Probing axion-like particles coupling to gluons at the LHC

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-(G) over tilde/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 machinelearning 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-(G) over tilde/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 14TeV center-of-mass energy and 3 ab(-1) integrated luminosity. Vast previously unprobed regions in the parameter space spanned by C-(G) over tilde/f(a) and ma are probed and the best upper limits on C-(G) over tilde/f(a) at 2-sigma significance are found to be around 1.0 x 10(-2) TeV-1 for m(a) similar to 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 m(a) is below about 1TeV at the HL-LHC. The effects of systematic uncertainties and validation of the EFT framework are also checked and discussed.