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 G ~ $$ {C}_{\overset{\sim }{G}} $$ /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 G ~ $$ {C}_{\overset{\sim }{G}} $$ /f a for ALP mass m a 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 G ~ $$ {C}_{\overset{\sim }{G}} $$ /f a and m a are probed and the best upper limits on C G ~ $$ {C}_{\overset{\sim }{G}} $$ /f a at 2-σ significance are found to be around 1.0 × 10−2 TeV−1 for m a ∼ 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 1 TeV at the HL-LHC. The effects of systematic uncertainties and validation of the EFT framework are also checked and discussed.