Flavor-changing phenomenology in a U(1) model

We study family-nonuniversal extensions of the SM, based upon gauge symmetry \(SU(3)_C \otimes SU(2)_L \otimes U(1)_X \otimes U(1)_N\), where the last two factors determine the hypercharge, \(Y=X+N\). We consider the third quark family transforming under the gauge symmetry differently from the first two quark families. The setup naturally induces small neutrino masses through the existence of right-handed neutrinos by anomaly cancellation. The nonuniversality of quark families leads to tree-level quark FCNCs coupled to the new gauge boson \(Z'\) and an extended Higgs sector. The FCNCs significantly modify neutral-meson mixings, rare meson decays, and CP-violating observables, which constrain the new physics at TeV. If the first or second quark family transforms differently from the rest of the quark families (opposite to the above case), the new physics contribution is extremely enhanced, bounding the new physics to be much beyond TeV. Hence, the discrepancy of the third quark family is favored, given that the new physics is at TeV. The approach also suggests a flipped model in which the family nonuniversality is translated to the lepton sector; that is, all quark families transform universally, which suppresses the quark FCNCs, whereas lepton families transform nonuniversally, implying lepton FCNCs. Hence, the FCNCs are also transmuted from the quark sector to the lepton sector. A novel aspect of the flipped model is that lepton flavor violation is coupled to the new \(Z'\) boson and new Higgs bosons at the tree level. Additionally, it provides suitable neutrino masses and dark matter candidates via a scotoseesaw scheme. The new physics in the flipped model is bounded at TeV, similar to the original model.