Observation of the ${\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}1}$ (3872) pK⁻ decay

Abstract

Using proton-proton collision data, collected with the LHCb detector and corresponding to 1.0, 2.0 and 1.9 fb⁻¹ of integrated luminosity at the centre-of-mass energies of 7, 8, and 13 TeV, respectively, the decay ${\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}1}$(3872)pK⁻ with χ_c1(3872) → J/ψ π⁺π⁻ is observed for the first time. The significance of the observed signal is in excess of seven standard deviations. It is found that (58 ± 15)% of the decays proceed via the two-body intermediate state χ_c1(3872)Λ(1520). The branching fraction with respect to that of the ${\Lambda}_{\mathrm{b}}^0$ → ψ(2S)pK⁻ decay mode, where the ψ(2S) meson is reconstructed in the J/ψ π⁺π⁻ final state, is measured to be:

$$ \frac{\beta \left({\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}1}(3872){\mathrm{pK}}^{-}\right)}{\beta \left({\Lambda}_{\mathrm{b}}^0\to \uppsi \left(2\mathrm{S}\right){\mathrm{pK}}^{-}\right)}\times \frac{\beta \left({\upchi}_{\mathrm{c}1}(3872)\to \mathrm{J}/\uppsi {\uppi}^{+}{\uppi}^{-}\right)}{\beta \left(\uppsi \left(2\mathrm{S}\right)\to \mathrm{J}/\uppsi {\uppi}^{+}{\uppi}^{-}\right)}=\left(5.4\pm 1.1\pm 0.2\right)\times {10}^{-2}, $$

where the first uncertainty is statistical and the second is systematic.