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Particle physics, or elementary particle physics, is the study of how the smallest building blocks of matter interact with each other through forces. Quarks and electrons are examples of elementary particles. The concept of what is "elementary,'' i.e. not built of anything else, changes as science progresses. Thirty years ago, it was believed that the proton was an elementary particle, but it has long since been known that it is built from three quarks.
It is possible that some of the particles that we now regard as elementary are actually built from something else. Because we have not been able to see any substructure yet means that, even if such substructure is discovered, we can correctly continue to describe the particle as elementary in all circumstances that we have already experienced. A model does not become incorrect just because one learns something more. It rather becomes a valid part of a new, better description.
Experiments in particle physics usually proceed by smashing together particles so hard that one manages either to jiggle some of the substructure and maybe kick it out, or else to create entirely new particles from pure energy. For this one uses gigantic accelerators to make the particles collide with as high speeds as possible.
Particle theorists work in two different ways. Either they attempt to explain particular experimental results with mathematical models, or they independently construct mathematical models guided by principles of naturalness and beauty. Such models can suggest new experiments to be done and have often been able to predict the outcome of experiments before they have been performed.
Cosmology is the study of the origin of the universe. Particle Cosmology has developed as a science after it became clear in the 1960's that the universe originates from a Big Bang, and therefore that the very early universe must have been dense, hot and filled with particles that would, under such circumstances, collide at least as hard as in the collisions that we create in particle accelerators.
Because of this correspondence, many aspects of the very early universe can be described with models of particle physics. Conversely, we are now beginning to learn things about particle physics through observations that use telescopes and other methods to look far into the universe and deep into its past. The recent abundance of results from such observations has made the subject of particle cosmology blossom.
Field theory in its various forms (classical, quantum, finite-temperature, conformal, topological) is a powerful mathematical tool that can provide solutions to physical problems in areas as diverse as the interactions of elementary particles, the birth of the universe, and microelectronics.
A field , or classical field, is a mathematical entity which can be thought of as a collection of numbers that tells you something about each point in space-time, such as the temperature at each point in a room. A quantum field is a more complicated mathematical entity which, in particular, can describe the creation and destruction of particles. Field theory is a collection of rules that determine how fields behave and evolve.
Quantum field theory has been exceedingly successful in explaining phenomena in particle physics involving the strong, weak, and electromagnetic forces, which together with gravity constitute the forces of Nature. For example, there is a particle called the muon which is a heavier cousin of the electron. The muon's magnetic dipole moment , i.e. ability to respond like a compass needle to a magnetic field, has been calculated so accurately using quantum field theory that the difference between the calculated value and the value measured in experiments is only a few parts in a million.
Field theory is successful not only for tiny particles, but can be applied to the universe as a whole. The best explanation of the observed expansion of our universe is that it originates from a point-like superdense state, the Big Bang. Using Einstein's field theory of gravitation, one can even prove that Big Bang must have happened, if one takes into account the fact that the universe today appears similar when viewed from any arbitrary point within it (homogeneity) and independent of the direction in which one chooses to look (isotropy).
Field theory in all its glory, there are numerous shortcomings of the present models. The Standard Model of particle physics fails to explain why the various particles that constitute matter (electron, proton, to name a few) have precisely the masses that we measure. For example, why is the proton 1836 times heavier than the electron, rather than some other number?
The Standard Model also cannot explain why there are three almost identical families of particles, when only the first family occurs in ordinary matter and the other two families consist of heavier cousins that can only be lured into existence through smashing particles together in gigantic particle accelerators.
More complicated models, based on the idea that the different forces of Nature become one and the same (unification) at very high energies, may give the answer to some of these questions. There is the problem, however, that many of these mathematical models contain parameters that must be fine-tuned to incredible precision in order to get the models to work, and this makes the models both unnatural and improbable.
The requirement of fine-tuning can be avoided through a symmetry that may exist in Nature, called supersymmetry , which would automatically enforce particular relations between the values of the parameters and so avoid the problem of fine-tuning. Supersymmetry is also a required ingredient in string theory , which is so far the only successful attempt to unify gravity with the other three forces of Nature.
Cosmological models also suffer from incompleteness and uncertainty. By observing the motion of distant galaxies one is able to calculate the mass density of the universe (i.e. "weigh'' it). On the other hand, we now know enough about nuclear physics that we can compute the maximal number of atomic nuclei of each kind that could have been assembled in the early universe, and this gives a mass which is only about 2 - 10 % of the mass that should be there according to the galaxies' motion. It seems that the galaxies, and also stars in our own galaxy, are being tugged by some mysterious "Dark Matter''.
