# Observation of an excited $B_c^+$ state

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## Abstract

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. Fig1.pdf [9 KiB] HiDef png [46 KiB] Thumbnail [12 KiB] *.C file Invariant-mass distribution of the selected $B _ c ^+$ candidates. % The fit results are overlaid. Fig2.pdf [30 KiB] HiDef png [438 KiB] Thumbnail [93 KiB] *.C file 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. Fig3.pdf [20 KiB] HiDef png [460 KiB] Thumbnail [111 KiB] *.C file 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. Fig4.pdf [36 KiB] HiDef png [456 KiB] Thumbnail [116 KiB] *.C file Animated gif made out of all figures. PAPER-2019-007.gif Thumbnail

## Tables and captions

 Results of the fit to the $\Delta M$ distribution. Uncertainties are statistical only. Table_1.pdf [54 KiB] HiDef png [86 KiB] Thumbnail [14 KiB] 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] HiDef png [384 KiB] Thumbnail [83 KiB] *C file

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