FIZIKA B 13 (2004)  2, 649-656

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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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