Measurement of the Spin-Density Matrix Elements in Exclusive Electroproduction of rho^0 Mesons at HERA

TL;DR

This paper reports measurements of spin-density matrix elements in exclusive rho^0 meson electroproduction at HERA, revealing small but significant helicity violations across different Q^2 ranges.

Contribution

It provides the first comprehensive measurement of all 15 spin-density matrix elements in rho^0 electroproduction at HERA, highlighting helicity conservation violations.

Findings

Small but significant violation of s-channel helicity conservation.

Measurement of all 15 spin-density matrix elements.

Evidence of longitudinal rho^0 production from transverse photons.

Abstract

Exclusive electroproduction of rho^0 mesons has been measured using the ZEUS detector at HERA in two Q^2 ranges, 0.25<Q^2<0.85 GeV^2 and 3<Q^2<30 GeV^2. The low-Q^2 data span the range 20<W<90 GeV; the high-Q^2 data cover the 40<W<120 GeV interval. Both samples extend up to four-momentum transfers of |t|=0.6 GeV^2. The distribution in the azimuthal angle between the positron scattering plane and the rho^0 production plane shows a small but significant violation of s-channel helicity conservation, corresponding to the production of longitudinally polarised (i.e. helicity zero) rho^0 mesons from transverse photons. Measurements of the 15 combinations of spin-density matrix elements which completely define the angular distributions are presented and discussed.