Single pion contribution to the hyperfine splitting in muonic hydrogen

A detailed discussion of the long-range one-pion exchange (Yukawa potential) contribution to the 2S hyperfine splitting in muonic hydrogen which had, until recently, been disregarded is presented. We evaluate the relevant vertex amplitudes, in particular $\pi^0\mu^+\mu^-$, combining low energy chiral expansions together with experimental data on $\pi^0$ and $\eta$ decays into two leptons. A value of $\Delta{E}^\pi_{HFS}= -(0.09 \pm 0.06)\ \mu\rm{eV}$ is obtained for this contribution.