Wing Sheung Chan

The Standard Model and lepton flavour violation 25 Table 1.9.: Current experimental limits on the branching fractions of various LFV processes. The limits are set at 90% confidence level for µ , τ or hadron decays and 95% for Z and H decays. The limits on the Z → `τ decays were the most stringent before the work in this thesis. More limits on LFV hadron decays can be found summarised in Reference [6] . Violation Process Upper limit on branching fraction Experiment e and µ lepton flavours µ → eγ 4 . 2 × 10 − 13 [48] MEG µ → eee 1 . 0 × 10 − 12 [49] SINDRUM π 0 → eµ 3 . 6 × 10 − 10 [50] KTeV J/ψ → eµ 1 . 6 × 10 − 7 [51] BESIII H → eµ 3 . 5 × 10 − 4 [52] CMS Z → eµ 7 . 3 × 10 − 7 [53] CMS e and τ lepton flavours τ → eγ 3 . 3 × 10 − 8 [54] BaBar τ → eee 2 . 7 × 10 − 8 [55] Belle τ → e ± µ ∓ µ ± 2 . 7 × 10 − 8 [55] Belle J/ψ → eτ 8 . 3 × 10 − 6 [56] BES H → eτ 4 . 7 × 10 − 3 [57] ATLAS Z → eτ 9 . 8 × 10 − 6 [58] OPAL µ and τ lepton flavours τ → µγ 4 . 4 × 10 − 8 [54] BaBar τ → e ± e ∓ µ ± 1 . 8 × 10 − 8 [55] Belle τ → µµµ 2 . 1 × 10 − 8 [55] Belle J/ψ → µτ 2 . 0 × 10 − 6 [56] BES H → µτ 2 . 5 × 10 − 3 [59] CMS Z → µτ 1 . 2 × 10 − 5 [60] DELPHI decays especially exciting, since it can be expected to either find evidence for New Physics or be able to set new constraints on relevant BSM models. Moreover, the B -anomalies (see Section 1.2.6) seem to hint at something different between the muon and the τ lepton that we have yet to understand, making the Z → µτ channel even more intriguing. Experimentally, a τ lepton in the final state is a challenge for reconstruction and identification due to its short life time (mean life = 290 fs , corresponding to a proper decay length of 86 . 9 µ m ) and many decay modes. But on the other hand, the neutrino(s) from the τ lepton decay, which escapes the detector and becomes missing energy in reconstruction, gives us a unique signature to identify the signal events. In the ATLAS (or CMS) detector, τ leptons produced at the collisions almost always decay before reaching the detector,