The project PHOBIDE has studied the decays of B mesons, hadrons made of a bottom quark and a lighter one. In particular it focused on semileptonic decays: processes in which the bottom quark inside the hadron decays into a lighter quark, a lepton and a neutrino (b→clν and b→ulν transition, where the lepton “l” stands for an electron, a muon or a tau). Measurements of semileptonic B decays lie at the core of the physics program of the LHCb and Belle II experiments in the next decade. Thanks to relatively large decay probability and clean experimental signatures, inclusive and exclusive semileptonic B decays offer a clean avenue for the determinations of two input parameters of the the Standard Model (SM) of particle physics: Vcb and Vub. These are elements of the Cabibbo-Kobayashi-Maskawa matrix (CKM) which parameterize the strength of the flavour-changing weak interaction among quarks.
These CKM elements are extracted based on the predictions obtained in the SM. However in recent years certain anomalies like R(D) and R(D*) have appeared in semileptonic decays of B mesons and have been interpreted as possible evidence of New Physics (NP) beyond the SM: indirect hints of non-standard interactions due to new particles exchanged in the process beyond those postulated by the SM.
PHOBIDE has studied inclusive semileptonic decays of B mesons, a type of decays where in the measurements one considers all possible hadronic final-states, independently on their composition. Physicists can make quantitative predictions for inclusive decays thanks to the heavy quark expansion (HQE): a method with which decay rate probabilities are computed as systematic double expansions in the strong coupling constant and inverse powers of the bottom quark mass. Constrains on generic NP effects in b→clν and b→ulν transitions have been derived from the exclusive modes (B→Dlν and B→D*lν). NP effects in inclusive decays have been merely explored, in spite of several precision measurements available from CDF, CLEO, Delphi, BABAR, Belle and Belle II experiments.
The central question of the project PHOBIDE has been to which extent the rich data set of kinematic distributions on inclusive decays can constrain non-standard interactions. To this end, the objectives have been to improve the SM predictions and to establish the theoretical fundations for the search of NP signals in inclusive semileptonic B decays. The project has focused on a comprehensive phenomenological analysis of inclusive decays, including for the first time all data from B factories on differential distributions. It has also studied the complementarity with the other decay modes, to better interpret the obtained constraints in specific NP scenarios, with particular attention to the Vub/Vcb puzzle.
