Reinterpreting the ATLAS bounds on heavy neutral leptons in a realistic neutrino oscillation model

TL;DR

This paper reinterprets ATLAS heavy neutral lepton search results within a realistic neutrino oscillation model, revealing that limits on mixing angles can vary significantly depending on model parameters, thus emphasizing the importance of reinterpretation for accurate constraints.

Contribution

The study demonstrates how experimental limits on heavy neutral leptons can differ in realistic models with multiple HNLs, highlighting the need for reinterpretation of search results.

Findings

Limits can vary by several orders of magnitude in realistic models.

Reinterpretation can weaken exclusion limits by up to 3 orders of magnitude.

Experimental results need detailed context for accurate reinterpretation.

Abstract

Heavy neutral leptons (HNLs) are hypothetical particles, motivated in the first place by their ability to explain neutrino oscillations. Experimental searches for HNLs are typically conducted under the assumption of a single HNL mixing with a single neutrino flavor. However, the resulting exclusion limits may not directly constrain the corresponding mixing angles in realistic HNL models -- those which can explain neutrino oscillations. The reinterpretation of the results of these experimental searches turns out to be a non-trivial task, that requires significant knowledge of the details of the experiment. In this work, we perform a reinterpretation of the latest ATLAS search for HNLs decaying promptly to a tri-lepton final state. We show that in a realistic model with two HNLs, the actual limits can vary by several orders of magnitude depending on the free parameters of the model. Marginalizing over the unknown model parameters leads to an exclusion limit on the total mixing angle which can be up to 3 orders of magnitude weaker than the limits reported in arXiv:1905.09787. This demonstrates that the reinterpretation of results from experimental searches is a necessary step to obtain meaningful limits on realistic models. We detail a few steps that can be taken by experimental collaborations in order to simplify the reuse of their results.