Measurement of the ${\bar{\nu }}_{\mu }-$Hydrogen Charged-Current Quasi-Elastic Cross Section using the NOvA Near Detector

We report a measurement of the total cross section for muon antineutrino charged-current quasi-elastic scattering on hydrogen, ${\bar{\nu }}_{\mu }\mathrm{H}\to {\mu }^{+}n$, in the NOvA near detector using a $1.2\times {10}^{21}$ proton-on-target exposure in the NuMI beam. A selection based on topological and kinematic constraints yields 35,509 signal events in the hydrogen-rich ($10.8\mathrm{\%}$) detector, providing the highest statistics of (anti)neutrino–hydrogen interactions measured to date. Backgrounds from (anti)neutrino interactions on heavier nuclei are constrained using dedicated data control samples, significantly reducing the related systematic uncertainties. We obtain a value $\sigma ({\bar{\nu }}_{\mu }\mathrm{H}\to {\mu }^{+}n)=0.538\pm 0.009(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.010(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\pm 0.055(\mathrm{f}\mathrm{l}\mathrm{u}\mathrm{x})\times {10}^{-38}$ cm${}^2$ for the total cross section at an average energy of 1.9 GeV, the most precise total cross-section measurement of this process to date. The combined statistical and non-flux systematic uncertainty is more than four times smaller than the flux uncertainty, allowing a future use of this measurement to constrain the absolute ${\bar{\nu }}_{\mu }$ flux.