Branching ratio of the electromagnetic decay of the ${\Sigma }^{\mathbf{+}}(1385)$

The CLAS detector was used to obtain the first ever measurement of the electromagnetic decay of the ${\Sigma }^{*+}(1385)$ from the reaction $\gamma p\to K^0{\Sigma }^{*+}(1385)$. A real photon beam with a maximum energy of 3.8 GeV was incident on a liquid-hydrogen target, resulting in the photoproduction of the kaon and ${\Sigma }^{*}$ hyperon. Kinematic fitting was used to separate the reaction channel from the background processes. The fitting algorithm exploited a new method to kinematically fit neutrons in the CLAS detector, leading to the measured decay widths ratio ${\Sigma }^{+}(1385)\to {\Sigma }^{+}\gamma /{\Sigma }^{+}(1385)\to {\Sigma }^{+}{\pi }^0=11.95\pm 2.21(\mathrm{stat}{)}_{-1.21}^{+0.53}(\mathrm{sys})\%$ and a deduced partial width of $250.0\pm 56.9(\mathrm{stat}{)}_{-41.2}^{+34.3}(\mathrm{sys})\mathrm{keV}$. A U-spin symmetry test using the SU(3) flavor-multiplet representation yields predictions for the ${\Sigma }^{*+}(1385)\to {\Sigma }^{+}\gamma$ and ${\Sigma }^{*0}(1385)\to \Lambda \gamma$ partial widths that agree with the experimental measurements.

Figure 1

The CLAS detector at Jefferson Lab showing the toroidal magnet, the drift chambers, the time-of-flight scintillators, the Čerenkov counter, and the electromagnetic calorimeter.

Figure 2

The $\Delta \beta$ distributions for ${\pi }^{+}$ (left) and ${\pi }^{-}$ (right). The cut of $-0.035\le \Delta \beta \le 0.035$ is shown as the dashed lined in each case.

Figure 3

Invariant mass of the ${\pi }^{+}\mathrm{\text{−}}{\pi }^{-}$ combination for the two different ${\pi }^{+}$ detected, prior to any ${\pi }^{+}$ organization.

Figure 4

Top left: the neutron $\Delta p$ distribution before correction, top right: the Gaussian mean from the binned fits, bottom left: the corrected distribution, and bottom right: the final corrected Gaussian mean from the binned fits.

Figure 5

The invariant mass of the ${\pi }_1^{+}\mathrm{\text{−}}{\pi }^{-}$ (upper left), the missing mass off the ${\pi }_1^{+}\mathrm{\text{−}}{\pi }^{-}$ (upper right), the $n\mathrm{\text{−}}{\pi }_2^{+}$ invariant mass (lower left), and the missing mass squared of all the detected particles (lower right). All distributions are before any kinematic constraints.

Figure 6

The invariant mass of the $\mathrm{n}\mathrm{\text{−}}{\pi }_2^{+}$ with the dotted lines showing the $|M({\pi }_2^{+}n)-M_{{\Sigma }^{+}}|<0.01\mathrm{GeV}$ cut (upper left), the missing mass off the $K^0$ (upper right), the total missing energy with the dotted line showing the cut used (lower left), and the missing mass squared of all the detected particles (lower right) after the $\pm 0.01\mathrm{GeV}$ cut on the $K^0$ peak.

Figure 7

The Monte Carlo missing energy distribution for the $\gamma p\to K^0{\Sigma }^{*+}\to K^0{\Sigma }^{+}\gamma$ reaction with the dotted line indicating the 0.24 GeV cut.

Figure 8

The missing mass off the ${\pi }_1^{+}\mathrm{\text{−}}{\pi }^{-}$ with three progressive cuts applied to isolate the ${\Sigma }^{*+}$ events. First $|M({\pi }_2^{+}n)-M_{{\Sigma }^{+}}|<0.01\mathrm{GeV}$, next shown are the events left over after the $E_x<0.24\mathrm{GeV}$ cut, and finally the dotted lines show the $\pm 0.03\mathrm{GeV}$ cut around the mass of the ${\Sigma }^{*+}$.

