The Search for Heavy Majorana Neutrinos

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.

• The paper demonstrates that heavy Majorana neutrinos can enhance lepton-number violating decays, offering a novel test for physics beyond the Standard Model.
• It employs theoretical models, including seesaw mechanisms and left-right symmetric frameworks, to calculate transition rates for 36 rare processes.
• The study correlates simulation results with collider and rare decay data, setting constraints on neutrino mass and mixing parameters for future experiments.

Overview of "The Search for Heavy Majorana Neutrinos"

The paper "The Search for Heavy Majorana Neutrinos" authored by Anupama Atre et al. focuses on exploring the existence and implications of heavy Majorana neutrinos beyond the Standard Model (SM) of particle physics. The primary objective is to investigate how heavy Majorana neutrinos can mediate certain lepton-number violating (LV) processes, which are absent in the SM and can only occur if neutrinos are of the Majorana type.

Theoretical Framework

Despite the SM predicting massless neutrinos, experimental evidence from neutrino oscillation experiments suggests that neutrinos do possess mass. This indicates a need for physics beyond the SM. A promising theoretical framework to incorporate neutrino masses is the seesaw mechanism, which naturally explains the smallness of neutrino masses by introducing heavy right-handed neutrinos. The study examines various theoretical models and extensions of the SM that predict Majorana neutrinos, such as the Left-Right symmetric models, SO(10) grand unification models, and models with additional Higgs representations.

Exploration of Lepton-Number Violating Processes

The key focus of this study is on LV processes that have the potential to confirm the Majorana nature of neutrinos. The authors identify specific LV processes involving tau lepton decays and rare meson decays as test cases. By calculating the transition rates and branching fractions for 36 such LV processes, the paper aims to establish theoretical predictions that can be confronted with experimental data from current and future collider experiments such as the Tevatron and the Large Hadron Collider (LHC).

Results and Implications

1. LV Decays and Experimental Bounds: The study highlights that the presence of heavy Majorana neutrinos could enhance the rates of LV decays through resonant contributions if the mass of the Majorana neutrino is kinematically accessible. By correlating theoretical predictions with existing experimental bounds, the researchers place stringent constraints on the mass and the mixing parameters of heavy neutrinos, particularly exploring the potential for LV processes in tau and meson decays.
2. Collider Searches and Sensitivity: At high-energy colliders, the authors explore possible signals of heavy Majorana neutrinos via like-sign dilepton production channels, which serve as a clean collider signature of LV. The paper discusses the sensitivity reach in terms of the mass and mixing parameters of Majorana neutrinos, highlighting that collider experiments have the capability to probe neutrino masses up to a few hundred GeVs.
3. Future Prospects and Theoretical Significance: The paper emphasizes the importance of directly testing these LV processes to distinguish between Dirac and Majorana neutrino scenarios. Success in observing these processes or improving experimental bounds would significantly deepen our understanding of neutrino properties and might reveal new symmetries of nature that extend beyond the SM.

Conclusion

Overall, the study by Atre et al. provides a comprehensive approach towards understanding the potential impact of heavy Majorana neutrinos through LV processes in both low-energy and collider physics contexts. The research underlines the importance of continued experimental investigation and deeper theoretical scrutiny to validate the existence of Majorana neutrinos, potentially reshaping our comprehension of particle physics and cosmology.