Spin Density Matrix Elements in Exclusive rho^0 Electroproduction on 1H and 2H Targets at 27.5 GeV Beam Energy

TL;DR

This paper reports on the measurement of Spin Density Matrix Elements in exclusive rho^0 electroproduction at HERMES, revealing hierarchies in helicity amplitudes, kinematic dependencies, and deviations from helicity conservation, indicating unnatural-parity exchanges.

Contribution

First measurement of SDMEs in rho^0 electroproduction on hydrogen and deuterium at 27.6 GeV, analyzing helicity amplitudes and exchange mechanisms.

Findings

Hierarchy of helicity amplitudes observed

Kinematic dependencies of SDMEs presented

Deviation from s-channel helicity conservation detected

Abstract

Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive rho^0 electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1 GeV^2 < Q^2 < 7 GeV^2, 3.0 GeV < W < 6.3 GeV, and -t < 0.4 GeV^2. Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q^2 and t are presented, as well as the longitudinal-to-transverse rho^0 electroproduction cross section ratio as a function of Q^2. A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism.