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Other hints and comments
- If you want to examine the picture of a collision very carefully – to find small angle kinks, for example – it is a good idea to print the picture and look at it from a very low angle.
Click here for two examples of low angle kinks!
- All charged particles visible in a bubble chamber have charges of +/- that of the electron - this makes it easy to use charge conservation.
- It is rare to be able to identify every particle in an event just by looking at the pictures – for example, many pions are produced in high energy collisions and they usually leave before they decay. Since the number of bubbles per centimetre (`ionisation density', ID) depends on speed , all charged particles moving close to the speed of light have the same ID and are indistinguishable. (The greater the speed, the lower the ionisation density : the impulse imparted to the electron depends on the time spent by the ionising particle in the vicinity of the electron; a
highly relativistic particle is thus `minimum ionising'; click here for full detailed discussion).
So, for energies greater than about 2 GeV, tracks that just leave the chamber can not be identified. If we are studying the strong interaction, such tracks are often referred to as `unknown hadrons'; they may be pions, kaons or protons, but they are usually pions because these are produced more copiously. Click
here for example
Proton targets often only receive a gentle glancing blow from the beam, and therefore move with speeds well below that of light; these will have dark tracks – high ID – and will often stop, making protons easy to identify. Click
here for example.
- For introduction to E2=p2c2 + m2c4, click
- Without access to actual measurements, it is possible to make intelligent estimates about the relative momenta of particles by comparing their curvatures. This can be done using this curvature template
Students should be made aware of the limitation of this method: the tracks are not actually in one plane.
- If you only have one view of a picture, it is sometimes difficult to tell what is going on because, for example, one track might overlap another. To be able to sort out such
problems (and, more importantly, to be able to reconstruct the event in 3 dimensions),
three or four pictures of each event are taken. Click
here for an example of the same event on two views.
- For connoisseurs: very occasionally, a negative track will look like a stopping proton. This is an example of `pion capture' in the reaction .
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