The first observation of the decays $\Lambda_b^0 \to \chi_{ c1} p K^-$ and $\Lambda_b^0 \to \chi_{ c2} p K^-$ is reported using a data sample corresponding to an integrated luminosity of $3.0$ fb$^{ -1}$, collected by the LHCb experiment in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV. The following ratios of branching fractions are measured \begin{ eqnarray*} \frac{ { \cal B}(\Lambda_b^0 \to \chi_{ c1} p K^-)}{ { \cal B}(\Lambda_b^0 \to J/\psi p K^-)} = 0.242 \pm 0.014 \pm 0.013 \pm 0.009, \frac{ { \cal B}(\Lambda_b^0 \to \chi_{ c2} p K^-)}{ { \cal B}(\Lambda_b^0 \to J/\psi p K^-)} = 0.248 \pm 0.020 \pm 0.014 \pm 0.009, \frac{ { \cal B}(\Lambda_b^0 \to \chi_{ c2} p K^-)}{ { \cal B}(\Lambda_b^0 \to \chi_{ c1} p K^-)} = 1.02 \pm 0.10 \pm 0.02 \pm 0.05, \end{ eqnarray*} where the first uncertainty is statistical, the second systematic and the third due to the uncertainty on the branching fractions of the $\chi_{ c1}\to J/\psi\gamma$ and $\chi_{ c2} \to J/\psi\gamma$ decays. Using both decay modes, the mass of the $\Lambda_b^0$ baryon is also measured to be $m_{ \Lambda_b^0} = 5619.44 \pm 0.28 \pm 0.26$ MeV/$c^2$, where the first and second uncertainties are statistical and systematic, respectively.
Categories: hep-ex