[Paper Review] Probing the Type I Seesaw Mechanism with Displaced Vertices at the LHC

A. M. Gago, Pilar Hernández+3 more|arXiv (Cornell University)|May 21, 2015

Particle physics theoretical and experimental studies1 references4 citations

TL;DR

This paper proposes a search for Higgs boson decays into heavy neutrinos within the Type I seesaw mechanism at the LHC, focusing on displaced vertices in the few-GeV mass range. Using 300 fb⁻¹ of 13 TeV data, it demonstrates that over 10 signal events remain after pseudorapidity cuts, and identifies kinematic distributions to optimize dedicated triggers despite limitations from conventional triggers.

ABSTRACT

The observation of Higgs decays into heavy neutrinos would be strong evidence for new physics associated to neutrino masses. In this work we propose a search for such decays within the Type I seesaw model in the few-GeV mass range via displaced vertices. Using 300 fb$^{-1}$ of integrated luminosity, at 13 TeV, we explore the region of parameter space where such decays are measurable. We show that, after imposing pseudorapidity cuts, there still exists a region where the number of events is larger than $\mathcal{O}(10)$. We also find that conventional triggers can greatly limit the sensitivity of our signal, so we display several relevant kinematical distributions which might aid in the optimization of a dedicated trigger selection.

Motivation & Objective

• To explore the viability of detecting Higgs decays into heavy neutrinos via displaced vertices in the Type I seesaw model at the LHC.
• To identify the region of parameter space in the few-GeV heavy neutrino mass range where such decays are measurable with current LHC luminosity.
• To assess the impact of conventional triggers on signal sensitivity and propose kinematic distributions for optimized trigger selection.
• To complement existing W-boson decay searches by probing a distinct combination of seesaw parameters through Higgs decays.

Proposed method

• Utilizes a minimal 3+2 neutrino model with an approximate U(1)ₗ lepton number symmetry to parametrize the seesaw mechanism.
• Applies constraints from neutrino masses, direct searches, neutrinoless double beta decay, and lepton flavor violation to restrict viable parameter space.
• Calculates Higgs branching ratios into heavy neutrinos using the parametrization of the mixing matrix and the effective neutrino mass matrix.
• Simulates Higgs production via gluon fusion at the LHC and models displaced vertex signatures from the decay chain h → νN followed by N → ℓqq′ or ℓ⁺ℓ⁻ν.
• Implements pseudorapidity cuts and evaluates signal yields using differential decay rates in the lab frame, derived from relativistic kinematics.
• Analyzes kinematic distributions (e.g., transverse momentum, decay vertex displacement) to guide trigger optimization for dedicated searches.

Experimental results

Research questions

• RQ1 Can Higgs decays into heavy neutrinos with masses in the few-GeV range produce a detectable displaced vertex signal at the LHC?
• RQ2 What is the expected number of observable events after applying realistic pseudorapidity cuts in the 300 fb⁻¹ dataset at 13 TeV?
• RQ3 How do conventional LHC triggers limit the sensitivity to displaced vertex signals from Higgs decays into heavy neutrinos?
• RQ4 Which kinematic distributions are most effective for designing a dedicated trigger to enhance signal detection efficiency?

Key findings

• After applying pseudorapidity cuts, the number of signal events for Higgs decays into heavy neutrinos in the few-GeV mass range remains above 𝒪(10) for 300 fb⁻¹ of integrated luminosity at 13 TeV.
• The signal is sensitive to the neutrino-Higgs coupling and provides a direct probe of the Type I seesaw mechanism, complementary to W-boson decay searches.
• Conventional triggers significantly limit sensitivity due to suppression of displaced vertex signatures, necessitating optimized trigger strategies.
• Kinematic distributions such as transverse momentum and decay vertex displacement are identified as critical for improving trigger efficiency and signal detection.
• The model allows for enhanced contributions to neutrinoless double beta decay when heavy neutrino masses are nearly degenerate and the Majorana phase is large, but this is constrained by existing experimental limits.
• The differential decay rate for h → νN is derived in the lab frame using relativistic kinematics, with momentum conservation and phase space constraints explicitly enforced.