Finding new particles history 6
In the second half of the twentieth century, scientists discovered hundreds of new particles. They found them in cosmic rays and through the use of accelerators that were producing higher energy collisions. Most of these particles were hadrons. This is the family of particles that feel the strong nuclear force and includes protons and neutrons. However, they also found evidence of the force-carrying particles called gauge bosons.
Pictures of particles
Results of a high energy collision in an accelerator
Figure 1. The tracks of colliding particles and their products are recorded by special sensors and computers.
To find new particles, physicists set up high-energy collisions in a particle accelerator. They monitor the tracks of any particles that are produced in the region of the collisions (figure 1). Notice that some of the tracks are curved. These are the tracks of charged particles as they move through a magnetic field. From the curvature of the tracks the physicists can work out what the particle is. They can then deduce which particles made the other tracks. Sometimes, they have to do thousands of experiments before the desired collision takes place. They use computers to analyse the results and discard any that fail to show anything new.
Too many particles
Hundreds of hadrons explained by a few quarks with roll over highlight
In the 1950s, the mounting number of hadrons was beginning to complicate the picture of fundamental particles. With so many types of particle, people began looking for patterns to see if they were made up from something smaller. Murray Gell-Mann and Yuval Ne’eman began putting the hadrons into families. In the same way that Mendeleev had looked for patterns in the elements, they looked for patterns in the properties of hadrons and for gaps that needed filling. They explained the layout of their families using a new type of particle – the quark. They needed only six quarks, which joined together in different combinations to make the hundreds of hadrons. The quarks are fundamental particles. They are one of the families of fundamental particle that make up the Standard Model.
Explaing the hadron families in terms of quarks
The story so far
There were hundreds of new particles discovered in the 1950s onwards
The quark theory was developed to explain the existence and properties of all these particles
Question H6

a) Where did physicists start finding new particles?

b) A muon has the same charge as an electron but 207 times its mass. Assuming that they both have the same energy, which will bend more in a magnetic field - a muon or an electron? Explain your answer.

c) Imagine a proton and an electron with the same energy going through a magnetic field. Give two differences about their curved paths.

The story so far
  • Rutherford had determined that the atom has a positive nucleus
  • The nucleus contains parotons and neutrons
  • Physicists in the 1950s were setting up high energy collisions in accelerators to look for new particles