Neutrino Masses and the LHC: Testing Type II Seesaw (0805.3536v2)

Abstract: We demonstrate how to systematically test a well-motivated mechanism for neutrino mass generation (Type-II seesaw) at the LHC, in which a Higgs triplet is introduced. In the optimistic scenarios with a small Higgs triplet vacuum expectation value vd < 10^{-4} GeV, one can look for clean signals of lepton number violation in the decays of doubly charged and singly charged Higgs bosons to distinguish the Normal Hierarchy (NH), the Inverted Hierarchy (IH) and the Quasi-Degenerate (QD) spectrum for the light neutrino masses. The observation of either H+ --> tau+ nubar or H+ --> e+ nubar will be particularly robust for the spectrum test since they are independent of the unknown Majorana phases. The H++ decays moderately depend on a Majorana phase Phi2 in the NH, but sensitively depend on Phi1 in the IH. In a less favorable scenario vd > 2 10^{-4} GeV, when the leptonic channels are suppressed, one needs to observe the decays H+ --> W+ H_1 and H+ --> t bbar to confirm the triplet-doublet mixing which in turn implies the existence of the same gauge-invariant interaction between the lepton doublet and the Higgs triplet responsible for the neutrino mass generation. In the most optimistic situation, vd approx 10^{-4} GeV, both channels of the lepton pairs and gauge boson pairs may be available simultaneously. The determination of their relative branching fractions would give a measurement for the value of vd.

• The paper introduces a testable Type II seesaw model that extends the Standard Model via an SU(2)L Higgs triplet to generate neutrino masses observable through charged Higgs decays.
• The paper details specific decay channels of doubly and singly charged Higgs bosons that differentiate between normal, inverted, and quasi-degenerate neutrino mass hierarchies.
• The paper incorporates neutrino oscillation data and existing collider limits while anticipating future LHC experiments to validate lepton number violation signatures.

Overview of "NEUTRINO MASSES AND THE LHC: TESTING TYPE II SEESAW"

This paper by Pavel Fileviez Pérez, Tao Han, Guiyu Huang, Tong Li, and Kai Wang introduces a compelling approach to understanding neutrino mass generation, with an emphasis on the Type II seesaw mechanism and its testability at the Large Hadron Collider (LHC). The authors extend the Standard Model's Higgs sector by incorporating an SU(2)$_L$ Higgs triplet, enabling the paper of neutrino masses through observable phenomena at energy scales accessible by modern colliders.

Key Elements of the Type II Seesaw Mechanism

In the Type II seesaw scenario, the SU(2)$_L$ Higgs triplet's vacuum expectation value plays a crucial role in the neutrino mass generation. This mechanism provides a framework to explore new physics by predicting the observable lepton number violation at the LHC. With a small triplet vacuum expectation value below $10^{-4}$ GeV, the paper highlights the potential for detecting lepton number violation via decays of doubly charged $H^{\pm\pm}$ and singly charged $H^{\pm}$ Higgs bosons.

Predictions and Observables at the LHC

The research delineates several scenarios that vary based on the triplet vacuum expectation value ($v_{\Delta}$), outlining clear methods to differentiate neutrino mass hierarchies:

• Normal Hierarchy (NH): Characterized by dominant $H^{++} \rightarrow \mu^+ \mu^+, \tau^+ \tau^+$ decay channels, given the neutrino oscillation parameters.
• Inverted Hierarchy (IH): Exhibits dominantly $H^{++} \rightarrow e^+ e^+$, with specific sensitivity to Majorana phases, especially $\Phi_1$.
• Quasi-Degenerate (QD) Spectrum: Exhibits nearly uniform branching fractions among different flavor combinations.

The paper introduces methodologies for interpreting the branching fractions of these Higgs decays to correlate with the neutrino mass parameters. It emphasizes the robustness of certain decay channels, such as $H^{+} \rightarrow \tau^+ \bar{\nu}$, which remain unaffected by unknown Majorana phases, reinforcing their role as pivotal indicators of the neutrino mass pattern.

Constraints and Implications

Fileviez Pérez et al. address experimental constraints from neutrino oscillation data, rare decay processes, and existing collider results, ensuring the viability of their model within these parameters. The authors highlight the necessity of further investigating the mass spectra by systematic experimental observation at the LHC, which would provide insights into distinguishing the different seesaw hierarchies.

For larger triplet vacuum values ($v_{\Delta} > 10^{-4}$ GeV), where leptonic decay modes are suppressed, the paper discusses relying on the observation of decays leading to gauge boson pairs (e.g., $H^{+} \rightarrow W^+ H_1, W^+ Z$) and the triplet-doublet mixing parameter ($\mu$) to imply the neutrino mass mechanism without directly observing lepton number violation.

Future Prospects and Conclusions

The paper presents a comprehensive theoretical approach to test the Type II seesaw mechanism at the LHC, predicting an intricate set of observables that can paint a detailed picture of the neutrino sector. The research underscores the potential for significant progress in neutrino physics and associated new physics beyond the Standard Model, contingent upon future collider experiments.

Aligning with the presented methodology, future developments at the LHC or other high-energy physics facilities might offer vital empirical evidence supporting this framework, bridging the gap between theoretical predictions and experimental confirmations in the domain of neutrino mass generation.