The Cabibbo angle anomaly highlights sterile neutrinos

The mixing angle of the quark sectors are constrained by the unitarity in the standard model. However, the recent data indicates the deviation from the unitarity in the mixing angle between 1st and 2nd generation down-type quarks, which is known as the Cabibbo angle anomaly. The observations include the charged-current weak decays of neutron, nuclei, kaon, and the hadronic decay of tau-lepton. Althogh this seems to be an anomaly in the quark sector, the modiffication of the neutrino sector may ameliorate the tension because all the observations involve neutrinos. We find a sterile neutrino of MeV mass is the most reasonable scenario. The current bounds and future prospect of this scenario are also discusssed.