Observation of a Narrow Charm-Strange MesonDsJ+(2632)→Ds+ηandD0K+

We report the first observation of a charm-strange meson $D_{sJ}^{+}(2632)$ at a mass of $2632.5\pm 1.7\mathrm{M}\mathrm{e}\mathrm{V}/{\mathrm{c}}^2$ in data from SELEX, the charm hadro-production experiment E781 at Fermilab. This state is seen in two decay modes, $D_s^{+}\eta$ and $D^0{K}^{+}$. In the $D_s^{+}\eta$ decay mode we observe a peak with 101 events over a combinatoric background of 54.9 events at a mass of $2635.4\pm 3.3\mathrm{M}\mathrm{e}\mathrm{V}/{\mathrm{c}}^2$. There is a corresponding peak of 21 events over a background of 6.9 at $2631.5\pm 2.0\mathrm{M}\mathrm{e}\mathrm{V}/{\mathrm{c}}^2$ in the decay mode $D^0{K}^{+}$. The decay width of this state is $<17\mathrm{M}\mathrm{e}\mathrm{V}/{\mathrm{c}}^2$ at 90% confidence level. The relative branching ratio $\Gamma (D^0{K}^{+})/\Gamma (D_s^{+}\eta )$ is $0.14\pm 0.06$. The mechanism that keeps this state narrow is unclear. Its decay pattern is also unusual, being dominated by the $D_s^{+}\eta$ decay mode.