There are no limits to curiosity. We think about the evolution of life and our origins and even ask questions about the origin and ultimate end of the Universe. We seek answers to such questions in religion, philosophy and, most recently, in science. Today evidence suggests that the Universe originated in a Big Bang – an enormous expansion beginning some fifteen thousand million years ago, when the size of the whole Universe was less than the size of an atom. We have also found that all matter in the Universe is made up from a small number of basic building blocks, governed by a few fundamental forces. Particle physics is the study of these basic particles of matter and the forces that act upon them. It also looks at how matter evolved in the Universe, especially in the first seconds of the Big Bang, when many of the governing features of the Universe today were established. In this article we set out the ideas behind the scientific research at different high-energy research organizations.
Types Of Matter?
The world around us and the Universe beyond is built from a huge diversity of materials, and these forms of matter were common in the first instants of the early Universe. Surprisingly, however, this wide variety of matter is made from relatively few simple building blocks.
All types of known matter in the Universe is built from nearly a hundred different type of atoms, each of which is made up of electrons having negative electric charge circulating around a positively charged nucleus. The nucleus further consists of nucleons-positive protons and neutral neutrons – each some 2000 times heavier than the electron. In this way the infinite variety of matter in the Universe is built from only three particles: the electron, the proton and the neutron. The electron seems to have no internal structure. However, the nucleons are composite particles, each containing three quarks. Like the electron, the quarks appear to have no internal structure. Only two types of quark, called “up” and “down”, are needed to build the proton and neutron. They have charges of +2/3 and -1/3 compared with the electron’s charge of -1. One more structure less particle must be added to complete the picture. This is a neutral and very light (possibly massless) particle called the electron neutrino, which behaves like an electron with no charge. It plays a vital role in reactions that convert neutrons to protons and vice versa. Such reactions allow matter to find its most stable form, in the processes of radioactivity, which lead to the elements we observe. They are also important in fuelling the Sun and other stars (where the lightest element, hydrogen, is converted first to helium, and then to successively heavier elements).
Early in the 20th century, when Theodore Wulf took radiation detectors up the Eiffel Tower, he discovered that detectors registered more radiation at the top than on the ground (where he knew that radioactivity in the rocks gives a natural radiation). Studies of this “cosmic” radiation with detectors on the mountain-tops carried by balloons, showed that it is due to showers of particles created when high energy atomic nuclei (mainly protons) from outer space collide with atoms at the top of the earth’s atmosphere. This research has revealed that the particles created, known as cosmic rays, consist not only of the electrons, protons and neutrons of familiar matter, but also new kinds of particle. Near the ground, the cosmic rays include muons. These are just like the electron but 210 times heavier. But unlike electrons, muons are short lived. After an average lifetime of 2.2 microseconds, a muon will change into an electron and converting its extra mass into kinetic energy shared between the electron and two neutral particles. One is an electron neutrino, associated to the electron, while the other is a muon neutrino related to muon. The cosmic-ray muons emerge mainly from the decays of other short-lived particles. Some of these particles, like protons and neutrons, are made from up (denoted by u) and down (d) quarks. However, others contain a third type of quark, called the “strange” quark(s). So to understand the matter that exists as cosmic rays, we need more components than we need to make atoms. In addition to the electron, electron neutrino, up quark and down quark, we need the muon, the muon neutrino and the strange quark.
High Energy Matter
Cosmic rays provide an interesting natural high-energy laboratory, but it is difficult to work in this laboratory. The cosmic ray particles arrive unpredictably from all directions and with a whole range of energies. To study high energy particle collisions under more controlled conditions, particle physicists use laboratories such as CERN, DESY, FNAL etc, where high-energy particle colliders imitate the actions of cosmic rays in the atmosphere. Nowadays, these experiments reach energies that were common in the Universe only in the first instants of its existence.
Experiments at high energies have revealed new fundamental particles of matter that are too short-lived for easy identification in cosmic ray studies. One of the first to be found was a third charged particle like the electron, but far heavier than the electron or its other relative, the muon. This new particle, called the tau, is 3550 times heavier than the electron and lives for only one third of a millionth of a millionth of a second (0.3 x 10-12 s). The tau can change into the lighter electron or muon or even to the particles known as pions. Whichever way it decays, it always produces its neutral lightweight counterpart, the tau neutrino. Other heavy particles produced in high-energy collisions are composites, built from quarks, like the proton. However, these particles are much heavier because they include heavy quarks, which can be produced only at the higher energies. There are three heavier quarks called “charm”, “bottom” and “top”, which bring the total number of quarks to six. Unlike the lighter “up”, “down” and “strange” quarks, the three heavier quarks are even more massive than composite particles like the proton. The charm quark is one and a half times heavier than the proton, the bottom quark nearly five times heavier, while the top quarks weighs in with nearly 200 times the proton’s mass.
For each of the basic particles of matter, there also exists an antiparticle with properties such as electric charge are reversed. The common electron, for example, has negative charge, whilst its antiparticle, called the positron, has positive charge. Similarly the positively charged proton has a negatively charged antiparticle, the antiproton. Like the proton, the antiproton is a complex particle, but built from three antiquarks, with opposite charges to the quarks that form the proton. Antiparticles are made in energetic process together with particles – whenever a particle is created, an antiparticle must also be made. This means that there must be sufficient initial energy to make all the mass of the particle and antiparticle, according to the equation E=mc2. All kinds of particle-antiparticle pair can be made in this way provided there is enough energy. Particle-antiparticle pairs can be made when particles collide together, as in the energetic collisions of cosmic rays in the atmosphere. Positrons created in this way in cosmic ray interactions were the first antiparticles to be seen. Particle-antiparticle pairs can also materialize from the high-energy radiation known as gamma rays, provided the gamma ray has enough energy to make the necessary mass. When a particle and antiparticle of the same kind meet they soon disappear into a burst of pure energy, in a process called annihilation. The energy released is equal to the total energy of the annihilating pair, including the mass-energy, given by the equation E=mc2.
Measurements in astronomy imply that up to 90% or more of the Universe is in the form of “dark matter”. This matter does not emit light or any other detectable radiation, so we cannot “see” it, but its presence is felt through its gravitational effects on the matter we can see. For example Stars in galaxies appear to be moving much faster than they would if only the visible matter in the galaxy influenced them. The gravitational lensing of a distant galaxy due to a foreground cluster of galaxies two billion light years away has allowed the reconstruction of the total mass of the foreground cluster and shows that the dark matter outweighs all the stars in the cluster’s galaxies by 250 times. The total mass is over 300 million million million times the mass of the Earth. Individual galaxies in the cluster appear as mass pinnacles. Some of the dark matter may be in the form of large planets or dead stars made from ordinary protons and neutrons. However cosmological theories imply that a large fraction of the dark matter must be of an entirely different form. Whatever its nature, it must be very weakly interacting, otherwise it would already have been detected. One possibility is that the weakly interacting particles called neutrinos could have a small mass, and make up dark matter, but the behaviour of neutrinos creates problem in theories of how galaxies formed in the Universe. Another possibility is that dark matter could be in the form of particles predicted by theories, but not yet seen. The idea of “super-symmetry” links matter particles with force-carrying particles, and implies the existence of heavy “super-particles”. The lightest of these super-particles could be stable; in which case large numbers of them created in the early Universe could now have clustered into structures of dark matter on the scale of galaxies.