Deeply virtual Compton scattering on longitudinally polarized protons

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

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

Sensitivity to GPDs of DVCS spin observables g f e’ e x= x. B/(2 -x. B) k=-t/4 M 2 Polarized beam, unpolarized target: ~ -k. F 2 E}df Ds. LU ~ sinf Im{F 1 H + x(F 1+F 2)H Unpolarized beam, longitudinal target: Ds. UL ~ sinf. Im{F 1~ H+x(F 1+F 2)(H + x. B/2 E) –xk. F 2 ~ E+…}df Polarized beam, longitudinal target: Ds. LL ~ (A+Bcosf)Re{F 1~ H+x(F 1+F 2)(H + x. B/2 E)…}df Unpolarized beam, transverse target: Ds. UT ~ sinf. Im{k(F 2 H – F 1 E) + …. . }df leptonic plane hadronic plane Proton N’ Neutron ~, E } Im{Hp, H p p ~ Im{Hn, En} ~ Im{Hp, Hp} ~ Im{Hn, En} ~ Re{Hp, Hp} Re{H , E~} n Im{Hp, Ep} Im{Hn} n n

Sensitivity to GPDs of DVCS spin observables g f e’ e x= x. B/(2 -x. B) k=-t/4 M 2 Polarized beam, unpolarized target: ~ -k. F 2 E}df Ds. LU ~ sinf Im{F 1 H + x(F 1+F 2)H Unpolarized beam, longitudinal target: Ds. UL ~ sinf. Im{F 1~ H+x(F 1+F 2)(H + x. B/2 E) –xk. F 2 ~ E+…}df Polarized beam, longitudinal target: Ds. LL ~ (A+Bcosf)Re{F 1~ H+x(F 1+F 2)(H + x. B/2 E)…}df Unpolarized beam, transverse target: Ds. UT ~ sinf. Im{k(F 2 H – F 1 E) + …. . }df leptonic plane hadronic plane Proton N’ Neutron ~, E } Im{Hp, H p p ~ Im{Hn, En} ~ Im{Hp, Hp} ~ Im{Hn, En} ~ Re{Hp, Hp} Re{H , E~} n Im{Hp, Ep} Im{Hn} n n

What we have learned from the published CLAS asymmetries CLAS p. DVCS BSAs F. -X. Girod et al, PRL. 100 162002 (2008) Model-independent fit at fixed x. B, t, Q 2 of DVCS observables Im. H has steeper t-slope than ~ Im. H: is axial charge more concentrated than the electromagnetic charge? M. Guidal, Phys. Lett. B 689, 156 -162 (2010) CLAS p. DVCS TSAs eg 1 (2000), not a DVCSdedicated experiment S. Chen et al, PRL 97, 072002 (2006)

The eg 1 -dvcs experiment at CLAS • Data taken from February to September 2009 • Beam energies = 4. 735, 5. 764, 5. 892, 5. 967 Ge. V • 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∙ 1034 cm-2 s-1 • Target polarization monitored by NMR • ~75 fb-1 on NH 3 (parts A, B), ~25 fb-1 on ND 3 (part C) Polarized ammonia Carbon Empty cell C. D. Keith et al. , NIM A 501 (2003) 327

p. DVCS (ep→e’p’g): particle ID Energy deposited in EC for negative tracks Db (DC/TOF) for positive 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 IC photon ID cuts: E >2. 5 Ge. V Geometrical 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)

p. DVCS: channel selection & coverage Kinematical and exclusivity cuts to select DVCS events: • Eγ>2. 5 Ge. V Q 2>1 Ge. V 2 W>2 Ge. V • Cone Angle (angle between detected and predicted γ) • MM 2 ep. X • Missing Energy • Coplanarity (angle between (γ*, p) and (γ, p) planes) • MM 2 epg. X • Missing Transverse Momentum (in reaction frame) Cone angle: before/after cuts

p. DVCS- Sanity check: Beam Spin Asymmetry eg 1 -dvcs F. -X. Girod et al, PRL. 100 162002 (2008) Integrated over all kinematics, only IC photons included Only eg 1 -dvcs part B data (~2/3) No p 0 background subtraction yet Beam polarization: ~ 83 %

p. DVCS: Target Spin Asymmetry Preliminary <x. B>~0. 3 <Q 2>=2. 3 (Ge. V/c 2)2 Dilution factor: f~ 0. 76 Target polarization: PT=-85%, +90% Only IC photons included only eg 1 -dvcs part B data No p 0 background subtraction yet Erin Seder, UConn Gary Smith, Glasgow

p. DVCS: Double (Beam-Target) Spin Asymmetry Gary Smith, Glasgow First bin in -t p 0 vs x. B ALL THESE RESULTS ARE VERY PRELIMINARY! Dilution factor: f~ 0. 76 Target polarization: PT=-85%, +90% Beam Polarizarion: PB= 83% Only IC photons included only eg 1 -dvcs part B data No p 0 background subtraction yet p 1 vs -t p 2 vs -t

DVCS on different targets Daria Sokhan, IPNO Free proton F. -X. Girod et al, PRL. 100 (2008) 162002 NH 3 H 2 n Free proton in nuclear medium Quasi-free proton in deuterium and in heavier nuclear medium Calculate DVCS on a “free” neutron ND 3 Quasi-free neutron in deuterium and in heavier nuclear medium

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

n. DVCS in ND 3 – channel selection B < 0. 95 (EC timing) p. X Standard PID cuts for electron and photon Exclusivity cuts:

n. DVCS ALU beam-spin asymmetry from ND 3 Very preliminary Daria Sokhan, IPNO Projections for 90 days of running with CLAS 12 <x. B>~0. 3 <Q 2>=2. 3 (Ge. V/c 2)2 Integrated over all kinematics No p 0 subtraction yet Statistics very low, but ALU≠ 0! AUL analysis also underway More data will be taken with CLAS 12 at 11 Ge. V, on liquid deuterium target

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 eg 1 -dvcs experiment combined the CLAS-DVCS setup (CLAS+IC) with polarized hydrogen and deuterium targets • Preliminary results for TSA for p. DVCS 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 n. DVCS (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 p. DVCS and n. DVCS on a wider phase space will come from CLAS 12 Thanks again to Erin Seder, Gary Smith, Daria Sokhan