An introduction to Particle Physics

 

Particle physics is the study of the basic elements of matter and the forcesacting among them. It aims to determine the fundamental laws that control themake-up of matter and the physical universe. RALis one of the leading laboratories investigating this.

Probing Particles

Experiments at particle accelerators, such as LEP, where sub-atomic particles collide at very high energies, reveal details of particlesand conditions that prevailed just after the Big Bang over 15 billion years ago.Most experiments involve large international collaborations and are performed atoverseas laboratories such as CERN in Geneva andDESY in Hamburg. These collaborations typically involve more than 300 people and the work at CERN is supported by 19 European countries.

Accelerators

The accelerator is the basic tool of particle physics. It allows us to createthe particle collisions that we want to study in our own laboratories. The highenergy collisions between particles that physicists are interested in do occurnaturally but the events are unpredictable and the number that can be observed(in cosmic rays) is low.Accelerators work by accelerating charged particles using electric fields. Alinear accelerator accelerates particles in a straight line: the biggest linearmachine, in Stanford, California, is two miles long. Circular machines are morecommon. As well as accelerating the particles using an electric field, circularaccelerators bend their p aths using a magnetic field. In a machine like LEP atCERN, where they have opposite charges, the particles being accelerated travelin opposite directions until they are forced to collide. The drawback is thatthe faster a particle travels, the harder it is to keep it moving in a circlebut, in the largest circles (LEP is the largest in the world with acircumference of 27km) less energy is wasted when accelerating particles to highspeeds.

Detectors

Detectors are used to examine tracks made by the new particles that are producedwhen accelerated particles collide. In the early days photographic film, sparkchambers and bubble chambers were used. Since the late 1960s electronicdetectors have taken over. There are two basic kinds - tracking detectors whichreveal the trajectories of individual charged particles, and calorimeters whichmeasure energies. A modern electronic detector is built like an onion, withlayers of trackers and calorimeters to give as much information as possibleabout the particles produced in each collision.

Antimatter

Antimatter is very much like ordinary matter, but it carries the oppositecharge. An anti-electron (a positively charged electron) is just another way ofdescribing a positron.Crashing matter and antimatter together is now a dailyoccurence in machines like LEP. The fact that the universe seems to be full ofmatter and not antimatter is one of the most baffling problems in modernphysics. At the time of the Big Bang, matter and antimatter are believed tohave been produced in equal quantities. What seems to have happened is that, ata somewhat later time, collisions between the two types have destroyed all theantimatter but left a little of the matter behind, from which our universe ismade. The reason may be due to a tiny asymmetry in the way particles of matter andantimatter decay, thereby creating an excess of matter.

Big Bang Science

It is thought that the universe began around 15 billion years ago in the Big Bangand that it has been cooling down and expanding ever since. For physicists, themost interesting time was within the very first moment (within 10^-34 seconds)where the conditions were so extreme that the laws of physics as we know themtoday didn't apply. After about 0.01 seconds, the universe was cold enough forquarks to stick together, forming protons and neutrons. These formed the firsthelium nuclei after 100 seconds, but the first atoms didn't appear for 100,000years. After a few billion years stars began to form, using hydrogen and heliumto build the heavier elements that make up the familiar world around us -elements heavier than helium owe their origin to stars.The Big Bang theory correctly predicts that about 75% of all visible matter ishydrogen and about 25% helium. (All other matter accounts f or less than 1%.)Another great success of the theory is the presence of background microwaveradiation in our universe, a relic of the Big Bang.

Dark Matter

We know from observing the rotation of galaxies that about 90% of the matterthey contain is invisible to us. The matter we can't see is called "missing" or"dark" matter. The amount of dark matter contained in the universe is crucialto its fate. If it is greater than a certain amount, the universe willeventually collapse. Below this, and it will keep on expanding for ever.There are many ideas about what dark matter might be, ranging from exotic newparicles to black holes. One idea says that the neutrino,an abundant fundamental particle which is thought to have zero mass, actuallyhas a tiny mass. However, neutrinos generally move about the universe quicklyand are not stuck together in clumps, as they would need to be to explain the rotation of the galaxies. The most recent explanations of dark matter therefore use a combination of "hot" matter, like neutrinos, and "cold" matterlike black holes. The true answer has yet to be found. Underground experimentson dark matter are taking place now.