So far, we’ve talked about how we detect particles that interact with the detector in some way, but these methods cannot be used when we want to study particles that do not interact with the detector (e.g., neutrinos, dark matter, etc.).
When we want to know something about these “invisible” particles, we can use one of the first principles we learn in physics: conservation of momentum. Since the incoming particles in the collision carry negligible momentum in transverse to the beamline, we know that the vector sum of the final state particles should have a transverse component that is zero. The way we use this is that after we reconstruct all of the other particles in the event, the missing transverse energy (MET) is calculated as the negative vector sum of those objects.
Since the reconstruction of MET explicitly relies on everything else in an event, we do not calculate it until the analysis step. After all of the necessary calibrations are applied to the objects in the event, we use the tools that the JetETMiss group have developed.
The process of calculating MET and determining the associated systematics is a very complex process with many “moving parts” that are beyond the scope of this tutorial. For more detailed information, please see the MET Reconstruction in Run II twiki page.