Studying the Collisons of Strongly Interacting Particles

Purpose

The purpose of the bubble chamber was to allow us to find out what happened when one strongly interacting particle (eg. p, p) collided (eg. p, n)  with another at high energies.

Simplest interaction

A bubble chamber filled with liquid hydrogen contains many proton targets and electrons that are sensitive to the passage of charged particles (‘detectors’). These may be knocked on to produce their own tracks.

Energy Conversion

Using our knowledge of the equation E = mc² we use beams of very high energies and collide them with target particles. The total energy at the start of the interaction must be the same as the total energy after the interaction by the law of conservation of energy. If we introduce enough kinetic energy into the system then particles may be produced and the energy stored as rest mass energy.

When looking at the particles produced in this way we want to know if particles of any mass can be produced or if there are constraints and only very specific masses of particles are produced. If this is the case is there any pattern that emerges which leads insight into the structure of these strongly interacting particles.

When two protons collide at very high energies the following tracks may be produced:

    The paths coming out of an interaction are commonly known as ‘prongs’ and the above interaction would be defined as ‘4 pronged’.

New Particles

We want to know about the properties of the new particles produced.

            Charge – obtained from direction of curvature

            Momentum – obtained form radius of curvature

Mass – the masses of the new particles can often be determined using knowledge of their momentum and energy. (Click here to see an example)

It was found that the masses of particles produced were not random. Only a certain number of particles were produced of definite masses.

These particles were initially identified by their charge and mass.

The different types of particles could also be determined visually by the different ways in which they decayed into other particles.

These decay patterns help to focus researchers’ attention to certain events when looking for the existence of new particles.

Categorising interactions

While the interactions of particles are usually indicated by the production of ‘prongs’, the decays of particles are usually indicated by ‘kinks’ and ‘vees’.

‘Kinks’

A ‘kink’ is caused when a charged particle decays into another charged particle and one or more neutral particles. We only see one track entering and leaving the event but the change in direction indicates the presence of one or more neutral particles in order of conserve momentum.

‘Vees’

A ‘vee’ is caused by the decay of a neutral particle into two charged particles.

  A special case of this is the K⁺ that sometimes decays into three pions producing a trident.

A special kind of signature is also produced by gamma rays that are often produced in a decay. If they have enough energy these may form spiralling electron positron pairs that have their own very characteristic signature.

These decay characteristics can be used to help identify the particles that have been produced in an interaction by inspection alone.

IMPORTANT

You must remember that not all particles will undergo their complete decay cycle within the bubble chamber, in fact very many of them won’t. So often you will only be able to narrow down the possibilities rather than obtain a definite answer.

Looking at the decays of particles

Initial Particle	Decay Products	Kinks	Vees	Trident	Gammas
p ±		Ö
K+	p+ p+ p -			Ö
K±	m± nm, p	Ö
S -	n p -	Ö
S +	n p+ or pp0	Ö
X -	L0 p -	Ö
W -	L0 K-	Ö
K0	p - p+		Ö
L0	pp-		Ö
p0	gg ® 2e+e-				Ö

This information can now be put together to form the characteristic signatures of the particles.