DOI: 10.1140/epjc/s2002-01118-x
The Q2-dependence of the generalised Gerasimov-Drell-Hearn integral for the deuteron, proton and neutron
The HERMES Collaboration(Received: 20 October 2002 / Published online: 15 January 2003 )
Abstract
The Gerasimov-Drell-Hearn (GDH) sum rule connects the anomalous
contribution to the magnetic
moment of the target nucleus with an energy-weighted integral of the difference
of the helicity-dependent photoabsorption cross sections. Originally conceived for real
photons, the GDH integral can be generalised to the case of photons with
virtuality
Q2. For spin-1/2 targets such as the nucleon, it then
represents the non-perturbative limit of the first moment
of the spin structure function
g1(x,Q2)
in deep inelastic scattering (DIS).
The data collected by HERMES with a deuterium target are presented together
with a re-analysis of previous measurements on the proton.
This
provides an unprecedented and complete
measurement of the generalised GDH integral for photon-virtuality ranging over
1.2<Q2<12.0 GeV
2 and
for photon-nucleon invariant mass squared
W2 ranging over
1<W2<45 GeV
2, thus covering simultaneously the nucleon-resonance and the deep inelastic
scattering regions.
These data allow the study of the
Q2-dependence of the full GDH integral, which is
sensitive to both the
Q2-evolution of the resonance form factors and
contributions of higher twist. The contribution of the nucleon-resonance region is seen to decrease
rapidly with increasing
Q2. The DIS contribution is
sizeable over the full measured range, even down to the lowest measured
Q2. As expected, at higher
Q2 the data are
found to be in
agreement with previous measurements of the first moment of
g1. From data on the deuteron
and proton, the GDH integral for the
neutron has been derived and the proton-neutron difference evaluated. This
difference is found to satisfy the fundamental Bjorken sum rule at
Q2 = 5 GeV
2.
© Società Italiana di Fisica, Springer-Verlag 2003
