eXTReMe Tracker

Superconducting cavities for the LEP accelerator

The ALEPH detector at LEP

The L3 detector at LEP

The OPAL detector at LEP

The DELPHI detector at LEP

Particle tracks in an old-style Bubble Chamber picture
Zeptoscale Physics: Particle Physics in Plain English.
High Energy Physics research is about matter, force, space, and time at the most fundamental level. It takes you on a trip down through the atom, inside the atomic nucleus, to the quantum world of subatomic particles and elementary forces. Or it can take you out beyond the Solar System, to Supernova explosions, gamma ray bursts, and supermassive black holes, even to cosmological distance scales of order the size of the known Universe; it can go off to higher-dimensional worlds of curled-up extra spatial dimensions; or it can be a journey in time, back to the Big Bang or to the far future; regardless of the direction, it is always, to me, a voyage of challenge and of wonder. It is also one of evolution, as new discoveries push back the edge of knowledge, and as new speculations form on what migth lie behind it. Fermilab
Wilson Hall at Fermilab.

 * Over the last century, in the wake of Einstein's theory of relativity and Bohr's quantum mechanics, came sea changes to our perception of almost all fundamental aspects of Nature. With the new fundamental discoveries came also a truly enormous number of successively more advanced applications that were to shape our future everyday lives, like semiconductors, electronics, lasers, computers, optical fibers, superconductivity, and so on.

On the practical side, all that is required to explore Nature at its most extreme, is a perception tuned to very small wavelengths (so we can see really tiny things). In physics, this is equivalent to using probes of extremely high energies. The proper equipment for the expedition thus normally consists of, firstly, the vehicle, a good-sized particle accelerator, secondly, the engine, an amount of electricity that otherwise might keep a small city comfortable, and, thirdly, the eye, a hi-tech wonder composed of gas, steel, lead, cryogenics, computer farms, and electronics, the detector. CDF
The CDF Detector at

Thus equipped, the tiny world of elementary particles, beginning at length scales below a millionth of a nanometre, may be opened to human investigation. (Theorists, of course, will undertake the same expedition without all the equipment and are therefore much cheaper, though somewhat less reliable.)

 * Today, particle accelerators are capable of reaching energy scales several billion times larger than those of ordinary chemical reactions. The highest energies in the world are currently achieved at the Tevatron accelerator at Fermilab, which collides protons with antiprotons at energies thousands of billions times larger than the energy released by the combustion of an octane molecule in a car engine. At these energies, the ordinary phases of matter (solids, liquids, and gasses) are no longer meaningful; we are in a domain where the effects of relativity and quantum mechanics are dominating; matter and antimatter can be created and annihilated from the vacuum, and particles are not localised but are described by fuzzy quantum wave functions, or `fields'. D0
The DØ Detector at Fermilab

In fact, this fundamental `fuzziness' of Nature is one of the most striking aspects of quantum physics. Observing the light from a faraway star, for instance, sensitive particles in our eyes are being gently swayed from side to side by small waves in the electromagnetic field, the crests of which are a few hundred nanometres apart. Due to the fuzziness of the light quanta, visible light could never tell us about structures smaller than that. Again, that is why `particle physics' and `high energy physics' are synonymous; to resolve things that are, at the largest, 100 million times smaller than the distance between two crests of a wave of visible light, we need probes of extremely short wavelengths. That is precisely what is obtained by accelerating matter to high energies. The higher the energy, the better our 'vision' in this 'fuzzy' quantum land.

 * Returning to antimatter, the discovery of the anti-electron, or positron, dates as far back as 1932. Today, antipartners have been found for all of the known matter particles. These antiparticles have exactly the same masses as their particle counterparts, but they carry diametrically opposite charges. The billion dollar question, of course, is why? In the quantum theory of fields, this phenomenon found a very elegant and natural explanation. Matter and antimatter appear simply as two different states of the same field; so you can't have one without the other. A more intuitive way of looking at antimatter is that antimatter in fact acts just like matter travelling backwards through time. Think about it. positron
The discovery of antimatter: positron track in a cloud chamber.
[C.D. Anderson, Phys. Rev. 43, 491 (1933).]

Matter and force are also intimately related - by symmetries. Among the true success stories of 20th century theoretical physics was the discovery of 'gauge forces'. These are forces that have no existence 'by themselves'. Instead, they arise, naturally and elegantly, almost as if by magic, as simple consequences of the symmetry properties of matter. These symmetries, called gauge symmetries, are not spatial symmetries, rather they are symmetries of the quantum charges carried by the matter fields, like the phase invariance of small electric circuits.

event in D0
A top quark event recorded by the DØ detector at Fermilab
 * My own principal interest lies with a very colourful but quite complicated force of nature, the so-called Strong Force. As just described, it is believed to arise as a consequence of symmetry properties of the matter quanta, here called quarks (eg nuclear matter, like protons and neutrons, is made up of quarks). However, the symmetry is not quite as simple as that governing the familiar electromagnetic force. Instead of just having positive and negative charge, there exist 3 different kinds of charge for the strong force, with corresponding anticharges.

These 3 different kinds of "strong charge" are suggestively labelled red, green, and blue colours (and anticolours: cyan, magenta, and yellow, respectively), since they mix with each other in a way similar to that of ordinary coloured light. Moreover, the force quanta, called gluons, themselves carry strong charge (and even more of it than the quarks!) --- a fact which causes the strong force to give us considerably larger headaches than simple electromagnetism.

