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Summary and conclusions
The beauty of bubble chamber pictures is that they provide detailed pictures of the motion of any charged particles that have appeared in a bubble chamber as a result of an interaction between an incoming beam particle and a nucleus of an atom in the liquid. The tracks we see are trails of bubbles caused by ionization. The momenta of these charged particles are obtained by measuring the curvatures of the tracks. If the particles stop in the bubble chamber, their ranges (distance travelled) give their kinetic energies.
Common features:
- In most bubble chamber pictures there will be some small spirals caused by knock-on electrons. These can be used to tell the direction of the magnetic field. (Remember: the current is in the opposite direction to the direction of motion of the electron)
- Dark tracks: these are usually slow protons that have received only a small amount of momentum during the collision.
In the tutorial we have concentrated mainly on the features that tell us about the decays of strange particles (link to ptleprop1). Since these are weak-interaction decays (like beta-decay), the particles have a long enough lifetime to travel a measurable distance before they decay. Visual analysis of bubble chamber pictures is mainly one of looking for.
- Kinks:
in hydrogen, a kink, for example, is a signature for the decay of a charged particle
into another charged particle of the same sign but with a
lower momentum. (Some momentum will be carried off by one or more neutral
particles.)
The chances of noticing a low angle kink are improved if you print off the event and examine the picture with a low angle of viewing.
In liquids other than hydrogen, we cannot be so sure that kinks signal decays because a charged particle bouncing off a neutron might look like a kink.
- Vees: a vee, for example, signifies the
decay of a neutral particle. An important part of the pattern recognition
is to check
where the vee is coming from; there
are three possibilities:
- From the main collision
- From a kink (for charged
or
)
- Somewhere downstream of a kink (neutral
)
Another source of useful information:
-
Electron-positron pairs: these tell us that a high energy photon
has
'materialised’ in
the electric field of a nucleus. Most 
pairs
come from the decay of a 
to
two photons.
This is an electromagnetic decay, with a very short lifetime,
and so the pairs
appear to come directly from collision points.
Occasionally, one finds two pairs.
Measurement can show if they could be from a decay or are just a random pair of photons.
It is very satisfying to be able to recognize particles as an exercise in pattern recognition; however, it is not always possible – for example, particles might leave the bubble chamber before decaying.
(It must also be remembered that, in a real experiment, tracks are then measured – on more than one view if we are to be able to reconstruct the event in 3-dimensions - with the aim of determining the energies and momenta of as many particles as possible.)
We end with three exercise events: