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

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) $N \to \nu h$ 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 $2\sigma$ exclusion limits and $5\sigma$ 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.