References - Bogoliubov Laboratory of Theoretical Physics - JINR

A 2
0.3
0.25
0.2
0.15
0.1
0.05
0
-0.05
-0.1
10 -2
HERMES preliminary
(10.0% scale uncertainty)
E155 Coll.
E143 Coll.
SMC Coll.
ww A2 10 -1
X
1
xg 2
0.08
0.06
0.04
0.02
0
-0.02
-0.04
-0.06
Figure 2: Left panel: the virtual photon asymmetry A p
2
function of the proton xg p
2
10 -1
10.0% scale uncertainty
HERMES preliminary
E155 Coll.
E143 Coll.
as a function of x. Right panel: the structure
as a function of x. HERMES data are shown together with data from E155,
E143 and SMC experiments. The lines correspond to predictions following Wandzura-Wilczek relation.
d 3 Δσ
dxdydφS
= −PBPT cos φS
xg 2 ww
1
X
e4 4π2Q2 γ � �
y
1 − y
2 g1(x, Q 2 )+g2(x, Q 2 �
) , (1)
where PB and PT are the beam and target polarizations, while φS is the azimuthal angle
about the beam direction between the lepton scattering plane and the “upwards” target
spin direction.
During the 2003–2005 running period, an average polarization value 〈PB · PT 〉 of 0.24 was
achieved. The experimental data are unfolded for detector smearing and radiative effects
analogously to the studies of the structure function g1(x, Q 2 ) [3]. The HERMES results are
presented in Fig. 2, where the virtual photon asymmetry A p
2 (left panel) and the structure
function xg p
2 (right panel) are shown as functions of x. Data from experiments at CERN
and SLAC are presented as well for comparison. HERMES results are in good agreement
with the most accurate measurement made by the E155 experiment, and consistent with
expectations based on the Wandzura-Wilczek term gWW 2 (x).
Two-photon exchange contribution. The interpretation of spin asymmetries in DIS
is based on the single-photon exchange assumption. Interference between one- and twophoton
exchange amplitudes would lead to a sin φS azimuthal asymmetry in inclusive DIS
off transversely polarized target. Here, φS is the same angle defined in previous section.
The asymmetry is expected to be proportional to the charge of the incident lepton and to
M/ � Q2 ,whereMis the nucleon mass. The Q2 range was divided into a “DIS region”,
with Q2 > 1GeV2 ,anda“low-Q2region”, with Q2 < 1GeV2 ,totestforapossible
enhancement of the asymmetry due to the factor M/ � Q2 sin φS
. The amplitudes AUT were
extracted [6] separately for electrons and positrons. The asymmetries are consistent with
zero for both the “DIS region” as well as for the “low-Q2 region”. As conclusion of the
study, no signal of the two-photon exchange was found within the uncertainty, which is
of order 10 −3 .
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