Exclusive meson production at HERMES
Sergey Manaenkov (on behalf of the HERMES collaboration)

TL;DR
This paper reports on exclusive meson production measurements at HERMES, analyzing spin-density matrix elements and helicity amplitudes for $ ho^0$ and $ ext{omega}$ mesons, revealing violations of helicity conservation and the dominance of unnatural-parity exchange.
Contribution
It provides the first extraction of nucleon-helicity-flip amplitude ratios for $ ho^0$ production using a transversely polarized target and compares results with a perturbative QCD-inspired model.
Findings
Violation of s-channel helicity conservation observed.
Unnatural-parity exchange dominates certain spin-density matrix elements.
Good agreement with Goloskokov-Kroll model including pion-pole contribution.
Abstract
The data were accumulated with the HERMES forward spectrometer using the 27.6 GeV longitudinally polarized electron or positron beam of HERA. Exclusive electroproduction of mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of GeV, 3.0 GeV 6.3 GeV, and GeV, while for -meson production on a transversely polarized hydrogen target GeV is used. Spin-density matrix elements for production are presented in projections of or , while the ratios of the helicity amplitudes for the reaction are obtained in the entire kinematic region. The usage of the transversely polarized target allows for the first time the extraction of the ratios of certain nucleon-helicity-flip amplitudes to the natural-parity exchange amplitude…
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