Harut Avakian (Jlab)Harut Avakian (Jlab)

DVCS results with unpolarized and polarized target

•Introduction•Event selection• MC simulations and radiative corrections•DVCS with unpolarised target•DVCS with longitudinally polarized target•Summary

1H.Avakian, Paris March 8

Deeply Virtual Compton Scattering ep->e’p’

•Different GPD combinations accessible as azimuthal moments of the total cross section.

DVCS

BH

LU~ sinIm{F1H + (F1+F2)H +kF2E}~

Polarized beam, unpolarized target:

Unpolarized beam, longitudinal target:

UL~ sinIm{F1H+(F1+F2)(H +.. }~

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

Kinematically suppressed

Kinematically suppressed

d4dQ2dxBdtd ~ |TDVCS + TBH|2DVCS BH

TBH : given by elastic form factorsTDVCS: determined by GPDs

GPD

UT~ cosm{k1(F2H-F1E ) +.. }

Unpolarized beam, transverse target:

Kinematically suppressed2

H.Avakian, Paris March 8

Electroproduction Kinematics

Ey /

)2/sin(4 '21

2 EEqQ

MQqpqxB 2/2/ 211

21

212 )( ppt

'EE

B

B

x

x

)q)(qp(p

)q(qξ

22 2121

221

))(1( min22 tt

require a finite longitudinal momentum transfer defined by the generalized Bjorken variable

e

p

•Define the procedure to extract GPDs from DVCS data •effect of finite bins (prefactor variations) ~10%

•Define background corrections•pion contamination ~10%•Radiative corrections

GPDs from ep->e’p’

Requirements for precision (<15%) measurements of GPDs from DVCS SSA:

A complete simulation of the whole chain from particle detection to GPD extraction, including the DVCS and background (counts, asymmetries) as well as extraction procedure (averaging over kinematic factors) required to ensure the reliability of measured GPDs.

4H.Avakian, Paris March 8

Main experiments in valence region

H.Avakian, Paris March 85

1)HERMES eX sample ~1000 events2)CLAS epX e1c,e1d e1f+e16(~2M events) Dominated by small t, small photon angles:

(<10)3) Hall A eX sample 1) Hall-B e1dvcs/e1dvcs2 ep2) Hall-B eg1dvcs ep3) Hall-A+Hall-B @ 12 GeV ep4)COMPASS

x~0.1

Target Spin Asymmetry: t- Dependence

Measurements with polarized target will constrain the polarized GPDs and combined with beam SSA measurements would allow precision measurement of unpolarized GPDs.

Unpolarized beam, longitudinal target:

Eg1dvcs provides order of magnitude more data compared to published eg1 data(5 CLAS days),

UL~ sinIm{F1H+(F1+F2)(H +.. }LL~ cosRe{F1H+(F1+F2)(H +.. }

~

Kinematically suppressed

~

6H.Avakian, Paris March 8

GPD extraction from DVCS data

H.Avakian, Paris March 87

Polarized data is crucial also for GPD-H extraction

M.GuidalPhys.Lett.B689:156-162,2010

MC vs Data

•Kinematic distributions in x,Q2,t consistent with the CLAS data

Region where BH totally dominates (small t, small photon LAB)•Negligible DVCS x-section, small 0

contamination•Rapidly changing prefactors, mainly small , hard to detect photons

Large angles•Uniform coverage in angle , photon measurement less challenging•DVCS x-section non negligible introduce some model dependence)0 dominates the single photon sample (in particular at low Q2 )

8H.Avakian, Paris March 8

-dependent amplitude

•Strong dependence on kinematics of prefactor -dependence, at t≈tcol,P1()→0•Radiative corrections may be significant

5.7 GeV

xMEQ

xMEQQtcol )2(

)2(2

22

9H.Avakian, Paris March 8

Radiative corrections

10H.Avakian, Paris March 8

z1/2m defined from minimum photon energy cut, x1/2-defined shifted kinematics

I. Akushevich

true x

-dependent amplitude

• Depending on the t the correction (the leading term of double bremsstrahlung x-section expanded over the electron mass ) can change the shape.

