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Observation of overlapping spin-1 and spin-3 $\overline{D}^0 K^-$ resonances at mass $2.86 {\rm GeV}/c^2$

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Abstract

The resonant substructure of $B_s^0 \rightarrow \bar{D}^0 K^- \pi^+$ decays is studied using a data sample corresponding to an integrated luminosity of $3.0\,{\rm fb}^{-1}$ of $pp$ collision data recorded by the LHCb detector. An excess at $m(\bar{D}^0 K^-) \approx 2.86 {\rm GeV}/c^2$ is found to be an admixture of spin-1 and spin-3 resonances. Therefore the $D^*_{sJ}(2860)^-$ state previously observed in inclusive $e^+e^- \rightarrow \bar{D}^0 K^- X$ and $pp \rightarrow \bar{D}^0 K^- X$ processes consists of at least two particles. This is the first observation of a heavy flavoured spin-3 resonance, and the first time that any spin-3 particle has been seen to be produced in $B$ decays. The masses and widths of the new states and of the $D^*_{s2}(2573)^-$ meson are measured, giving the most precise determinations to date.

Figures and captions

Fit to the $ B ^0_ s \rightarrow \overline{ D }{} {}^0 K ^- \pi ^+ $ candidate invariant mass distribution. Data points are shown in black, the result of the fit as a solid blue line and the components as detailed in the legend.

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Dalitz plot distribution of $ B ^0_ s \rightarrow \overline{ D }{} {}^0 K ^- \pi ^+ $ candidates in the signal region. The effect of the $ D ^0$ veto can be seen as an unpopulated horizontal band.

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Projections of the data and the Dalitz plot fit result onto (a) $m( K ^- \pi ^+ )$, (b) $m(\overline{ D }{} {}^0 K ^- )$ and (c) $m(\overline{ D }{} {}^0 \pi ^+ )$, with zooms into $m(\overline{ D }{} {}^0 K ^- )$ around (d) the $D^{*}_{s2}(2573)^{-}$ resonance and (e) the $D^{*}_{sJ}(2860)^{-}$ region. The data are shown as black points, the total fit result as a solid black curve, and the contributions from different resonances as detailed in the legend (small components, including the background contributions, are not shown).

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Projections of the data and Dalitz plot fit results with alternative models onto the cosine of the helicity angle of the $\overline{ D }{} {}^0 K ^- $ system, $\cos\theta(\overline{ D }{} {}^0 K ^- )$, for $2.77 < m(\overline{ D }{} {}^0 K ^- ) < 2.91 {\mathrm{\,GeV\!/}c^2} $, where $\theta(\overline{ D }{} {}^0 K ^- )$ is the angle between the $\pi ^+ $ and the $\overline{ D }{} {}^0 $ meson momenta in the $\overline{ D }{} {}^0 K ^- $ rest frame. The data are shown as black points, with the fit results with different models as detailed in the legend. The dip at $\cos\theta(\overline{ D }{} {}^0 K ^- ) \approx -0.6$ is due to the $\overline{ D }{} {}^0 $ veto. Comparisons of the data and the different fit results in the 50 bins of this projection give $\chi^2 $ values of 47.3, 214.0 and 150.0 for the default, spin-1 only and spin-3 only models, respectively.

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Animated gif made out of all figures.

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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.