Observation of a Narrow Charm-Strange Meson $D_{s J}^{+} (2632) \to D_{s}^{+} η$ and $D^{0} K^{+}$

We report the first observation of a charm-strange meson

D_{s J}^{+} (2632)

at a mass of

2632.5 \pm 1.7 M e V / c^{2}

in data from SELEX, the charm hadro-production experiment E781 at Fermilab. This state is seen in two decay modes,

D_{s}^{+} η

and

D^{0} K^{+}

. In the

D_{s}^{+} η

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 M e V / c^{2}

. There is a corresponding peak of 21 events over a background of 6.9 at

2631.5 \pm 2.0 M e V / c^{2}

in the decay mode

D^{0} K^{+}

. The decay width of this state is

< 17 M e V / c^{2}

at 90% confidence level. The relative branching ratio

Γ (D^{0} K^{+}) / Γ (D_{s}^{+} η)

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}^{+} η

decay mode.

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