References - Bogoliubov Laboratory of Theoretical Physics - JINR

3 Exclusive DVCS Events
The HERMES experiment [3] exploited longitudinally polarized 27.6 GeV electron or
positron beam at HERA storage ring at DESY together with longitudinally or transversely
polarized or unpolarized gas targets (H, D or heavier nuclei). Exclusive events were
selected requiring the detection of the scattered lepton and one photon. In addition,
as the recoiling proton has not been detected, the calculated missing mass was required
to match the proton mass within the resolution of the spectrometer, which defines the
“exclusive region”, see Fig. 1b.
Without the detection of the recoil proton it is not possible to separate the elastic
DVCS and BH events from the “associated” process, where the nucleon in the final state
is excited to a resonant state. Within the exclusive region, its contribution is estimated
from Monte Carlo simulation to be about 12 %, which is taken as part of signal. The main
background contribution of about 3% is originating from semi-inclusive π 0 production and
is corrected for. The contribution from exclusive π 0 production is estimated to be less
than 0.5%. The systematic uncertainties are obtained from a Monte Carlo simulation
estimating the effects of limited acceptance, smearing, finite bin width and alignment of
the detectors with respect to the beam. Other sources are background contributions and
a shift of the position of the exclusive missing mass peak between the data taken with
different beam charges.
4 Beam-Charge and Beam-Spin Asymmetries
In Figs. 1 and 2 results obtained on the hydrogen target are shown [4]. The first four
rows of Fig. 1 represent different cosine amplitudes of the BCA, whereas the last row
shows the fractional contributions of the associated BH process. In the first column the
integrated result is shown, in the other columns the amplitudes are binned in −t, xB and
Q2 variables. The error bars show the statistical and the bands represent the systematic
uncertainties. The magnitudes of the first two cosine moments A
cos 0φ
C
and A
cos φ
C
increase
with increasing −t, while having opposite signs, they are in agreement with theoretical
calculations. At HERMES kinematic conditions, both mainly relate to the Compton
Form Factor (CFF) which is a convolution of GPD function H with the hard scattering
amplitude [4]. The constant term there is suppressed relative to the first moment. The
second cosine moment appears in twist-3 approximation and is found to be compatible
with zero like the third cosine moment, which is related to gluonic GPDs. Fraction of
“associated” production is presented in the fifth raw for each kinematic point.
The first sine moment A
sin φ
LU,I
is large and negative in HERMES kinematic conditions,
see Fig. 2. This amplitude relates to the imaginary part of the CFF. The results of the
GPD model calculations [5,6] based on the framework of double distributions [7] are also
shown there. The model includes a Regge-inspired t-ansatz and a factorized t-ansatz with
the calculation of the contribution of D-term [6]. We note, that the calculations performed
without the contribution of D-term describe well HERMES results on BCA, see Fig. 2.
Both BSA model calculations fail to describe the data except for small −t.
Results obtained on the deuteron target (not shown) are compatible with the data
on a proton for almost all amplitudes in the same kinematic bins [8]. The nuclear-mass
dependence of beam-helicity azimuthal asymmetries has been measured for targets rang-
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