Evidence for the rare decay $B^{+} \to \bar Λp μ^{+} μ^{-}$

TL;DR

This LHCb study searches for the rare baryonic semileptonic decay B+ → anti-Lambda p mu+ mu-. It uses a normalization channel B+ → J/psi anti-Lambda p and performs a simultaneous fit across long and downstream Lambda categories to extract partial branching fractions in two Lambda p mass regions. An excess is observed in the low-mass region with about 3.5 sigma significance, yielding a partial branching fraction near 1.70×10^-8, while the high-mass region provides only an upper limit of 2.8×10^-9 at 90% CL. The combined branching fraction over the full mass range is about 1.53×10^-8, hinting at threshold enhancement in b → s mu mu decays with a baryon-antibaryon final state and informing baryonic form-factor studies.

Abstract

A search for the rare decay $B^{+} \to \bar Λp μ^{+} μ^{-}$ is performed using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of $\sqrt{s}= 13$ TeV, corresponding to an integrated luminosity of 5.4 fb$^-1$. An excess of events is found with respect to the background-only expectation, with a signal significance of 3.5 standard deviations, in the low invariant-mass region of $m(\bar Λp)<2.8$ GeV/$c^2$. The branching fraction is measured to be ${\cal B}_{\rm low}({B^{+}}\to {\bar Λp μ^{+} μ^{-}})=\left(1.70 ^{+0.65}_{-0.56}(\rm stat) \pm 0.17(\rm syst) \pm 0.14(\rm ext)\right) \times 10^{-8}$, where the last uncertainty is due to external inputs on ${\cal B}(B^+\to{J/ψ\bar Λp})\times {\cal B}({J/ψ}\to{μ^+μ^-})$. With no significant signal observed in the high $m(\bar Λp)$ region above 2.8 GeV/$c^2$, an upper limit is set to be ${\cal B}_{\rm high}(B^{+}\to {\bar Λp μ^{+} μ^{-}})<2.8\,(3.7) \times 10^{-9}$ at the $90\%$ ($95\%$) confidence level.

Figures (4)

• Figure 1: Invariant-mass distributions of (left) $\hbox{$B^{+}\!\rightarrow\xspace {{\mathrm{J}\xspace / \uppsi\xspace}}\xspace {\overline{ \Lambda\xspace}}\xspace p$}\xspace$ and $\hbox{$B^{+}\!\rightarrow\xspace {\overline{ \Lambda\xspace}}\xspace p \mu^{+} \mu^{-}$}\xspace$ candidates with (middle) $m({\overline{ \Lambda\xspace}}\xspace p\xspace)<2.8\text{\,Ge V\!/}c^2\xspace$ and (right) $m({\overline{ \Lambda\xspace}}\xspace p\xspace)>2.8\text{\,Ge V\!/}c^2\xspace$, after applying all selection criteria described in the text. Candidates from the long and downstream${\overline{ \Lambda\xspace}}\xspace\xspace$ categories are combined for illustration purposes only. The combined fit functions are shown together with the individual fit components.
• Figure 2: Projection of log-likelihood ratio $-2 \Delta \ln {\cal L}$ when determining the yield of $\hbox{$B^{+}\!\rightarrow\xspace {\overline{ \Lambda\xspace}}\xspace p \mu^{+} \mu^{-}$}\xspace$ decays in the $m({\overline{ \Lambda\xspace}}\xspace p\xspace)<2.8\text{\,Ge V\!/}c^2\xspace$ region.
• Figure 3: Results from the CL$_s$ scan used to obtain the upper limit on the partial branching fraction of $\hbox{$B^{+}\!\rightarrow\xspace {\overline{ \Lambda\xspace}}\xspace p \mu^{+} \mu^{-}$}\xspace$ decay in the $m({\overline{ \Lambda\xspace}}\xspace p\xspace)>2.8\text{\,Ge V\!/}c^2\xspace$ region. The background-only expectation is shown by the dashed black line, with the 1$\sigma$ and 2$\sigma$ bands shown as green and yellow bands, respectively. The observed values are displayed as a solid black line with red markers, and the solid red line intersecting with the observation indicates the limit at the 90% confidence level.
• Figure 4: (Left) Invariant-mass distribution of $\hbox{$B^{+}\!\rightarrow\xspace {\overline{ \Lambda\xspace}}\xspace p \mu^{+} \mu^{-}$}\xspace$ candidates, showing the fit result along with the fit model components. (Right) Projection of the likelihood ratio $-2 \Delta \ln {\cal L}$ used to determine the $\hbox{$B^{+}\!\rightarrow\xspace {\overline{ \Lambda\xspace}}\xspace p \mu^{+} \mu^{-}$}\xspace$ yield in the full $m({\overline{ \Lambda\xspace}}\xspace p\xspace)$ region.