Speaker
Description
In the Standard Model, the flavor-changing neutral currents of the $b \to s$ and $b \to d$ transitions appear as vacuum effects, at the one-loop level. Rare semileptonic decays of $B$-mesons, originating by these currents, are extremely useful tools for testing the Standard Model and searching for a possible physics beyond the Standard Model. Differential branching fractions of semileptonic $B$-decays and angular distributions in some of them are experimentally measured by LHCb, ATLAS and CMS collaborations at LHC as well as by BaBar and Belle at $B$-factories. Here, we consider the rare $B^+ \to P \ell^+ \ell^-$ decay, where $P$ is a pseudoscalar meson and $\ell = e, \mu$ is a charged lepton. For example, we present results on the dilepton invariant-mass spectrum and decay rate for $B^+ \to \pi^+ \ell^+ \ell^-$ based on the effective Hamiltonian approach for the $b \to d \ell^+ \ell^-$ transitions in two cases — with taking into account weak annihilation diagrams and without this contribution. Our prediction for total branching fraction of $B^+ \to \pi^+ \mu^+ \mu^-$ is in a good agreement with the LHCb result (Aaij R. et al.. LHCb Collab,. JHEP. 10 (2015) 34) within experimental uncertainties. Мoreover, accounting weak annihilation contributions allow us to obtain a better agreement with the experimental data on the distribution in the muon-pair invariant mass squared $q^2$ in the entire kinematically allowed region and, in particular, in its lowest $q^2$-part. This differs from the previous analysis, where the low-$q^2$ experimental peak was obtained through the long-distance contributions from light vector mesons.
