References - Bogoliubov Laboratory of Theoretical Physics - JINR

cos 0φ
C
A
φ
cos
C
A
cos 2φ
C
A
cos 3φ
C
A
Res. frac
0.2
0
−0.2
0.2
0
0.1
0
−0.1
0.1
0
−0.1
0.4
0.3
0.2
0.1
0
overall
2
0
.2
2
0
1
0
.1
1
0
.1
4
.3
.2
.1
HERMES PRELIMINARY 2 VGG Regge, D
±
± e + p → e + p + γ
Accep & smear → sys error
VGG Regge, no D
0 0.2 0.4 0.6
0 0.2 0.4 0.6
0 0.2 0.4 0.6
0
0 0.2 0.4 0.6
2
−t[GeV ]
0
.2
2
0
1
.1
1
4
.1
3
2
.1
0
0.1 0.2 0.3
0.1 0.2 0.3
0.1 0.2 0.3
0.1 0.2 0.3
2
0
.2
2
0
1
.1
1
.1
4
3
2
.1
0
xB
2 4 6 8 10
2 4 6 8 10
2 4 6 8 10
2 4 6 8 10
2 2
Q [GeV ]
Figure 2: The amplitudes of the beam-charge asymmetry extracted from hydrogen data (red squares) [4].
The error bars (bands) represent the statistical (systematic) uncertainties. The curves are predictions of
a double-distribution GPD model [5, 6].
ing from hydrogen to xenon [9]. For hydrogen, krypton and xenon, data were taken with
both beam charges. For both DVCS and BH, coherent scattering occurs at small values of
−t and rapidly diminished with increasing |t|. Coherent and incoherent-enriched samples
are selected according to a −t threshold that is chosen to vary with the target such that
for each sample approximately the same kinematic conditions are obtained for all target
gases. The nuclear-mass dependence of the BCA and BSA is presented separately for the
coherent and incoherent-enriched samples in Fig. 4 on the left and right panels, respectively.
The cos φ amplitude of the BCA is consistent with zero for the coherent-enriched
samples for all three targets, while it is about 0.1 for the incoherent-enriched samples.
The sin φ amplitude of the beam-helicity asymmetry shown in Fig. 4 (right) has values
of about −0.2 for both the coherent and incoherent-enriched samples. No nuclear-mass
dependence of the BCA and BSA asymmetries is observed within experimental uncertainties.
This is in agreement with models that approximate nuclear GPDs by nucleon GPDs
neglecting bound state effects. The data do not support the enhancement of nuclear
asymmetries compared to the free proton asymmetries for coherent scattering in spin-0
and spin- 1
2
nuclei as anticipated by various models [10–12]. They also contradict the pre-
dicted strong A dependence of the beam-charge asymmetry resulting from a contribution
of meson exchange between nucleons to the scattering amplitude [12].
182