Universe_Man said:
By what means do scientists detect the presence of elementary particles? How do we know what we are looking at through these means of detection?
Straight into my profession.
A good place to start, apart from the good, already mentionned link, is:
http://en.wikipedia.org/wiki/Particle_detector
There's a long list of different techniques to detect particles, all of which, by today, are electronic.
The basic principle of any particle detector today is the interaction of CHARGED particles with matter, which takes on two forms:
-excitation
-ionisation
Indeed, point charges which traverse matter at high speed cause these two phenomena to happen. Once these phenomena took place in matter, you still have to have a means of observing them. Excitation can give rise to light emission, in which case, you try to detect the light (and in the end, try to ionise matter one way or another, like a photocathode on a PM). In the end, you're after ionisation. Why ?
Because the application of an electric field let's you guide the hence obtained charges (ions but preferably electrons), and there is an effect which is often exploited: HIGH electric fields give rise to avalanches.
The electrons accelerate in the high electric field, and become ionizing charges in themselves. This has the potential of AMPLIFYING the original ionisation charge. In the end, you try to observe the movement of all these charges by placing electrodes somewhere: moving charges induce CURRENTS in nearby electrodes.
Almost all particle detectors operate in one way or another on this principle: the initial charged particle ionises matter, this gives (often) rise to SEVERAL electrical charges, which are then guided and accelerated by an electrostatic field, which can give rise to secondary ionisations and which ultimately give rise to moving charges inducing electrical currents in electrodes.
Amplifiers connected to these electrodes then amplify these (tiny) currents, and this goes into a data acquisition system.
A few examples:
*) gas detectors (the old working horse). Fast charges create little clouds of ions and electrons in the gas. These drift along the lines of an electrostatic field. Sometimes, this is enough: the movement of these charges induces currents in electrodes: we have an ionisation chamber.
Sometimes, the E-field is very strong, and we get gas amplification (the initial electron becomes itself an ionising particle, etc..). We have now proportional detectors, or streamer tubes, or geiger counters
Wires or strips or other electrodes configure the E-field and serve as induction electrodes.
*) photomultipliers: the photon liberates an electron of the photocathode which is then accelerated in an E-field in vacuum, hitting a "dynode" where several electrons are liberated, to the next dynode etc... until a very large number of electrons arrive at an anode: their movement induces a current pulse in the anode.
*) silicon detectors: very similar to gas detectors, but the medium is now silicon, and the charged particles are electrons and holes.
Now, what happens if we want to observe *neutral* particles ? Well, we need a fundamental interaction with the neutral particle to generate light, or charged particles. For instance, gamma radiation needs to interact with matter (photo-electric effect, compton effect, or pair production) which liberates electrons. Fast neutrons can hit elastically a nucleus, which becomes a charged particle (the fast-moving nucleus).
Slow neutrons need to interact through a nuclear interaction with matter (for instance, fission in U-235).
Neutrinos are harder.
