Ratios of helicity amplitudes for exclusive ρ⁰ electroproduction on transversely polarized proton

National Research Centre “Kurchatov Institute”, Petersburg Nuclear Physics Institute, Russia

^* e-mail: manaenkov_si@pnpi.nrcki.ru

Published online: 27 March 2017

Abstract

Exclusive ρ⁰-meson electroproduction is studied by the HERMES experiment, using the 27.6 GeV longitudinally polarized electron/positron beam of HERA and a transversely polarized hydrogen target, in the kinematic region 1.0 GeV² < Q² < 7.0 GeV², 3.0 GeV < W < 6.3 GeV, and −t′ < 0.4 GeV². Using an unbinned maximum-likelihood method, 25 parameters are extracted. They determine the real and imaginary parts of the ratios of certain helicity amplitudes describing ρ⁰-meson production by a virtual photon, where the denominator is the dominant amplitude $$F_{0\frac{1}{2}}{}_{0\frac{1}{2}}$$. The latter is the nucleon-helicity-non-flip amplitude, which describes the production of a longitudinal ρ⁰ meson by a longitudinal virtual photon. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to $$F_{0\frac{1}{2}}{}_{0\frac{1}{2}}$$. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis.