Redo problem 3 of homework 5 for extra credit. (Pair creation problem.)
Interactions of Particles with Bulk Matter
| Process | Description |
|---|---|
ionization | passing particle ionizes an atom, leaving behind an electron and positive ion. |
scintillation | passing particle excites (or ionizes) an atom which emits light (visible or u.v.) as it returns to an unexcited state |
showers (electromagnetic and hadronic) | cascade of secondary particles created when particles that interact electromagnetically and strongly pass through high A/Z material. |
Cerenkov radiation | emission of light (visible and u.v.) when the speed of a particle, βc, exceeds the speed (phase velocity) of light in a medium, c/n where n is the index of refraction (analogy of sonic boom for a vehicle exceeding the speed of sound) |
transition radiation | emission of light (visible and u.v.) when a particle passes from one medium to another of differing EM properties |
Most of the hundreds of particles are extremely short lived. If produced in a collision they will decay after traveling a very short distance, basically undetectable for our purposes. While the interaction of these particles with nuclei or electrons may be of importance, it is not part of the present topic. Which particles are we generally concerned with detecting? We will concentrate on a small subset of particles that can travel macroscopic distances through matter, and have interact electromagnetically or strongly (so neutrinos are ignored).
| particle | lifetime |
|---|---|
| e+, e- | infinite |
| p, pbar | infinite |
| γ | infinite |
| μ+, μ- | 2.2×10-6 s |
| π+, π- | 2.6×10-8 s |
| K+, K- | 1.2×10-8 s |
| K0S | 0.89×10-10 s |
| K0L | 5.2×10-8 s |
| Λ, Λbar | 2.6×10-10 s |
These are the particles we are concerned with measuring. Other particles that we may be interested in -- such as W±, Z0, t quark, D0, B0 -- eventually decay into the particles above, sometimes after several intermediate stages. We learn about the unstable particles by reconstructing them from their decay products.
Before getting started with calculations, let's consider a few examples from actual experiments to understand how these processes are used to detect particles.
Let's start with tracking of particles. This is easy to visualize, as shown by the figures I've handed out. In particle physics, tracking is the process of following the path of a charged particle inside of the experimental apparatus. It is not presently feasible to track neutral particles. Tracking generally relies on the ionization of nearby atoms by the passing particle. The ionizations are detected, linked together to form a "track", and the parameters of the track are determined.
The first figure shows tracks in nuclear emulsion. Nuclear emulsion is simply high quality film, generally with extra thick gel, that can be stacked together to create a detection apparatus. The emulsion is exposed to cosmic rays or a particle beam, then the film is developed and scanned -- this was normally done by hand with microscopes! It is normal for a particle to pass from one piece of film to the next in the stack. The track has to be followed from layer to layer, hence the mosaic pattern in these pictures from "pasting" together the pieces from different layers.
In Fig. 4.8.1 you see a pion entering the emulsion, slowing, and decaying to a muon and an unseen muon neutrino. For scale, the width of the entire figure is only about 100 microns. The resolution in emulsions is excellent, and you can readily link the ionizations together to form a track. Additionally, it is easy to locate the point where the pion decays and the muon originates.
In Fig. 4.8.2 you again see a pion entering the emulsion, slowing, and decaying to a muon and an unseen muon neutrino. Now we can also see the decay of the muon into an electron and two unseen neutrinos. It is easy to see where the pion and muon decay, and where the muon and electron originate.
Now take a look at the second drawing. This is an event in the CDF detector. The CDF detector has cylindrical geometry in the main section, and we are seeing a view looking down the axis of the cylinder. The proton and anti-proton beams travel along this axis and collide in the center of the detector. All the tracks that you see emerge from the central collision point. To set the scale, the central part of the detector where the tracks appear is about 2m in diameter.
CDF is an electronic detector. Some of the ionizations left by a particle are turned into electronic signals which I'll generically call "hits" -- I won't discuss how this is done in class, but you are free to talk to me about it outside of class if you are interested. Both the reconstructed tracks and the hits are shown in the figure. The first thing you notice is that there are alot more tracks than in the emulsion figures. The protons and anti-protons collide with nearly 2TeV (= 2000 GeV = 2×106 MeV) of energy in the CM, and this energy can be converted into mass for many particles.
You will see many more hits than the displayed tracks can account for.
The detection of particles relies on a transfer of energy from the incoming particle to the bulk medium of detection.