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In the Standard Model,  b  →  s  and  b  →  d  flavor-changing neutral currents (FCNCs) are not allowed at the tree level and are induced by loop effects. We consider the rare semileptonic  B +  →  π + ℓ + ℓ −  decay, where  ℓ  =  e, μ  is a charged lepton. The dilepton invariant-mass spectrum and decay rate for  B +  →  π + ℓ + ℓ −  are calculated in the effective Hamiltonian approach in two cases — by taking into account the weak annihilation diagrams and without this contribution. Our predictions for the branching fraction of the  B +  →  π + ℓ + ℓ −  decay, being dependent on the choice of the  B  →  π  form factors, are in agreement with the existing LHCb results within experimental uncertainties. Moreover, accounting for the weak annihilation contributions allows us to get a better agreement with the experimental data on the entire  q 2 -distribution in the kinematically allowed region, in particular, in its lowest  q 2 -part. This provides an alternative description of the data than a previous analysis, in which the low- q 2  enhancement seen experimentally was obtained through the long-distance contributions from the light vector mesons.

Impact of weak annihilation contribution on rare semileptonic B + → π + ℓ + ℓ − decay

Impact of weak annihilation contribution on rare semileptonic B ⁺ → π ⁺ ℓ ⁺ ℓ ⁻ decay