Improved measurement of the branching fractions for $J\mathrm{/}\psi \to \gamma {\pi }^0$, $\gamma \eta$ and $\gamma {\eta }^{\mathrm{\prime }}$

Using a data sample of $(1.0087\pm 0.0044)\times {10}^{10}$ $J\mathrm{/}\psi$ events collected with the BESIII detector, the decays of $J\mathrm{/}\psi \to \gamma {\pi }^0(\eta ,{\eta }^{\mathrm{\prime }})\to \gamma \gamma \gamma$ are studied. Newly measured branching fractions are $\mathcal{B}$$(J\mathrm{/}\psi \to \gamma {\pi }^0)$=$(3.34\pm 0.02\pm 0.09)\times {10}^{-5}$, $\mathcal{B}$$(J\mathrm{/}\psi \to \gamma \eta )$=$(1.096\pm 0.001\pm 0.019)\times {10}^{-3}$ and $\mathcal{B}$$(J\mathrm{/}\psi \to \gamma {\eta }^{\mathrm{\prime }})$=$(5.40\pm 0.01\pm 0.11)\times {10}^{-3}$, where the first uncertainties are statistical and the second are systematic. These results are consistent with the world average values within two standard deviations. The ratio of partial widths $\mathrm{\Gamma }(J\mathrm{/}\psi \to \gamma {\eta }^{\mathrm{\prime }})\mathrm{/}\mathrm{\Gamma }(J\mathrm{/}\psi \to \gamma \eta )$ is measured to be $4.93\pm 0.13$. The singlet-octet pseudoscalar mixing angle ${\theta }_P$ is determined to be ${\theta }_P=-(22.11\pm 0.26{)}^{\circ }$ or $-(19.34\pm 0.34{)}^{\circ }$ with two different phenomenological models.