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Study of the $\psi_2(3823)$ and $\chi_{c1}(3872)$ states in $B^+\to (J/\psi \pi^+\pi^-)K^+$ decays

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Abstract

The decays $B^+\rightarrow J/\psi \pi^+ \pi^- K^+$ are studied using a data set corresponding to an integrated luminosity of 9 fb$^{-1}$ collected with the LHCb detector in proton-proton collisions between 2011 and 2018. Precise measurements of the ratios of branching fractions with the intermediate $\psi_2(3823)$, $\chi_{c1}(3872)$ and $\psi(2S)$ states are reported. The decay of $B^+\rightarrow \psi_2(3872)K^+$ with $\psi_2(3823)\rightarrow J\psi\pi^+\pi^-$ is observed for the first time with a significance of 5.1 standard deviations. The mass differences between the $\psi_2(3823)$, $\chi_{c1}(3872)$ and $\psi(2S)$ states are measured to be \begin{eqnarray*} m_{\chi_{c1(3872)}} - m_{\psi_2(3823)} &= & \phantom{0}47.50 \pm 0.53 \pm 0.13 \mathrm{MeV/}c^2 , \\ m_{\psi_2(3823)} - m_{\psi(2S)} &= & 137.98 \pm 0.53 \pm 0.14 \mathrm{MeV/}c^2 , \\ m_{\chi_{c1}(3872)} - m_{\psi(2S)} &= & 185.49 \pm 0.06 \pm 0.03 \mathrm{MeV/}c^2 , \end{eqnarray*} resulting in the most precise determination of the $\chi_{c1}(3782)$ mass. The width of the $\psi_2(3823)$ state is found to be below 5.2 MeV at 90% confidence level. The Breit--Wigner width of the $\chi_{c1}(3872)$ state is measured to be $$ \Gamma^{\mathrm{BW}}_{\chi_{c1}(3872)} = 0.96^{ + 0.19}_{ - 0.18}\pm0.21 \mathrm{MeV} , $$ which is inconsistent with zero by 5.5 standard deviations.

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Distribution for the $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ \pi ^- K ^- $ mass for selected $ B ^+$ candidates (points with error bars). A fit, described in the text, is overlaid.

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Distributions of the (left) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ $\pi ^+$ $\pi ^-$ $ K ^+$ and (right) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu}$ $\pi ^+$ $\pi ^-$ mass for selected (top) $ B ^+ \rightarrow \psi {\rm{(2S)}} K ^+ $ , (middle) $ B ^+ \rightarrow \psi _2(3823) K ^+ $ and (bottom) $ B ^+ \rightarrow \chi _{ c 1} (3872) K ^+ $ candidates shown as points with error bars. A fit, described in the text, is overlaid.

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Likelihood profiles for the Breit--Wigner width of (left) $\psi _2(3823)$ and (right) $\chi _{ c 1} (3872)$ states.

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Parameters of interest from the simultaneous unbinned extended maximum-likelihood two-dimensional fit. Results and statistical uncertainties are shown for the three fit regions.

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Relative systematic uncertainties (in %) for the ratios of branching fractions $\mathcal{R}^{\mathrm{X}}_{\mathrm{Y}}$.

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Systematic uncertainties (in $\text{ Me V /}c^2$ ) for the mass splitting between the $\psi _2(3823)$ , $\chi _{ c 1} (3872)$ and $\psi {\rm{(2S)}}$ states.

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Supplementary Material [file]

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Created on 16 October 2020.