They give very precise information, but they are expensive, so we concentrate them closest to the beam where they don't have to cover so much area. Outside the silicon, D0 has an outer tracker made using scintillating fibers, which produce photons of light when a particle passes through. Calorimeter Outside the tracker is a dense absorber to capture particles and measure their energies. This is called a calorimeter. The outermost layer of the detector detects muons.
Muons are unstable particles but they live long enough to leave the detector.
High energy muons are quite rare and a good sign of interesting collisions. Unlike most common particles they don't get absorbed in the calorimeter so by putting particle detectors outside it, we can identify muons. Because the muon system has to surround all of the rest of the detector, it ends up being very large, and it is the first thing that you see when looking at D0. Trigger system Proton-antiproton collisions happen inside the detector 2.5 million times every second. The trigger is the system of fast electronics and computers than has to decide, in real time, whether an event is interesting enough to be worth keeping.