Distinguishing Dirac and Majorana Heavy Neutrinos at Lepton Colliders

The future muon collider has the potential to discover feebly interacting particles in a wide range of masses above the electroweak scale. It is particularly suitable to search for heavy neutral leptons (HNLs), as their production cross section $$ \sigma \sim {m}_W^{-2} $$ σ ∼ m W − 2 is not suppressed by the new physics scale. We demonstrate that with the capacity to observe up to 105 events in the previously unexplored TeV mass range, the muon collider provides the means to measure the fraction of lepton number violating (LNV) processes with precision at the level of a percent. This capability enables elucidating the nature of HNLs, allowing us to differentiate between Majorana, Dirac, and non-minimal scenarios featuring multiple degenerate HNLs. We link the observed fraction of LNV processes to the parameters of the model with three degenerate HNLs, which could be responsible for generating baryon asymmetry in the Universe. Additionally, we present a simple estimate for the number of signal events, as well as analyze the feasibility of vector boson fusion processes in searches for HNLs.

Measuring lepton number violation in heavy neutral lepton decays at the future muon collider

Mikulenko,

Marinichenko

2024

Phenomenology of the simplest linear seesaw mechanism

Batra,

Bharadwaj,

Mandal

et al. 2023

The linear seesaw mechanism provides a simple way to generate neutrino masses. In addition to Standard Model particles, it includes quasi-Dirac leptons as neutrino mass mediators, and a leptophilic scalar doublet seeding small neutrino masses. Here we review its associated physics, including restrictions from theory and phenomenology. The model yields potentially detectable μ → eγ rates as well as distinctive signatures in the production and decay of heavy neutrinos (Ni) and the charged Higgs boson (H±) arising from the second scalar doublet. We have found that production processes such as e+e−→ NN, e−γ → NH− and e+e−→ H+H− followed by the decay chain $$ {H}^{\pm}\to {\ell}_i^{\pm }N $$ H ± → ℓ i ± N , $$ N\to {\ell}_j^{\pm }{W}^{\mp } $$ N → ℓ j ± W ∓ leads to striking lepton number violation signatures at high energies which may probe the Majorana nature of neutrinos.

Heavy neutrino-antineutrino oscillations at the FCC-ee

Antusch,

Hajer,

Oliveira

2023

We discuss the impact of heavy neutrino-antineutrino oscillations ($$ \textrm{N}\overline{\textrm{N}}\textrm{Os} $$ N N ¯ Os ) on heavy neutral lepton (HNL) searches at proposed electron-positron colliders such as the future circular e+e− collider (FCC-ee). During the Z pole run, HNLs can be produced alongside a light neutrino or antineutrino that escapes detection and can decay into a charged lepton or antilepton together with an off-shell W boson. In this case, signals of lepton number violation (LNV) only show up in the final state distributions. We discuss how $$ \textrm{N}\overline{\textrm{N}}\textrm{Os} $$ N N ¯ Os , a typical feature of collider-testable low-scale seesaw models where the heavy neutrinos form pseudo-Dirac pairs, modify such final state distributions. For example, the forward-backward asymmetry (FBA) of the reconstructed heavy (anti)neutrinos develops an oscillatory dependence on the HNL lifetime. We show that these oscillations can be resolvable for long-lived HNLs. We also discuss that when the $$ \textrm{N}\overline{\textrm{N}}\textrm{Os} $$ N N ¯ Os are not resolvable, they can nevertheless significantly modify the theory predictions for FBAs and observables such as the ratio of the total number of HNL decays into ℓ− over ones into ℓ+, in an interval of the angle θ between the HNL and the beam axis. Our results show that $$ \textrm{N}\overline{\textrm{N}}\textrm{Os} $$ N N ¯ Os should be included in collider simulations of HNLs at the FCC-ee.