[Paper Review] Experimental Constraints on Higgs Boson Decays to TeV-scale Right-Handed Neutrinos

Michael L. Graesser|arXiv (Cornell University)|May 15, 2007

Particle physics theoretical and experimental studies33 references23 citations

TL;DR

This paper investigates the Higgs boson's decay into TeV-scale right-handed neutrinos via higher-dimensional operators, leading to cascade decays into six final-state particles with two displaced vertices, each violating lepton number. Experimental constraints from the Tevatron suggest that short decay lengths (≤5 cm) and branching fractions of order 1 are already excluded, while longer decay lengths remain unconstrained but within current detection reach.

ABSTRACT

The existence of neutrino masses strongly suggests that right-handed neutrinos exist, but the data do not favor any particular scale for the Majorana mass parameters. Here I explore the possibility that these particles exist at the electroweak scale along with additional new physics at the TeV scale . Higher dimension operators involving right-handed neutrinos and the Higgs boson can introduce new decay modes of the Higgs boson, significantly modifying its phenomenology if it is light. With minimal flavor violation the Higgs boson cascade decays to 6 particles containing two highly displaced vertices. Each displaced vertex produces an odd number of leptons, leading to a dramatic signature of overall lepton violation at each vertex. I discuss the limits from the Tevatron, and find that they are close to exploring interesting regions of parameters, while limiting others. Moving beyond minimal flavor violation, cascade decays of the Higgs boson into as many as 14 particles can occur.

Motivation & Objective

• To investigate the phenomenology of Higgs boson decays into right-handed neutrinos when higher-dimensional operators are induced by new physics at the TeV scale.
• To assess the detectability of such decays at the Tevatron, particularly through displaced vertices and lepton number violation.
• To derive model-independent branching fractions for right-handed neutrino decays into various final states, including leptons and quarks.
• To evaluate the reach of existing Tevatron searches in constraining the parameter space of this scenario.
• To identify regions of parameter space—especially long decay lengths—still accessible to current and future collider experiments.

Proposed method

• Uses effective field theory to model higher-dimensional operators involving the Higgs boson and right-handed neutrinos, induced by new physics at the TeV scale.
• Derives model-independent branching fractions for right-handed neutrino decays into lepton pairs, quark pairs, and neutrino final states using Fermi's constant and couplings.
• Applies the Fierz transformation and computes amplitudes for charged and neutral current decays, including quantum interference effects.
• Evaluates the decay width of right-handed neutrinos into final states involving charged leptons, neutrinos, and quarks, using the formula $ \Gamma = \frac{G_F^2 M^5}{192\pi^3} |D_{JI}|^2 \times \text{kinematic factors} $.
• Considers the impact of finite W and Z boson widths on decay rates, especially in the on-shell region.
• Analyzes the kinematic reach of the Tevatron, focusing on signatures with two displaced vertices and high lepton multiplicity.

Experimental results

Research questions

• RQ1 What are the dominant decay modes of the Higgs boson into right-handed neutrinos when higher-dimensional operators are present at the TeV scale?
• RQ2 How do displaced vertices from long-lived right-handed neutrinos produce signatures of lepton number violation in Higgs cascade decays?
• RQ3 What are the model-independent branching fractions for right-handed neutrino decays into various final states, including leptons and quarks?
• RQ4 To what extent do existing Tevatron searches constrain the parameter space of Higgs decays into TeV-scale right-handed neutrinos?
• RQ5 Which regions of the parameter space—particularly those with long decay lengths—remain experimentally accessible?

Key findings

• Tevatron searches already constrain Higgs boson decays into right-handed neutrinos with average decay lengths of 2–5 cm and branching fractions of order 1, suggesting such decays are likely subdominant.
• Decay lengths greater than 10 cm are not yet constrained by Tevatron data, but production rates in this region are close to the current detection threshold.
• The Higgs boson can cascade decay into six final-state particles via two displaced vertices, each producing an odd number of leptons, leading to overall lepton number violation.
• The decay width of right-handed neutrinos into charged lepton pairs and neutrinos is enhanced by quantum interference between charged and neutral current amplitudes, increasing the rate by approximately 10% at 30 GeV and 50% at 50 GeV.
• The total decay rate for $ N_I \to l_J^- l_J^+ \nu_J $ is proportional to 0.59 in the low-energy limit, accounting for interference, which is significantly less than the incoherent sum of 1.07.
• Right-handed neutrino decays into same-flavor leptons are suppressed due to interference, while decays into different-flavor leptons and quarks are governed by the combination $ (g_L^{(K)})^2 + (g_R^{(K)})^2 $, affecting branching fraction distributions.