Hunting for Long-Lived ALPs in Top Events via Displaced Dielectron Pairs at the LHC

Abstract

A generator-level sensitivity study is presented for long-lived axion-like particles (ALPs) produced in association with top-quark pairs and decaying to displaced electron-positron pairs at the CERN LHC. The prompt objects from the top quark-antiquark system provide independent handles for event selection, allowing the event to be selected independently of the displaced electron pair and extending the search to ALP masses below the dimuon threshold. A search strategy motivated by the CMS detector is developed for ALP masses between 17 MeV and 10 GeV and proper decay lengths between 0.1 mm and 1 m. Expected upper limits at 95% confidence level are computed for $σ(pp\to t\bar t a)\times\mathcal{B}(a\to e^+e^-)$ as a function of the ALP mass and proper decay length. In the most sensitive region of the parameter space considered, values of the cross section times branching fraction of order $10^{-3}$ pb can be probed for an integrated luminosity of 500 fb$^{-1}$ at $\sqrt{s}=13$ TeV. These projections do not include detector-level displaced-electron reconstruction or instrumental backgrounds.