Threshold $\pi^-$ Photoproduction on the Neutron

Abstract

Recent data from the PIONS$@$MAX-lab Collaboration, measuring the total cross section of the pion incoherent photoproduction $\gamma d\to\pi^-pp$ near threshold, have been used to extract the E$_{0+}$ multipole and total cross section of the reaction $\gamma n\!\to\!\pi^-p$, also near threshold. These are the first measurements of the reaction $\gamma d\to\pi^-pp$ in the threshold region. The value of E$_{0+}$ is extracted through a fit to the deuteron data in a photoproduction model accounting for final-state interactions. The model takes an $S$-wave approximation for the elementary reaction $\gamma n\!\to\!\pi^-p$ with E$_{0+}\! = $ const in the threshold region. The obtained value E$_{0+} = -31.86\pm 0.8$ (in $10^{-3}/m_{\pi^+}$ units) is in agreement with other existing results. Model predictions for the total cross section $\sigma(\gamma n\!\to\!\pi^-p)$ are also given.