The most immediate consequence may be described as anti-screening. Drop a coloured particle in vacuum, and fix it in place. Then watch what happens as you move away from it, carrying an anticharge with you. Anti-screening means that the colour charge will seem to increase steadily as you withdraw (more precisely, as the resolution scale is decreased), causing a stronger and stronger attraction. In fact, you won't reach many femtometres away before the colour field between the two charges narrows into a string-like flux tube of high energy density that pulls at you in a tight, elastic-like manner.

That can't go on forever. If you are not very strong, you'll give up and let the charges stay together. If you are more determined and keep pulling harder and harder, at some point your continued exertions will have pumped enough energy into the field to set the vacuum boiling! Drawing on the colour field energy, virtual quark-antiquark pairs living as quantum fluctuations in the vacuum may make the transition to become real in a manner that screens and severs the connection between the two original charges. This mechanism effectively confines colour charges to the realm of sub-femtometre distances, since any attempt to separate charges by more than that will immediately be neutralised by the creation of compensating charges from the vacuum. Hence quarks and gluons are hadronized at ordinary energies, ie they are locked up inside colour neutral composite particles, like the proton and neutron, collectively called hadrons.

 * This process of hadronization is extremely hard to calculate. In fact, it is so hard it has not even been rigorously proven mathematically to exist in the theory. Some of my work has focussed on how hadronization occurs in certain interesting topologies of the colour field, for instance the field you would get if you could put a stick of dynamite inside a proton, blow it up, and watch how the fragments subsequently hadronizes into blobs of hadronic matter, mostly pions, kaons, and nucleons. In fact, it is something along these lines that occurs inside the collision halls at the Tevatron accelerator right here at Fermilab as you read this, several times every microsecond.
The Tevatron and Main Injector at Fermilab

Such collisions also easily involve hundreds of (calculable) interfering subprocesses, in addition to the goings-on just mentioned. Highly complicated physics models and extensive computer algorithms are often needed to make the calculations tractable. This work, which also takes a lot of my time, has the added bonus that it not only allows us to study theoretical predictions in detail, it also provides high-accuracy computer simulations of particle collisions which can be used by experimenters for detector design and performance studies, and for calibration of the final experiment.

 * Finally, who knows what may be discovered tomorrow? From the experimental point of view, the search is on for anything that might not agree with the predictions of the so-called Standard Model of Particle Physics, which incorporates the state of the art of our present day knowledge. Scores of measurements are continually being carried out, testing one or another facet of the Standard Model. Meanwhile, within the limits permitted by existing data, theorists speculate.
The Large Hadron Collider at CERN, Geneva, scheduled to turn on in 2007.

Despite its many successes the Standard Model is not likely to be `the end of the line'; there are missing pieces and internal inconsistencies in the model, which point towards the existence of hitherto undiscovered properties of Nature, some of them may be just around the corner! Naturally, it is exciting to speculate on what these properties might be, and how best to search for them. The game is basically to come up with an alternative theory that
  • agrees with present data,
  • `solves' one or more problems in the Standard Model, preferably without introducing new ones,
  • gives predictions which are testable at one or more future experiments,
  • is the one for which evidence is found at those experiments, this last condition being the one required to go to Stockholm.

The LEP tunnel, where the
LHC is currently being built.
 * At the most general level, the open-minded model-builder has several starting options available. It is possible, for instance, that there could exist new kinds of fundamental matter. Some possibilities are the existence of something called a 4th generation of fermions, and models with more Higgs bosons (or other -ons). It is also possible that what we think of as being fundamental matter, is not. Ideas along these lines are compositeness (substructure in quarks and leptons), technicolour (compositeness of the Higgs boson), and even string theory (where point-like particles are replaced by strings or even membranes).

There could also exist a `fifth force' in Nature, i.e. a new gauge symmetry. Familiar examples are here models with Z' bosons, so-called Left-Right symmetric models, and also technicolour models. The known forces might also turn out all to be but different aspects of one 'original' force, as in Grand Unified Theories. And finally, our concepts of space--time are not inviolate either. There could exist a new kind of space--time symmetry (called supersymmetry). Or relativistic (Lorentz) invariance could be violated to some small extent. Also, when thinking of space and time it is natural to ask why the universe appears to have exactly one time and three space dimensions. The last decade has seen an explosion of activity on this question, with the emergence of many new theories incorporating one or several extra dimensions, which are then curled up in more or less sophisticated ways.

 * I have focussed mainly on the consequences of a hypothetical new type of space-time symmetry called supersymmetry, or SUSY for short. If this symmetry really is a property of Nature, there should exist a large variety of hitherto unseen exotic particles, which should be detectable in experiments at forthcoming particle accelerators. Conversely, if the experiments at some point do reveal that such particles exist in Nature, we may have discovered a completely new type of symmetry of space and time!
LHC experiments: ATLAS (left) and CMS (right).

Another topic which has caught my interest in the past is what happens when you 'invert' space (sort of like turning it inside out by performing a point reflection through the origin) and simultanously exchange matter by antimatter. This 'parity-plus-charge-conjugation' operation, called CP for short, is almost a symmetry of the theory of the natural forces, but not quite. For instance, violation of CP symmetry shows up in decays of heavy b (beauty) quarks living inside bound states, so-called B mesons. One of the ways of exploring physics beyond the Standard Model, is by observing the magnitude of this violation, and checking whether the Standard Model alone is capable of explaining all the data, or whether 'something else' is required. This 'something else' might well arise if, for instance, the Universe is supersymmetric.

 * To end on a philosophical note, it is worth thinking about the fact that the matter-antimatter asymmetry is actually a very important aspect of Nature to us, since we, and all the stars and galaxies that we can see, are made of matter, not antimatter. If the laws of Nature were exactly matter-antimatter symmetric, for instance, it is difficult to see how we could have come to exist at all.

You may also want to look at: