Observation of an excited $B_c^+$ state

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Using $pp$ collision data corresponding to an integrated luminosity of $8.5 \mathrm{fb}^{-1}$ recorded by the LHCb experiment at centre-of-mass energies of $\sqrt{s} = 7$, $8$ and $13\mathrm{ Te V}$, the observation of an excited $B_c^+$ state in the $B_c^+\pi^+\pi^-$ invariant-mass spectrum is reported. The observed peak has a mass of $6841.2 \pm 0.6 { \rm (stat) } \pm 0.1 { \rm (syst) } \pm 0.8 (B_c^+) \mathrm{ MeV}/c^2$, where the last uncertainty is due to the limited knowledge of the $B_c^+$ mass. It is consistent with expectations of the $B_c^{*}(2^{3}S_{1})^+$ state reconstructed without the low-energy photon from the $B_c^{*}(1^{3}S_{1})^+ \to B_c^+ \gamma$ decay following $B_c^{*}(2^{3}S_{1})^+ \to B_c^{*}(1^{3}S_{1})^+ \pi^+ \pi^-$. A second state is seen with a global (local) statistical significance of $2.2 \sigma$ ($3.2 \sigma$) and a mass of $6872.1 \pm 1.3 { \rm (stat) } \pm 0.1 { \rm (syst) } \pm 0.8 (B_c^+) \mathrm{ MeV}/c^2$, and is consistent with the $B_c(2^{1}S_{0})^+$ state. These mass measurements are the most precise to date.

Figures and captions

Transitions among the $ B _ c ^{(*)}(2S)^+ $ and $B_c^{(*)+}$ states.

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Invariant-mass distribution of the selected $ B _ c ^+ $ candidates. % The fit results are overlaid.

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Invariant-mass $M( B _ c ^+ \pi ^+ \pi ^- )$ distributions for the data and same-sign samples with the distribution of the $ B _ c ^+ $ mass sidebands overlaid.

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Distribution of $\Delta M = M( B _ c ^+ \pi ^+ \pi ^- ) - M( B _ c ^+ )$ with the fit results overlaid. The same-sign distribution has been normalized to the data in the $ B _ c ^{(*)}(2S)^+ $ sideband region.

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Animated gif made out of all figures.


Tables and captions

Results of the fit to the $\Delta M$ distribution. Uncertainties are statistical only.

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tex code

Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [92 KiB]

This ZIP file contains supplemetary material for the publication LHCb-PAPER-2019-007. The files are: Supplementary.pdf : An overview of the extra figure *.pdf, *.png, *.eps : The figure in variuous formats

Fig1.pdf [29 KiB]
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*C file

Created on 14 June 2019.Citation count from INSPIRE on 14 June 2019.