Search for Orthopositronium Decay into Four Photons as a Test of Charge-Conjugation Invariance

We report the preliminary results of an experiment designed to search for the $C$-nonconserving decay of ground-state orthopositronium ($1{}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$) into four photons. In terms of the branching ratio ${F_T}^{4\gamma }=\frac{{R_T}^{4\gamma }({}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 4\gamma )}{R_{3\gamma }({}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 3\gamma )}$, where ${R_T}^{4\gamma }({}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 4\gamma )$ is the $C$-nonconserving four-photon decay rate and $R_{3\gamma }({}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}\to 3\gamma )$ is the $C$-conserving three-photon decay rate of free orthopositronium in our sample, we find ${F_T}^{4\gamma }<8\mathrm{}\times {10}^{-6\mathrm{}}$ (68% confidence).