Deeply Virtual Compton Scattering and Meson Production at JLab/CLAS

Transcription

1 Deeply Virtual Compton Scattering and Meson Production at JLab/CLAS Hyon-Suk Jo for the CLAS collaboration IPN Orsay PANIC 2011 M.I.T. Cambridge - July 25, th Particles & Nuclei International Conference

2 Outline Introduction Deeply Virtual Compton Scattering (DVCS) Deeply Virtual Meson Production (DVMP) JLab 12GeV: DVCS/DVMP at CLAS12 Conclusions

3 Deeply Virtual Compton Scattering (DVCS) handbag diagram (high Q 2, small t, fixed x B ) DVCS DVMP BH fully calculable in QED 2 (epep) = + GPDs + H(x,,t), E(x,,t) ~ ~ H(x,,t), E(x,,t) DVCS and H(x,,t), Bethe-Heitler E(x,,t) (BH) experimentally undistinguishable ~ ~ H(x,,t), E(x,,t) interference between the 2 processes dq 2 4 d dx dtd B T DVCS with T BH I T 2 DVCS T T * BH DVCS is the most straightforward, theoretically cleanest reaction allowing to access the GPDs DVCS 2 T T * DVCS interference term BH T 2 BH I

4 Extracting GPDs from DVCS observables ReH ImH q q 2 e q BSA P e 2 q : 1 0 q q 1 1 H ( x,, t) H ( x,, t) dx H A LU (,, t) H = x B /(2-x B ) k = t/4m 2 q d d d d x (,, t) LU d d Polarized beam, Unpolarized target ~ LU ~ sin Im{F 1 H + (F 1 +F 2 )H -kf 2 E}d x Unpolarized beam, Longitudinally polarized target ~ ~ UL ~ sin Im{F 1 H+(F 1 +F 2 )(H + x B /2E) kf 2 E+ }d Unpolarized beam, Transversely polarized target q ep e e * leptonic plane hadronic plane p ep Proton Neutron Im{H p, H ~ p, E p } ~ Im{H n, H n, E n } ~ Im{H p, H p } ~ Im{H n, E n, E n } Im{H p, E p } UT ~ cos Im{k(F 2 H F 1 E) +.. }d Im{H n } Polarized beam, Longitudinally polarized target ~ Re{H p, H p } ~ LL ~ (A+Bcos Re{F 1 H+(F 1 +F 2 )(H + x B /2E) }d Re{H n, E n, E ~ n }

5 Jefferson Lab (Newport News, Virginia, USA) CEBAF : Continuous Electron Beam Accelerator Facility Duty cycle 100% E max 5.8 GeV P max 80% Hall B CLAS detector in Hall B

6 A typical DVCS/BH event in the CLAS detector epe p The DVCS/BH photon is mostly emitted at forward angles SC DC e CC EC (EC) (CC) p DVCS/BH photon not detected with CLAS alone (DC) (SC) Need an electromagnetic calorimeter at forward angles for DVCS experiments!

7 The e1-dvcs experiment (first experiment with CLAS dedicated to DVCS) with the CLAS detector + DVCS electromagnetic calorimeter + Solenoid Part 1 of the e1-dvcs experiment: Data taken from March 11 until May 27, 2005 Beam energy ~ GeV Beam current = na Polarization ~ 76-82% Integrated luminosity ~ 3.33 x 10 7 nb -1 CLAS Target LH 2 Solenoid DVCS electromagnetic calorimeter shielding the detectors from the Møller electrons 424 lead tungstate crystals + APD readout

8 The e1-dvcs experiment (first experiment with CLAS dedicated to DVCS) with the CLAS detector + DVCS electromagnetic calorimeter + Solenoid Part 1 of the e1-dvcs experiment: Data taken from March 11 until May 27, 2005 Beam energy ~ GeV Beam current = na Polarization ~ 76-82% Integrated luminosity ~ 3.33 x 10 7 nb -1 CLAS Target LH 2 Solenoid DVCS electromagnetic calorimeter The e1-dvcs data can be used to extract DVCS and DVMP Beam Spin Asymmetries and Cross Sections shielding the detectors from the Møller electrons 424 lead tungstate crystals + APD readout

9 Kinematic coverage of the e1-dvcs data The kinematics of the DVCS reaction is defined by 4 independent variables : Q 2, x B, t and 4-dimensional bins = (Q 2, x B, -t, ) e e * leptonic plane hadronic plane p

10 DVCS Beam Spin (A LU ) and Longitudinal Target Spin (A UL ) asymmetries A LU integrated over t F.X. Girod et al. (CLAS Collaboration), Phys. Rev. Lett. 100, (2008)

11 DVCS Beam Spin (A LU ) and Longitudinal Target Spin (A UL ) asymmetries A LU JLab Hall A results VGG twist-3 VGG twist-2 A UL <Q 2 >=1.82, <-t>=0.31, <>=0.16 Regge (JML) Previous CLAS results F.X. Girod et al. (CLAS Collaboration), Phys. Rev. Lett. 100, (2008) S. Chen et al. (CLAS Collaboration), Phys. Rev. Lett. 97, (2006) VGG model: Vanderhaeghen,Guichon,Guidal

