Study of spin-density matrix in exclusive electroproduction of ω meson at HERMES

S I Manaenkov

doi:10.1088/1742-6596/678/1/012043

Abstract

Exclusive electroproduction of ω mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of Q2 > 1.0 GeV2, 3.0 GeV < W < 6.3 GeV, and −t' < 0.2 GeV2. The data were accumulated with the HERMES forward spectrometer during the 1996-2007 running period using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. Spin-density matrix elements are presented in projections of Q2 or −t'. Violation of s-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is established for special combinations of spin-density matrix elements. The determination of the virtual-photon longitudinal-to- transverse cross-section ratio reveals that a dominant part of the cross section arises from transversely polarized photons. Good agreement is found between the HERMES proton data and results of a pQCD-inspired Goloskokov-Kroll model that includes pion-pole contributions.

Highlights

Exclusive electroproduction of vector mesons on nucleons gives information both on the reaction mechanisms and on the nucleon structure [1, 2]
The electroproduction at high energies can be considered to consist of three subprocesses: i) the incident lepton emits a virtual photon γ∗, which dissociates into a quark-antiquark pair; ii) this qqpair interacts strongly with the nucleon; iii) the observed vector meson is formed from the scattered qqpair
In perturbative quantum chromodynamics, the interaction of the qqpair with the nucleon can proceed via two-gluon exchange or qqexchange, where the former corresponds to the pomeron exchange and the latter to the exchange of other reggeons

Summary

Introduction

Exclusive electroproduction of vector mesons on nucleons gives information both on the reaction mechanisms and on the nucleon structure [1, 2]. In the framework of pQCD valid at large photon virtuality Q2 and high photon-nucleon center-of-mass (CM) energy W , the nucleon structure can be studied through hard exclusive meson production as the process amplitude contains Generalized Parton Distributions (GPDs) [3, 4, 5]. At high Q2, W and for leading twist, the GPDs Hf and Ef are sufficient to describe exclusive vector-meson production on a nucleon, where f denotes a quark of flavor f or a gluon. The presented HERMES data can be well described in the GK model if the pion exchange is included into account

Formalism The ω meson is produced and decays in the following reactions:

Results

B: Interference γ*