YETI 2021: The Future of Lepton Colliders - Shared screen with speaker view
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quick answer to question above. the tau's from the Z are highly boosted, and the neutrinos are therefore nearly 100% aligned with the tau's and the leptonic (or hadronic) decay products. assuming that the system has more or less 0 transverse momentum allows to solve stuff pretty well ... (of course you can further refine this simple argument).