Observation of the $\Xi_b^- \to J/\psi\Lambda K^-$ decay

Abstract

The observation of the decay $\Xi_b^- \to J/\psi\Lambda K^-$ is reported, using a data sample corresponding to an integrated luminosity of 3 fb$^{−1}$, collected by the LHCb detector in ppcollisions at centre-of-mass energies of 7and 8 TeV. The production rate of $\Xi_b^-$ baryons detected in the decay $\Xi_b^- \to J/\psi\Lambda K^-$ is measured relative to that of $\Lambda^0_b$ baryons using the decay $\Lambda^0_b \to J/\psi\Lambda$. Integrated over the $b$-baryon transverse momentum $p_T < 25 GeV/c$ and rapidity $2.0 < y < 4.5$, the measured ratio is
$\frac{f_{\Xi^-_b}}{f_{\Lambda^0_b}}
\frac{\mathscr{B}(\Xi^-_b \to J/\psi\Lambda K^−)}
{\mathscr{B}(\Lambda^0_b \to J/\psi\Lambda)} = (4.19 \pm 0.29(stat) \pm 0.15(syst)) \times 10^{−2}$,
where $f_{\Xi^-_b}$ and $f_{\Lambda^0_b}$ are the fragmentation fractions of $b \to \Xi^-_b$ and $b \to \Lambda^0_b$ transitions, and $\mathscr{B}$ represents the branching fraction of the corresponding b-baryon decay. The mass difference between $\Xi^-_b$ and $\Lambda^0_b$ baryons is measured to be
$M(\Xi^-_b) − M(\Lambda^0_b) = 177.08 \pm 0.47(stat) \pm 0.16(syst) MeV/c^2$.