Spin Density Matrix Elements in Exclusive Production of Omega Mesons at HERMES

Hrachya Marukyan

doi:10.1142/s2010194516600636

Abstract

Exclusive electroproduction of [Formula: see text] mesons on unpolarized hydrogen and deuterium targets is studied at HERMES in the kinematic region of [Formula: see text], [Formula: see text], and [Formula: see text]. The data were accumulated during the 1996-2007 running period using the [Formula: see text] longitudinally polarized electron or positron beams at HERA. The determination of the virtual-photon longitudinal-to-transverse cross-section ratio shows that a considerable part of the cross section arises from transversely polarized photons. Spin density matrix elements are derived and presented in projections of [Formula: see text] or [Formula: see text]. Violation of s-channel helicity conservation is observed for some of these elements. A sizable contribution from unnatural-parity-exchange amplitudes is found and the phase shift between those amplitudes that describe transverse [Formula: see text] production by longitudinal and transverse virtual photons is determined for the first time. Good agreement is found between the HERMES proton data and results of a pQCD-inspired phenomenological model that includes pion-pole contributions.

Highlights

Spin Density Matrix Elements Exclusive electroproduction of vector mesons on nucleons provide a rich source of information on the structure of the nucleon and the meson production mechanism
The angular distribution of vector mesons in this reactions depends on Spin Density Matrix Elements (SDMEs), which describe the final spin state of produced mesons
HERMES has measured the values of 23 SDMEs in exclusive electroproduction of ω mesons on unpolarized hydrogen and deuterium targets with longitudinally polarized electron or positron beams at HERA.[1]

Summary

Introduction

SDMEs are expressed by the helicity amplitudes describing the transition of longitudinal or transverse virtual photons to the vector meson with given helicity. HERMES has measured the values of 23 SDMEs in exclusive electroproduction of ω mesons on unpolarized hydrogen and deuterium targets with longitudinally polarized electron or positron beams at HERA.[1] They are determined from the process e + p(d) → e + p (p n ) + ω → (π+π−π0(→ 2γ)) by fitting the 3-dimensional angular distribution, which depends on the angle between the lepton scattering and production planes and the polar and azimuthal angles of a unit vector normal to the decay plane in the rest frame of ω, to the experimental angular distribution of decay pions.

Results

Conclusion