5. Probing matter page 21
Synchrotrons are now the standard particle accelerator for high energy collisions. They are extremely flexible in their applications. They can be used for positve or negative particles and for matter and antimatter.

The largest synchrotron is is being built at the European Laboratory for Particle Physics (known as CERN). It runs through a tunnel under the border between France and Switzerland. It has a 27 km circumference and can give an effective accelerating voltage of 100 GV (a hundred thousand million volts).

The principle of the synchrotron is quite straightforward – though their implementation is not so simple. A synchrotron is like a linear accelerator that has been twisted into a circle, forming a kind of doughnut shape.

Swapping voltages in a synchrotorn animation
Picture 5.8. This is a simplified picture of a synchrotron with only a few electrodes and coils. The voltages are swapped to make sure that the positive protons are always leaving a positive electrode. The coils provide a magnetic field that keeps the particles in orbit.
Synchronised spinning
Let's look at how we can accelerate (and keep accelerating) protons in a synchrotron. The protons are positive so we have to make sure they are always leaving a positive electrode and travelling towards a negative one.

Each time the protons go around the circle, the electrodes give them a push and accelerate them (just like the linear accelerator). The voltages on these electrodes have to be continually swapped over so that the protons are always travelling from a positive electrode to a negative one (picture 5.8). However, the rate at which the voltages swap over cannot be constant. As the protons speed up, they will spend less time between the electrodes so the voltages have to be swapped more rapidly. Also, the magnetic field has to be increased because a bigger force is needed to keep the faster electrons in the same orbit.

Both the increasing frequency of the voltage and the increasing magnetic field have to be synchronised with the speeding protons. Hence this accelerator is called a synchrotron.

Results of a high energy collision in an accelerator
Picture 5.9. The tracks of colliding particles and their products are recorded by special sensors and computers.
Experiments in accelerators
The aim of particle accelerators is to probe deeper and deeper into matter. Sometimes, the experimenters are trying to find new particles that have been proposed by theory. When the high energy particles collide in an accelerator, they break apart and form new particles.

The physicists monitor the tracks of any particles that are produced in the region of the collisions (picture 5.9). 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 particles are. 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.

Why so much voltage?
One reason for using such large amounts of energy is to try to turn that energy into the mass of new particles. At the beginning of the 20th century, Albert Einstein showed that mass and energy are equivalent and are related by the equation E=mc2 (see page 25).

When two particles crash into each other, they will break up and often form new particles. Amazingly, the new particles can sometimes have a bigger mass than the crashing particles. This is because some of the energy of the collision has become part of the mass of the new particles.

This is rather like firing two eggs at each other and producing a three-egg omelette (the combined kinetic energies of the eggs would have to be enormous – equivalent to the mass of an extra egg multiplied by the speed of light squared – about 1016 joules).

Searching for W
For example, the theory of the weak force includes field particles called W particles. These particles have a mass about 90 to 100 times that of a proton. In nature, they exist as virtual particles for an extremely short time during a beta decay (when a neutron turns into a proton). The challenge to physicists was to try to create these particles as real particles in an accelerator. They would do this by crashing protons into antiprotons with huge energies.

By using targets that were moving in opposite directions, they were able to increase the energy of the collision. So, in the egg analogy, if the energy of the eggs is doubled, then it would be possible to produce a four-egg omelette.

The kinetic energy in the collision had to be equivalent to the mass-energy of the W particles. They succeeded in producing W (and Z) particles in the early 1980s. This confirmed the theory that had predicted their existence and added to the understanding of particle physics.

Now they are in search of the more elusive Higgs particle.

The Big Bang (nearly)
Another reason for increasing the energy of the particles is to recreate the conditions that existed in the early seconds of the Universe, soon after the Big Bang. At this time there was an extremely hot and very dense soup of particles, called the primordial soup. The temperature of this soup meant that the particles had an enormous kinetic energy. The high energy collisions at this time produced lots of new particles. It is these conditions and these particles that physicists are now trying to make in accelerators. This will help them to understand the beginnings of the Universe.
Click here to open a new window with an interactive journey through time (this is a large Flash file).
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Question 20
The kinetic energy of particles in a gas is related to the temperature of the gas. As an estimate, the KE is equal to kT, where k is the Boltzman constant (k = 1.38 x 10-23 J K-1.

a) What is the energy (in joules) of a 100 GeV particle?

b) What temperature would a gas need to be for its particles to have this energy?

c) Why is this relevant to the work of particle physicists and cosmologists?