Observation of the Baryonic Flavor-Changing Neutral Current Decay ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$

We report the first observation of the baryonic flavor-changing neutral current decay ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$ with 24 signal events and a statistical significance of 5.8 Gaussian standard deviations. This measurement uses a $p\overline{p}$ collisions data sample corresponding to $6.8{\mathrm{fb}}^{-1}$ at $\sqrt{s}=1.96\mathrm{TeV}$ collected by the CDF II detector at the Tevatron collider. The total and differential branching ratios for ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$ are measured. We find $\mathcal{B}({\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-})=[1.73\pm 0.42(\mathrm{stat})\pm (\mathrm{syst})]\times {10}^{-6}$. We also report the first measurement of the differential branching ratio of $B_s^0\to \varphi {\mu }^{+}{\mu }^{-}$, using 49 signal events. In addition, we report branching ratios for $B^{+}\to K^{+}{\mu }^{+}{\mu }^{-}$, $B^0\to K^0{\mu }^{+}{\mu }^{-}$, and $B\to K^{*}(892){\mu }^{+}{\mu }^{-}$ decays.

Figure 1

Invariant mass of (a) ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$, (b) $B_s^0\to \varphi {\mu }^{+}{\mu }^{-}$, (c) $B^{+}\to K^{+}{\mu }^{+}{\mu }^{-}$, (d) $B^0\to K^{*0}{\mu }^{+}{\mu }^{-}$, (e) $B^0\to K_S^0{\mu }^{+}{\mu }^{-}$, (f) $B^{+}\to K^{*+}{\mu }^{+}{\mu }^{-}$, with fit results overlaid. The histograms are the data. Solid, dashed-dotted, and dotted curves show the total fit, the signal PDF, and the background PDF, respectively.

Figure 2

Differential branching ratios of (a) ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$, (b) $B_s^0\to \varphi {\mu }^{+}{\mu }^{-}$, (c) $B^{+}\to K^{+}{\mu }^{+}{\mu }^{-}$, and (d) $B^0\to K^{*0}{\mu }^{+}{\mu }^{-}$. The points are the fit result. The solid curves are the SM expectation [1, 10, 21]. The dashed line in the ${\Lambda }_b^0\to \Lambda {\mu }^{+}{\mu }^{-}$ plot is the SM prediction normalized to our total branching ratio measurement. The hatched regions are the charmonium veto regions.