Silvia Niccolai, IPN Orsay, for the CLAS Collaboration QNP2012, Palaiseau, April 19 th 2012 Deeply virtual Compton scattering on longitudinally polarized protons and neutrons at CLAS k k’ q’ GPDs NN’

Deeply Virtual Compton Scattering and GPDs e’ t (Q 2 ) e L*L* x+ξ x-ξ H, H, E, E (x,ξ,t) ~ ~  p p’ « Handbag » factorization valid in the Bjorken regime: high Q 2,  (fixed x B ), t<<Q 2 Q 2 = - (e-e’) 2 x B = Q 2 /2M  =E e -E e’ x+ξ, x-ξ longitudinal momentum fractions t = (p-p’) 2  x B /(2-x B )   0,x  ),(Ex q  2 1 Hxdx q  J G =  2 1 J q  1 1  )0,,( Quark angular momentum (Ji’s sum rule) X. Ji, Phy.Rev.Lett.78,610(1997) Vector: H (x,ξ,t) Tensor: E (x,ξ,t) Axial-Vector: H (x,ξ,t) Pseudoscalar: E (x,ξ,t) ~ ~ conserve nucleon helicity flip nucleon helicity «3D» quark/gluon image of the nucleon 4 GPDs for each quark flavor

Polarized beam, longitudinal target:  LL ~ (A+Bcos  )Re{F 1 H+x(F 1 +F 2 )(H + x B /2E)…}d  ~ Re{ H p, H p } ~ Im{ H n, E n, E n } x= x B /(2-x B ) k=-t/4M 2   leptonic plane hadronic plane N’N’ e’ e  LU ~ sin  Im{F 1 H + x(F 1 +F 2 )H -kF 2 E}d  ~ Polarized beam, unpolarized target: Im{ H p, H p, E p } ~  UL ~ sin  Im{F 1 H+x(F 1 +F 2 )(H + x B /2E) –xkF 2 E+…}d  Unpolarized beam, longitudinal target: ~ Im{ H p, H p } ~ Unpolarized beam, transverse target:  UT ~ sin  Im{k(F 2 H – F 1 E) + ….. }d  Im{ H p, E p } Sensitivity to GPDs of DVCS spin observables Im{ H n, H n, E n } ~ Proton Neutron ~ Re{ H n, E n, E n } ~ Im{ H n } ~

Polarized beam, longitudinal target:  LL ~ (A+Bcos  )Re{F 1 H+x(F 1 +F 2 )(H + x B /2E)…}d  ~ Re{ H p, H p } ~ Im{ H n, E n, E n } x= x B /(2-x B ) k=-t/4M 2   leptonic plane hadronic plane N’N’ e’ e  LU ~ sin  Im{F 1 H + x(F 1 +F 2 )H -kF 2 E}d  ~ Polarized beam, unpolarized target: Im{ H p, H p, E p } ~  UL ~ sin  Im{F 1 H+x(F 1 +F 2 )(H + x B /2E) –xkF 2 E+…}d  Unpolarized beam, longitudinal target: ~ Im{ H p, H p } ~ Unpolarized beam, transverse target:  UT ~ sin  Im{k(F 2 H – F 1 E) + ….. }d  Im{ H p, E p } Sensitivity to GPDs of DVCS spin observables Im{ H n, H n, E n } ~ Proton Neutron ~ Re{ H n, E n, E n } ~ Im{ H n } ~

CLAS pDVCS BSAs CLAS pDVCS TSAs eg1 (2000), not a DVCS- dedicated experiment What we have learned from the published CLAS asymmetries Model-independent fit at fixed x B, t, Q 2 of DVCS observables S. Chen et al, PRL 97, (2006) M. Guidal, Phys. Lett. B 689, (2010) ImH has steeper t-slope than ImH: is axial charge more concentrated than the electromagnetic charge? ~ F.-X. Girod et al, PRL (2008)

