Exploring Heavy Neutral Leptons with Transition Magnetic Moments at Neutrino Experiments

Heavy neutral leptons (HNLs) are additional electroweak-singlet fermions with masses significantly above the mass scale of the Standard Model neutrinos. They show up frequently in extensions of the Standard Model, including the seesaw mechanism—an elegant solution to the small observed masses of SM neutrinos. HNLs with masses in the MeV to GeV regime are the subject of many current and planned experimental searches. In this talk, we focus on HNLs with a transition magnetic moment coupling to active neutrinos, hereafter referred to as "neutrissimos". In this talk, we explore an explanation of the $4.8sigma$ MiniBooNE anomaly based on neutrissimo decay to a single photon. Specifically, we evaluate the consistency of such a model with the energy and angular distributions of the MiniBooNE excess. We also derive world-leading constraints on neutrissimos using MINERvA elastic scattering data. The implications of this result with regard to the neutrissimo-based explanation of the MiniBooNE anomaly are discussed. Finally, we examine the sensitivity of upcoming neutrino experiments to neutrissimos, with a particular emphasis on the Coherent CAPTAIN-Mills (CCM) experiment at the Los Alamos Neutron Science Center (LANSCE). CCM uses a light-based liquid argon detector to search for the interactions of neutrinos and potential dark sector particles produced when 800 MeV protons from the LANSCE accelerator collide with the tungsten target at the Lujan facility. Neutrissimos can also be produced in large abundance in such an environment—thus, CCM will be able to set world-leading constraints on neutrissimos using data collected from its ongoing three-year run.