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Study of $B_c^+ \rightarrow \chi_c \pi^+$ decays

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Abstract

A study of $B_c^+ \rightarrow \chi_c \pi^+$ decays is reported using proton-proton collision data, collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9fb$^{-1}$. The decay $B_c^+ \rightarrow \chi_{c2} \pi^+$ is observed for the first time, with a significance exceeding seven standard deviations. The relative branching fraction with respect to the $B_c^+ \rightarrow J/\psi \pi^+$ decay is measured to be $$ \frac{\mathcal{B}_{B_c^+ \rightarrow \chi_{c2} \pi^+}} {\mathcal{B}_{B_c^+ \rightarrow J/\psi \pi^+}} = 0.37 \pm 0.06 \pm 0.02 \pm 0.01 , $$ where the first uncertainty is statistical, the second is systematic, and the third is due to the knowledge of the $\chi_c \rightarrow J/\psi \gamma$ branching fraction. No significant $B_c^+ \rightarrow \chi_{c1} \pi^+$ signal is observed and an upper limit for the relative branching fraction for the $B_c^+ \rightarrow \chi_{c1} \pi^+$ and $B_c^+ \rightarrow \chi_{c2} \pi^+$ decays of $$ \frac{\mathcal{B}_{B_c^+ \rightarrow \chi_{c1} \pi^+}} {\mathcal{B}_{B_c^+ \rightarrow \chi_{c2} \pi^+}} < 0.49 $$ is set at the 90% confidence level.

Figures and captions

Mass distribution for selected $ B ^+ \rightarrow \chi _ c K ^+ $ candidates with a $\chi _{ c 1} $ massdash-constraint. The result of the fit, described in the text, is overlaid.

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Mass distribution for selected $ B _ c ^+ \rightarrow \chi _ c \pi ^+ $ candidates with a $\chi _{ c 2} $ mass constraint. The result of the fit, described in the text, is overlaid.

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Mass distributions for selected $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ $ candidates. The result of the fit, described in the text, is overlaid.

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Background-subtracted $ { J \mskip -3mu/\mskip -2mu\psi } \gamma $ mass distribution from (left) the $ B _ c ^+ \rightarrow \chi _{ c 2} \pi ^+ $ decays and (right) $ B ^+ \rightarrow \chi _{ c 1} K ^+ $ decays. Projections of the fit, described in the text, are overlaid.

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Comparison of upper limit on the branching fraction ratio between $ B _ c ^+ \rightarrow \chi _{ c 1} \pi ^+ $ and $ B _ c ^+ \rightarrow \chi _{ c 2} \pi ^+ $ decays set in this analysis with theoretical predictions.

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Animated gif made out of all figures.

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Tables and captions

Parameters of interest from the fit to the $\chi _ c K ^+ $ mass spectrum: the yields of the $ B ^+ \rightarrow \chi _{ c 1} K ^+ $ and $ B ^+ \rightarrow \chi _{ c 2} K ^+ $ decays, the $ B ^+$ mass bias parameter $\updelta m_{ B ^+ }$ and the resolution scale factor $s_{ B ^+ }$.

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Relative systematic uncertainties (in %) for the ratio of branching fractions $\mathcal{R}^{\chi _{ c 2} }_{ { J \mskip -3mu/\mskip -2mu\psi } }$ and the ratio of efficiencies for the $ B _ c ^+ \rightarrow \chi _{ c 1} \pi ^+ $ and $ B _ c ^+ \rightarrow \chi _{ c 2} \pi ^+ $ decays. The total uncertainty is the quadratic sum of individual contributions.

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Created on 17 May 2024.