How strange is pion electroproduction?

TL;DR

This paper investigates pion production in parity-violating electron scattering to explore strangeness contributions in nucleons and updates the dispersive gamma-Z box correction estimates for the proton's weak charge, enhancing precision in fundamental measurements.

Contribution

It introduces a partial wave analysis framework with unitarity for PVES pion production, linking it to nucleon strangeness and refining gamma-Z correction estimates for proton weak charge measurements.

Findings

Sensitivity of asymmetry to strange form factors can reach 20% near threshold.

Threshold pion production can constrain strangeness contributions effectively.

Updated gamma-Z box correction estimates improve the precision of weak charge measurements.

Abstract

We consider pion production in parity-violating electron scattering (PVES) in the presence of nucleon strangeness in the framework of partial wave analysis with unitarity. Using the experimental bounds on the strange form factors obtained in elastic PVES, we study the sensitivity of the parity-violating asymmetry to strange nucleon form factors. For forward kinematics and electron energies above 1 GeV, we observe that this sensitivity may reach about 20\% in the threshold region. With parity-violating asymmetries being as large as tens p.p.m., this study suggests that threshold pion production in PVES can be used as a promising way to better constrain strangeness contributions. Using this model for the neutral current pion production, we update the estimate for the dispersive $\gamma Z$-box correction to the weak charge of the proton. In the kinematics of the Qweak experiment, our new prediction reads Re$\,\Box_{\gamma Z}^V(E=1.165\,{\rm GeV}) = (5.58\pm1.41)\times10^{-3}$, an improvement over the previous uncertainty estimate of $\pm2.0\times10^{-3}$. Our new prediction in the kinematics of the upcoming MESA/P2 experiment reads Re$\,\Box_{\gamma Z}^V(E=0.155\,{\rm GeV}) = (1.1\pm0.2) \times 10^{-3}$.