While theorists routinely work with the most fundamental degrees of freedom in their calculations, the world of an experimentalist, i.e., the real world, is quite different. Experimenters must cope with particles that decay too quickly to be observed; particles that don’t exist freely by themselves but only in bound, hadronized, and fragmented states that keep showering into even more particles; and particles that cannot be detected at all. And we theorists expect them to relate the pile of stuff they observe to our fundamental degrees of freedom, and to get it right, every time. Experimenters accomplish this feat with what looks like a huge pile of semi-organized chunks of metal, liquid, gas, wire, and cable, called a detector. It’s amazing, really. I am always in awe when I visit a detector! In order to interpret LHC (or Tevatron or B-Factory) data, a theorist must have at least a rudimentary knowledge of how a detector works.
Modern collider detectors are known as 4pi, or hermetic, detectors. (Burt Richter led the team which built the first 4pi detector in the 1970’s at SLAC.) A hermetic detector is simply one which completely surrounds the collision point, i.e., in more technical language, it covers all 4pi steradians of solid angle around the collision. The shape of a collider detector is essentially cylindrical and one can think of it as a can, with the interaction point being located at the center of the can. The can has a barrel piece, which has the beampipe as its axis, and two endcap pieces which fit in snugly to the barrel region. The snug fit is incredibly important, and the degree of snugness is called hermeticity. A detector whose components do not have a snug fit is not hermetic – it would allow particles to escape undetected. If this happens, not only do you lose the particles that you should see, but you have over counted the production rate of the ones you shouldn’t see. That’s obviously a bad situation. Unfortunately, no detector is ever perfectly hermetic — they all have cracks and experimenters learn how to fold that into their data analysis.
These cylindrical detectors are a series of subcomponents, each wrapped around each other, and each performing a specific job. A general all-purpose collider detector has the following components (looking at a slice view and starting at the interaction point and going out):
Vertex detector: This is a very tiny component which surrounds the interaction region as closely as possible. It is basically built of layers of silicon and if a particle has a lifetime of order a picosecond (10-12 seconds) then its decay vertex (i.e., the track of the single particle splits into the 2-3 tracks of its decay products) can be observed. Bottom quarks, and sometimes charm quarks, can be identified in this manner. The vertex detector is the first component to get fried if something goes even slightly amiss with the beam. Tracking Chamber: This component determines the trajectory of charged particles. The electromagnetic energy loss (via interactions with the medium in the tracking chamber) and momentum of a charged particle can be measured. It works by tracing the helix of the charged particle as it traverses the chamber in a magnetic field. The chamber is made of layers of finely segmented material, usually silicon. Electromagnetic Calorimeter: High energy electrons and photons interact with the material in the ECAL and create showers of particles — this process occurs at an exponential rate allowing for the ECAL to absorb all their energy and they finally come to rest. The energy of electrons and photons is thus measured. Various materials are used (lead crystals are popular), but the calorimeter is usually transversely segmented. Hadronic Calorimeter: Hadronic particles (jets of particles made from hadronized quarks and gluons), usually protons, neutrons, pions, and Kaons, interact with the material in the HCAL, creating showers, coming to a stop and thus depositing all their energy in the HCAL. This usually requires a fairly dense medium (steel scintillators are popular) and is also generally transversely segmented. Muon Chambers: These are huge chunks of iron that surround the outside of the detector. Muons are heavy and relatively long-lived — thus they traverse the rest of the detector without stopping and track through to the outside chambers. Supposedly, muons are the only particle that can travel through the detector without showering and stopping in the calorimeters, but sometimes very energetic pions can make it through the hadronic calorimeter and punch through into the muon chambers. This is known as pion punch-through and gives a fake muon signal.
As you can tell from the above list, every detector also has a magnet, in order to track charged particles. The size and shape of the magnetic can vary quite dramatically, but it is usually quite large with state of the art magnetic field strength.
A nice graphic illustrating these various detector subcomponents and the particles they are designed to identify is:
The solid lines illustrate when a particle leaves a track in a detector component, and the showers depositing all their energy in the ECAL and HCAL are also shown. Neutrinos, by the way, sail smoothly through the detector unnoticed, and unbothered by all the material placed in their path.
This is a very basic and rudimentary description of collider detectors, but one that I give on the first day of class when I teach a Collider Physics course. A nice set of lectures, written for theorists, describing detectors and collider physics in more detail can be found in hep-ph/0508097 by my good friend Tao Han at the University of Wisconsin. I admit to stealing the above graphics from these lecture notes.
As you can see, detectors are very complicated and finely tuned instruments and my hat is off to the experimenters who make them work and give us the data!