Motivated by models proposed to explain the Standard Model anomalies, and the
unprecedented τ+τ− data to be collected by the Belle~II
experiment during the next years, we study the kinematics of tau pair decays
and propose a new method to search for lepton flavor violating processes in tau
lepton decays to invisible beyond Standard Model particles, such as τ→ℓα, where ℓ is either an electron or a muon, and α is a
massive particle that escapes undetected. The new method improves by one order
of magnitude the expected upper limit on the τ→ℓα production
in 3x1 prong tau decays and establishes the possibility of performing this
search in 1x1 prong tau decays which has not been previously considered.Comment: 6 pages, 7 figure