Electromagnetic decay of the ${\Sigma }^0(1385)$ to $\Lambda \gamma$

The electromagnetic decay ${\Sigma }^0(1385)\to \Lambda \gamma$ was studied using the CLAS detector at the Thomas Jefferson National Accelerator Facility. A real photon beam with a maximum energy of 3.8 GeV was incident on a proton target, producing an exclusive final state of $K^{+}{\Sigma }^{*0}$. We report the decay widths ratio ${\Sigma }^0(1385)\to \Lambda \gamma /{\Sigma }^0(1385)\to \Lambda {\pi }^0=1.42\pm 0.12(\mathrm{stat}{)}_{-0.07}^{+0.11}(\mathrm{sys})\%$. This ratio is larger than most theoretical predictions by factors ranging from 1.5–3, but is consistent with the only other experimental measurement. From the reported ratio we calculate the partial width and electromagnetic transition magnetic moment for ${\Sigma }^0(1385)\to \Lambda \gamma$.

Figure 1

Missing mass squared $(M_x^2)$ for the reaction $\gamma p\to p{\pi }^{+}{\pi }^{-}(X)$, where the ${\pi }^{+}$ was a potentially misidentified kaon. Events above the dotted line at $0.01{\mathrm{GeV}}^2$ were kept.

Figure 2

The distance of closest approach between the proton and ${\pi }^{-}$ shown in cm. The cut used at 5 cm is indicated by the dotted line.

Figure 3

The invariant mass of the $p\mathrm{\text{−}}{\pi }^{-}$ showing the $\Lambda$ peak with a Gaussian fit giving a $\sigma =1.3\mathrm{MeV}$. A cut of $\pm 0.005\mathrm{GeV}$ around the Particle Data Group mass of the $\Lambda$ is used indicated by the dotted lines.

Figure 4

Missing mass for the reaction $\gamma p\to K^{+}(X)$, for events passing the cut on the $\Lambda$ mass. The dotted lines show the 1.34 GeV to 1.43 GeV cut used to select the $\Sigma (1385)$.

Figure 5

Missing mass for the reaction $\gamma p\to \Lambda (X)$ for events passing cuts on the $\Lambda$ and ${\Sigma }^{*}$ masses. The dotted line at 0.55 GeV shows the cut used to remove the $\gamma p\to K^{+}\Lambda$ channel. (A looser timing cut was used to illustrate that these accidentals are at the $K^{+}$ mass.)

Figure 6

Missing mass squared for the reaction $\gamma p\to K^{+}p{\pi }^{-}(X)$ after all kinematic cuts.

Figure 7

(a) Transverse missing momentum and (b) transverse missing momentum squared for the reaction $\gamma p\to K^{+}\Lambda (X)$. The dashed line shows the cut used at $P_{xy}^2>0.0009{\mathrm{GeV}}^2$.

Figure 8

(a) ${\chi }^2$ distribution and (b) confidence level distribution for a missing ${\pi }^0$ hypothesis in the kinematic fit.

Figure 9

(a) The ${\chi }^2$ distribution and (b) the confidence level distribution for a missing $\gamma$ hypothesis in the kinematic fit before the two-step kinematic fit. (c) The ${\chi }^2$ distribution and the (d) the confidence level distribution for a missing $\gamma$ hypothesis in the kinematic fit after the $P_{\pi }^a({\chi }^2)<1\%$ cut.

Figure 10

(a) The $n_{\pi }$ counts extracted using the confidence level cuts $P_{\gamma }^a<1\%$ and $P_{\pi }^b>10\%$. (b) The $n_{\gamma }$ counts extracted using the confidence level cuts $P_{\pi }^a<1\%$ and $P_{\gamma }^b>10\%$. (c) The counts $n_{\pi }$ and $n_{\gamma }$ shown in the spectrum before any kinematic fit.

Figure 11

Missing mass of $\gamma p\to K^{+}\Lambda (X)$ for data (points with error bars) and Monte Carlo simulations for the $\gamma p\to K^{+}\Lambda (1405)$ reaction (histogram) normalized to the data.

Figure 12

Missing mass off the $\Lambda$ fit with a Gaussian for the mass off of the $K^{+}$ in the window of 1.35–1.5 GeV.