There are two parts to be understood in particle physics. The first part is about the particles, and the second part about forces. Both are briefly described in this section. It will quickly become obvious that the two are intertwined.
In the particle physics view, the universe contains four "fundamental" forces; in order of strength these are the strong and weak nuclear forces, electromagnetism, and gravity. Although gravity is the force which we notice most in our day-to-day lives, it is too weak on the scales involved in high energy physics to have a noticable effect on the interactions we observe. These forces act on certain properties of matter; gravity acts on the mass, and electromagnetism acts on the charge of a particle, for example.
The picture of these forces is that two particles interact by means of an intermediate particle called a boson. This boson is specific to the force involved in the interaction. It can be pictured as a ball thrown between two ice-skaters. The act of throwing the ball (cf emitting the particle) will nudge the first skater off course slightly. Likewise, the act of catching the ball (cf absorbing the particle) will change the course of the second skater. The analogy is not perfect, but it serves to help provide a mental picture of these forces. In the case of electromagnetism, this "ball" is a photon. For the strong force, which acts only on very small scales, say about the size of an atomic nucleus, the particle is a gluon. The weak force has two associated particles, one with no charge, and the other which can have either positive or negative charge. The chargeless particle is the Z, while the charged particle is the W. The graviton has been postulated as the mediatory particle for gravity, but as yet it hasn't been detected.
So, bosons are the particles associated with forces. But there are, of course, other particles, which form the matter in our bodies, our homes and our world! These are split into two groups: hadrons, which are made up of quarks held together by the strong force (ie gluons), and leptons. Leptons and quarks are fundamental particles, or at least are believed to be so far! This means that they cannot be split into even smaller particles. (Incidentally, the word atom was derived from the Greek word meaning indivisible, just another one of the many mistaken assumptions we've made in the past!) However, while leptons have a charge of 1, quarks have fractional charges, and whereas we frequently encounter leptons isolated from other particles (electrons and neutrinos are examples of leptons), we have never discovered an isolated quark. It is believed that quarks cannot exist on their own. [Incidentally, that atoms can exist at all is quite amazing. It's the colour charge which determines the quarks that make up a particle, not the charge, so isn't it amazing that the proton (up,up,down quarks of red, green and blue colours - it doesn't matter which quark is which colour) has the same charge as the electron? After all, the electron isn't made of quarks!]
The leptons are divided into three pairs of particles; consisting of the particle and its corresponding neutrino. So we have the electron, and the electron neutrino, the muon and the muon neutrino, and the tau and the tau neutrino. Neutrinos are very difficult to observe, and there are huge experiments dedicated to measuring their properties.
Quarks come in six flavours, and are also divided into three pairs, consisting of one positively and one negatively charged quark. So we have the (up, down), (charm, strange), and (top, bottom) pairings. (The last pair are often more poetically referred to as truth and beauty).
Hadron is the name for any particle which consists of quarks. These are further split into two groups, called mesons and baryons. A baryon is a particle which consists of 3 quarks. Quarks have three properties that are important. The colour charge is the property of the quarks that the gluons couple to. You could say that it's the fact that quarks have colour which attracts the gluons (gluons also have colour). The flavour refers to the type of quark. There are six flavours: up, down, charm, strange, top and bottom (sometimes called truth and beauty). Each flavour can possess one of the three colours, and any particle we can actually observe will be white: that is to say made up of all three. The flavours all have different masses. The third property is the charge, which isn't important on nuclear scales, since the strong force dominates, but is important on scales the size of an atom. all different "colours". Each colour has an anti-colour, which allows mesons to exist. A meson is made up of one quark and one anti-quark, not necessarily of the same flavour, existing as one particle bound by gluons. Most of the matter we come into contact with on a daily basis is made up of baryons, since the proton and neutron in the nucleus are baryons, containing up and down quarks. Mesons are not often found in the world in general, or rather rarely noticed.
Forces |
Strong | Acts within a nucleus. Only acts on particles which possess a "colour charge" - hadrons. Is transmitted by gluons. Works on very small ranges and actually increases with distance. |
| Weak | Is responsible for radioactive beta decay. Transmitted by massive particles (W and Z), which means it's short range. | |
| Electromagnetic | Acts on charged particles, and is transmitted by photons. Long range. Is responsible for chemistry. | |
| Gravity | Acts on the mass of a particle. Not fully integrated with the rest of particle physics or quantum theory because it is incredibly weak on small scales. Better described by Einstein and General Relativity. | |
Particles |
Hadrons | Baryons: Baryons are composed of 3 quarks, each possessing a different colour charge. They are held together by gluons. The protons and neutrons in the nucleus of an atom are baryons. |
| Mesons: Mesons are made up of a quark and an anti-quark, not necessarily the same flavour (up, down, charm, strange, top, bottom). While one quark has a certain colour, the other will have the anti-colour, so that all hadrons are colourless. | ||
| Leptons | Leptons are fundamental particles. There are six leptons, the electron, the muon and the tau and their corresponding neutrinos. In matter, the electron, muon and tau are negatively charged, and their anti-particles are positively charged. The neutrinos are always neutral. |
As I'm sure you can imagine, these things are very difficult to prove. We don't yet have a microscope capable of "seeing" these particles. So we have designed large experiments which allow us to infer by experiment both the existance of these particles and the ways in which they interact.
