A Family-nonuniversal U(1)′ model for excited beryllium decays

Abstract

Excited beryllium has been observed to decay into electron-positron pairs with a 6.8 sigma anomaly. The process is properly explained by a 17 MeV proto-phobic vector boson. In present work, we consider a family-nonuniversal U(1)' that is populated by a U(1)' gauge boson Z ' and a scalar field S, charged under U(1)' and singlet under the Standard Model (SM) gauge symmetry. The SM chiral fermion and scalar fields are charged under U(1)' and we provide them to satisfy the anomaly-free conditions. The Cabibbo-Kobayashi-Maskawa (CKM) matrix is reproduced correctly by higher-dimension Yukawa interactions facilitated by S. The vector and axial-vector current couplings of the Z ' boson to the first generation of fermions do satisfy all the bounds from the various experimental data. The Z ' boson can have kinetic mixing with the hypercharge gauge boson and S can directly couple to the SM-like Higgs field. The kinetic mixing of Z ' with the hypercharge gauge boson, as we show by a detailed analysis, generates the observed beryllium anomaly. We find that beryllium anomaly can be properly explained by a MeV-scale sector with a minimal new field content. The minimal model we construct forms a framework in which various anomalous SM decays can be discussed.