characteristics: Very high luminosity (10 37 cm -2 s -1 ) Well defined kinematics (high resolution detection of e - and γ) Very high statistics for Q 2 up to 2.5 GeV 2 at fixed x B =0.35 Method: Measure weighted cross-section differences ( purer than asymmetries) as function of ϕ angle. Deduce Q 2 dependence and relative importance of twist-2 and higher twists. Test factorization of DVCS Measure linear combinations of GPD s One of three bins at Q 2 =2.5GeV 2

26 Experimental Setup: Modified CLAS Configuration Increase luminosity to 2 x10 34 cm -2 s ο 8 ο Replace the mini-torus shield with a solenoidal magnetic field. Exclusivity of DVCS event by event PbWO 4 crystal array for the detection of high-energy photons at small lab angles.

27 Complete Photon Detection 430 crystals, 15x15x160mm 3, pointing geometry, APD readout γ γ 4GeV/c π 0 at 65 cm distance (symmetric decay)

28 W > 2 and E = 6 GeV CLAS Kinematical coverage in Q 2, t, and x B

29 DVCS and Generalized Parton Distributions (GPDs) ~ ~ GPDs: H, E unpolarized, H, E polarized γ γ e.g. H(ξ, t, Q 2 ) = Hq (x, ξ, t, Q 2 )dx x-ξ + iε H q (x, ξ, t, Q 2 )dx = x-ξ 10/8/2002 real part + iπh q (ξ, ξ, t, Q 2 ) imaginary part ~ ~ P ( x + ξ ) P( x ξ ) GPD s cross section difference d 3 σ dq 2 dx B dt ~[ah(ξ, t, Q2 )+be(ξ, t, Q 2 )+ch(ξ, t, Q 2 )+de(ξ, t, Q 2 )] 2 H q : Probability amplitude for N to emit a parton q with x+ξ and N to absorb it with x- ξ. Latifa Elouadrhiri - DVCS at 6 GeV with CLAS 31

30 CLAS 4.3 GeV - Kinematical Coverage

31 DVCS: Beam Spin Dependence The imaginary part of the DVCS amplitude can be accessed with longitudinally polarized beam: d 4 σ + dq 2 dx B dtdφ - d 4 σ - dq 2 dx B dtdφ Im(TDVCS )T BH a ImM 1,1 sinφ + b ImM 0,1 sin2φ + O(1/Q 2 ) The leading term (twist-2) has a sinφ modulation. dσ + - dσ - = - 16(2 y) 1- x 1- yx - 2 Q 2 sinφim{f 1 H+ ~ x 2 - x (F 1 + F 2 )H - 2 4M 2 F 2E}dM The twist-3 term has sin2φ modulation

32 Measurements to extract all twist-2 and twist-3 Compton Form Factors. polarized beam polarized target - longitudinal - transverse e - /e + beam beam target observed experiment e + /e unpol Re C I unp?/e1-dvcs pol long. Re C I LP EG1 pol trans. Re C I TP+/-? pol unpol Im C I unp E1/E1-DVCS unpol long. Im C I LP EG1 unpol trans. Im C I TP+/-? Fourier components cosφ, cos2φ,cos3φ 1, cosφ, cos2φ cosφ,cos2φ sinφ, sin2φ sinφ, sin2φ,sin3φ Σ = 10 (13) x B C I unp = F 1 H + (F 1 +F 2 )H x B 4M 2 F 2 E H = dxc(ξ,x)h(x,ξ) -1 +1

33 Total cross section for γ*p p π + π Comparison to world W ~ 2GeV σ ( B) * µ γ p pππ CLAS DESY Cornell p r e l i m i n a r y

34 ep e'px - Missing Mass Analysis Radiative eleastic events Exclusive ep e'pγγ For DVCS kinematics ep e'p(γ) ep e'pπ o Exclusive ep e'pγ

35 Expected Results ep e'pγ (DVCS + BH) at 6 GeV 60 days of beam time at luminosity L = 2x10 34 cm -2 sec -1 Q 2 =1 Q 2 = 2 φ= 90 o t= Q 2 =3

36 Expected t Dependence φ= 90 o x B = 0.22

37 Expected Φ Dependence t = x B = 0.22

38 Expected CLAS DVCS Results at 11 GeV Expected Kinematical Coverage

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40 Measurement of exclusive DVCS/BH with CLAS Beam Spin Asymmetry g CLAS Data g First Measurement of the beam spin asymmetry in fully exclusive DVCS process gthe measured asymmetry: α = ± stat ± sys Α(φ) = αsinφ + βsin2φ S. Stepanyan et al. Phys. Rev. Lett. 87 (2001) g From the recent calculation CLAS data are consistent with antiquark-gluon-quark correlation in the twist-3 GPDs