Observation of a Narrow Charm-Strange Meson $D_{sJ}^{+}(2632)\to D_s^{+}\eta$ and $D^0{K}^{+}$

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.

Figure 1

$M(\gamma \gamma )$ distribution for photon pairs in the $\eta$ mass region from 0.1% of the data sample. Results for the fit shown are in Table 1. The inset shows the background subtracted $\eta$ signal. The dark points indicate the $\eta$ signal region.

Figure 2

$M(KK{\pi }^{\pm }\eta )$-$M(KK{\pi }^{\pm })$ mass-difference distribution. Charged conjugates are included. The darker shaded region is the event excess used in the estimation of signal significance. The lighter shaded region is the event-mixed combinatoric background described in the text. The inset shows the difference of the two with a Gaussian fit to the signal. Results for the fit shown are in Table 1.

Figure 3

(a) $D_s(2632)\to D^0{K}^{+}$ mass-difference distribution. Charged conjugates are included. The shaded regions are the event excesses used in the estimation of signal significances. Results for the fit shown are in Table 1. (b) Wrong-sign background $D^0{K}^{-}$ events, as described in the text.