Loop effects with a vector mediator in coherent neutrino-nucleus scattering

The observation of coherent elastic neutrino-nucleus scattering (CE nu NS) provides us opportunities to explore a wide class of new physics. In the Standard Model (SM), the CE nu NS process arises from the vector and axial-vector neutral currents through the exchange of Z bosons, and the axial-vector current contribution turns out to be subdominant. It is, thus, natural to consider the extra contributions to CE nu NS from more generic new physics beyond the SM with (axial-)vector interactions associated with a new vector mediator Z'. Besides ordinary CE nu NS, the active neutrinos can convert into a new exotic fermion chi through the process nu N -> chi N mediated by Z' without violating coherence. It would be interesting to consider the implication of this conversion for the new fermion sector beyond the SM. We consider the framework of a simplified neutrino model in which a new Dirac fermion chi interacts with active neutrinos and a leptophobic vector mediator Z'. We evaluate both the tree-level and loop-level contributions to CE nu NS, and, in particular, the loop diagrams produce active neutrino elastic scattering process nu N -> nu N with the fermion chi inside the loops. When the interaction between Z' and the SM quarks is vector type and axial-vector type, the CE nu NS processes are, respectively, dominated by the tree-level and loop-level contributions. We investigate the constraints on the model parameters by fitting to the COHERENT data, assuming a wide range of m(chi). The parameter space with m(chi) larger than the maximal energy of incoming neutrinos can be constrained by including the loop-level contribution. More importantly, the inclusion of loop diagrams can place a constraint on axial-vector interaction whose tree-level process is absent in the coherent neutrino-nucleus scattering.