Lepton Number Violation: Seesaw Models and Their Collider Tests (1711.02180v2)

Abstract: The Majorana nature of neutrinos is strongly motivated from the theoretical and phenomenological point of view. A plethora of neutrino mass models, known collectively as Seesaw models, exist that could generate both a viable neutrino mass spectrum and mixing pattern. They can also lead to rich, new phenomenology, including lepton number non-conservation as well as new particles, that may be observable at collider experiments. It is therefore vital to search for such new phenomena and the mass scale associated with neutrino mass generation at high energy colliders. In this review, we consider a number of representative Seesaw scenarios as phenomenological benchmarks, including the characteristic Type I, II, and III Seesaw mechanisms, their extensions and hybridizations, as well as radiative constructions. We present new and updated predictions for analyses featuring lepton number violation and expected coverage in the theory parameter space at current and future colliders. We emphasize new production and decay channels, their phenomenological relevance and treatment across different facilities in $e^+e^-$, $e^-p$ and $pp$ collisions, as well as the available Monte Carlo tools available for studying Seesaw partners in collider environments.

• The paper demonstrates that Seesaw models generate small neutrino masses and predict observable lepton number violation with sensitivity reaching |VₗN|² ~ 10⁻³ at colliders.
• It analyzes Type I, II, and III mechanisms by detailing right-handed neutrino mixings and scalar triplet constraints, with mass limits above 800 GeV established by current data.
• The study highlights future collider prospects, urging enhanced detection and statistical methods to further probe neutrino mass generation and lepton number violation.

Overview of "Lepton Number Violation: Seesaw Models and Their Collider Tests"

The paper "Lepton Number Violation: Seesaw Models and Their Collider Tests" provides a detailed review of neutrino mass generation models and their implications for collider experiments. It focuses on the Majorana nature of neutrinos, driven by the theoretical and phenomenological motivations underlying the Seesaw mechanisms. These mechanisms are crucial for explaining small neutrino masses and offer potential signals for lepton number violation detectable in high-energy colliders.

Two significant aspects of the paper are the notion that neutrino masses necessarily imply lepton number violation and the exploration of Seesaw models that predict distinct particle signatures at current and future colliders. The Seesaw framework encompasses three primary mechanisms, Type I, Type II, and Type III, each introducing new particles and interactions capable of mediating lepton number violating processes.

Key Insights and Numerical Results

1. Type I Seesaw and Extensions:
   • Type I Seesaw involves right-handed neutrinos that mix with the Standard Model left-handed neutrinos, leading to the possibility of observable heavy Majorana neutrinos in collider experiments.
   • The analysis details parameter space constraints and expected collider signatures, with particular focus on $pp$ and $ep$ colliders. Constraints are placed on mixing parameters, with a sensitivity reach extending to $\vert V_{\ell N}\vert^2 \sim 10^{-3}$ for neutrino masses around 500 GeV at the LHC.
2. Type II Seesaw:
   • This mechanism introduces scalar triplets that mediate lepton number violation via doubly charged Higgs bosons.
   • Current collider searches have constrained the masses of these triplets to be above 800 GeV. Branching fraction predictions for decays to charged leptons help distinguish neutrino mass hierarchies.
3. Type III Seesaw:
   • Characterized by triplet fermions that couple to the electroweak sector, allowing for potentially large production cross sections at hadron colliders.
   • These fermions can be produced via Drell-Yan processes, with exclusion limits from LHC data extending to masses of about 800 GeV.
4. Radiative Neutrino Mass Models:
   • Discusses the Zee-Babu and related models where neutrino masses are generated via loop-level processes.
   • These models predict unique signatures, such as the pair production of doubly charged scalars, offering distinct channels to explore at the LHC, though challenging LFV constraints push the predicted masses well above 1 TeV.

Theoretical and Practical Implications

From a theoretical perspective, the paper underscores the robustness of Seesaw models in extending the Standard Model to incorporate small neutrino masses. Practically, it suggests that ongoing and future collider experiments are positioned to probe these models further, potentially distinguishing between different neutrino mass generation scenarios.

The paper discusses the complementarity of collider experiments with neutrino oscillation data, emphasizing the potential for lepton number violation to confirm the Majorana nature of neutrinos. This synthesis of phenomenology forms the basis for a rich experimental program at the LHC and beyond, including prospective 100 TeV colliders.

Future Directions

Future developments hinge on refining experimental techniques to probe higher mass scales and smaller mixing angles. The paper suggests that advancements in detector technology and statistical methods will enhance sensitivity, potentially revealing the intricate mechanisms responsible for neutrino mass generation. Additionally, addressing challenges in identifying new channels and reducing theoretical uncertainties in predictions remains critical for maximizing the discovery potential of future experiments.

In conclusion, this paper makes significant strides in consolidating theoretical frameworks with experimental strategies, offering valuable guidance for the future of neutrino physics research.