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Search for excited $B_c^+$ states

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Abstract

A search is performed in the invariant mass spectrum of the $B_c^{+}\pi^{+}\pi^{-}$ system for the excited $B_c^{+}$ states $B_c(2^{1}S_{0})^+$ and $B_c(2^{3}S_{1})^+$ using a data sample of $pp$ collisions collected by the LHCb experiment at the centre-of-mass energy of $\sqrt{s} = 8 \,{\mathrm{TeV}}$, corresponding to an integrated luminosity of $2 \,{\mathrm{fb^{-1}}}$. No evidence is seen for either state. Upper limits on the ratios of the production cross-sections of the $B_c(2^{1}S_{0})^+$ and $B_c(2^{3}S_{1})^+$ states times the branching fractions of ${B_c(2^{1}S_{0})^+} \to {B_c^{+}\pi^{+}\pi^{-}}$ and ${B_c(2^{3}S_{1})^+} \to {B_c^{*+}\pi^{+}\pi^{-}}$ over the production cross-section of the $B_c^{+}$ state are given as a function of their masses. They are found to be between 0.02 and 0.14 at $95\%$ confidence level for $B_c(2^{1}S_{0})^+$ and $B_c(2^{3}S_{1})^+$ in the mass ranges $[6830, 6890] \,{\mathrm{MeV}}/c^{2}$ and $[6795,6890] \,{\mathrm{MeV}}/c^{2}$, respectively.

Figures and captions

Invariant mass distribution of the selected $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ $ candidates. The points with error bars represent the data. The blue solid line is the fit to data. The red cross-hatched area shows the signal. The green shaded area represents the $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K^{+}$ background. The violet dash-dotted line is the combinatorial background.

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Mass distributions of the selected $ B _ c ^+ \pi ^+ \pi ^- $ candidates in the range $[6795,6890] {\mathrm{\,MeV\!/}c^2} $ for the four MLP categories.

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The $ M( B _ c ^+ \pi ^+ \pi ^- )$ distributions in the same-sign (darkgreen shaded areas) and data (points with error bars) samples in the range $[6600,7300] {\mathrm{\,MeV\!/}c^2} $ with the background model (blue solid line) overlaid, for the four MLP categories. The areas between the two vertical red lines are the signal regions.

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The upper limits on the ratio $\mathcal{R}( B _ c ^{(*)}(2S)^+ )$ at $95\%$ and $90\%$ confidence levels under different mass splitting $\Delta M$ hypotheses.

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Animated gif made out of all figures.

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Tables and captions

Efficiencies for the $ B _ c ^{(*)}(2S)^+ $ states in the regions $ p_{\mathrm{ T}} \in [0,20] {\mathrm{\,GeV\!/}c} $ and $y \in [2.0,4.5]$ for each MLP category. The efficiencies obtained before applying the MLP classifier are $0.0091 \pm 0.0002$ and $0.0086 \pm 0.0001$ for $ B _ c (2S)^+ $ and $ B _ c ^{*}(2S)^+ $, respectively. The uncertainties are statistical only, and are due to the limited size of the simulated sample.

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Summary of the systematic uncertainties entering the upper limit calculation for the four MLP categories.

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Comparison of the $\mathcal{R}$ value between the LHCb upper limits at 95% CL and the ATLAS measurement [18], where $0 < \varepsilon_{7,8} \leq 1$ are the relative efficiencies of reconstructing the $ B _ c ^{(*)}(2S)^+ $ candidates with respect to the $ B _ c ^+ $ signals for the 7 and $8 {\mathrm{\,TeV} }$ data, respectively.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2017-042. The files are: Supplementary.pdf : An overview of the extra figure and contents *.pdf, *.png, *.eps, *.C : The figure in variuous formats

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.