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FIZIKA B 13 (2004) 2, 649-656
 
HARD EXCLUSIVE ELECTROPRODUCTION OF RESONANT AND NON-RESONANT PION
PAIRS AT HERMES
ALEXANDER BORISSOV
on behalf of the HERMES collaboration
Department of Physics and Astronomy, University of Glasgow, Glasgow,
G128QQ, United Kingdom
Received 27 October 2003; Accepted 30 August 2004
Online 22 November 2004
A selection of recent HERMES results is presented on the hard exclusive
electroproduction of resonant and non-resonant p+ p- pairs. The ratio of r0
electroproduction cross sections per nucleon on 14N as compared to that on 1H,
known as the nuclear transparency, was found to increase (decrease) with increasing
coherence length for coherent (incoherent) r0
electroproduction. For a fixed coherence length, a rise of the nuclear transparency with Q2
is observed for both coherent and incoherent r0
production, which is in agreement with theoretical calculations that include the effect of
color transparency. The first-order intensity density for exclusive electroproduction of p+ p- pairs has been
studied as a function of the invariant mass Mpp in
the range 0.3 < Mp+p-
< 1.5 GeV. The derived quantities show a dependence of the p+
p- invariant mass, which can be understood as due to
the interference between pion pairs in relative P-wave (isovector channel) and S,D-waves
(isoscalar channel) states.
PACS numbers: 25.30.-c,13.60.Le,21.30.Fe
UDC 539.12
Keywords: resonant and non-resonant p+ p- pairs, cross sections, hard exclusive
electroproduction, generalized parton distributions
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