The eg1-dvcs experiment at CLAS Data taken from February to September 2009 Beam energies = 4.735, 5.764, 5.892, GeV Beam polarizaton ~ 85% CLAS+IC to detect forward photons Target: longitudinally polarized via DNP (5 Tesla, 1 Kelvin, 140 Ghz microwaves) NH 3 (~80%) and ND 3 (~30%) – Luminosity ~ 5∙10 34 cm -2 s -1 Target polarization monitored by NMR ~75 fb -1 on NH3 (parts A, B), ~25 fb -1 on ND3 (part C) Polarized ammonia Carbon Empty cell C.D. Keith et al., NIM A 501 (2003) 327

pDVCS (ep→e’p’  ): particle ID Energy deposited in EC for negative tracks Electron ID cuts: Charge: < E/p < 0.4 (energy deposited in EC) E in > 0.06 (energy deposited in inner EC) p > 0.8 Nphe(CC)>2 Geometrical matching between EC, SC, CC z vertex cut EC, DC fiducial cuts Proton ID cuts: Charge > 0 z vertex cut ¦Δβ¦ < DC fiducial cuts  (DC/TOF) for positive tracks ++ protons deuterons IC photon ID cuts: E >2.5 GeV Geometrical fiducial cuts EC photons not yet included in the analysis (<10% events)

pDVCS: channel selection & coverage Kinematical and exclusivity cuts to select DVCS events: E γ >2.5 GeV Q 2 >1 GeV 2 W>2 GeV Cone Angle (angle between detected and predicted γ) MM 2 epX Missing Energy Coplanarity (angle between (γ*,p) and (γ,p) planes) MM 2 ep  X Missing Transverse Momentum (in reaction frame) Cone angle: before/after cuts

pDVCS- Sanity check: Beam Spin Asymmetry Integrated over all kinematics, only IC photons included Only eg1-dvcs part B data (~2/3) No  0 background subtraction yet Beam polarization: ~ 83 % F.-X. Girod et al, PRL (2008) eg1-dvcs

pDVCS: Target Spin Asymmetry Dilution factor: f~ 0.76 Target polarization: P T =-85%, +90% Only IC photons included only eg1-dvcs part B data No  0 background subtraction yet ~0.3 =2.3 (GeV/c 2 ) 2 Preliminary Erin Seder, UConn Gary Smith, Glasgow

pDVCS: Double (Beam-Target) Spin Asymmetry Dilution factor: f~ 0.76 Target polarization: P T =-85%, +90% Beam Polarizarion: P B = 83% Only IC photons included only eg1-dvcs part B data No  0 background subtraction yet Gary Smith, Glasgow First bin in -t p 0 vs -t p 1 vs -t p 2 vs -t p 0 vs x B ALL THESE RESULTS ARE VERY PRELIMINARY!

DVCS on different targets H2 Free proton NH3 Free proton in nuclear medium ND3 Quasi-free proton in deuterium and in heavier nuclear medium ND3 Quasi-free neutron in deuterium and in heavier nuclear medium Calculate DVCS on a “free” neutron n F.-X. Girod et al, PRL. 100 (2008) Daria Sokhan, IPNO

Sanity check: A LU – proton in NH 3 /ND 3 Raw beam-spin asymmetries No  0 background subtraction Good agreement between the two analyses Daria Sokhan, IPNO

nDVCS in ND 3 – channel selection  < 0.95 (EC timing) Standard PID cuts for electron and photon pXpX Exclusivity cuts:

nDVCS A LU beam-spin asymmetry from ND 3 Integrated over all kinematics No  0 subtraction yet Statistics very low, but A LU ≠0! A UL analysis also underway More data will be taken with CLAS12 at 11 GeV, on liquid deuterium target ~0.3 =2.3 (GeV/c 2 ) 2 Daria Sokhan, IPNO Very preliminary Projections for 90 days of running with CLAS12

Summary and outlook Combining various DVCS spin observables for proton and neutron targets is necessary to provide constraints for model-independent extractions of Compton Form Factors (→GPDs) The eg1-dvcs experiment combined the CLAS-DVCS setup (CLAS+IC) with polarized hydrogen and deuterium targets Preliminary results for TSA for pDVCS are in good agreement with existing data, and the statistics with respect to previous CLAS data has been improved by more than a factor 5 Preliminary results for double-spin asymmetries show dominance of the constant term Very preliminary results for nDVCS (very low statistics) hint to non-zero beam-spin asymmetries A lot of work (mainly on background subtraction) still needs done Much more data for both pDVCS and nDVCS on a wider phase space will come from CLAS12 Thanks again to Erin Seder, Gary Smith, Daria Sokhan