The LHC as an Axion-Photon Collider

TL;DR

This work demonstrates that the LHC can function as a high-energy ALP collider via ultraperipheral collisions, exploiting ALP fluxes from protons and ions to probe ALP-nucleon and top-quark couplings. The authors derive forward ALP and photon fluxes, implement an ALP EFT in Monte Carlo tools, and show that proton–lead UPCs (pPb) provide the most promising channel for ALP–photon interactions, especially in top-quark final states. Their analysis indicates that sub-GeV ALP flavor-violating couplings to the top quark, $c_{tc}$ and $c_{tu}$, can be probed at competitive levels, with ALP–photon collisions offering a leading sensitivity. They also provide a concrete path to test these ideas using existing 2016 $p$Pb datasets, enabling a novel, collider-based probe of ALP physics that complements low-energy searches.

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 Weizsäcker-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.

Figures (2)

• Figure 1: Normalized differential luminosities of the $aa$, $a\gamma$, $\gamma\gamma$ collisions triggered from the $pp$ and $p$Pb UPCs occurring at the LHC. $\sqrt{s_{NN}}$ is the center-of-mass energy per nucleon.
• Figure 2: Pseudo-rapidity distributions of the final states from the collisions $a\gamma\to t\bar{t}$, $\gamma\gamma\to t\bar{t}$ (left) and $a\gamma\to t j$ (right) in the $p$Pb mode.