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.
In the Standard Model, the → and → flavor-changing neutral currents (FCNC) are induced by loop effects. Rare semileptonic -meson decays originated by these currents are standard channels for testing the Standard Model precisely and in searching for possible physics beyond it. Here, we consider the rare + → + ℓ + ℓ − decay, where ℓ = , is a charged lepton, and present its dilepton invariant-mass spectrum and decay rate based on the effective electroweak Hamiltonian approach for the → ℓ + ℓ − transitions in the Standard Model, taking into account weak annihilation and long-distance contributions, of which the later is from the light vector mesons. Our prediction for the total branching fraction of + → + + − agrees with the latest LHCb result within the experimental and theoretical uncertainties. Moreover, including the annihilation diagrams and contributions from the 0 -and -resonances together gives better agreement with the experimental data in the low 2 -part of the dimuon mass spectrum. We also present theoretical predictions for the total and partial branching fractions for + → + + − in the Standard Model. These results are potentially useful in testing the lepton flavor universality in the FCNC → ℓ + ℓ − decays.
Among experimental processes for testing precisely the Standard Model (SM) and searching for possible physics beyond the SM, rare bottom-hadron decays induced by the → and → flavor-changing neutral currents attract a lot of attention. Radiative and semileptonic -meson decays with 0 , , and mesons in the final state like 0 → 0 ( ) , 0 → 0 ( )ℓ + ℓ − , 0 → , and 0 → ℓ + ℓ − , being mainly of pure annihilation-type topology, are of significant interest as in the SM they are extremely suppressed and New Physics effects can increase substantially their decay widths. Experimental searches of some of them were undertaken at the KEKB and LHC colliders. The upper limit on the radiative decay branching fraction, B ( 0 → ) < 1.0 × 10 −7 , obtained by the Belle collaboration in 2016, was the only one for quite some time. In 2022, the LHCb collaboration put the upper limit on its semileptonic counterpart, B ( 0 → + − ) < 3.2 × 10 −9 . Here, we consider a theory of the annihilation-type semileptonic 0 → ℓ + ℓ − decays, where ℓ is a charged lepton, and present SM theoretical predictions for their branching fractions based on the Effective Electroweak Hamiltonian approach for the → ℓ + ℓ − transitions. An impact of theoretical models for the -meson distribution amplitudes on these decays is also discussed.
In the Standard Model (SM), the 𝑏 → 𝑠 (𝑑) flavor-changing neutral currents, being loop-induced, are standard experimental channels for testing the SM precisely and searching for possible physics beyond the SM. Pure annihilation 𝐵-meson decays originating by these currents are of significant interest as they are extremely suppressed in the SM and New Physics effects can increase substantially their decay widths. Typical examples of these annihilation processes are radiative and semileptonic decays with 𝜌 0 -, 𝜔-, and 𝜙-production like 𝐵 0 𝑠 → 𝜌 0 (𝜔) 𝛾, 𝐵 0 𝑠 → 𝜌 0 (𝜔) ℓ + ℓ − , 𝐵 0 → 𝜙 𝛾, and 𝐵 0 → 𝜙 ℓ + ℓ − , where ℓ = 𝑒, 𝜇 is the charged lepton. At the beginning of 2022, the LHCb Collaboration presented the upper limit on the 𝐵 0 → 𝜙𝜇 + 𝜇 − decay branching fraction B exp (𝐵 0 → 𝜙𝜇 + 𝜇 − ) < 3.2 × 10 −9 , and it is important to have a precise SM prediction for this decay. Here, we present theoretical predictions for 𝐵 0 → 𝜙 ℓ + ℓ − branching fraction in the lepton-pair invariant mass range 1 GeV 2 < 𝑞 2 < 8 GeV 2 , so far without taking into account 𝜔 − 𝜙 mixing effect. The main goal is to study a dependence of the branching fraction on the choice of the 𝐵-meson distribution amplitude model. Theoretical prediction for the total branching fraction B th (𝐵 0 → 𝜙ℓ + ℓ − ) ∼ 10 −12 , being an order of magnitude estimate, is far below the LHCb experimental limit.
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