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Study of charmonium and charmonium-like contributions in $B^+ \rightarrow J/\psi \eta K^+$ decays

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Abstract

A study of $B^+ \rightarrow J/\psi \eta K^+$ decays, followed by $J/\psi \rightarrow \mu^+ \mu^-$ and $\eta \rightarrow \gamma \gamma$, is performed using a dataset collected with the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 9 fb$^{-1}$. The $J/\psi\eta$ mass spectrum is investigated for contributions from charmonia and charmonium-like states. Evidence is found for the $B^+\rightarrow \left( \psi_2(3823) \rightarrow J/\psi \eta \right) K^+$ and $B^+\rightarrow \left( \psi(4040) \rightarrow J/\psi \eta \right) K^+$ decays with significance of 3.4 and 4.7 standard deviations, respectively. This constitutes the first evidence for the $\psi_2(3823) \rightarrow J/\psi \eta$ decay.

Figures and captions

Mass distribution of selected $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \eta K ^+ $ candidates. The result of the fit, described in the text, is overlaid.

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Background-subtracted $ { J \mskip -3mu/\mskip -2mu\psi } \eta $ mass distribution from $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \eta K ^+ $ decays in four $ { J \mskip -3mu/\mskip -2mu\psi } \eta $ mass regions. The results of the fits without contributions from a narrow $ X$ state, described in the text, are overlaid.

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Background-subtracted $ { J \mskip -3mu/\mskip -2mu\psi } \eta $ mass distribution from $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \eta K ^+ $ decays in the vicinity of the conventional (top row) $\psi (3770)$, $\psi _2(3823)$, (middle row) $\psi _3(3842)$, $\psi (4040)$, (bottom row) $\psi (4160)$ and $\psi (4415)$ charmonium states. The results of the fits, described in the text, are overlaid. The red open point with horizontal error bars indicates the mass and width of the resonance assumed in the fits.

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Background-subtracted $ { J \mskip -3mu/\mskip -2mu\psi } \eta $ mass distribution from $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \eta K ^+ $ decays in the vicinity of the charmonumdash-like (top row) $ R (3760)$, $ R (3790)$, (middle row) $ Z (3900)^0$, $\psi (4230)$, (bottom row) $\psi (4360)$ and $\psi (4390)$ states. The results of the fits, described in the text, are overlaid. The red open point with horizontal error bars indicates the mass and width of the resonance assumed in the fits.

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Background-subtracted $ { J \mskip -3mu/\mskip -2mu\psi } \eta $ mass distribution from $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \eta K ^+ $ decays in the vicinity of the hypothetical (left) $ X ^{\prime}_{ C }$ and (right) $ Z _{ c }(4430)^0$ states. The results of the fits, described in the text, are overlaid. The red open point with horizontal error bars indicates the mass and width of the resonance used in the fits.

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Upper limit (90% CL) on the ratio of branching fractions $F_{ X }$ as a function of the mass of the hypothetical narrow $ X$ state. The median expected upper limit together with the expected CL bands corresponding to 1, 2 and 3 standard deviations are also shown.

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Upper limit (90% CL) on the product of branching fractions $B_{ X }$ as a function of the mass of the hypothetical narrow $ X$ state. The median expected upper limit together with the expected CL bands corresponding to 1, 2 and 3 standard deviations are also shown.

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Animated gif made out of all figures.

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Tables and captions

Relative systematic uncertainties for the efficiency ratio $R_{\varepsilon}$. When an uncertainty is found to be dependent on the $ { J \mskip -3mu/\mskip -2mu\psi } \eta $ mass, the corresponding range is shown. The total uncertainty is obtained as the quadratic sum of the individual contributions.

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Upper limits at 90% CL for the ratio of branching fractions $F_{ X }$ and product of branching fractions $B_{ X }$ for different conventional charmonia, charmoniumdash-like and hypothetical states.

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Created on 22 March 2024.