Loop effect with vector mediator in the coherent neutrino-nucleus scattering

Abstract

The active neutrinos can convert into a new exotic fermion χ through coherent neutrino-nucleus scattering νN → χN and thus the ν − χ interaction leads to novel recoil spectrum in the neutrino scattering experiments. We study the general neutrino interactions by evaluating both the tree-level and loop-level contributions to the coherent elastic neutrino-nucleus scattering (CEνNS). The loop diagrams produce active neutrino elastic scattering process νN → νN with the fermion χ inside the loops. The neutrino interaction is illustrated by the framework of a simplified neutrino model in which a new Dirac fermion χ interacts with active neutrinos and a leptophobic vector mediator Z . For the Z q q coupling being vector type and axialvector type, the CEνNS processes are dominated by the tree-level and loop-level contribution, respectively.We investigate the constraints on the couplings between Z and the new particle χ or the Standard Model quarks by fitting to the COHERENT data. The parameter space with m χ larger than the maximal energy of incoming neutrinos can also be constrained by including the loop-level contribution. More importantly, the inclusion of loop diagrams can place constraint on axial-vector interaction whose the tree-level process is absent in the coherent neutrino-nucleus scattering.