12 Extraction of GPDs from fitting A LU and A UL CLAS data Having both the beam-spin and longitudinal target-spin asymmetries, a nearly model-independent GPD analysis in leading twist was achieved fitting simultaneously A LU and A UL extracted with CLAS at 3 values of t and fixed x B ImH ImH ~ M. Guidal, Phys. Lett. B 689, (2010) ImH : VGG model predictions reproduce the shape of the data but overestimate it, especially at lower t ~ ImH : VGG model predictions tend to underestimate the data ~ ImH shows a steeper t-slope than ImH which would suggest that the axial charge is more concentrated than the electromagnetic charge

13 Extraction of GPDs from fitting world data red circles: fit with only H Indication that we can not neglect H ~, [Kumericki and Mueller] (blue and green curves, black triangles) K. Kumericki and D. Mueller, Proceedings of 4th Workshop on Exclusive Reactions at High Momentum Transfer, Newport News, Virginia, May 2010

14 DVCS cross sections Q , e 0.1 x 45, p B e 0.58, t 3 0.8, W 2 4-dimensional bins = (Q 2, x B, -t, ) The fast variation of the BH cross section: Around Φ=0 (where lies the BH singularity), there can be a factor ~2 between neighbouring kinematics Q 2 x B The very fast variations of the BH cross section make the DVCS cross section analysis particularly difficult

15 DVCS cross sections Q , e 0.1 x 45, p B e 0.58, t 3 0.8, W 2 4-dimensional bins = (Q 2, x B, -t, ) PRELIMINARY Q 2 x B 2 independent analyses in progress JLab / IPN Orsay

16 Deeply Virtual Meson Production (DVMP) handbag diagram (high Q 2, small t, fixed x B ) DVMP Factorization proven only for longitudinally polarized virtual photons 1 T ~ L 2 Q ~ H(x,,t), E(x,,t) H(x,,t), E(x,,t) H(x,,t), E(x,,t) ~ ~ H(x,,t), E(x,,t) conserve nucleon helicity ~ H (x,ξ,t) H (x,ξ,t) ~ E (x,ξ,t) E (x,ξ,t) flip nucleon helicity Vector mesons ( w ) sensitive to H and E Pseudoscalar mesons ( ) sensitive to H ~ and E ~ ρ 0 ω ρ u+d 2u-d u-d 2u+d u-d 2u-d DVMP allows quark flavor decomposition

17 Deeply Virtual Meson Production (DVMP) Vector mesons: exclusive 0, w and electroproduction on the proton at CLAS 6 GeV: K. Lukashin et al., Phys. Rev. C 63, , 2001 (@4.2 GeV) e1-b C. Hadjidakis et al., Phys. Lett. B 605, , 2005 ( GeV) (1999) L. Morand et al., Eur. Phys. J. A 24, , 2005 (w@5.75gev) J. Santoro et al., Phys. Rev. C 78, , 2008 (@5.75 GeV) S. Morrow et al., Eur. Phys. J. A 39, 5-31, 2009 ( A. Fradi, Orsay Univ. PhD thesis GeV) e1-6 ( ) e1-dvcs (2005) There are also results on exclusive pseudoscalar meson electroproduction on the proton at CLAS 6 GeV: R. De Masi et al., Phys. Rev. C 77, (R), 2008 ( K. Park et al., Phys. Rev. C 77, , 2008 GeV) I. Bedlinskiy et al., paper in preparation (

18 Comparison between 0 w and 0 w

19 Comparison between 0 w and b increases with W: valence quarks in the center of the nucleon and sea quarks at the periphery of the nucleon b decreases with Q 2 : by increasing the resolution of the probe, smaller objects in the nucleon can be seen

20 Longitudinal cross section L ( L p p L0 ) As a function of W, at fixed Q 2, L first drops (W<5 GeV) and then slightly rises (W>5 GeV) S. Morrow et al., Eur. Phys. J. A 39, 5-31, 2009

21 Longitudinal cross section L ( L p p L0 ) The GPD models fail to reproduce the behavior at low W (W < 5 GeV) VGG GPD model GK GPD model Valence q Sea q and gluons VGG: Vanderhaeghen,Guichon,Guidal S. Morrow et al., Eur. Phys. J. A 39, 5-31, 2009 GK: Goloskokov,Kroll

22 L + Comparison between and 0 L p p GK: Goloskokov,Kroll L p p L 0 Gluon GPDs Quark GPDs + Gluon GPDs GPD models fail to reproduce the behavior at low W (W < 5 GeV) for 0 w but succeed for which is only sensitive to gluon GPDs

23 JLab upgrade to 12 GeV E= 2.2, 4.4, 6.6, 8.8, 11 GeV Beam polarization P e > 80% Add new hall 12 GeV CHL-2 Continuous Electron Beam Accelerator Facility CLAS12 Upgrade of the instrumentation of the existing Halls

