Testing tree level TeV scale seesaw scenarios in $\mu$TRISTAN

TL;DR

This paper explores TeV-scale seesaw mechanisms at future muon colliders, analyzing heavy neutrino and scalar boson production to improve detection prospects and constrain neutrino mass models.

Contribution

It provides new estimates of heavy neutrino mixing limits and explores the potential of muon colliders to probe neutrino mass ordering and triplet fermion production.

Findings

Heavy neutrino mixing angles can be constrained two orders of magnitude better than current data.

Neutrino mass ordering can be probed at 5σ significance via $e^+ e^+$ final states.

Discovery potential for triplet fermions at 1 TeV with projected luminosity.

Abstract

We investigate TeV scale seesaw scenarios at $\mu^+ e^-$ and $\mu^+ \mu^+$ colliders in the $\mu$TRISTAN experiment. In minimal type-I seesaw scenario we consider two generations of Standard Model (SM) singlet heavy Majorana type Right Handed Neutrinos (RHNs) which couples with SM gauge bosons through light-heavy neutrino mixing. We discuss the prospects of probing heavy neutrinos via the processes such as $\mu^+e^-\to \nu N_i\to e^+ j j\nu$ or $\mu^- j j\nu$ for $\sqrt{s}=346$~GeV and $1\text{ ab}^{-1}$ luminosity. Studying these process, we estimate limits on the light-heavy neutrino mixing angles as a function of heavy neutrino mass, which could be two orders of magnitude stronger than electroweak precision data. Further, we study the effect of doubly charged scalar boson $(H^{++})$ from the type-II seesaw scenario in $\mu^+ \mu^+$ collision at $\sqrt{s}=2$ TeV. In this case we consider $\mu^+ \mu^+ \to \ell_i^+ \ell_j^+$ and $\mu^+ \mu^+ \to H^{++} Z/ \gamma$ processes followed by the same sign dilepton decay of $H^{++}$. We find that events involving $e^+ e^+$ among these final states can probe the neutrino mass ordering in $\mu$TRISTAN experiment at 5$\sigma$ significance. In addition to that we study the production of positively charged triplet fermion in $\mu$TRISTAN following $\mu^+ \mu^+ \to \mu^+ \Sigma^+$ process where $\Sigma^+$ decays into $\mu^+ jj$ mode through $Z$ boson exchange. Considering a triplet at 1 TeV and studying SM backgrounds we estimate the discovery potential of $\mu^+ \mu^+ jj$ signal at $\mu$TRISTAN with respect to projected luminosity.