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Evidence for the decay $X(3872)\rightarrow\psi(2S)\gamma$

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Abstract

Evidence for the decay mode $X(3872)\rightarrow\psi(2S)\gamma$ in $B^+\rightarrow X(3872)K^+$ decays is found with a significance of 4.4 standard deviations. The analysis is based on a data sample of proton-proton collisions, corresponding to an integrated luminosity of 3fb$^{-1}$ collected with the LHCb detector, at centre-of-mass energies of 7 and 8TeV. The ratio of the branching fraction of the $X(3872)\to\psi(2S)\gamma$ decay to that of the $X(3872)\rightarrow J/\psi \gamma$ decay is measured to be $$ \frac{Br(X(3872)\rightarrow\psi(2S)\gamma)}{Br(X(3872)\rightarrow J/\psi\gamma)} = 2.46\pm0.64\pm0.29, %\pm0.06,$$ where the first uncertainty is statistical and the second is systematic. The measured value agrees with expectations for a pure charmonium interpretation of the $X(3872)$ state and a mixture of charmonium and molecular interpretations. However, it does not support a pure $D\bar{D}^{*}$ molecular interpretation of the $X(3872)$state.

Figures and captions

a) Distribution of the ${ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \gamma K ^+ }$ invariant mass with fit projection overlaid, restricted to those candidates with ${ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \gamma }$ invariant mass within $\pm 3\sigma$ from the $ X (3872)$ peak position. b) Distribution of the ${ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \gamma }$ invariant mass with fit projection overlaid, restricted to those candidates with ${ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \gamma K ^+ }$ invariant mass within $\pm 3\sigma$ from the $ B ^+ $ peak position. The total fit (thick solid blue) together with the signal (thin solid green) and background components (dash-dotted orange for the combinatorial, dashed magenta for the peaking component and long dashed blue for their sum) are shown.

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a) Distribution of the ${\psi {\mathrm{(2S)}} \gamma K ^+ }$ invariant mass with fit projection overlaid, restricted to those candidates with ${\psi {\mathrm{(2S)}} \gamma }$ invariant mass within $\pm 3\sigma$ from the $ X (3872)$ peak position. b) Distribution of the ${\psi {\mathrm{(2S)}} \gamma }$ invariant mass with fit projection overlaid, restricted to those candidates with ${\psi {\mathrm{(2S)}} \gamma K ^+ }$ invariant mass within $\pm 3\sigma$ from the $ B ^+ $ peak position. The total fit (thick solid blue) together with the signal (thin solid green) and background components (dash-dotted orange for the combinatorial, dashed magenta for the peaking component and long dashed blue for their sum) are shown.

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Animated gif made out of all figures.

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Tables and captions

Parameters of the signal functions of the fits to the two-dimensional mass distributions of the ${ B ^+ \rightarrow X (3872) K ^+ }$ decays followed by ${ X (3872) \rightarrow \psi \gamma }$. Uncertainties are statistical only.

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Relative systematic uncertainties on the ratio of branching fractions ($R_{\psi \gamma }$).

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Created on 26 April 2024.