Study of Electromagnetic Decays of Orbitally Excited $\Xi_c$ Baryons

Using 980 $fb^-1$ of data collected with the Belle detector operating at the KEKB asymmetric-energy e^+e^- collider, we report a study of the electromagnetic decays of excited {charmed baryons} $\Xi_c(2790)$ and $\Xi_c(2815)$. A clear signal (8.6 standard deviations) is observed for $\Xi_c(2815)^0 \to \Xi_c^0\gamma$, and we measure: $B[\Xi_c(2815)^0 \to \Xi_c^0\gamma]/B[\Xi_c(2815)^0 \to \Xi_c(2645)^+\pi^- \to \Xi_c^0\pi^+\pi^-] = 0.41 \pm 0.05 \pm 0.03$. We also present evidence (3.8 standard deviations) for the similar decay of the $\Xi_c(2790)^0$ and measure: $B[\Xi_c(2790)^{0}\to\Xi_c^{0}\gamma]/B[\Xi_c(2790)^0\to\Xi_c^{\prime +}\pi^{-}\to\Xi_c^{+}\gamma \pi^-] = 0.13 \pm 0.03 \pm 0.02$. The first quoted uncertainties are statistical and the second systematic. We find no hint of the analogous decays of the $\Xi_c(2815)^+$ and $\Xi_c(2790)^+$ baryons and set upper limits at the 90% confidence level of: $B[\Xi_c(2815)^{+}\to\Xi_c^{+}\gamma]/B[\Xi_c(2815)^+\to\Xi_c(2645)^0\pi^+\to\Xi_c^+\pi^-\pi^+] < 0.09,$ and $B[\Xi_c(2790)^{+}\to\Xi_c^{+}\gamma]/B[\Xi_c(2790)^+\to\Xi_c^{\prime 0}\pi^{+}\to\Xi_c^{0}\gamma \pi^+] < 0.06.$ Approximate values of the partial widths of the decays are extracted, which can be used to discriminate between models of the underlying quark structure of these excited states.