Phenomenology of GeV-scale Heavy Neutral Leptons

Abstract: We review and revise phenomenology of the GeV-scale heavy neutral leptons (HNLs). We extend the previous analyses by including more channels of HNLs production and decay and provide with more refined treatment, including QCD corrections for the HNLs of masses $\mathcal{O}(1)$ GeV. We summarize the relevance of individual production and decay channels for different masses, resolving a few discrepancies in the literature. Our final results are directly suitable for sensitivity studies of particle physics experiments (ranging from proton beam-dump to the LHC) aiming at searches for heavy neutral leptons.

Overview of "Phenomenology of GeV-scale Heavy Neutral Leptons"

This essay presents an expert analysis of the paper "Phenomenology of GeV-scale Heavy Neutral Leptons," which revises the current understanding of GeV-scale Heavy Neutral Leptons (HNLs), focusing on their production and decay mechanisms. The paper offers an extensive examination of HNLs, particularly relevant to Beyond Standard Model (BSM) phenomena such as neutrino oscillation, baryon asymmetry, and dark matter.

Key Insights into HNL Production and Decay Mechanisms

The authors provide a detailed description of various HNL production channels, emphasizing secondary decays from mesons produced in initial collision processes. The role of charmed and beauty mesons as the leading sources of HNL production is notably highlighted for mass ranges above kaon thresholds. The paper resolves discrepancies in meson decay width computations by revisiting form factors and incorporating recent lattice calculations, ensuring consistency across studies.

Regarding decay processes, HNLs predominantly undergo semileptonic decays mediated by charged-current interactions. The decay properties are sensitive to mixing angles and mass scales, with significant branching into mesons such as $D$, $B$, and $\tau$ leptons, depending on the HNL's mass. The paper explores multi-meson decay channels, addressing the importance of hadronic contributions at certain mass thresholds.

Numerical Results and Predictions

The paper provides a comprehensive list of decay channels relevant for cosmological models and particle physics experiments. For instance, the branching ratio for HNL decays into $\nu_{\alpha} \nu_{\beta} \bar{\nu}_{\beta}$ reaches 100%, showing its dominance at lower mass ranges. Production probabilities are explicitly calculated for fixed-target experiments, leading to predictions about event rates that will be pivotal in experimental scenarios such as SHiP and MATHUSLA.

Implications and Future Outlook

The paper's results are pivotal for sensitivity studies in current and forthcoming particle physics experiments aiming to detect GeV-scale HNLs. The refined theoretical framework supports experimental designs to search for these elusive particles, enhancing our understanding of their role in resolving BSM phenomena. Future research should explore improved QCD corrections for HNL decays and explore novel experimental signatures of heavier HNLs, especially above the GeV scale.

The authors not only reconcile previous discrepancies but also open a path for clearer experimental strategies by accurately modeling the production and decay dynamics of HNLs. Their findings bridge the gap between theoretical predictions and experimental implementations, setting a higher standard for studies focused on sterile neutrinos and related BSM research.

Conclusion

Overall, "Phenomenology of GeV-scale Heavy Neutral Leptons" provides a meticulously detailed revision of HNL characteristics, offering essential insights for researchers focusing on the phenomenology of sterile neutrinos. The paper integrates theoretical advancements with practical implications, ensuring its relevance for the development of future experimental initiatives targeting BSM physics.