Figure 9

The missing mass squared of all detected particles after all analysis cuts. A Gaussian fit gives a mean of $0.018\pm 0.0002\mathrm{GeV}$.

Figure 10

The missing mass of the ${\Sigma }^{+}$.

Figure 11

The Monte Carlo and data distributions for the missing mass off the ${\pi }_2^{+}n$ combination with the data showing a $\omega$ peak. The Monte Carlo shown is for the reaction $\gamma p\to K^0{\Sigma }^{*+}\to {\pi }^{+}{\pi }^{-}{\pi }^{+}n{\pi }^0$, indicating the expected missing mass off the ${\pi }_2^{+}n$ combination if no background was present. The dashed lines indicate the known masses of the $\omega$ and $K^{*0}$.

Figure 12

The Monte Carlo (lines) and data (points) distributions for the missing mass off the ${\pi }_2^{+}n$ combination (as shown in Fig. 11) after the $P({\chi }^2)<1\%$ cut showing that the distributions now match in the $\omega$ mass region.

Figure 13

The momentum distributions for the ${\pi }^{+}$, ${\pi }^{-}$, and neutron are shown in the first 3 panels, with the cosine of the kaon lab frame angular distribution in the last panel, for the reaction $\gamma p\to K^0{\Sigma }^{*+}\to {\pi }^{-}{\pi }^{+}n{\pi }^{+}{\pi }^0$. The data and Monte Carlo are shown as the histogram and points, respectively, which closely overlap.

Figure 14

Plot (A) shows the ${\chi }^2$ distribution for the Monte Carlo channel $\gamma p\to K^0{\Sigma }^{*+}\to {\pi }^{+}{\pi }^{-}{\pi }^{+}n{\pi }^0$ under the radiative hypothesis, displaying a highly distorted ${\chi }^2$ distribution. Plot (B) shows the fit results for $\gamma p\to K^0{\Sigma }^{*+}\to {\pi }^{+}{\pi }^{-}{\pi }^{+}n\gamma$, displaying a reasonable ${\chi }^2$ distribution with the ideal $P_1$. The radiative hypothesis kinematic fit ${\chi }^2$ distribution and distribution fit to data before any $P_{\pi }^a({\chi }^2)$ cut is applied is shown in (C) and the same distribution after a $P_{\pi }^a({\chi }^2)<0.01\%$ cut is applied is shown in (D).

Figure 15

(A) The confidence level distribution for a fit to the data under the radiative hypothesis before the $P_{\pi }^a({\chi }^2)<0.01\%$ cut (solid line) and after (dotted line). (B) The confidence level distribution for a fit to the data under the missing ${\pi }^0$ hypothesis before the $P_{\gamma }^a({\chi }^2)<0.01\%$ cut (solid line) and after (dotted line).

Figure 16

The missing mass off of all detected particles. The plot shows the final radiative candidates at zero missing mass after the $P_{\pi }^a({\chi }^2)<0.01\%$ and $P_{\gamma }^b({\chi }^2)>10\%$ cuts. Also shown are the final ${\pi }^0$ candidates after the $P_{\gamma }^a({\chi }^2)<0.01\%$ and $P_{\pi }^b({\chi }^2)>10\%$ cuts.

Figure 17

The missing mass off the ${\pi }_2^{+}\mathrm{\text{−}}n$ combination with the $|M({\pi }_1^{+}{\pi }^{-})-M_{K^0}|<0.01\mathrm{GeV}$, $|M({\pi }_2^{+}n)-M_{{\Sigma }^{+}}|<0.01\mathrm{GeV}$, and $|M_x^2-M_{{\pi }^0}^2|<0.0175{\mathrm{GeV}}^2$ cuts. The resulting $K^{*0}$ and $\omega$ peaks are fit with Breit-Wigner line shapes, while the background is fit with a polynomial function.