Some of this dark matter could be comprised of particles that would exist in the supersymmetric models of particle physics that were mentioned above. Other dark matter could simply be small, compact objects of ordinary matter that do not shine (such as a small, dark sun). The number of such objects is currently being measured by viewing the light from distant light sources in the universe and studying how the light is lensed (bent) by the gravitational field around massive objects that pass between us and the light source.
According to Einstein's field theory of gravitation, matter causes space-time to curve around it. In addition to matter, Einstein's theory permits another source of curvature called the cosmological constant. Unlike ordinary matter, which gets less dense when it is spread out, the cosmological constant represents a kind of matter with the curious property that it remains at constant density when it is stretched. Recent observations of light from bright stellar explosions called supernovae seem to indicate that space-time actually is curved in such a way that the curvature appears to be caused in part by a cosmological constant.
In Einstein's theory, the constant can take any value it pleases, but if one is to explain it with particle physics or field theory, as we would like to do, the constant should be either incredibly large or zero! The fact that it seems to be rather small and non-zero is one of the great remaining puzzles of particle physics and cosmology.
In particle accelerators today it is just as easy to produce antimatter as it is to produce matter. Very small quantities of antimatter are also produced inside your body every second, but not enough to harm you. Antimatter is essentially the same as matter, but with some properties opposite. In ordinary matter, for example, negatively charged electrons "orbit'' a positive nucleus and together they build up an atom. In antimatter , however, the atom has a positive electron and a negative nucleus.
The universe today contains a noticable excess of matter versus antimatter. To see this, try to shake hands with the person next to you. If he or she is made of antimatter, you will both annihilate in a big flash. Be brave now!
The reason for the observed asymmetry between matter and antimatter has so far not been resolved, but it is believed that the origin is cosmological, i.e. that it results from some particle-physics process in the very early universe.
Observations of light from spiral galaxies, and other observations of our own galaxy the Milky Way, show that galaxies are permeated by a weak magnetic field (about 100,000 times weaker than the magnetic field of the earth). Many astrophysicists now believe that this magnetic field has evolved from a primordial field, i.e. one that existed before the galaxies even formed. If so, it is possible that the magnetic fields were created in a "phase transition'' in the early universe. The next section talks about such phase transitions.
Phase transitions are common phenomena in everyday life and happen, for example, when humidity (steam) condenses to form water on a cold surface, or when water freezes to form ice. In both cases H2O remains the same type of matter, but its phase changes.
In models of particle physics, phase transitions are necessary in order to explain how some elementary particles, such as the W boson, could receive its mass and yet not introduce devastating mathematical infinities (such as 1 divided by 0) in the quantum field theory.
Phase transitions happen naturally in the early universe, because it cools when it expands. When the universe goes through a phase transition it changes appearance, just as water does when it turns to ice, but essentially remains the same universe.
A phase transition can leave behind traces in the form of "topological defects''. Cosmic strings are an example of such defects and can be thought of as slithering tubes - piercing the universe - inside which some part of the old phase got trapped and never got converted. Such strings would interact with matter and could have important consequences for the clumping of matter in the early universe and the formation of galaxies.
Cosmic strings would also give rise to a very particular kind of irregularity in the cosmic microwave background radiation . This is the radiation left over from Big Bang which is still reaching us from all corners of Cosmos. A cosmic string would show up as a rather sharp silhouette against this background, and such a signature will be detectable in either of two very precise experiments, called MAP and the Planck Surveyor, that will soon be operating on satellites in orbit around the earth.
Several gigantic particle accelerators are currently being used to smash together small particles, such as electrons and protons, with higher and higher speeds. Amongst these are LEP in Switzerland, DESY in Germany, and Fermilab in the Unites States. The experiments are so difficult to carry out that it is not unusual to have between 200 and 1000 people from many nations collaborate on a single experiment. The aim of these experiments is to try to find new particles that are predicted to exist, such as the Higgs particle predicted by the Standard Model and other particles predicted by the supersymmetric extensions of the Standard Model (see above).
Two new laboratories are now being built for carrying out a new type of experiment. The Large Hadron Collider (in Switzerland) and RHIC (in the United States) will smash much heavier particles together, such as nuclei of heavy elements each consisting of many protons and neutrons. Such collisions will probe the nature of the colour force ("glue'') that binds quarks together in a proton. This will lead to new insights into the theory that describes these phenomena, Quantum ChromoDynamics (QCD).