Axion-Like Particles in Composite Higgs Models

Abstract

This thesis investigates an extension of the Standard Model that includes an axion-like particle (ALP). The Lagrangian characterizing interactions between this ALP and Standard Model particles is presented and the decay channels into gluons, photons and fermion pairs are studied. This includes the possibility of flavor-violating ALP-quark interactions, yielding decay widths comparable to those of rare loop processes in the Standard Model. Relations between the photonic, gluonic and flavor universal fermionic ALP couplings are found analytically. Under the assumption that the local diphoton and ditau excesses reported by experiments at the LHC are the result of production via an ALP of mass 95 GeV, the associated coupling constants can be estimated. This indicates how future analyses could proceed if the excesses are confirmed. The fermionic and photonic couplings are constrained by a fixed ratio, and upper and lower bounds are obtained for both in the limits where the ALP interacts weakly and strongly with the gluons, respectively. The resulting total decay width of the ALP is bounded from below, showing that the ALP must be short-lived. Using the numerical tools FeynRules and MadGraph5, together with the above-mentioned relations between couplings, the expected cross sections for dimuon production via an ALP are computed. While the cross section for this process is suppressed by the muon mass compared to the dominant Standard Model background processes, it is bounded from below. This establishes the minimum expected cross section scale for future dimuon searches at 95 GeV.