Type II Seesaw at LHC: the Roadmap (1108.4416v4)

Abstract: In this Letter we revisit the type-II seesaw mechanism based on the addition of a weak triplet scalar to the standard model. We perform a comprehensive study of its phenomenology at the LHC energies, complete with the electroweak precision constraints. We pay special attention to the doubly-charged component, object of collider searches for a long time, and show how the experimental bound on its mass depends crucially on the particle spectrum of the theory. Our study can be used as a roadmap for future complete LHC studies.

• The paper demonstrates that non-degenerate triplet mass splittings can lower Δ++ mass bounds from 250–300 GeV to around 100 GeV under specific conditions.
• It details the collider phenomenology of the type-II seesaw model, focusing on decay modes and lepton number violation observable via same-sign dilepton signatures.
• The analysis connects theoretical predictions with experimental strategies by highlighting impacts on Higgs decay channels and proposing enhanced search techniques at the LHC.

Type II Seesaw at LHC: A Comprehensive Examination

The paper "Type II Seesaw at LHC: the Roadmap" revisits the type-II seesaw model characterized by the incorporation of a weak isospin triplet scalar with hypercharge of $Y=2$ into the Standard Model (SM) framework. The establishment of neutrino mass through the seesaw mechanism is rooted in left-right symmetric theories, which can either be implemented at a low-energy scale or be subsumed within grand unified theories, such as SO(10). The type-II seesaw model postulates that neutrino masses arise from the vacuum expectation value (vev) of the triplet scalar $\Delta^0$.

Detailed Phenomenological Analysis

The analysis presented in the paper is oriented towards understanding the collider phenomenology of the type-II seesaw model at the Large Hadron Collider (LHC) energies, taking into account electroweak precision constraints. The primary focus is on the doubly-charged component, $\Delta^{++}$, due to its significant implications for lepton number violation (LNV), observable through its decay into same-sign charged leptons. Such decays are analogous to neutrinoless double beta decay processes, providing a probe into LNV at high energies. It is shown that the current experimental bounds on the mass of $\Delta^{++}$ are significantly influenced by the triplet's particle spectrum.

Collider Constraints and Spectrum Implications

The CMS collaboration previously placed constraints on mass bounds assuming degenerate triplet spectra. The paper challenges this assumption, conducting a thorough examination of triplet mass splittings and decay modes. It concludes that the lower limit on $\Delta^{++}$ mass of approximately 250$-$300 GeV can diminish to about 100 GeV under specific conditions, specifically with mass splits around 20$-$30 GeV. The triplet mass splittings impact EWPT and collider phenomenology significantly, notably altering the expected experimental constraints.

Key Modes and Model Implications

The decay rates of $\Delta^{++}$ are examined, revealing that its decay into charged leptons probes neutrino masses and mixings. Importantly, the paper illuminates how moderate mass splits between triplet components can lead to cascade decay modes, thereby obscuring straightforward detection channels like same-sign dilepton pairs.

Moreover, the paper pertains to the potential Higgs physics implications within the type-II seesaw. Couplings between the triplet and SM Higgs can alter Higgs decay pathways, influencing current constraints from Higgs searches in the context of non-degenerate spectra.

Future Directions and Recommendations

From the perspective of enhancing experimental analysis at the LHC, the paper proposes several directions:

• Investigate channels involving missing energy for scenarios (case B) where the $\Delta^{++}$ mass constraints are less stringent.
• Optimize searches to distinguish potential displaced vertex signatures in conjunction with $WW$ decay channels.

In conclusion, the paper provides a systematic approach toward understanding the type-II seesaw mechanism's phenomenology at hadron colliders, specifically the LHC, serving as a valuable guide for subsequent investigations into neutrino mass generation mechanisms within the particle physics community. This exploration sets a foundation for extensive research into elucidating the fundamental nature of neutrino masses and their ramifications for theoretical models beyond the Standard Model.