Target and beam-target spin asymmetries in exclusive pion electroproduction for $Q^2>1$ GeV$^2$. II. $e p \rightarrow e \pi^0 p$

TL;DR

This study measures beam-target and target single-spin asymmetries in exclusive neutral pion electroproduction at high momentum transfer, providing new data to constrain nucleon resonance models at moderate to large Q^2 values.

Contribution

It presents the first comprehensive measurement of spin asymmetries in e p → e π^0 p at Q^2 > 1 GeV^2, expanding understanding of nucleon resonances and their amplitudes.

Findings

Beam-target asymmetries are generally positive with modest phi dependence.

Target asymmetries show strong phi dependence and sign change between low and high W.

Results agree with previous models at low W but differ significantly at higher W.

Abstract

Beam-target double-spin asymmetries and target single-spin asymmetries were measured for the exclusive $\pi^0$ electroproduction reaction $\gamma^* p \to p \pi^0$, expanding an analysis of the $\gamma^* p \to n \pi^+$ reaction from the same experiment. The results were obtained from scattering of 6 GeV longitudinally polarized electrons off longitudinally polarized protons using the CEBAF Large Acceptance Spectrometer at Jefferson Lab. The kinematic range covered is $1.1<W<3$ GeV and $1<Q^2<6$ GeV$^2$. Results were obtained for about 5700 bins in $W$, $Q^2$, \cthcm, and $\phi^*$. The beam-target asymmetries were found to generally be greater than zero, with relatively modest \phicmsp dependence. The target asymmetries exhibit very strong \phicmsp dependence, with a change in sign occurring between results at low $W$ and high $W$, in contrast to $\pi^+$ electroproduction. Reasonable agreement is found with phenomenological fits to previous data for $W<1.6$ GeV, but significant differences are seen at higher $W$. When combined with cross section measurements, as well as $\pi^+$ observables, the present results will provide powerful constraints on nucleon resonance amplitudes at moderate and large values of $Q^2$, for resonances with masses as high as 2.4 GeV.