References - Bogoliubov Laboratory of Theoretical Physics - JINR

e
γ*
e’
x+ ξ x− ξ
A A’
e e’
γ
γ*
A A’
γ
e
γ*
e’
x+ ξ x− ξ
A A’
e e’
γ*
A A’
γ
γ
DIS
1000N/N
+
0.3 e data
-
e data
0.2
0.1
0
MC sum
elastic BH
associated BH
semi-inclusive
0 10 20 30
2 2
M (GeV )
(a) (b)
Figure 1: (a) Leading order diagrams for DVCS (top) and Bethe-Heitler process (bottom). (b) The
measured distributions of electroproduced real-photon events versus the squared missing mass M 2 X from
positron (open circles) and electron beam (closed circles). The dashed line represents a Monte Carlo
simulation including coherent DVCS and BH processes. The “associated” BH process with the excitation
of resonant final states is presented in shaded area. The semi-inclusive background is presented as dotted
line. Solid line represents the sum of all processes. The region between the two vertical lines indicates
the selected “exclusive region”.
2 Azimuthal Asymmetries for DVCS
The cross section for a longitudinally polarized lepton beam scattered off an unpolarized
proton target σLU can be related to the unpolarized cross section σUU by
where A I LU
�
σLU(φ, PB,CB) =σUU · 1+PBA
DV CS
LU (φ)+CBPBA I LU (φ)+CBAC(φ)
x
�
, (2)
CS
(ADV LU ) is the charge (in)dependent beam-helicity asymmetry (BSA) and AC
is the beam charge asymmetry (BCA). CB(PB) denotes the beam charge (polarization). In
the analysis, effective asymmetry amplitudes are extracted which include φ dependences
from the BH propagators and the unpolarized cross section. Each asymmetry can be
expanded in a Fourier harmonics of φ angular distributions:
A I DV CS(φ) =
A I LU (φ) =
2�
n=0
2�
n=0
AC(φ) =
A sin(nφ)
LU sin(nφ)+
A sin(nφ)
LU,DV CS sin(nφ)+
3�
n=0
1�
n=0
1�
n=0
A cos(nφ)
LU,I cos(nφ), (3)
A cos(nφ)
LU,DV CS cos(nφ), (4)
ACS cos(nφ) cos(nφ)+A sin(φ)
C sin(φ). (5)
By combining the data taken with different beam charges and helicities, the amplitudes
were fit simultaneously using a Maximum Likelihood method described in detail in Ref. [2].
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