  The tau lepton plays important role in the correlation between the low-energy
neutrino oscillation data and the lepton flavor structure in heavy neutrino
decay. We investigate the lepton flavor signatures with tau lepton at hadron
collider through lepton number violating (LNV) processes. In the Type I Seesaw
with U$(1)_{\rm B-L}$ extension, we study the pair production of heavy
neutrinos via a $Z'$ resonance. We present a detailed assessment of the search
sensitivity to the channels with tau lepton in the subsequent decay of heavy
neutrinos. For the benchmark model with $Z'$ only coupled to the third
generation fermions, we find that the future circular collider (FCC-hh) can
discover the LNV signal with tau lepton for $M_{Z'}$ up to 2.2 (3) TeV with the
gauge coupling $g'=0.6$ and the integrated luminosity of 3 (30) ab$^{-1}$. The
test on the flavor combinations of SM charged leptons would reveal the specific
nature of different heavy neutrinos.

High Energy Physics - Phenomenology