Z bosons decay into a lepton (electron, muon, or tau particle) and its anti-particle. Those decay products pass through a magnetic field and eventually collide with a material that absorbs all of their energy. Measuring the path of the leptons (in particular, the deflection caused by the magnetic field) allows you to determine their momentum. The intensity of the collision with the stopping material is used to determine the energy.

Knowing the energy and momentum allows one to calculate the path. Gravitational forces are also acting on those particles, and any changes in their paths caused will change the final calculated mass.