Charged Lepton Flavour Violation benchmarks in FCCee

Leptons are elementary particles which do not interact via strong interactions [1]. In the Standard Model (SM) of particle physics, there are three charged and three neutral leptons, grouped in three generations: (e, νe), (µ, νµ) and (τ, ντ ). The lepton flavour (L) is defined as a quantum number which takes +1 value for leptons, -1 value for the corresponding antileptons and 0 for the other types of particles. The lepton number was introduced to express the conservation of the overall difference in the number of leptons and antileptons in a given process, which was observed experimentally [2]. The phenomenon of Lepton Flavour Violation (LFV) has been predicted in many Beyond the Standard Model theories (BSM), namely in supersymmetry (SUSY) [3], little Higgs models [4], seesaw scenarios [5] as well as in models allowing fourth generation of leptons [6]. Research in this area may explain some major drawbacks of the SM, including neutrino oscillations and mechanism of baryogenesis [2]. Moreover, there are no known symmetries which do not allow LFV decays, for instance l → l′γ or l → 3l′ [7, 8]. It is argued that due to neutrino oscillations, such decays are admissible in the SM but in the presence of extremely small branching fractions [9]. LFV has already been observed in the sector of neutral leptons, nevertheless, no evidence for the LFV in the sector of charged leptons has been observed experimentally [2]. The searches for Charged Lepton Flavour Violation (CLFV) processes usually provide upper limits on the branching fraction for the given decay channel or limits on the specific cross section. In case of τ → 3µ, the best experimental exclusion region on the branching fraction is set by Belle experiment, with a value of B → 3µ < 2.1 · 10−8 at 90% confidence level [10]. The report is organized as follows. The Future Circular Collider (FCC) experiment is briefly described in Sec. 2, the discussion of data samples and specification of the examined processes are included in Sec. 3 and finally the summary and further outlook are presented in Sec. 4.