5.7 GeV

xMEQ

xMEQQtcol )2(

)2(2

22

11H.Avakian, Paris March 8

I. Akushevich

1212

CLAS configuration with longitudinally pol. targetCLAS configuration with longitudinally pol. target

e

Polarizations: Beam: ~70% NH3 proton ~70%

Target position -55cmTorus +/-2250Beam energy ~5.7 GeV

Longitudinally polarized target

ep→e’X

H.Avakian, Paris March 813

CLAS DVCS experiments (eg1-dvcs/e1dvcs2)

Helium tube

Polarized target

Inner Calo

DVCS solenoid

15o

18o

Extended cell

IC simulation and fiducial cuts

H.Avakian, Paris March 814

Photons in IC

Detailed simulation (Ahmed) is crucial for the x-section analysis

All single photons

DVCS MC

DVCS data

•Angular cut on difference between calculated and measured photons used to identify DVCS events

15H.Avakian, Paris March 8

epDVCS

ep

DVCS identification cuts

16H.Avakian, Paris March 8

Missing energy

•Angular cut on difference between calculated and measured photons practically eliminates the background

F.X. Girod

DVCS identification cuts

JLab DPWG, May 1917

Nuclear background

NH3

Carbon

Dilution for X<1 degree f=0.87

ep0

ep

ep

eg1-dvcs

Angular cut cleans up also the nuclear background

DVCS: 0 –background

Use ep to estimate the contribution of 0 in the epX, ep sample.

ep → ep

~70000 exclusive0s

•contamination by π0 photons •π0 SSA.

DVCS kinematics

H.Avakian, Paris March 819

F.X. Girod

DVCS x-sections from e1dvcs

H.Avakian, Paris March 820

F.X. Girod

Hyon-Suk JoAlex Kubarovski

CLAS PRELIMINARY

F.X. Girod

Radiative corrections and 0 contamination accounted, waiting for cross check

Radiative corrections comparison

H.Avakian, Paris March 821

F.X Girod

I. Akushevich

Good agreement for the leading contribution

Polarized DVCS kinematics

H.Avakian, Paris March 8

E. Seder

Longitudinal target SSA will be extracted in bins in x and t

H.Avakian, Paris March 823

Summary

CLAS e1dvcs experiment 1/2 provides precision data, crucial for extraction of GPDs in a wide kinematical range.

CLAS experiment with longitudinally polarized NH3 and ND3 targets (eg1dvcs) provides superior sample of events allowing for detailed studies of single and double spin asymmetries using multidimensional bins.

Combination of DVCS measurements with unpolarized and polarized targets would allow precision measurement of GPDs H and H~.

Radiative corrections are important for precision measurement of CFFs from final observables

H.Avakian, Paris March 824

Support slides….

Radiative corrections

25H.Avakian, Paris March 8

26H.Avakian, Paris March 8

BH cos moment

BH cos moment can generate ~3% sin2in the ALU

BH

BHBHII

BHBHBH

I

LU c

ccss

ccc

sA

0

0122

010

2 2sin)2/(sin

)cos/1(

sin

27H.Avakian, Paris March 8

Collinearity kinematics

Strong dependence of collinearity kinematics changes region of enhanced t as afunction of beam energy

xMEQ

xMEQQtcol )2(

)2(2

22

yxtQ

tQycol

2

2

HERMES CLAS-5.7

28H.Avakian, Paris March 8

All single photons

DVCS MC

DVCS data

MC vs Data

•Exclusive photon production simulated using a realistic MC•Kinematic distributions in x,Q2,t consistent with the CLAS data

29H.Avakian, Paris March 8

H.Avakian Deep Processes

Meeting

March 3

JLab

30

-dependent amplitude

Strong dependence on kinematics of prefactor -dependence, at y=ycol P1()=0 Fraction of pure DVCS increases with t and

=0

=45

=90

BH

DVCS

xMEQ

xMEQQtcol )2(

)2(2

22

yxtQ

tQycol

2

2

x=0.25

5.7 GeV

H.Avakian, Paris March 831

eg1-dvcs: Monitoring polarizations

Monitoring the time dependence of the product of target and beam polarizations using the elastic asymmetry

Monitoring the time dependence of the beam polarization using the single spin asymmetry in ep→e’X

HWP→IN

HWP→OUT

Deeply Virtual Compton Scattering ep→e’p’

Interference responsible for SSA, contain the same lepton propagator P1() as BH

Way to access to GPDs

GPD combinations accessible as azimuthal moments of the total cross section.

32H.Avakian, Paris March 8

•Define the procedure to extract GPDs from ALU •effect of finite bins ~10%

•Define background corrections•pion contamination ~10%•radiative background

GPDs from ep->e’p’

Requirements for precision (<10%) measurements of GPDs from DVCS SSA:

0 dominates the single photon sample at low Q2 in the kinematics where BH is small

VGG-99

33H.Avakian, Paris March 8