Enhancing the Sensitivity to Seesaw Predictions in Gauged $B-L$ Scenarios

Abstract

New gauge bosons coupled to heavy neutral leptons (HNLs) are simple and well-motivated extensions of the Standard Model. In searches for HNLs in proton fixed-target experiments, we find that a large population of gauge bosons ($Z^{\prime }$) produced by proton bremsstrahlung may decay to HNLs, leading to a significant improvement in existing bounds on the ($m_{HNL},U_{\hat{I}}\pm$), where $U_{\hat{I}}\pm$ represent the mixing between HNL and the active neutrinos with flavor $\hat{I}\pm$. We study this possibility in fixed target experiments with the 8 GeV proton beams, including SBND, MicroBooNE, and ICARUS, as well as DUNE and DarkQuest at 120 GeV. We find the projected sensitivities to additional $Z^{\prime }$-mediated HNL production can bring the seesaw mechanism of the neutrino masses within a broadened experimental reach.

Accepted and Published in PRD. 12 pages, 8 Figures