Precision Measurement of the Decay ${\mathrm{\Sigma }}^{+}\to p\gamma$ in the Process $J/\psi \to {\mathrm{\Sigma }}^{+}{\overline{\mathrm{\Sigma }}}^{-}$

Using $(10087\pm 44)\times {10}^6$ $J/\psi$ events collected with the BESIII detector, the radiative hyperon decay ${\mathrm{\Sigma }}^{+}\to p\gamma$ is studied at an electron-positron collider experiment for the first time. The absolute branching fraction is measured to be $(0.996\pm 0.021_{\mathrm{stat}}\pm 0.018_{\mathrm{syst}})\times {10}^{-3}$, which is lower than its world average value by 4.2 standard deviations. Its decay asymmetry parameter is determined to be $-0.652\pm 0.056_{\mathrm{stat}}\pm 0.020_{\mathrm{syst}}$. The branching fraction and decay asymmetry parameter are the most precise to date, and the accuracies are improved by 78% and 34%, respectively.

Figure 1

Fit to the $M_{\mathrm{rec}}$ distribution of ST candidates in data. Points with error bars represent data. The blue solid curve is the total fit result. The red dashed curve, magenta dotted curve, and blue dash-dotted curve show the shapes of signal, $J/\psi \to \mathrm{\Delta }(1232{)}^{+}(\to \text{anything})\overline{\mathrm{\Delta }}(1232{)}^{-}(\to \overline{p}{\pi }^0)$ background and other nonpeaking background components, respectively.

Figure 2

Fits to the $P_p$ distributions for (a) ${\mathrm{\Sigma }}^{+}\to p\gamma$ and (b) ${\overline{\mathrm{\Sigma }}}^{-}\to \overline{p}\gamma$ candidate events. Points with error bars are data. The red solid curves are the total fit results. The blue dashed, orange dotted, and green dash-dotted curves show the shapes of signal, ${\mathrm{\Sigma }}^{+}\to p{\pi }^0$ background, and other background, respectively. The insets show the details of the fit in the signal region.

Figure 3

Moment distributions for (a),(b) ${\overline{\mathrm{\Sigma }}}^{-}\to \overline{p}\gamma$ and (c),(d) ${\mathrm{\Sigma }}^{+}\to p\gamma$. The black points with error bars are data. The cyan dashed curves are the phase space MC simulation. The red solid curves show the MC simulation based on the fit results. For the comparison between charge conjugate channels, plots (c),(d) adopt the polar angle of ${\overline{\mathrm{\Sigma }}}^{-}$ hyperon as the abscissa variable.

Figure 4

Distribution of ${\alpha }_{\gamma }$ versus BF of the ${\mathrm{\Sigma }}^{+}\to p\gamma$ decay. The black star denotes the results measured by this work and the orange contours correspond to the $68\%/95\%$ confidence level of the results. The red diamond represents the PDG values [8] of the BF and ${\alpha }_{\gamma }$. The cyan colored line shows the predicted ${\alpha }_{\gamma }$ as a function of BF cited from Ref. [14]. Other symbols in blue stand for the results predicted by the vector meson dominance model (VMD) [31], pole model (PM I refers to Ref. [10] and PM II refers to Ref. [11]), nonrelativistic constituent quark model (NRCQM) [12], broken SU(3) model [BSU(3)] [32] and another ChPT model [33].