Question: What is the evidence for the presence of 3 bosons for the weak interaction when all the other fundamental forces only have 1 each?

David Freeborn answered on 21 Jun 2013:

Hi alexlw18,

Good question- but not an easy one to answer. I’ll have a go.

But the first thing I should say is this isn’t quite true. You’re right that the weak force has 3 bosons, but it’s not true to say the others only have one. Electromagnetism has only 1 boson: the photon, but the strong force has 8. This is because there are 8 different kinds of gluon, each with different combinations of “colour charge”.

The easy answer is to say that we’ve discovered the three weak bosons (called W+, W- and Z) in experiments. The first sign came from the Gargamelle Bubble Chamber at CERN in 1973. A bubble chamber allows physicists to see the tracks left by charged particles in a magnetic field: by studying the curves of the tracks, and the watching these tracks split, they are able to work out the mass and charge of particles and what they decay into. Gaps in the track can be used to deduce electronically neutral particles too, and that was how physicists first spotted the Z boson. You can see the original bubble-chamber photograph here:

The formal discovery was announced by CERN four years later in 1983, at a particle accelerator called the Super Proton Synchrotron. Physicists were able to measure the energy, mass, charge and momentum of the particles that the W and Z bosons decay into, and from this they “discovered” the particles.

But of course, physicists had predicted these particles in advance, and they had done that using the “mathematical symmetries” of the forces. Each of the different forces in nature corresponds to a different type of mathematical symmetry, and this is one reason why physicists consider these theories to be very beautiful.

A symmetry tells us how many times we can change things, whilst still keeping it looking the same way. So a reflection symmetry tells us we can turn an object over, and it still looks the same; a rotational symmetry tells us we can turn an object round, and it still looks the same. These mathematical symmetries tell us that we should be able to perform mathematical operations on these particles, and they still behave the same way. But there are only a limited number of ways to “construct” a particle that will still behave the same way under symmetry.

The different number of bosons of each of the forces (1 for electromagnetism, 3 for the weak*, and 8 for the strong) are just the number of ways we can construct a particle that is symmetric in the right way for each force.

*(Really, the weak interaction and electromagnetic interaction are both part of the same interaction, called “electroweak”, with 4 different bosons. But that doesn’t really matter for this explanation.)