Measurement of pion photoproduction at threshold onLi6

We remeasured with improved techniques the positive-pion photoproduction yield on ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ near threshold and up to 7.2 MeV above it. If the total cross section of the reaction $\gamma +p\to {\pi }^{+}+n$ is expressed as ${\sigma }_p=\frac{\mathrm{aq}}{k}$, our result for the reaction $\gamma +{}_{}{}^6{}_{}{}^{}\mathrm{Li}\to {\pi }^{+}+{}_{}{}^6{}_{}{}^{}\mathrm{He}_{\mathrm{g}.\mathrm{s}.\mathrm{}}^{}{}_{}{}^{}$ near threshold is ${\sigma }_{\mathrm{Li}}=(0.098\mathrm{}\pm 0.004)\left(\frac{\mathrm{aq}}{k}\right)\left\{\frac{\left(\frac{12.5}{q}\right)}{\left[\exp \right(\frac{12.5}{q})-1]}\right\},$ where $q$ is the pion and $k$ the photon c.m. momenta expressed in units of MeV/c. Deviations from this expression above threshold and the contribution of $\gamma +{}_{}{}^6{}_{}{}^{}\mathrm{Li}\to {\pi }^{+}+{}_{}{}^6{}_{}{}^{}\mathrm{He}$ (1.8 MeV) were also observed. The full understanding of this result requires further theoretical work and complementary experiments on the $A=6\mathrm{}$ nuclear system.