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Observation of a new excited $D_s^+$ meson in $B^0\to D^+D^-K^+\pi^-$ decays

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Abstract

Using $pp$ collision data corresponding to an integrated luminosity of $5.4 {\rm fb}^{-1}$ collected with the LHCb detector at a centre-of-mass energy of $13 {\rm TeV}$, 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 $m_R=2591\pm6\pm7 {\rm MeV}$, $\Gamma_R=89\pm16\pm12 {\rm MeV}$ 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 $D_s(2^1{\rm S}_0)^+$ state, the radial excitation of the pseudoscalar ground-state $D_s^+$ meson.

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

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Dalitz plot of the $ D ^+ D ^- $ versus $ D ^+ K ^+ \pi ^- $ masses-squared for $ B ^0$ candidates with masses within $\pm20\text{ Me V} $ around the known $ B ^0$ mass [10] in the low $ K ^+ \pi ^- $ mass region.

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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 [40]. 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.

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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 [40]. Fit results are overlaid as a solid red histogram, with individual contributions shown as dotted histograms.

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Tables and captions

Fit fractions for the three $ D ^+_ s $ components in the low $ K ^+$ $\pi ^-$ mass region ( $m( K ^+ \pi ^- )<0.75\text{ Ge V} $). The interference fraction between the $ D _{s0}(2590)^+$ and NR components is also given. There is no net interference between any other pair of components due to the orthogonality of the Wigner small-$d$ matrices for different spins.

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Systematic uncertainties on the pole mass and width of the $ D _{s0}(2590)^+$ state, and fit fractions of the three $ D ^+_ s $ components. Here, IF denotes the interference fraction between the $ D _{s0}(2590)^+$ and NR components. The individual sources are added in quadrature to obtain the total uncertainty.

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Supplementary Material [file]

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2020-034. The files are: supplementary.pdf : An overview of the extra figures *.pdf : The figures in pdf format

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Created on 22 January 2021.