A review of its role in the ANTIMATTER universe and its applications THE DISCOVERY OF NATURE’S SYMMETRIES ntimatter plays an intrinsic role in our Aunderstanding of the subatomic world THE UNIVERSE THROUGH THE LOOKING-GLASS C.D. Anderson, Anderson, Emilio VisualSegrè Archives C.D. The beginning of the 20th century or vice versa, it absorbed or emitted saw a cascade of brilliant insights into quanta of electromagnetic radiation the nature of matter and energy. The of definite energy, giving rise to a first was Max Planck’s realisation that characteristic spectrum of bright or energy (in the form of electromagnetic dark lines at specific wavelengths. radiation i.e. light) had discrete values The Austrian physicist, Erwin – it was quantised. The second was Schrödinger laid down a more precise that energy and mass were equivalent, mathematical formulation of this as described by Einstein’s special behaviour based on wave theory and theory of relativity and his iconic probability – quantum mechanics. The first image of a positron track found in cosmic rays equation, E = mc2, where c is the The Schrödinger wave equation could speed of light in a vacuum; the theory predict the spectrum of the simplest or positron; when an electron also predicted that objects behave atom, hydrogen, which consists of met a positron, they would annihilate somewhat differently when moving a single electron orbiting a positive according to Einstein’s equation, proton. However, the spectrum generating two gamma rays in the featured additional lines that were not process. The concept of antimatter explained. In 1928, the British physicist was born. Paul Dirac realised that because the electron was very light and moved very THE IMPORTANCE OF DIRAC’S IDEAS quickly, special relativity needed to Today, it is accepted that all subatomic Paul Dirac's equation of quantum mechanics (right) be incorporated into the equation to particles can have a “mirror” or predicted the existence of antimatter explain this fine structure. antimatter counterpart with opposite with velocities close to the speed Dirac produced a more complicated charge and right- or left-handed spin. of light. The third was Niels Bohr’s wave equation that revealed the Furthermore, our understanding of ground-breaking description of matter electron’s true character. First, it particles and forces – the fundamental as composed of atoms consisting predicted that electrons had spin, building blocks of the universe – is built of lightweight, negatively charged which could be right- or left-handed. on these kinds of mirror symmetries, electrons orbiting a central, positively The second, more bizarre characteristic which are reflected in the powerful charged nucleus. The key feature of the was that the electron could have a mathematical framework of quantum model was that the electrons occupied range of negative as well as positive mechanics. The concept of symmetry is a series of energy levels or quantum energies. This suggested that this now used throughout physics to explain states. When an electron jumped from concept predicted the existence of a and classify phenomena. a lower quantum state to a higher one positively charged electron – Another important idea to emerge 2 Antimatter | A review of its role in the universe and its applications THE UK ROLE The UK has always been a world-leader in particle physics. It has participated in most of the major international experiments involving antimatter, particularly through its membership The first detection in 1983 of a carrier of the weak force, the Z particle, in CERN's ground-breaking proton–antiproton collider (the Super Proton Synchrotron), which was later superseded by the Large of CERN. The UK has invested in Electron–Positron collider, LEP (top right) technologies for future collider designs and experiments. It plans to become an Associate Member of FAIR. from Dirac’s equation is that the from its momentum and negative vacuum is not empty space but a charge, and show that it annihilated sea of virtual particles. According to in a characteristic way when it hit a quantum probability, particles can proton or neutron in ordinary matter. pop in and out of existence, allowing particle–antiparticle pairs to be THE ROLE OF ANTIMATTER IN generated from the vacuum according PARTICLE PHYSICS to Einstein’s equation. Experiments employing or generating antimatter particles have played a key role in formulating ideas about The high-energy FAIR facility in Germany will generate THE DISCOVERY OF ANTIPARTICLES intense antiproton beams for many experiments The existence of positrons was the fundamental particles and confirmed when they were discovered forces. Particle collisions at very composed of the two lightest quarks. in cosmic rays using a cloud