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In 1928, the British theorist Paul Dirac combined quantum theory and special relativity in his theory of the electron. The theory produced an equation with two solutions: one described the electron (with a negative charge); the other a similar particle but with a positive charge. The positive particle was found in 1932, in cosmic-ray showers, and it became known as the antielectron, or positron. Dirac's theory applies to all quarks and leptons, and their antiparticles are collectively known as antimatter. When a particle meets its antimatter partner they annihilate in a blaze of high-energy radiation. This property is used in particle-antiparticle collider experiments to create new particles and study their behaviour. Physicists usually designate an antiparticle by putting a bar over the symbol for the particle - for instance, a bottom, or b, quark has an antipartner , called b-bar.
What happened to antimatter?
Our understanding of how particles formed just after the Big Bang suggests that matter particles and their antimatter partners were created in equal amounts. They should have annihilated each other but we know that the visible Universe is made of matter, so what happened to all the antimatter? The Russian theorist Andrei Sakharov suggested that the answer lay in a subtle effect called CP-violation. The antimatter and most of the matter would have annihilated, but CP- violation means that matter and antimatter don’t always behave in the same way, resulting in a one in a billion imbalance in favour of ordinary matter.
Symmetry is an important mathematical concept used in fundamental physics to describe particle properties - right and left-handed gloves are symmetrical - one is a mirror-image of the other - and you can change one to another by flipping over the glove. Particles can be described in a similar way.
Antiparticles mirror their related particles by having the opposite sign for several properties, particularly electric charge. Particle theory expresses this relationship in terms of a mathematical operator, or ‘mirror’, designated C, which changes the signs of the charge and other properties. In this way, operating on a particle with the C mirror yields an antiparticle.
Another mathematical ‘mirror’, P, reverses particle interactions in space, rather like flipping the right-handed glove into the left-handed one. P changes the sign of a property called parity which, according to Dirac's equation is opposite for particles and antiparticles. In a particle interaction, if the signs for C and P totalled over the particles involved are the same before and after the interaction, then C and P are each said to be conserved.
Now as it happens, C and P are not always conserved and this is called CP-violation. The first evidence for CP-violation came to light in 1956 when it was found that ‘strange’ mesons called neutral kaons occasionally decay into two pions, a CP-violating process. Recently, various experiments have been investigating the phenomenon.
The NA-48 experiment
The NA-48 experiment at CERN, which has been operating since 1998, measures the decays of neutral kaons . Beams of the particles are made by firing high-energy protons at a target using the Super Proton Synchrotron (SPS). The idea is to look for tiny differences in the decay rates between kaons and antikaons. In 2001, a minute difference was indeed found.
Another experiment at CERN, CPLEAR, looked at neutral kaon decay from another point of view. As well as charge (C) and parity (P), there is another mathematical ‘mirror’ that reflects particles is time (T). The idea is that if any particle process is reversed, it should look like the mirror image of the forward process. Studies of neutral kaons and antikaons which interchange over time indicated that antikaons decayed faster than their normal-matter partners, showing that the CPT-mirror is broken.
Kaons are not the only particles to show CP-violation. Physicists realised that heavier mesons made from bottom (also called beauty) quarks - B-mesons - should show similar effects. UK researchers are currently participating in an experiment at the Stanford Linear Accelerator Center (SLAC) called BaBar (after B-bar). Electrons and positrons are injected into a particle accelerator, PEP-II,s which stores the particle beams in two separate concentric rings, 2 kilometres across When they meet and annihilate, they produce B and B-bar mesons. UK research groups helped to build the BaBar detector which detects B and B-Bar decays. Again, data taken so far are furnishing the first evidence of CP-violation in B-mesons.
The LHCb experiment being planned at the LHC will also study CP-violation in B-mesons, providing stringent tests of how CP-violation fits into the Standard Model of Particle Physics. UK research groups have major responsibilities for some of the LHCb detector components.
Neutron and electron dipole experiments
An entirely different approach is to make precise measurements on certain particle properties at very low energies. Two UK experiments are looking for tiny differences in how electric charge is distributed (the electric dipole moment) in both the neutron and the electron. These asymmetries would be a signature of CPT-violation.
The experiments rely on extremely sensitive measurements of the interaction of electron and neutron spins in strong, varying electric fields. In the case of the electron experiment, the electron sits in the intense electric field supplied by its parent molecule (ytterbium fluoride), and its spin then interacts with the oscillating electric field from a laser beam. The neutron measurements are made at ultra-cold temperatures using the neutron source at the Institute Laue Langevin in Grenoble, France.
Antihydrogen experiment at CERN-
A further low-energy experiment is being carried out by the Athena antihydrogen collaboration at CERN. UK researchers are heavily involved. Antiprotons are made, slowed down, and then chilled in a ‘trap’ of electric and magnetic fields to about 4 degrees about absolute zero. Positrons are introduced with the idea of making antihydrogen. In September 2002, the collaboration announced that they had made large numbers of antihydrogen atoms. If precise comparisons can be made of the energy levels of hydrogen and antihydrogen using a laser, then this will provide a stringent test of the CPT mirror.
Studies are under way on building colliders that would create beams of electron and muon neutrinos and antineutrinos - neutrino factories. CP-violating effects are also suspected to manifest themselves when neutrinos oscillate from one flavour to another. Looking further ahead, if large amounts of Higgs boson could be made, say in a muon collider, their decay could reveal new types of CP-violation.
Importance to theory
CP-violation not only explains the lack of antimatter in the Universe but also provides a uniquely subtle probe of particle physics theory. The more we understand its mechanism, the more we will learn about the ultimate workings of Nature.
Id 1502: For more information about this page contact Jane Butt. Last updated 28 October 2003