Requirements on common solutions to the LSND and MiniBooNE excesses: a post-MicroBooNE study

Abstract

The strong statistical significance of an observed electron-like event excess in the MiniBooNE (MB) experiment, along with an earlier similar excess seen in the Liquid Scintillator Neutrino Detector (LSND), when interpreted in conjunction with recent MicroBooNE results may have brought us to the cusp of new physics discoveries. This has led to many attempts to understand these observations, both for each experiment individually and in conjunction, via physics beyond the Standard Model (SM). We provide an overview of the current situation, and discuss three major categories under which the many proposals for new physics fall. The possibility that the same new, non-oscillation physics explains both anomalies leads to new restrictions and requirements. An important class of such common solutions, which we focus on in this work, consists of a heavy O(MeV−sub-GeV) sterile neutral fermion produced in the detectors, (via up-scattering of the incoming muon neutrinos), and subsequently decaying to photons or e + e − pairs which mimic the observed signals. Such solutions are subject to strong demands from a) cross section requirements which would yield a sufficient number of total events in both LSND and MB, b) requirements imposed by the measured energy and angular distributions in both experiments and finally, c) consistency and compatibility of the new physics model and its particle content with other bounds from a diverse swathe of particle physics experiments. We find that these criteria often pull proposed solutions in different directions, and stringently limit the viable set of proposals which could resolve both anomalies. Our conclusions are relevant for both the general search for new physics and for the ongoing observations and analyses of the MicroBooNE experiment.

White Paper on Light Sterile Neutrino Searches and Related Phenomenology

“…Overall, as detailed in [234], energy distributions in LSND and MB, the angular distribution in MB, when combined with the stringent constraints on light singlet scalars, all suggest the use of a scalar doublet, with one light and one moderately heavy partner. We find this leads to a degree of angular isotropy while allowing a large number of events in the forward direction, consistent with observations.…”

“…We compare how scalar and vector mediators perform in helping to achieve a simultaneous understanding of both anomalies. Our treatment is necessarily brief, and for details and a full set of references on all that follows the reader is referred to [234].…”

“…The excess in MB is distributed over all directions but is moderately forward. The cross section responsible for the production of N 2 as a function of the cosine of the angle between the momentum direction of N 2 and the beam direction has been studied in a bin-wise manner in [234] for both the coherent and incoherent contributions. It was found that when m Z /H = 50 MeV, almost all the produced N 2 are in the most forward bin for both mediators.…”

“…From this, at first it appears that using a single scalar mediator and adjusting its mass to an intermediate value, as well as the mass of N 2 will allow us to find a common solution to the two anomalies as well as match the angular distribution in MB. However, further examination based on considerations related to the energy distributions in LSND and MB (for details, see [234]) reveals that this is not the case if good fits to both anomalies are desired.…”

Neutrinos from Stored Muons (nuSTORM)

“…Besides, some of the proposed explanations of the MiniBooNE anomaly involve the production of a heavy (1-100 MeV) neutrino via electromagnetic (γ mediator), weak (Z) or BSM (Z ) interactions, leading to a signal in the single photon or e + e − channels (see reference [42] for a recent review). While recent MicroBooNE results disfavour some explanations of the MiniBooNE anomaly, the full range of possible solutions is still unexplored [43].…”