24 Hall B at JLab 12GeV: CLAS12 CLAS12 Design luminosity L ~ cm -2 s -1 Central Detector High luminosity Large acceptance Large kinematic coverage Forward Detector PCAL

25 Kinematic coverage of CLAS12 Sea/gluon region Valence region H1, ZEUS JLab 12 GeV Upgrade is well matched for GPD studies in the valence quark regime

26 GPD program at CLAS12 DVCS beam-spin asymmetry A LU on the Proton DVCS longitudinal target-spin asymmetry A UL on the Proton DVCS transverse target-spin asymmetry A UT on the Proton DVCS on the Neutron DVCS unpolarized and polarized cross sections DVMP: pseudoscalar mesons DVMP: vector mesons

27 Projections of DVCS A LU on the Proton with CLAS12 ep ep 80 days Luminosity VGG model A LU Large acceptance: measurements in large Q 2, x B, t ranges simultaneously A LU (Q 2,x B,t) Δσ(Q 2,x B,t) σ(q 2,x B,t)

28 Projections of DVCS A LU on the Proton with CLAS12 ep ep 80 days Luminosity VGG model A LU Large acceptance: measurements in large Q 2, x B, t ranges simultaneously A LU (Q 2,x B,t) Δσ(Q 2,x B,t) σ(q 2,x B,t)

29 Projections of DVCS A LU on the Neutron with CLAS12 Total of 588 bins in t, Q 2, x B, Combined analysis of DVCS on Proton and Neutron allows flavor separation of GPDs t = GeV 2, Δt = 0.3 GeV 2 Q 2 = 2.75 GeV 2, ΔQ 2 = 1.5 GeV 2 x B = 0.225, Δx B = 0.15 This program requires adding a Central Neutron Detector (CND) to the CLAS12 base equipment DVCS on Neutron is sensitive to the GPD E n and the d-quark content of the nucleon spin CND

30 Conclusions CLAS has the largest set ever of data for DVCS and exclusive vector meson production in the valence region GPD models fairly agree with the DVCS asymmetry data at high Q 2 but fail to reproduce it at lower Q 2 GPD models describe well the exclusive vector meson data for W>5 GeV (sea quarks and/or gluons) which seem to be interpretable in terms of (k perp -modified) leading order handbag diagram (quark/gluon GPDs) BUT fail by large for W<5 GeV (valence region) except for which is only sensitive to gluon GPDs We need to go to higher Q 2 (while staying in the valence region) to verify scaling for DVCS on a wider Q 2 range, and to approach GPD validity regime for DVMP JLab 12GeV will provide high luminosity for high accuracy measurements to test models on a large x B scale and thus will be the ideal facility to study GPDs in the valence region and CLAS12 will be perfectly suited for a rich experimental GPD program Thank you

31 Backup slides

32 Generalized Parton Distributions (GPDs) GPDs provide a correlation between the transverse position AND the longitudinal momentum of quarks in the nucleon 3D image of the nucleon Form factors: transverse position of quarks in the nucleon Parton distributions: longitudinal momentum of quarks in the nucleon

33 GPDs and exclusive reactions handbag diagrams (high Q 2, small t, fixed x B ) * DVCS DVMP H(x,,t), E(x,,t) ~ ~ H(x,,t), E(x,,t) H(x,,t), E(x,,t) ~ ~ H(x,,t), E(x,,t) conserve nucleon helicity ~ H (x,ξ,t) H (x,ξ,t) ~ E (x,ξ,t) E (x,ξ,t) flip nucleon helicity Quark angular momentum (Ji s sum rule) J q 2 1 J G = xdx H 1 q q ( x,, 0 ) E ( x,,0) X. Ji, Phy.Rev.Lett.78,610(1997)

34 Deeply Virtual Compton scattering and GPDs At leading order real part imaginary part T DVCS 1 1 H( x,, t) H( x,, t) ~ dx ~ P dx i H(,, t) x i x 1 1 x Cross-section measurement and beam-charge asymmetry (ReT ) integrate GPDs over x Beam- or target-spin asymmetries are proportional to ImT, therefore to GPDs at x = and x = - VGG model: Vanderhaeghen,Guichon,Guidal

35 The CLAS detector (CEBAF Large Acceptance Spectrometer) Angular coverage (DC) (SC) (EC) (CC) charged particles : 8 < <140 (DC + SC) neutral particles : 8 < <45 (EC) Resolution dp/p1% d, dmrad

36 Interpretation with Regge theory (Laget model) L ( L p p L0 ),F 2 Pomeron Regge (Laget) Decent agreement for Q 2 up to 3.6 GeV 2 Off by an order of magnitude at higher Q 2

37 Longitudinal cross section L ( L p p L0 ) DDs + meson exchange DDs w/o meson exchange (VGG) meson exchange

38 Interpretation with Regge theory: Laget model p p 0 p pw p p

39 DVCS A LU on the Neutron with CLAS12 Combined analysis of DVCS on Proton and Neutron allows flavor separation of GPDs This program requires adding a Central Neutron Detector (CND) to the CLAS12 base equipment DVCS on Neutron is sensitive to the GPD E n and the d-quark content of the nucleon spin CND