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

TL;DR

This paper explores how new gauge bosons in gauged $B-L$ models can enhance the detection sensitivity of heavy neutral leptons in fixed-target experiments, potentially expanding the experimental reach of the seesaw mechanism.

Contribution

It demonstrates that $Z'$ gauge bosons produced in proton bremsstrahlung can significantly improve bounds on HNL parameters in fixed-target experiments.

Findings

Projected sensitivities for $Z'$-mediated HNL production in various experiments.

Potential to broaden the experimental reach of the neutrino mass seesaw mechanism.

Enhanced detection prospects for heavy neutral leptons via gauge boson decay channels.

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 ($m_{HNL}, U_{\alpha}$), where $U_\alpha$ represent the mixing between HNL and the active neutrinos with flavor $\alpha$. 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.