[Paper Review] GeV Seesaw, Accidentally Small Neutrino Masses, and Higgs Decays to Neutrinos

André de Gouvêa|ArXiv.org|Jun 12, 2007

Particle physics theoretical and experimental studies17 citations

TL;DR

This paper proposes that neutrino masses could be 'accidentally small' due to cancellations in the seesaw mechanism, allowing GeV-scale right-handed neutrinos to mix significantly with active neutrinos—up to several percent—despite small active neutrino masses. If so, the Higgs boson (below 130 GeV) could decay dominantly into an active and a sterile neutrino, leading to final states with a charged lepton, jets, and missing energy, potentially observable at the LHC.

ABSTRACT

If the Standard Model particle content is extended by gauge singlet fermions (right-handed neutrinos), active neutrinos generically acquire (Majorana) masses, in agreement with our current understanding of the lepton sector. If the right-handed neutrino masses are of the same order as the electroweak symmetry breaking scale, it is usually expected that these will not mediate any experimentally observable effects. Here, I explore the fact that this is not necessarily the case. If neutrino masses are "accidentally small", active-sterile mixing angles can, according to current experimental bounds, be as large as several percent. If this is the case, I argue that the dominant decay mode of light (M_H less than 130 GeV) Higgs bosons could be into an active and a sterile neutrino. The sterile neutrino decays promptly into a charge lepton and an on- or off-shell W-boson, so that the dominant Higgs boson decay mode is H to leptons + jets + missing transverse energy.

Motivation & Objective

• To explore the phenomenological consequences of a seesaw mechanism where neutrino masses are accidentally small due to cancellations in the Yukawa and Majorana mass terms.
• To investigate whether right-handed neutrinos with masses near the electroweak scale (GeV range) can lead to observable effects despite the conventional expectation that such states are invisible.
• To assess the viability of large active–sterile mixing angles (up to several percent) in the context of current experimental constraints on neutrino masses and lepton flavor violation.
• To examine the possibility that Higgs bosons below 130 GeV decay predominantly into active-sterile neutrino pairs, leading to distinctive collider signatures with missing energy and charged leptons.
• To evaluate the implications for Higgs physics and lepton number violation at the Tevatron and LHC if the seesaw scale is at the weak scale and neutrino masses are accidentally small.

Proposed method

• Uses the type-I seesaw mechanism with a general neutrino mass matrix: $ m_{\nu} = \begin{pmatrix} 0 & yv \\ (yv)^T & M \end{pmatrix} $, where $ y $ are Yukawa couplings, $ v = 246 $ GeV, and $ M $ is the Majorana mass matrix of right-handed neutrinos.
• Applies the seesaw approximation assuming $ yvM^{-1} \ll 1 $, leading to light neutrino masses $ m_a \sim y^2 v^2 / M $, and active–sterile mixing angles $ \Theta \sim yv / M $.
• Identifies conditions under which $ m_a $ is accidentally small (due to cancellations in $ yvM^{-1} $) while $ \Theta $ remains large (up to ~3%), violating the naive $ \Theta \sim \sqrt{m_a / M} $ scaling.
• Evaluates experimental bounds on active–sterile mixing from neutrinoless double-beta decay, lepton flavor violation, and LEP searches for neutral heavy leptons.
• Calculates the branching ratio for $ H \to \ell + \text{jets} + E_T^\text{miss} $ via $ H \to \ell N \to \ell W^* \ell \ell $, showing it can exceed the SM $ H \to b\bar{b} $ rate for $ M_H \lesssim 130 $ GeV.
• Considers the role of flavor symmetries and horizontal symmetries as possible dynamical explanations for the accidental smallness of neutrino masses.

Experimental results

Research questions

• RQ1 Can neutrino masses be accidentally small even when the right-handed neutrino masses are at the GeV scale, leading to large active–sterile mixing?
• RQ2 What are the phenomenological constraints on active–sterile mixing angles for GeV-scale right-handed neutrinos, and can they reach values of several percent?
• RQ3 Under what conditions can the Higgs boson decay dominantly into an active and a sterile neutrino, and what would be the resulting collider signature?
• RQ4 Could such a scenario lead to a branching ratio for $ H \to \ell + \text{jets} + E_T^\text{miss} $ that exceeds the SM $ H \to b\bar{b} $ rate for light Higgs bosons?
• RQ5 How do constraints from lepton flavor violation, neutrinoless double-beta decay, and LEP searches limit the viability of this model?

Key findings

• Accidentally small neutrino masses can arise when cancellations occur in the $ yvM^{-1} $ term, allowing large active–sterile mixing angles ($ \Theta \sim 3\% $) even for $ m_a \sim 0.1 $ eV.
• For $ M_{ij} \gtrsim 1 $ GeV, current experimental bounds allow active–sterile mixing angles up to several percent, depending on the specific model parameters.
• If active–sterile mixing is large and right-handed neutrinos are at the GeV scale, the Higgs boson decay width into $ \nu_a N_i $ can be comparable to or larger than the SM $ H \to b\bar{b} $ decay width for $ M_H \lesssim 130 $ GeV.
• The dominant Higgs decay mode would then be $ H \to \ell + \text{jets} + E_T^\text{miss} $, where the final state includes a charged lepton from the decay $ N_i \to \ell W^{(*)} $, making it a distinctive LHC signature.
• Such a scenario leads to significant lepton number violation and could be probed at the Tevatron and LHC through rare decays and missing energy signatures.
• The model remains viable if the accidental smallness of neutrino masses is protected by a hidden horizontal symmetry, which suppresses large contributions to the neutrino mass matrix while allowing large mixing angles.