The LHC as an Axion-Photon Collider

Abstract: Assuming the existence of an axion-like particle (ALP), beams of relativistic particles emit fluxes of quasi-real ALPs, analogous to the photon fluxes described by Weizs\"acker-Williams-type approximations. Consequently, ALP-ALP and ALP-photon collisions can occur at the LHC. We initiate the study of the LHC as an ALP collider, and show that ALP collisions provide competitive probes of certain ALP couplings. We show that ALP fluxes from heavy ions are suppressed relative to those from protons, unlike their photon counterpart. As a result, the most likely processes are ALP-photon collisions occurring in the proton-ion ($p$A) ultraperipheral collisions. Using our implementation of ALP fluxes in simulation tools, we show that ALP-photon collisions in $p$Pb efficiently probe ALP couplings to third generation fermions. LHC data with realistic $p$Pb luminosity can constrain the product of ALP couplings to nucleons and top quarks at the level of $O(0.01$ TeV$^{-1})$. Notably, ALP-photon collisions naturally provide the leading probe of ALP flavor-violating couplings to the top quark. We suggest that the 2016 $p$Pb dataset collected at CMS, ATLAS, and LHCb should be explored for evidence of such collisions.