Observation of a New Excited $D_s^{+}$ Meson in $B^0\to D^{-}{D}^{+}{K}^{+}{\pi }^{-}$ Decays

Using $pp$ collision data corresponding to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ collected with the LHCb detector at a center-of-mass energy of 13 TeV, the $B^0\to D^{-}{D}^{+}{K}^{+}{\pi }^{-}$ decay is studied. A new excited $D_s^{+}$ meson is observed decaying into the $D^{+}{K}^{+}{\pi }^{-}$ final state with large statistical significance. The pole mass and width, and the spin parity of the new state are measured with an amplitude analysis to be $m_R=2591\pm 6\pm 7\mathrm{MeV}$, ${\mathrm{\Gamma }}_R=89\pm 16\pm 12\mathrm{MeV}$, and $J^P=0^{-}$, where the first uncertainty is statistical and the second systematic. Fit fractions for all components in the amplitude analysis are also reported. The new resonance, denoted as $D_{s0}(2590{)}^{+}$, is a strong candidate to be the $D_s(2{{}^{1}S}_0{)}^{+}$ state, the radial excitation of the pseudoscalar ground-state $D_s^{+}$ meson.

Figure 1

Mass distribution of the selected $B^0$ candidates in the low $K^{+}{\pi }^{-}$ mass region. The fit result is overlaid.

Figure 2

Dalitz plot of the $D^{+}{D}^{-}$ versus $D^{+}{K}^{+}{\pi }^{-}$ masses squared for $B^0$ candidates with masses within $\pm 20\mathrm{MeV}$ around the known $B^0$ mass [10] in the low $K^{+}{\pi }^{-}$ mass region.

Figure 3

Mass projections of (a) $D^{+}{K}^{+}{\pi }^{-}$, (b) $K^{+}{\pi }^{-}$, (c) $D^{+}{D}^{-}$, and (d) $D^{+}{\pi }^{-}$ systems. Data points are shown in black with the background subtracted statistically using the sPlot method [33]. Results of the fit with the $D_{s0}(2590{)}^{+}$ ($J^P=0^{-}$) model are overlaid as a solid red histogram, and individual contributions shown as dotted histograms.

Figure 4

Comparison of $\cos {\theta }_{D_s}$ distributions for the spin parity of the $D_{s0}(2590{)}^{+}$ state assumed to be (a) $0^{-}$, (b) $1^{+}$, and (c) $2^{-}$. The $J^P=0^{-}$ model is the most consistent with data. Data points are shown in black with the background subtracted statistically using the sPlot method [33]. Fit results are overlaid as a solid red histogram, with individual contributions shown as dotted histograms.