Probing HNL-ALP couplings at colliders

TL;DR

This paper explores collider signatures of axion-like particles and heavy neutral leptons, proposing a clean four-jet and two-lepton final state process to improve sensitivity to their couplings, especially for TeV-scale masses.

Contribution

It identifies a novel collider process involving ALPs and HNLs, providing a method to probe their couplings with enhanced sensitivity and independence from certain mixing parameters.

Findings

Current LHC sensitivity can reach below the Seesaw scale.

Signal strength is significant for HNL masses below a few TeVs.

Joint limits on ALPs and HNLs can surpass individual constraints.

Abstract

Axion-like particles (ALPs) and heavy neutral leptons (HNLs) are both well-motivated extensions of the Standard Model. As ALPs couple to on-shell fermions proportionally to their masses, processes involving both types of particles may give rise, for TeV HNLs, to relevant phenomenology at colliders. In this work, we point out a particularly clean process, whose final state consists of four jets and two charged leptons, and we estimate its current and future sensitivity at the LHC. For on-shell HNLs, i) there is no dependence on the overall scale of the mixing between HNLs and the active neutrinos, making this process sensitive down to the type-I Seesaw line; ii) the signal strength of the process is sizable only as far as the HNL masses are below a few TeVs, contrary to what the proportionality to masses of the ALP couplings may suggest. Although ALPs and HNLs have been mainly studied independently in the literature, considering their interplay may lead to joint limits that are much stronger than those on the individual particles taken separately. This concise study paves the way for similar searches at colliders and other experiments.