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.
"final calculated [weight]" may be a better way to word it to avoid any confusion with the actual mass of the particle, as opposed to the calculation of how gravity at a given moment effects it.
So it's just the gravitational forces acting on those Z bosons that affect the measurement of weight? If so, I'm just blown away. Physics never ceases to make my day better.
At least in my last physics class, weight was defined to be the gravitational force of the earth acting on an object. Perhaps you're blown away because you're equating weight and mass?
To be precise, weight is defined as the magnitude of the force one must apply to an object in a gravitational field in order to hold it at rest.
If you're on the moon, the gravitational force of the earth acting on an object is not what matters. It's the gravitational force of the moon that matters.
They're not measuring weight anyway -- they are measuring mass. Maybe I'll write up a blog post with a picture of how mass spectrometry works. It's a fairly common homework problem in intro EM courses anyway.