Firstly, it's important to realise that on very small scales, matter does not necessarily act as we expect from day-to-day living. In fact, what we see is an average effect of millions of billions of particles interacting. So we had to watch and learn gradually.
The first particle physics experiments were small, low-budget affairs, mainly relying on cosmic rays to provide new particles. Detectors were often sent up in hot-air balloons, so that the particles didn't interact with the atmosphere before they could be observed.
With the computer age, particle physics has become large-scale. Experiments worth millions of pounds are running, attracting researchers from across the globe. Now, we artificially create out own particles, by accelerating and colliding beams of particles. The well-known relationship E=mc^2 comes in to play here; the higher energy the colliding particles have, the more massive are the particles we can create.
A particle physicist uses computers to search through the many electronic traces a particle leaves in the detector. Usually, an expertimentalist will be good at at least two of the three main disciplines: computer programming, physics, and hardware and electronics. In general, they will need to be competent in all three.
To give an insight into what it is like to be a particle physicist, we will leave aside matters of hardware and concentrate on how we get the physics.
First, you must settle on a type of physics you'd like to study, and a specific analysis. You may do this by reading papers written by theorists or by a different experiment. You may know of an old analysis that should be extended taking into account new knowledge. You read up on it, and find out as much as you can about the physics.
Now, you need to work out what this physics will look like in your detector. Are you looking for muons? For jets? Do you expect to find photons? And just as important, what other physics processes are there which look like your process, but are not? And how can you distinguish them?
You now write a computer program that will search through your experiment's memory of "interesting" events, and pass on to you all the information about these events that you need to know. You can use this information to produce graphs and plots. If there are any similar "background" processes going on, you have to find out how to get rid of them. We often use "Monte Carlo" programs for this.
A Monte Carlo program is a large program that is capable of simulating physics events. They are very useful, because although there's nothing actually happening, you have two pieces of information from them; what you put in, and what you get out. This is a useful way of simulating the background processes and of confirming that the subtraction process is effective. Monte Carlo programs are also used for comparing real data with expected results. In experiments which are not actually running due to an upgrade, for example, people will often do Monte Carlo studies of interesting physics to find out what they might expect.
So, you've written your program, and you've run on both data and Monte Carlo. You've subtracted your background, went over your results in a lot of detail to eliminate programming errors, and you're fairly confident that your results are accurate. You've got some pretty pictures, and you think you understand what's going on. You've calculated your error bars, and in general done everything you and anyone you're working with can think of.
At this point, it's over to the collaboration. Particle physics is in general made up of large international experiments including hundreds of people, and each paper has all of their names on it. So everyone is generally given a chance to contribute to the paper. Each collaboration has a different policy on this, so I can't describe it here. This process usually takes several months.
Finally, you have a paper. You may want to start a new analysis at this point....
It's a strange truth that studying the very smallest pieces of matter in the universe requires a very large piece of equipment. Particle accelerators exist in various places around the world. They take the form of large, circular-ish tunnels lined with magnets and electronics. And when I say large, they're often several kilometres round! At specific points around this tunnel will be located a detector. And this detctor is usually several metres high, and even more metres long.
A detector is specially designed for each experiment, taking into account the types of particles being accelerated, the energy of these particles, and the type of physics the experiment wants to study. It is built up of many parts. Each part will respond to different particles or to different properties of the particles. This must be translated to an electronic signal which can be saved on computer.
But, of course, it's not as simple as that, because I've left out the bad stuff. We don't have the technology to control individual particles, so we work in bunches of particles. Some of them don't interact the way we'd like them to. Some of them do all the boring stuff that we're not interested in. And we're also accepting a huge amount of information. We somehow need to sort out the good stuff from the bad before we store it. This is done via a complicated trigger system. This is mainly electronics which look at a very fast rate at the data, and if it passes certain conditions, they save the data. Usually there's more than one level of trigger - so very fast and basic decisions at the first level moves on to slower and more detailed decisions.
All of the hardware must be maintained, and this is a large part of the work involved in a collaboration.
So, here are some of the components that you might find in a particle physics detector:
It never ceases to amaze me that it's possible at all to get every individual component in a detector to work together. It's amazing that the information can be timed so precisely that components that are several metres apart can provide one snapshot of any given event.
Particle physics is a complex discipline, requiring insight into a lot of different ideas. An experimentalist in particular has to be aware of three things on three different levels at all times:
Page written by: Joanna Hamilton, May 2003.