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Observation of the $B_c^+ \to J/\psi \pi^+ \pi^0$ decay

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Abstract

The first observation of the $B_c^+ \to J/\psi \pi^+ \pi^0$ decay is reported with high significance using proton-proton collision data, corresponding to an integrated luminosity of 9fb$^{-1}$, collected with the LHCb detector at centre-of-mass energies of 7, 8, and 13 TeV. The ratio of its branching fraction relative to the $B_c^+ \to J/\psi \pi^+$ channel is measured to be $$ \frac{ {\cal{B}}( B_c^+ \to J/\psi \pi^+\pi^0 ) } { {\cal{B}}( B_c^+ \to J/\psi \pi^+ ) } = 2.80 \pm 0.15 \pm 0.11 \pm 0.16 , $$ where the first uncertainty is statistical, the second systematic and the third related to imprecise knowledge of the branching fractions for $B^+ \to J/\psi K^{*+}$ and $B^+ \to J/\psi K^+$ decays, which are used to determine the $\pi^0$ detection efficiency. The $\pi^+\pi^0$ mass spectrum is found to be consistent with the dominance of an intermediate $\rho^+$ contribution in accordance with a model based on QCD factorisation.

Figures and captions

Diagram for the decays of the $ B _ c ^+$ meson into the $ { J \mskip -3mu/\mskip -2mu\psi }$ meson and light hadrons.

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

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Mass distribution for selected $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } K ^{*+} $ candidates with the result of the fit described in the text.

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Mass distribution for selected $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^0 $ candidates with the result of the fit, described in the text.

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Background-subtracted $\pi ^+ \pi ^0 $ mass distribution from the $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^0 $ decays. The orange line shows the result of the twodash-component fit described in the text. The blue dashed line corresponds to the component parameterised by Eq. 3, which vanishes in the fit.

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The measured ratio of the branching fractions of the $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^0 $ and $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ $ decays in comparison with theory predictions. The colored band shows the measured ratio with corresponding uncertainties added in quadrature.

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Animated gif made out of all figures.

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Tables and captions

Yields $N$, mass biases $\updelta m_ B $ and resolution scale factors $s$ from the fits to the $ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ $, $ { J \mskip -3mu/\mskip -2mu\psi } K ^+ \pi ^0 $ and $ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^0 $ mass spectra. The uncertainties for $\updelta m_{ B }$ include those from the known masses of the $ B _ c ^+$ and $ B ^+$ mesons. The mass bias and resolution scale parameters for the $ B _ c ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^0 $ channel are constrained by those from the $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } K ^+ \pi ^0 $ control channel.

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Relative systematic uncertainties for the ratio ${\mathcal{R}}$. The total systematic uncertainty is the quadratic sum of individual contributions. The systematic uncertainty of the $\pi ^0$ reconstruction efficiency due to imprecise knowledge of the branching fraction of the $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } K ^{*+} $ and $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi } K ^+ $ decays is treated separately.

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Created on 31 May 2024.