Probing axion-like particles coupling to gluons at the LHC

Abstract

Assuming ALPs couple to gluons only, they can be produced via the pp → aj process and decay into two jets at the LHC. When the coupling parameter, {C}_{overset{sim }{G}} /fa, 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}_{overset{sim }{G}} /fa for ALP mass ma in the range 5–2300 GeV at 2-, 3- and 5-σ 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}_{overset{sim }{G}} /fa and ma are probed and the best upper limits on {C}_{overset{sim }{G}} /fa at 2-σ significance are found to be around 1.0 × 10−2 TeV−1 for ma∼ 500 GeV. The ALP mass is reconstructed from the kinematics of final state jets and we find that it is measurable with this method when ma 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.