Measurement of the parity-violating asymmetry in inclusive electroproduction of ${\pi }^{-}$ near the ${\Delta }^0$ resonance

Abstract

The parity-violating (PV) asymmetry of inclusive ${\pi }^{-}$ production in electron scattering from a liquid deuterium target was measured at backward angles. The measurement was conducted as a part of the G0 experiment, at a beam energy of 360 MeV. The physics process dominating pion production for these kinematics is quasi-free photoproduction off the neutron via the ${\Delta }^0$ resonance. In the context of heavy-baryon chiral perturbation theory (HB$\chi$PT), this asymmetry is related to a low energy constant $d_{\Delta }^{-}$ that characterizes the parity-violating $\gamma$N$\Delta$ coupling. Zhu et al. calculated $d_{\Delta }^{-}$ in a model benchmarked by the large asymmetries seen in hyperon weak radiative decays, and predicted potentially large asymmetries for this process, ranging from $A_{\gamma }^{-}=-5.2$ to $+5.2$ ppm. The measurement performed in this work leads to $A_{\gamma }^{-}=-0.36\pm 1.06\pm 0.37\pm 0.03$ ppm (where sources of statistical, systematic and theoretical uncertainties are included), which would disfavor enchancements considered by Zhu et al. proportional to $V_{ud}/V_{us}$. The measurement is part of a program of inelastic scattering measurements that were conducted by the G0 experiment, seeking to determine the $N-\Delta$ axial transition form-factors using PV electron scattering.

5 pages, 3 figures