chamber, high energies may generate particle– Each particle has an antimatter in 1932 by Carl Anderson at the antiparticle pairs, which provide partner. The four fundamental forces, California Institute of Technology. information about the characteristics the electromagnetic force, strong Then in 1955, a team led by Owen of the elementary units of which the and weak forces, and gravity, are Chamberlain and Emilio Segrè at the universe is composed. also described as being mediated by University of California at Berkeley Europe’s main particle physics particles, known as bosons (although uncovered the antiproton using a new research laboratory, CERN in Geneva, gravity is still not incorporated into the type of accelerator called the Bevatron. as well as laboratories in the US, used Standard Model). They were able to identify the particle beams of antiparticles to confirm the In the 1980s, experiments at so-called Standard Model of Particle CERN colliding beams of protons and Physics. In this quantum-based antiprotons confirmed the existence of picture, matter encompasses six types W and Z bosons, which carry the weak - of lightweight particles: the electron, force. These new particles were later π the muon and tau lepton, together studied throughout the 1990s using with their corresponding neutrinos; the Large Electron–Positron collider, η π + and also six heavy particles called LEP. A new electron-positron collider quarks: up, down, strange, charm, is being planned to probe the Higgs- π + bottom and top (in order of increasing boson-like particle that has now been π o - μ mass). Protons and neutrons are discovered in the Large Hadron Collider (LHC) at CERN. γ γ Another high-energy project at Darmstadt in Germany, FAIR (Facility for Antiproton and Ion Research), will μ + e- υ υ e- use high-intensity antiproton beams to investigate the strong force, and how matter first formed and evolved in + the universe. e e+ 3 NATURE’S SYMMETRIES BROKEN? e live in a universe made mostly Wof matter, so where did all the antimatter go? LOST WORLDS OF ANTIMATTER Physicists now believe that the particles “right- or left-handedness”; Universe was born some 13.75 billion and the reversibility of their behaviour years ago in a unified high-energy state in time (T) – the film of a particle that rapidly expanded and cooled, interaction looks the same whether run coalescing into the matter and force forwards or backwards. Particles and carriers responsible for our existence. their antiparticles with opposite charge Matter and antimatter particles and handedness should behave in the should have been created in equal same way, that is – symmetrically. numbers and should have annihilated. To search for any asymmetry, Somehow, some matter – but no researchers probe C, P and T, either antimatter – survived; observations separately or combined (CP or CPT), indicate that all the stars and galaxies by comparing how particles and their we see are made of the same version anti-versions behave. of matter as our own solar system The CPLEAR experiment at CERN investigated the and ourselves. CP mirror "breaking" of the CP mirror (CP-violation) in the Theoretical physicists think that early 1990s, using particles called kaons an imbalance between particles and decays of exotic composite particles antiparticles must have developed known as neutral kaons (K0). These during the primordial expansion, and consist of a down quark and a strange that there must be some inherent quark (p3) but exist in two forms that asymmetrical difference in the are each “quantum mixtures” of both behaviour of matter and antimatter the matter and antimatter versions. that favoured the survival of matter. The two forms decay at different rates, However, no one is sure what those + - and are known as K-long and K-short. processes were and how they Tiny discrepancies in their decay worked. Identifying subtle effects patterns indicated that the constituent associated with the matter–antimatter quarks and antiquarks were behaving asymmetry is now one of the main slightly differently in terms of their research areas in fundamental physics. Symmetry inherent in quantum mechanics shows parity, and were in fact “violating CP how the CP (charge–parity) "mirror" (a mathematical symmetry”. More precise experiments operation) reverses the basic attributes of SEARCHING FOR ASYMMETRY fundamental particles like electrons: the electric at CERN using the Low Energy As explained on p2, physicists classify charge transforms from negative to positive Antiproton Ring (LEAR) and at other the quantum properties of particles or vice-versa; and the direction of the particle's laboratories confirmed that CP and T according to various symmetries: in motion reverses with respect to the sense of its spin