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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’. q’. k. N. N’. GPDs. Silvia Niccolai, IPN Orsay, for the CLAS Collaboration QNP2012, Palaiseau, April 19 th 2012. ~. e. t. g. H, H, E, E (x,ξ,t). g L *. (Q 2 ). x+ξ. x-ξ. ~. ~.

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Silvia Niccolai, IPN Orsay, for the CLAS Collaboration QNP2012, Palaiseau, April 19 th 2012

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

  2. ~ e t g H, H, E, E (x,ξ,t) gL* (Q2) x+ξ x-ξ ~ ~ Axial-Vector:H (x,ξ,t) Pseudoscalar:E (x,ξ,t) ~ [ ] 1 1 1 ò = - JG = x + x q q q J xdx H ( x , , 0 ) E ( x , , 0 ) 2 2 - 1 Deeply Virtual Compton Scattering and GPDs e’ • Q2= - (e-e’)2 • xB = Q2/2Mn n=Ee-Ee’ • x+ξ, x-ξ longitudinal momentum fractions • t = (p-p’)2 • x  xB/(2-xB) 4 GPDs for each quark flavor p’ p conserve nucleon helicity « Handbag » factorization valid in the Bjorken regime: high Q2 ,  (fixed xB), t<<Q2 Vector:H (x,ξ,t) Tensor:E (x,ξ,t) flip nucleon helicity Quark angular momentum (Ji’s sum rule) «3D» quark/gluon image of the nucleon X. Ji, Phy.Rev.Lett.78,610(1997)

  3. Sensitivity to GPDs of DVCS spin observables g ~ Polarized beam, unpolarized target: Im{Hp, Hp, Ep} f e’ ~ DsLU~ sinfIm{F1H+ x(F1+F2)H -kF2E}df e leptonic plane N’ ~ ~ Unpolarized beam, longitudinal target: Polarized beam, longitudinal target: hadronic plane Re{Hp, Hp} Im{Hp, Hp} ~ ~ DsLL ~ (A+Bcosf)Re{F1H+x(F1+F2)(H+ xB/2E)…}df DsUL ~ sinfIm{F1H+x(F1+F2)(H+ xB/2E) –xkF2E+…}df Unpolarized beam, transverse target: Im{Hp, Ep} DsUT~ sinfIm{k(F2H – F1E) + …..}df x= xB/(2-xB) k=-t/4M2 ProtonNeutron ~ Im{Hn, Hn, En} ~ ~ Im{Hn, En, En} ~ Re{Hn, En, En} Im{Hn}

  4. Sensitivity to GPDs of DVCS spin observables g ~ Polarized beam, unpolarized target: Im{Hp, Hp, Ep} f e’ ~ DsLU~ sinfIm{F1H+ x(F1+F2)H -kF2E}df e leptonic plane N’ ~ ~ Unpolarized beam, longitudinal target: Polarized beam, longitudinal target: hadronic plane Re{Hp, Hp} Im{Hp, Hp} ~ ~ DsLL ~ (A+Bcosf)Re{F1H+x(F1+F2)(H+ xB/2E)…}df DsUL ~ sinfIm{F1H+x(F1+F2)(H+ xB/2E) –xkF2E+…}df Unpolarized beam, transverse target: Im{Hp, Ep} DsUT~ sinfIm{k(F2H – F1E) + …..}df x= xB/(2-xB) k=-t/4M2 ProtonNeutron ~ Im{Hn, Hn, En} ~ ~ Im{Hn, En, En} ~ Re{Hn, En, En} Im{Hn}

  5. What we have learned from the published CLAS asymmetries CLAS pDVCS TSAs eg1 (2000), not a DVCS- dedicated experiment CLAS pDVCS BSAs S. Chen et al, PRL 97, 072002 (2006) F.-X. Girod et al, PRL. 100 162002 (2008) Model-independent fit at fixed xB, t, Q2 of DVCS observables ImH has steeper t-slope than ImH: is axial charge more concentrated than the electromagnetic charge? ~ M. Guidal, Phys. Lett. B 689, 156-162 (2010)

  6. The eg1-dvcs experiment at CLAS • Data taken from February to September 2009 • Beam energies = 4.735, 5.764, 5.892, 5.967 GeV • Beam polarizaton ~ 85% • CLAS+IC to detect forward photons • Target: longitudinally polarized via DNP (5 Tesla, 1 Kelvin, 140 Ghz microwaves) NH3 (~80%) and ND3(~30%) – Luminosity ~ 5∙1034 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

  7. pDVCS (ep→e’p’g): particle ID Db (DC/TOF) for positive tracks Energy deposited in EC for negative tracks p+ protons deuterons Electron ID cuts: Charge: -1 0.2 < E/p < 0.4 (energy deposited in EC) Ein > 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¦ < 0.035 DC fiducial cuts EC photons not yet included in the analysis (<10% events) IC photon ID cuts: E >2.5 GeV Geometrical fiducial cuts

  8. pDVCS: channel selection & coverage Kinematical and exclusivity cuts to select DVCS events: Cone angle: before/after cuts • Eγ>2.5 GeV Q2>1 GeV2 W>2 GeV • Cone Angle (angle between detected and predicted γ) • MM2 epX • Missing Energy • Coplanarity (angle between (γ*,p) and (γ,p) planes) • MM2 epgX • Missing Transverse Momentum (in reaction frame)

  9. pDVCS- Sanity check: Beam Spin Asymmetry eg1-dvcs Integrated over all kinematics, only IC photons included Only eg1-dvcs part B data (~2/3) No p0 background subtraction yet F.-X. Girod et al, PRL. 100 162002 (2008) Beam polarization: ~ 83 %

  10. Erin Seder, UConn Gary Smith, Glasgow pDVCS: Target Spin Asymmetry Preliminary <xB>~0.3 <Q2>=2.3 (GeV/c2)2 Dilution factor: f~ 0.76 Target polarization: PT=-85%, +90% Only IC photons included only eg1-dvcs part B data No p0 background subtraction yet

  11. Gary Smith, Glasgow pDVCS: Double (Beam-Target) Spin Asymmetry p0 vs xB p0 vs -t First bin in -t ALL THESE RESULTS ARE VERY PRELIMINARY! Dilution factor: f~ 0.76 Target polarization: PT=-85%, +90% Beam Polarizarion: PB= 83% p1 vs -t p2 vs -t Only IC photons included only eg1-dvcs part B data No p0 background subtraction yet

  12. H2 n NH3 ND3 ND3 DVCS on different targets DariaSokhan, IPNO Calculate DVCS on a “free” neutron Free proton F.-X. Girod et al, PRL. 100 (2008) 162002 Free proton in nuclear medium Quasi-free neutron in deuterium and in heavier nuclear medium Quasi-free protonin deuterium and in heavier nuclear medium

  13. Sanity check: ALU – proton in NH3/ND3 DariaSokhan, IPNO Raw beam-spin asymmetries No p0 background subtraction Good agreement between the two analyses

  14. nDVCS in ND3 – channel selection B < 0.95 (EC timing) pX Standard PID cuts for electron and photon Exclusivity cuts:

  15. nDVCS ALU beam-spin asymmetry from ND3 Very preliminary DariaSokhan, IPNO Projections for 90 days of running with CLAS12 <xB>~0.3 <Q2>=2.3 (GeV/c2)2 Integrated over all kinematics No p0 subtraction yet Statistics very low, but ALU≠0! AUL analysis also underway More data will be taken with CLAS12 at 11 GeV, on liquid deuterium target

  16. 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

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