Elastic Electroproduction of rho Mesons at HERA

TL;DR

This study investigates elastic electroproduction of rho mesons at HERA, analyzing cross sections, spin density matrix elements, and helicity flip amplitudes across various Q^2 and W ranges, revealing a stronger W dependence at high Q^2.

Contribution

It provides detailed measurements of rho meson production, including spin density matrices and helicity flip amplitudes, highlighting deviations from traditional W dependence expectations.

Findings

Helicity flip amplitude is about 8% of non-flip amplitudes.

The gamma* p cross section shows a stronger W dependence at high Q^2.

The rho mass distribution shape varies with Q^2.

Abstract

The elastic electroproduction of rho mesons is studied at HERA with the H1 detector for a photon virtuality in the range 1 < Q^2 < 60 GeV^2 and for a hadronic centre of mass energy in the range 30 < W < 140 GeV. The shape of the pipi mass distribution in the rho resonance region is measured as a function of Q^2. The full set of rho spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of 8 +- 3 % of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on Q^2, W and t (the four-momentum transfer squared to the proton). They suggest that, especially at large Q^2, the gamma^*p cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron-hadron cross sections.