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Observation of the $B^0_s\rightarrow \chi_{c1}(3872)\pi^+\pi^-$ decay

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Abstract

The first observation of the $B^0_s \rightarrow \left( \chi_{c1}(3872) \rightarrow J/\psi\pi^+\pi^-\right) \pi^+ \pi^-$ decay is reported using proton-proton collision data, corresponding to integrated luminosities of 1, 2 and 6fb$^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13TeV, respectively. The ratio of branching fractions relative to the $B^0_s \rightarrow \left( \psi(2S) \rightarrow J\psi\pi^+\pi^- \right) \pi^+ \pi^-$ decay is measured to be $$ \frac{ \mathcal{B} \left( B^0_s \rightarrow \chi_{c1}(3872) \pi^+\pi^-\right) \times \mathcal{B} \left( \chi_{c1}(3872) \rightarrow J\psi\pi^+\pi^-\right)} { \mathcal{B} \left( B^0_s \rightarrow \psi(2S) \pi^+ \pi^- \right) \times \mathcal{B} \left( \psi(2S) \rightarrow J\psi\pi^+\pi^-\right) } = \left( 6.8 \pm 1.1 \pm 0.2 \right) \times 10^{-2} , $$ where the first uncertainty is statistical and the second systematic. The mass spectrum of the $\pi^+\pi^-$ system recoiling against the $\chi_{c1}(3872)$ meson exhibits a large contribution from $B^0_s \rightarrow \chi_{c1}(3872) \left( f_0(980) \rightarrow \pi^+ \pi^-\right)$ decays.

Figures and captions

Distributions of the (left) $ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^+ \pi ^- \pi ^- $ and (right) $ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^- $ mass of selected (top) $ B ^0_ s \rightarrow \psi {(2 S )} \pi ^+ \pi ^- $ and (bottom) $ B ^0_ s \rightarrow \chi _{ c 1} (3872)\pi ^+ \pi ^- $ candidates. Projections from the fit, described in the text, are overlaid. The $ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^+ \pi ^- \pi ^- $ mass distributions are shown for the $ B ^0_ s \rightarrow \psi {(2 S )} \pi ^+ \pi ^- $ and $ B ^0_ s \rightarrow \chi _{ c 1} (3872)\pi ^+ \pi ^- $ candidates within narrow $ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^- $ mass ranges, $3.679<m_{ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^- }<3.693\text{ Ge V /}c^2 $ and $3.864<m_{ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^- }<3.880\text{ Ge V /}c^2 $, respectively. The $ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^- $ mass distributions are shown for the $ B ^0_ s $ candidates within a narrow $ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^+ \pi ^- \pi ^- $ mass range, $5.35<m_{ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^+ \pi ^- \pi ^- }<5.38\text{ Ge V /}c^2 $.

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The background-subtracted mass spectra for the dipion system recoiling against the $\chi _{ c 1} (3872)$ or $\psi {(2 S )} $ states for (left) $ B ^0_ s \rightarrow \psi {(2 S )} \pi ^+ \pi ^- $ and (right) $ B ^0_ s \rightarrow \chi _{ c 1} (3872)\pi ^+ \pi ^- $ decays. The results of the fit, described in the text, are overlaid.

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Animated gif made out of all figures.

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Tables and captions

Signal yields, hadron masses, and detector resolution scale factors from the simultaneous fit described in the text. The parameters $m_{ B ^0_ s }$, $s_{ B ^0_ s }$ and $s_{ { J \mskip -3mu/\mskip -2mu\psi } \pi ^+ \pi ^- }$ are shared in the fit among the two mass ranges. The uncertainties are statistical only.

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Relative systematic uncertainties (in %) for the ratio of branching fractions. The sources are described in the text.

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Created on 19 April 2024.