Illuminating Dark Sectors:A Prospective Dark Photon search at the FCC-ee

Sammanfattning

Dark sectors are an interesting and promising way of parameterizing physics Beyond the StandardModel, because they could naturally solve some of the biggest problems in particle physics.This thesis in particular focuses on the vector portal mediated by the dark photon, which couldpotentially be observed because of its kinetic mixing with the Standard Model photon. Asensitivity analysis is performed for the signal scenario 𝑒+𝑒− → 𝑍 ℎ, 𝑍 → 𝑞 ¯𝑞, ℎ → 𝑍𝑑 𝑍𝑑 , 𝑍𝑑 →𝜇+ 𝜇− at a center-of-mass energy of 240 GeV. This signal features the exotic Higgs boson decayinto a long-lived dark photon pair, for dark photon masses in the range 0.36-10 GeV, and kineticmixing parameter 𝜀 in the range 6.7×10−5-8.7×10−8. The analysis is performed within theFCCAnalyses framework, where the signal is generated using MadGraph, Pythia8 and theIDEA Winter2023 Delphes card. It is first investigated whether or not the dark photon in thisphase space could be observed at the FCC-ee. Then, an event selection is applied, requiring atleast two jets with 𝑝𝑇 >5 GeV, 4 muons of opposite charge, a Higgs candidate with invariantmass ∈ [120, 130] GeV, and two dark photon candidates with invariant mass <11 GeV. Thisselection efficiently reduces the Standard Model background to zero. The results suggest that theproposed FCC-ee offers excellent prospects for probing long-lived dark photons produced in theHAHM model. Indeed, the 95% CL exclusion limits derived in this work cover a parameterspace that is not yet constrained by existing collider experiments