Invisible dark gauge boson search in top decays using a kinematic method

TL;DR

This paper proposes a novel kinematic method using on-shell constrained $M_2$ variables to detect invisible dark gauge bosons produced in rare top decays at the LHC, distinguishing asymmetric signals from symmetric $tar{t}$ backgrounds.

Contribution

It introduces a new search strategy employing the $M_2$ variables to identify invisible $Z'$ bosons in top decays, leveraging event topology asymmetry.

Findings

The method can effectively differentiate signal from background in simulated LHC conditions.

Monte Carlo simulations show promising sensitivity for detecting light $Z'$ bosons.

The approach is robust against realistic experimental effects.

Abstract

We discuss the discovery potential of a dark force carrier ($Z'$) of very light mass, $m_{Z'} \lesssim {\cal O}(1-10)$ GeV, at hadron colliders via rare top quark decays, especially when it decays invisibly in typical search schemes. We emphasize that the top sector is promising for the discovery of new particles because top quark pairs are copiously produced at the Large Hadron Collider. The signal process is initiated by a rare top decay into a bottom quark and a charged Higgs boson ($H^\pm$) decaying subsequently into a $W$ and one or multiple $Z'$s. The light $Z'$ can be invisible in collider searches in various scenarios, and it would be hard to distinguish the relevant collider signature from the regular $t\bar{t}$ process in the Standard Model. We suggest a search strategy using the recently proposed on-shell constrained $M_2$ variables. Our signal process is featured by an $\textit{asymmetric}$ event topology, while the $t\bar{t}$ is $\textit{symmetric}$. The essence behind the strategy is to evoke some contradiction in the relevant observables by applying the kinematic variables designed under the assumption of the $t\bar{t}$ event topology. To see the viability of the proposed technique, we perform Monte Carlo simulations including realistic effects such as cuts, backgrounds, detector resolution, and so on at the LHC of $\sqrt{s}=14$ TeV.