First Measurement Using Elliptically Polarized Photons of the Double-Polarization Observable $E$ for $\ensuremath{\gamma}p\ensuremath{\rightarrow}p{\ensuremath{\pi}}^{0}$ and $\ensuremath{\gamma}p\ensuremath{\rightarrow}n{\ensuremath{\pi}}^{+}$

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First Measurement Using Elliptically Polarized Photons of the Double-Polarization Observable $E$ for $γ p \to p π^{0}$ and $γ p \to n π^{+}$

F. Afzal et al. (A2 Collaboration at MAMI)

Phys. Rev. Lett. 132, 121902 – Published 19 March 2024

Abstract

We report the measurement of the helicity asymmetry $E$ for the $p π^{0}$ and $n π^{+}$ final states using, for the first time, an elliptically polarized photon beam in combination with a longitudinally polarized target at the Crystal Ball experiment at MAMI. The results agree very well with data that were taken with a circularly polarized photon beam, showing that it is possible to simultaneously measure polarization observables that require linearly (e.g., $G$ ) and circularly polarized photons (e.g., $E$ ) and a longitudinally polarized target. The new data cover a photon energy range 270–1400 MeV for the $p π^{0}$ final state (230–842 MeV for the $n π^{+}$ final state) and the full range of pion polar angles, $θ$ , providing the most precise measurement of the observable $E$ . A moment analysis gives a clear observation of the $p η$ cusp in the $p π^{0}$ final state.

Received 18 July 2023
Revised 31 January 2024
Accepted 2 February 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.121902

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society