Searches for heavy neutrinos at 3 TeV CLIC in fat jet final states

TL;DR

This study assesses heavy Majorana neutrino production in a minimal Type-I seesaw at a 3 TeV CLIC, focusing on two boosted decay channels: $N\to \ell W$ with $W$ hadronic and $N\to \nu h$ with $h\to b\bar{b}$. Using a full detector-level pipeline (MadGraph5_aMC@NLO, Pythia, Delphes with CLICdet, and VLC jet reconstruction), the authors optimize selection strategies for $1\ell+J_W+\slashed{E}_T$ and $J_h+\slashed{E}_T$ final states. They derive $2\sigma$ exclusion and $5\sigma$ discovery contours in the $m_N$–$|V_{\ell N}|^2$ plane and 95% CL upper limits on $|V_{\ell N}|^2$, showing that a 3 TeV CLIC can improve sensitivity by about two orders of magnitude over projected hadron colliders and remains competitive with other lepton-collider channels. The analysis highlights the impact of beam polarization, with a ~1.8× enhancement in signal and background, and positions CLIC as a powerful probe of heavy neutrino mixing and Majorana nature in the TeV mass range.

Abstract

The type-I seesaw mechanism provides an elegant explanation for the smallness of neutrino masses via the introduction of heavy Majorana neutrinos (N), which also constitute a well-motivated extension of the Standard Model. In this work, we explore the production and detection prospects of TeV-scale heavy neutrinos ($m_N \gtrsim 1$ TeV) at a future 3 TeV Compact Linear Collider (CLIC). We focus on two distinct decay topologies: (i) $N \to \ell^\pm W^\mp$ with hadronic $W$ boson decay, leading to a final state with one charged lepton, a hadronic fat-jet $J_W$, and missing transverse energy ($1\ell + J_W + \slashed{E}_T$); and (ii) $N \to νh$ with subsequent Higgs decay $h \to b\bar{b}$, yielding a Higgs-tagged fat-jet $J_h$ and $\slashed{E}_T$. Based on comprehensive detector-level simulations and background analysis, we present both $2σ$ exclusion limits and $5σ$ discovery reaches in the $m_N$-$|V_{\ell N}|^2$ plane. We further extract 95\% confidence level upper limits on the mixing parameter $|V_{\ell N}|^2$, and perform a detailed comparison with existing constraints from direct searches at future colliders and indirect global fits. Our findings demonstrate that a 3 TeV CLIC can improve the sensitivity to $|V_{\ell N}|^2$ by about two orders of magnitude compared to the projected reaches of future hadron colliders, while remaining competitive with other CLIC search channels.

Figures (6)

• Figure 1: The production cross section of $e^{+}e^{-}\to \nu N$ as a function of $m_N$ at the CLIC with $|V_{\ell N}|^2 = |V_{e N}|^2 = |V_{\mu N}|^2=10^{-4}$.
• Figure 2: The Feynman diagrams of processes for Case 1 and Case 2.
• Figure 3: Normalized distributions for the four signals (with $m_N = 1000$ , 1500, 2000, and 2500 GeV and relevant SM backgrounds for Case 1.
• Figure 4: Normalized distributions for the signals and relevant SM backgrounds for Case 2.
• Figure 5: Exclusion limits ($2\sigma$) and discovery reach ($5\sigma$) for (a): Case 1, and (b): Case 2 in the $m_N$-$|V_{\ell N}|^2$ parameter space.