Prospects of Heavy Neutrino Searches at Future Lepton Colliders

TL;DR

This paper evaluates the potential of future lepton colliders to detect heavy neutrinos, exploring different production channels and analysis techniques to improve sensitivity beyond current experimental limits.

Contribution

It introduces new search channels and analysis methods for heavy neutrinos at future lepton colliders, including the use of multivariate techniques to enhance detection prospects.

Findings

Probing $|V_{eN}|^2$ down to ~10^{-4} for heavy neutrino masses up to 400 GeV.

Multivariate analysis improves signal significance by up to an order of magnitude.

Future colliders can outperform current limits from electroweak precision tests and LHC projections.

Abstract

We discuss the future prospects of heavy neutrino searches at next generation lepton colliders. In particular, we focus on the planned electron-positron colliders, operating in two different beam modes, namely, $e^+e^-$ and $e^-e^-$. In the $e^+e^-$ beam mode, we consider various production and decay modes of the heavy neutrino ($N$), and find that the final state with $e+2j+{E\!\!\!/}_T$, arising from the $e^+e^-\to N\nu$ production mode, is the most promising channel. However, since this mode is insensitive to the Majorana nature of the heavy neutrinos, we also study a new production channel $e^+e^-\to Ne^\pm W^\mp$, which leads to a same-sign dilepton plus four jet final state, thus directly probing the lepton number violation in $e^+e^-$ colliders. In the $e^-e^-$ beam mode, we study the prospects of the lepton number violating process of $e^-e^-\to W^-W^-$, mediated by a heavy Majorana neutrino. We use both cut-based and multivariate analysis techniques to make a realistic calculation of the relevant signal and background events, including detector effects for a generic linear collider detector. We find that with the cut-based analysis, the light-heavy neutrino mixing parameter $|V_{eN}|^2$ can be probed down to $\sim 10^{-4}$ at 95\% C.L. for the heavy neutrino mass up to $400$ GeV or so at $\sqrt s=500$ GeV with $100 \rm{fb}^{-1}$ of integrated luminosity. For smaller mixing values, we show that a multivariate analysis can improve the signal significance by up to an order of magnitude. These limits will be at least an order of magnitude better than the current best limits from electroweak precision data, as well as the projected limits from $\sqrt s=14$ TeV LHC.