= Solution
Use a high-multiplicity hadronic mode with only one invisible <neutrino>, and reconstruct the total visible <four-momentum> $p_h$. In the <tau lepton> rest frame,
$$
E_h^*=\frac{m_\tau^2+m_h^2-m_{\nu_\tau}^2}{2m_\tau},\qquad m_h\leq m_\tau-m_{\nu_\tau}.
$$
Fit the endpoint of the hadronic <invariant mass> distribution, preferably together with its laboratory <energy> boundary at known beam <energy>. The two-dimensional boundary uses more information than the single largest observed <mass>. Detector smearing, unseen neutral products, backgrounds, beam radiation and hadronic spectral modeling must be included. Absence of a displaced endpoint generally sets an upper bound rather than a positive <neutrino mass> measurement. Strictly the flavour state contains several <mass eigenstates>; an unresolved endpoint constrains the corresponding weighted mass spectrum.
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