# Ratios of helicity amplitudes for exclusive $\rho^0$ electroproduction on transversely polarized protons

## TL;DR

This study measures helicity amplitude ratios in exclusive $
ho^0$ electroproduction on transversely polarized protons, providing new insights into nucleon-helicity-flip processes and supporting models based on generalized parton distributions.

## Contribution

It is the first to extract ratios of nucleon-helicity-flip amplitudes using transversely polarized targets in $
ho^0$ electroproduction, aligning experimental results with handbag approach predictions.

## Key findings

- Ratios of nucleon-helicity-non-flip amplitudes agree with previous analyses.
- First extraction of nucleon-helicity-flip amplitude ratios with transverse polarization.
- Data support a positive $
ho- ext{pion}$ transition form factor.

## Abstract

Exclusive $\rho^0$-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 GeV$^2$<Q$^2$<7.0 GeV$^2$, 3.0 GeV<W<6.3 GeV, and -t'<0.4 GeV$^2$. Using an unbinned maximum-likelihood method, 25 parameters are extracted. These determine the real and imaginary parts of the ratios of several helicity amplitudes describing $\rho^0$-meson production by a virtual photon. The denominator of those ratios is the dominant amplitude, the nucleon-helicity-non-flip amplitude $F_{0\frac{1}{2}0\frac{1}{2}}$, which describes the production of a longitudinal $\rho^{0}$-meson by a longitudinal virtual photon. The ratios of nucleon-helicity-non-flip amplitudes are found to be in good agreement with those from the previous HERMES analysis. The transverse target polarization allows for the first time the extraction of ratios of a number of nucleon-helicity-flip amplitudes to $F_{0\frac{1}{2}0\frac{1}{2}}$. Results obtained in a handbag approach based on generalized parton distributions taking into account the contribution from pion exchange are found to be in good agreement with these ratios. Within the model, the data favor a positive sign for the $\pi-\rho$ transition form factor. By also exploiting the longitudinal beam polarization, a total of 71 $\rho^0$ spin-density matrix elements is determined from the extracted 25 parameters, in contrast to only 53 elements as directly determined in earlier analyses.