The Search for Heavy Majorana Neutrinos

TL;DR

This paper investigates lepton-number violating processes involving Majorana neutrinos, calculating their rates, comparing with experimental bounds, and exploring detection prospects at colliders like the Tevatron and LHC.

Contribution

It provides the first comprehensive analysis of 36 lepton-number violating decays, deriving bounds on Majorana neutrino parameters and assessing collider detection sensitivities.

Findings

Current experiments set stringent bounds on Majorana neutrino mass and mixing.

Tevatron and LHC can potentially detect resonant Majorana neutrinos in specific mass ranges.

Collider signals, especially in muon modes, could be observable with sufficient luminosity.

Abstract

The Majorana nature of neutrinos can be experimentally verified only via {\it lepton-number} violating processes involving charged leptons. We study 36 lepton-number violating ($\lv$) processes from the decays of tau leptons and pseudoscalar mesons. These decays are absent in the Standard Model but, in presence of Majorana neutrinos in the mass range $\sim 100 \mev$ to $5 \gev$, the rates for these processes would be enhanced due to their resonant contribution. We calculate the transition rates and branching fractions and compare them to the current bounds from direct experimental searches for $\dl=2$ tau and rare meson decays. The experimental non-observation of such $\lv$ processes places stringent bounds on the Majorana neutrino mass and mixing and we summarize the existing limits. We also extend the search to hadron collider experiments. We find that, at the Tevatron with $8 {fb}^{-1}$ integrated luminosity, there could be $2\sigma$ ($5\sigma$) sensitivity for resonant production of a Majorana neutrino in the $\mu^\pm \mu^\pm$ modes in the mass range of $\sim 10 - 180 {\gev} (10 - 120 {\gev})$. This reach can be extended to $\sim 10 - 375 {\gev} (10 - 250 mbox{\gev})$ at the LHC of 14 TeV with $100 {fb}^{-1}$. The production cross section at the LHC of 10 TeV is also presented for comparison. We study the $\mu^\pm e^\pm$ modes as well and find that the signal could be large enough even taking into account the current bound from neutrinoless double-beta decay. The signal from the gauge boson fusion channel $W^+ W^+\to \ell^+_1 \ell^+_2$ at the LHC is found to be very weak given the rather small mixing parameters. We comment on the search strategy when a $\tau$ lepton is involved in the final state.