First Determination of the Spin and Parity of a Charmed-Strange Baryon, ${\mathrm{\Xi }}_c(2970{)}^{+}$

We report results from a study of the spin and parity of ${\mathrm{\Xi }}_c(2970{)}^{+}$ using a $980\mathrm{f}{\mathrm{b}}^{-1}$ data sample collected by the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The decay angle distributions in the chain $\Xi_{c}(2970)^+ \to \Xi_c(2645)^{0}\pi^{+} \to \Xi_c^{+}\pi^{-}\pi^{+}$ are analyzed to determine the spin of this charmed-strange baryon. The angular distributions strongly favor the ${\mathrm{\Xi }}_c(2970{)}^{+}$ spin $J=1\mathrm{/}2$ over $3\mathrm{/}2$ or $5\mathrm{/}2$, under an assumption that the lowest partial wave dominates in the decay. We also measure the ratio of ${\mathrm{\Xi }}_c(2970{)}^{+}$ decay branching fractions $R={$\mathcal{B}$[ \Xi_{c}(2970)^+ \to \Xi_c(2645)^{0}\pi^{+} ]} / { $\mathcal{B}$[ \Xi_{c}(2970)^+ \to \Xi_c^{\prime0}\pi^{+} ]} =1.67 \pm 0.29$(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})$^{ +0.15}_{ -0.09}$(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})$ \pm 0.25$(\mathrm{I}\mathrm{S})$$, where the last uncertainty is due to possible isospin-symmetry-breaking effects. This $R$ value favors the spin-parity $J^P=1\mathrm{/}{2}^{+}$ with the spin of the light-quark degrees of freedom $s_l=0$. This is the first determination of the spin and parity of a charmed-strange baryon.