Hadron Tracking

Three types of charged hadrons (protons, pions, and kaons) leave similar signatures in both the tracking system (vertex detector and drift chamber) and the calorimeter.

Since the three particle types have different masses, they travel at different speeds for a given energy or momentum. However, for energies large compared to all three mass types, all three particles then travel at close to the speed of light¸ and the differences in speed are more difficult to measure.

Time of Flight

If the energy is low enough that the speeds differ significantly, then the particles can be distinguished by a detector placed just outside the drift chamber that records their time of arrival relative to the collision time.

Cerenkov Detectors

A Cerenkov detector is a more accurate velocity-measuring device that can be used for somewhat higher energy particles. These detectors are an extra layer or two, placed between the tracking system and the calorimeter.

The phenomenon of Cerenkov radiation is somewhat like a sonic boom, except it is light instead of sound. A plane emits a sonic boom in a cone around its direction of travel when it travels through air at a speed faster than the speed of sound in the air. A charged particle emits Cerenkov radiation -- light -- in a cone around its direction of travel when it travels through any medium at a speed faster than the speed of light in that medium. (Cerenkov -- pronounced Cherenkov -- is the name of the scientist who first recognized the nature of this effect and its possible use for distinguishing particle types.)

Although the speed of light in a vacuum is the fastest speed that any particle or light can have, in a medium of any type light travels more slowly than this speed because of its interactions with the electric fields of the atoms in the medium. The refractive index of a medium is a measure of the ratio of the speed of light in the medium to the speed of light in a vacuum.

If a particle has sufficient energy it can thus travel faster than light in a particular medium. The angle of the cone of Cerenkov radiation about its track is then a measure of its velocity. A Cerenkov ring imaging detector measures the size of the ring formed when the cone of light hits a light-sensitive detector. This information, together with the knowledge of where the light comes from, can be used to calculate the angle of the cone and thus the particle velocity.

Two Layer Cerenkov Detector

A simpler type of Cerenkov detector can be used in cases where the energy of the particles of interest is well known. Then a medium, such as gas or very low density material such as aerogel, can be selected so that the pions travel faster than the speed of light in it, but the protons and kaons do not. Then a second layer can be used in which pions and kaons travel faster than light but protons do not. Now we can clearly tell the three types:

• Cerenkov light in both layers ==> pion
• Cerenkov light in the second layer only ==> kaon
• Cerenkov light in neither layer ==> proton.