CERN Accelerating science

This talk will cover the latest CMS results on three rare decays: the new $B_s\to\mu\mu$  decay measurement, including the determination of the $B_s\to\mu\mu$  effective lifetime, a search for the $B^0\to\mu\mu$ decay, and the first observation of the $\Lambda_b\to J/\psi \Lambda\phi$ decay. The measurement of the $B_s\to\mu\mu$  branching fraction yields the value of $[2.9^{+0.7}_{-0.6} (\mathrm{exp}) \pm 0.2 (\mathrm{frag})] \times 10^{-9}$, while the $B_s\to\mu\mu$ effective lifetime is determined to be $1.70^{+0.61}_{-0.44}$ ps. Both results are consistent with the standard model expectations. A new upper limit on the $B^0\to\mu\mu$ branching fraction is set at $3.6\times 10^{-10}$ at 95\% CL, which is about 3.5 times the standard model prediction. The decay $\Lambda_b\to J/\psi(\mu\mu) \Lambda(p\pi^-)\phi(K^+K^-)$ is observed for the first time, and its branching fraction relative to that for the $\Lambda_b \to \psi(2S)\Lambda$ decay is measured to be $8.26 \pm 0.90 (\mathrm{stat}) \pm 0.68 (\mathrm{syst})\pm 0.11 (\mathrm{BF})$ percent.