Probing axion-like particles coupling to gluons at the LHC

TL;DR

This study investigates the detection prospects of axion-like particles (ALPs) coupling to gluons at the LHC, including both prompt and displaced jet signatures, using machine learning techniques to improve sensitivity and explore new parameter space regions.

Contribution

It introduces a comprehensive analysis of ALPs with gluon coupling at the LHC, incorporating displaced jets and advanced multivariate methods for the first time.

Findings

Projected upper limits on coupling parameter $C_{ ilde{G}}/f_a$ around $1.0 imes 10^{-2} \,\, \mathrm{TeV^{-1}}$ for $m_a \sim 500$ GeV.

ALP mass can be reconstructed and measured below about 1 TeV at the HL-LHC.

Significant new parameter space regions are accessible with the proposed analysis.

Abstract

Assuming ALPs couple to gluons only, they can be produced via the $p p \to a j$ process and decay into two jets at the LHC. When the coupling parameter, $C_{\tilde{G}} / f_a$, is small, the lifetime of ALPs can be long enough leading to displaced final state jets. In this paper, we consider the signal including both the prompt and long-lived cases of ALPs by employing a specialized Delphes module to handle displaced jets. Relevant background processes are generated and simulated at the detector level, and multivariate analyses based on machine-learning are performed to discriminate signal and background events and achieve the best sensitivities. Based on the data accumulated for this study, we forecast the expected upper limits on $C_{\tilde{G}}/f_a$ for ALP mass $m_a$ in the range 5$-$2300 GeV at 2-, 3- and 5-$\sigma$ significances at the High Luminosity-LHC with 14 TeV center-of-mass energy and 3 ab$^{-1}$ integrated luminosity. Vast previously unprobed regions in the parameter space spanned by $C_{\tilde{G}}/f_a$ and $m_a$ are probed and the best upper limits on $C_{\tilde{G}}/f_a$ at 2-$\sigma$ significance is found to be around $1.0 \times 10^{-2} \,\, {\rm TeV^{-1}}$ for $m_a \sim 500$ GeV. The ALP mass is reconstructed from the kinematics of final state jets and we find that it is measurable in this method when $m_a$ is below about 1 TeV at the HL-LHC. The effects of systematic uncertainties and validation of the EFT framework are also checked and discussed.