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Observation of new excited $B_s^0$ states

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Abstract

A structure is observed in the $B^+K^-$ mass spectrum in a sample of proton--proton collisions at centre-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding to a total integrated luminosity of 9 fb${}^-1$. The structure is interpreted as the result of overlapping excited $B_s^0$ states. With high significance, a two-peak hypothesis provides a better description of the data than a single resonance. Under this hypothesis the masses and widths of the two states, assuming they decay directly to $B^+K^-$, are determined to be $m_1 = 6063.5 \pm 1.2 \text{ (stat)} \pm 0.8\text{ (syst) MeV},$ $\Gamma_1 = 26 \pm 4 \text{ (stat)} \pm 4\text{ (syst) MeV},$ $m_2 = 6114 \pm 3 \text{ (stat)} \pm 5\text{ (syst) MeV},$ $\Gamma_2 = 66 \pm 18 \text{ (stat)} \pm 21\text{ (syst) MeV}.$ Alternative values assuming a decay through $B^{*+}K^-$, with a missing photon from the $B^{*+} \rightarrow B^+\gamma$ decay, are also determined. The ratio of the total production cross-section times branching fraction of the new states relative to the previously observed $B_{s2}^{*0}$ state is determined to be $0.87 \pm 0.15 \text{ (stat)} \pm 0.19 \text{ (syst)}$.

Figures and captions

The $ B ^+$ $ K ^-$ mass difference distributions in data, overlaid with the fit models: (left column) one-peak hypothesis and (right column) two-peak hypothesis. In each column, the rows are from top to bottom for candidates with the prompt kaon $ p_{\mathrm{T}}$ : $0.5 < p_{\mathrm{T}} < \SI{1}{\giga\evolt}$, $1 < p_{\mathrm{T}} < \SI{2}{\giga\evolt}$, and $ p_{\mathrm{T}} > \SI{2}{\giga\evolt}$. The legend in the top row applies for each column. The associated production (AP) background is described by a second-degree polynomial in each fit. The combinatorial background shape is fixed from a fit to the $ B ^+$ $ K ^+$ mass difference distributions.

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Tables and captions

Sources of systematic uncertainty on the peak parameters assuming a two-peak structure decaying either directly to $ B ^+ K ^- $ or through $B^{*+} K ^- $. The parameter differences with respect to the nominal fit result are given in $\text{ Me V}$ .

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Summary of the systematic uncertainties on the production ratio. The effect of the unknown $ p_{\mathrm{T}}$ distribution is separated from other uncertainties affecting the relative efficiency.

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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-2020-026. The files are: supplementary.pdf : An overview of the extra figures supplementary.tex : Latex source to make supplementary.pdf *.pdf, *.png, *.eps, *.C : The figures in variuous formats

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Created on 19 February 2021.