Silvia Niccolai, IPN Orsay, for the CLAS Collaboration

QNP2012, Palaiseau, April 19th 2012

Deeply virtual Compton scattering on longitudinally polarized protons and neutrons at CLAS

k

k’

q’

GPDsN N’

Deeply Virtual Compton Scattering and GPDs

e’t

(Q2)

eL*

x+ξ x-ξ

H, H, E, E (x,ξ,t)~~

p p’

« Handbag » factorization validin the Bjorken regime:

high Q2 , (fixed xB), t<<Q2

• Q2= - (e-e’)2

• xB = Q2/2M=Ee-Ee’

• x+ξ, x-ξ longitudinal momentum fractions• t = (p-p’)2

• xB/(2-xB)

0,x ),( Ex q21 Hxdx qJG =

21J 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/gluonimage of

the nucleon

4 GPDs for each quark flavor

Polarized beam, longitudinal target:

LL ~ (A+Bcos)Re{F1H+x(F1+F2)(H + xB/2E)…}d~ Re{Hp, Hp}~

Im{Hn, En, En}

x= xB/(2-xB) k=-t/4M2

leptonic planehadronic

planeN’

e’

e

LU ~ sin Im{F1H + x(F1+F2)H -kF2E}d~Polarized beam, unpolarized target: Im{Hp, Hp, Ep}

~

UL ~ sinIm{F1H+x(F1+F2)(H + xB/2E) –xkF2 E+…}dUnpolarized beam, longitudinal target:

~ Im{Hp, Hp}~

Unpolarized beam, transverse target:

UT ~ sinIm{k(F2H – F1E) + ….. }d Im{Hp, Ep}

Sensitivity to GPDs of DVCS spin observables

dxxx

txHtxHPRe qq

1

0

q

11),,(),,(

2

qe H

),,(),,(q tHtHIm qq 2qe H

Im{Hn, Hn, En}

~

Proton Neutron

~

Re{Hn, En, En}~

Im{Hn}

~

Polarized beam, longitudinal target:

LL ~ (A+Bcos)Re{F1H+x(F1+F2)(H + xB/2E)…}d~ Re{Hp, Hp}~

Im{Hn, En, En}

x= xB/(2-xB) k=-t/4M2

leptonic planehadronic

planeN’

e’

e

LU ~ sin Im{F1H + x(F1+F2)H -kF2E}d~Polarized beam, unpolarized target: Im{Hp, Hp, Ep}

~

UL ~ sinIm{F1H+x(F1+F2)(H + xB/2E) –xkF2 E+…}dUnpolarized beam, longitudinal target:

~ Im{Hp, Hp}~

Unpolarized beam, transverse target:

UT ~ sinIm{k(F2H – F1E) + ….. }d Im{Hp, Ep}

Sensitivity to GPDs of DVCS spin observables

dxxx

txHtxHPRe qq

1

0

q

11),,(),,(

2

qe H

),,(),,(q tHtHIm qq 2qe H

Im{Hn, Hn, En}

~

Proton Neutron

~

Re{Hn, En, En}~

Im{Hn}

~

CLAS pDVCSBSAs

CLASpDVCSTSAseg1 (2000),not a DVCS-dedicatedexperiment

What we have learned from the published CLAS asymmetries

Model-independent fitat fixed xB, t, Q2

of DVCS observables

S. Chen et al, PRL 97, 072002 (2006)

M. Guidal, Phys. Lett. B 689, 156-162 (2010)

ImH has steeper t-slope than ImH: is axial charge more

concentrated than the electromagnetic charge?

~

F.-X. Girod et al, PRL. 100 162002 (2008)

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

pDVCS (ep→e’p’): particle IDEnergy deposited in EC for negative tracks

Electron ID cuts:Charge: -10.2 < E/p < 0.4 (energy deposited in EC)Ein > 0.06 (energy deposited in inner EC)p > 0.8Nphe(CC)>2Geometrical matching between EC, SC, CC z vertex cutEC, DC fiducial cuts

Proton ID cuts:Charge > 0z vertex cut¦Δβ¦ < 0.035

DC fiducial cuts

(DC/TOF) for positive tracks

22pTOF

TOF

mp

p

ct

d

+

protons

deuterons

IC photon ID cuts:E >2.5 GeVGeometrical fiducial cuts

EC photons not yet included inthe analysis (<10% events)

pDVCS: channel selection & coverage

Kinematical and exclusivity cuts to select DVCS events:

• 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 epX• Missing Transverse Momentum (in reaction frame)

Cone angle: before/after cuts

pDVCS- Sanity check: Beam Spin Asymmetry

)(

NNP

NNALU

Integrated over all kinematics,only IC photons includedOnly eg1-dvcs part B data (~2/3)No 0 background subtraction yet

Beam polarization: ~ 83 %

cos1

sin

1

0

p

pFit

F.-X. Girod et al, PRL. 100 162002 (2008)

eg1-dvcs

pDVCS: Target Spin Asymmetry

Dilution factor: f~ 0.76Target polarization: PT=-85%, +90%

Only IC photons includedonly eg1-dvcs part B data No 0 background subtraction yet

)(

NNPf

NNA

TLU

<xB>~0.3

<Q2>=2.3 (GeV/c2)2

Preliminary

Erin Seder, UConnGary Smith, Glasgow

pDVCS: Double (Beam-Target) Spin Asymmetry

Dilution factor: f~ 0.76Target polarization: PT=-85%, +90%Beam Polarizarion: PB= 83%

Only IC photons includedonly eg1-dvcs part B data No 0 background subtraction yet

)()(

)()(

NNNNPPf

NNNNA

TBLL

Gary Smith, Glasgow

2coscos 210 pppFit

First bin in -tp0 vs -t

p1 vs -t p2 vs -t

p0 vs xB

ALL THESE RESULTS ARE VERY PRELIMINARY!

DVCS on different targets

H2Free proton

NH3Free proton in nuclear medium

ND3Quasi-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” neutronn

F.-X. Girod et al, PRL. 100 (2008) 162002

Daria Sokhan, IPNO

Sanity check: ALU – proton in NH3/ND3

Xpeep '' Xpeed ''

cos1

sin

1

0

p

pA

Raw beam-spin asymmetriesNo 0 background subtraction

Good agreement between the two analyses

Daria Sokhan, IPNO

nDVCS in ND3 – channel selection

< 0.95 (EC timing)

cGeVpGeVQ

cGeVWGeVE

X /4.01

/21

22

2

5

8

/ 4.0

)/( 15.0 222

cGeVp

cGeVm

n

Xepep

Standard PID cuts for electron and photon

Xneed ''pX

Exclusivitycuts:

nDVCS ALU beam-spin asymmetry from ND3

cos1

sin

1

0

p

pA

Xneed ''

Integrated over all kinematicsNo 0 subtraction yetStatistics very low, but ALU≠0! AUL analysis also underway

More data will be taken with CLAS12at 11 GeV, on liquid deuterium target

<xB>~0.3

<Q2>=2.3 (GeV/c2)2

Daria Sokhan, IPNOVery 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 polarizedhydrogen 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