  The very high luminosity run foreseen at the $Z$-pole for the FCC-ee will
allow the detection in $Z$ decays of new particles with very low couplings to
the Standard Model. These particles will have measurable flight paths before
they decay. If the timing and the position of the decay vertex can be measured
with high precision, the mass of such particles can be measured by exploiting
the constrained kinematics of an $e^+e^-$ collider. The mass resolution
achievable with this technique is studied through a detailed analysis in the
framework of a parametrised simulation of the performance of the IDEA detector.
The adopted benchmark model is the production of Heavy Neutral Leptons, which
is one of the key channels for new physics discovery at the FCC-ee.
