The ALPs from the Top: Searching for long lived axion-like particles from exotic top decays

TL;DR

This paper proposes a novel search strategy for long-lived axion-like particles produced in exotic top decays at the LHC, focusing on their decay signatures in calorimeters to improve detection sensitivity.

Contribution

It introduces a dedicated search method for long-lived ALPs from top decays, utilizing calorimeter energy ratios and track vetoes, with potential to significantly improve detection sensitivity.

Findings

Can probe exotic top branching ratios below 10^{-4} at the LHC.

Method effectively suppresses Standard Model backgrounds.

Measurements of single top production can constrain these branching ratios.

Abstract

We propose a search for long lived axion-like particles (ALPs) in exotic top decays. Flavour-violating ALPs appear as low energy effective theories for various new physics scenarios such as t-channel dark sectors or Froggatt-Nielsen models. In this case the top quark may decay to an ALP and an up- or charm-quark. For masses in the few GeV range, the ALP is long lived across most of the viable parameter space, suggesting a dedicated search. We propose to search for these long lived ALPs in $t\bar{t}$ events, using one top quark as a trigger. We focus on ALPs decaying in the hadronic calorimeter, and show that the ratio of energy deposits in the electromagnetic and hadronic calorimeters as well as track vetoes can efficiently suppress Standard Model backgrounds. Our proposed search can probe exotic top branching ratios smaller than $10^{-4}$ with a conservative strategy at the upcoming LHC run, and potentially below the $10^{-7}$ level with more advanced methods. Finally we also show that measurements of single top production probe these branching ratios in the very short and very long lifetime limit at the $10^{-3}$ level.