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DVCS results with unpolarized and polarized target. Harut Avakian (Jlab). Introduction Event selection MC simulations and radiative corrections DVCS with unpolarised target DVCS with longitudinally polarized target Summary. d 4 . ~ | T DVCS + T BH | 2. dQ 2 dx B dtd . - PowerPoint PPT Presentation
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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