December 14, 2006
JoAnne Hewett has a nice general description of modern collider detectors over at Cosmic Variance. To provide some value-added, let me relate it to CLEO-c:
- No detector can be perfectly hermetic (cover all of solid angle) because the colliding particles need to get in somewhere and there is accelerator machinery associated with focusing the beams. In CLEO’s case, the tracking system covers 93% of 4π solid angle. The LHC general-purpose detectors do somewhat better, because the products of their collisions will be closer to the beamline than ours are, making “forward” tracking and calorimetry more important.
- We have a “vertex detector” (the ZD) but it serves a different role: instead of finding the displaced vertices of long-lived particles, it tracks particles near the primary vertex (where the electron-positron collision occured). At our energies, the particles that would normally have displaced vertices don’t have enough relativistic oomph, so their lifetimes aren’t stretched out long enough for us to see their flight distance — the exceptions being the short-lived neutral kaon and the Λ baryon. The ZD is not made out of silicon, since we don’t need that kind of resolution; it is instead a drift chamber.
- Our main tracking chamber is also a drift chamber. Contrary to the original article, very few detectors rely primarily on silicon tracking for most of the volume (the only one I can think of being CMS); usually there is a gas detector component.
- Our electromagnetic calorimeter is made out of crystals of cesium iodide (doped with a pinch of thallium to increase the light output). Electrons and photons hitting it create flashes of light whose intensity increases with the incident particle’s energy. There are a whole bunch of different technologies that are used — although crystals are great, they’re also very expensive! The only recent experiment I can think of that uses lead crystals, though, is CMS.
- We have no hadronic calorimeter. This doesn’t hurt us all that much; we can individually track charged hadrons through our detector (so we really miss only neutrons and long-lived neutral kaons), while at higher energies the tracks merge together into a blob and the only way to measure their (sum) energy reliably is with a calorimeter.
- Our muon chambers work, but muons need a high momentum to get out there, and they tend not to have it. (These things were designed back in the higher-energy days.
- Unlike the “energy frontier” experiments, we have an additional system for distinguishing types of particles – the Ring Imaging Cherenkov detector, which uses the Cherenkov light from charged particles travelling quickly through lithium fluoride to measure their velocities. Since we measure their momenta in the tracking system, we can deduce the masses of the particles, and thus whether they are pions or kaons. This kind of additional particle identification system is common at lower-energy experiments.