# Observation of the $\Lambda_b^0\rightarrow \chi_{c1}(3872)pK^-$ decay

## TL;DR

This paper reports the first observation of the decay b1_b^0  b1_{c1}(3872) p K^- using LHCb data, with a significance over seven standard deviations, and measures its branching fraction relative to b1_b^0  \u03c8(2S) p K^-.

## Contribution

First observation of the b1_b^0  b1_{c1}(3872) p K^- decay mode with detailed branching fraction measurement.

## Key findings

- Decay observed with >7 sigma significance.
- 58% of decays proceed via b1_{c1}(3872) b1(1520) intermediate state.
- Branching fraction ratio measured as (5.4 b1 1.1 b1 0.2) d 10^{-2}.

## Abstract

Using proton-proton collision data, collected with the LHCb detector and corresponding to 1.0, 2.0 and 1.9fb$^{-1}$ of integrated luminosity at the centre-of-mass energies of 7, 8, and 13 TeV, respectively, the decay $\Lambda_b^0\to \chi_{c1}(3872)pK^-$ with $\chi_{c1}\to J/\psi\pi^+\pi^-$ is observed for the first time. The significance of the observed signal is in excess of seven standard deviations. It is found that $(58\pm15)\%$ of the decays proceed via the two-body intermediate state $\chi_{c1}(3872)\Lambda(1520)$. The~branching fraction with respect to that of the $\Lambda_b\rightarrow\psi(2S)p K^{-}$ decay mode, where the $\psi(2S)$~meson is reconstructed in the $J/\psi \pi^+\pi^-$ final state, is measured to be: \begin{equation*} \frac{\Lambda_b^0\to\chi_{c1}(3872)pK^-}{\Lambda_b\to\psi(2S)p K^-} \times \frac{\mathcal{B}(\chi_{c1} \to J/\psi \pi^+\pi^-)}{\mathcal{B}(\psi(2S)\to J/\psi \pi^+\pi^-)} = \left(5.4 \pm 1.1 \pm 0.2\right)\times 10^{-2}\,, \end{equation*} where the first uncertainty is statistical and the second is systematic.