Abstract
Following updated and extended measurements of the full angular distribution of the decay $\Lambda_b\to \Lambda(\to p\,\pi^-)\mu^+\mu^-$ by the LHCb collaborations, as well as a new measurement of the $\Lambda \to p \pi^-$ decay asymmetry parameter by the BESIII collaboration, we study the impact of these results on searches for non-standard effects in exclusive $b\to s\mu^+\mu^-$ decays. To this end, we constrain the Wilson coefficients $\mathcal{C}_{9}$ and $\mathcal{C}_{10}$ of the numerically leading dimension-six operators in the weak effective Hamiltonian, in addition to the relevant nuisance parameters. In stark contrast to previous analyses of this decay mode, the changes in the updated experimental results lead us to find very good compatibility with both the Standard Model and with the $b\to s\mu^+\mu^-$ anomalies observed in rare $B$-meson decays. We provide a detailed analysis of the impact of the partial angular distribution, the full angular distribution, and the $\Lambda_b\to \Lambda\mu^+\mu^-$ branching fraction on the Wilson coefficients. In this process, we are also able to constrain the size of the production polarization of the $\Lambda_b$ baryon at LHCb.
Highlights
The persistent anomalies in the rare flavor-changing decays of B mesons, which arise in analyses of branching fractions, angular distributions and lepton flavor universality tests, have sparked considerable interest in constructing candidate theories to replace the Standard Model (SM) of particle physics; see for example Ref. [1] for a comprehensive guide
We carry out the first beyond the Standard Model (BSM) analysis of the measurements of the full angular distribution in Λb → Λð→ pπÞμþμ− decays. In this analysis we challenge the available data in four fit scenarios, corresponding to the absence of beyond the SM (BSM) effects [scenario SMðν-onlyÞ]; BSM effects only in operators present in the SM [scenarios (9) and (9,10)]; and BSM effects in allvector operators [scenario ð9; 10; 90; 100Þ]
Our results supersede those of a previous analysis of this decay mode in Ref. [13], due to updates to various experimental results and a correction in the numerical code
Summary
The persistent anomalies in the rare flavor-changing decays of B mesons, which arise in analyses of branching fractions, angular distributions and lepton flavor universality tests, have sparked considerable interest in constructing candidate theories to replace the Standard Model (SM) of particle physics; see for example Ref. [1] for a comprehensive guide. [1] for a comprehensive guide If these anomalies are a hint of physics beyond the SM (BSM), we should see signs of similar deviations in the baryonic partners of these rare B meson decays, e.g., in Λb → Λð→ pπ−Þμþμ−. The decay mode Λb → Λð→ pπ−Þμþμ− is quite appealing from a theoretical point of view. A previous analysis of the constraints of Λb → Λð→ pπ−Þμþμ− on the b → sμþμ− Wilson coefficients [13] using——outdated experimental inputs found a central value of C9 shifted in the opposite direction from the SM point compared to the B-meson findings. In this paper we confront this previous analysis with new, updated, and reinterpreted experimental results, and constrain BSM effects in b → sμþμ− operators
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