Threshold

Recent data from the PIONS@MAX-lab Collaboration, measuring the total cross section of the incoherent pion photoproduction reaction, $$\gamma d\rightarrow \pi ^- pp$$, near threshold, have been used to extract the $$E_{0+}$$ multipole and total cross section of the reaction $$\gamma n\rightarrow \pi ^-p$$, also near threshold. These are the first measurements of the reaction $$\gamma d\rightarrow \pi ^- pp$$ 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 $$\gamma n\rightarrow \pi ^-p$$ with $$\hbox {E}_{0+} = $$ const in the threshold region. The fit over all the 6 deuteron data points gives the value $$E_{0+} = -\,31.86\pm 0.8$$ (in units $$10^{-3}/m_\pi $$). We explore the dependence of our results on the choice of data subsets included in the fit. The obtained values of $$E_{0+}$$, for different subsets, have overlapping errors and agree with previous determinations