The Standard Model by Christine Sutton In May 1983, the central detector of the UA1 experiment at CERN's proton-antiproton collider showed the tell-tale signature of the long-awaited Z particle as it decays into an electron-positron pair (arrowed). As the electrically neutral carrier of the weak force, the Z° plays a vital role in the Standard Model. The initial evidence from Fermilab (see previous article) for the long awaited sixth ('top') quark puts another rivet in the already firm structure of today's Standard Model of physics. Analysis of the Fermilab CDF data gives a top mass of 174 GeV with an error of ten per cent either way. This falls within the mass band predicted by the sum total of world Standard Model data and underlines our understanding of physics in terms of six quarks and six leptons. Model encompasses all the elemen­ their interactions emerge. Instead it is In this specially commissioned tary particles we now know and three an amalgam of the best theories we overview, physics writer Christine of the fundamental forces. The basic have, which we can bolt together Sutton explains the Standard building blocks are two sets or because they have enough in com­ Model. "families" or "matter particles" - the mon to suggest an underlying unity, quarks and the leptons (see page 5). although due to our ignorance the These particles interact with each joins still clearly show. t is nearly 100 years since the other through the exchange of force The structure as a whole rests on a I discovery of the first subatomic carriers or "messengers". (These single theoretical framework known particle, the electron, which we still messengers are also particles, but as quantum field theory. This has its recognize as one of the basic build­ they are distinct from matter particles roots in attempts to understand the ing blocks of matter. Since then as we shall see.) The three forces of most familiar of the three forces of research has revealed a rich the Standard Model are the electro­ the Standard Model, the electromag­ "microworld" of particles, from pro­ magnetic force, which acts only on netic force, which acts upon anything tons and neutrons to quarks, gluons, charged particles; the strong force, with an electric charge. The charge is and W and Z particles. The field has which acts only on quarks and is the source of an electromagnetic flourished particularly during the past ultimately responsible for binding field, and it is our understanding of 40 years, culminating today in what protons and neutrons within the how this field works at a fundamental we call the Standard Model of parti­ nucleus; and the weak force which level that has led to quantum field cle physics. acts upon all quarks and leptons, theory, and the concept of the mes­ Standard models arise in many including those with no electric senger particles. different parts of science. They charge, and which underlies radioac­ Quantum field theory treats the provide a basis for understanding the tive beta-decay. (A fourth force, electromagnetic field as a sea of tiny behaviour of a particular system. The gravity, remains outside the Standard lumps of energy, or photons, the Ancient Greeks, for example, had Model, but this does not invalidate "particles" of light. In electromagnetic their own standard model of matter the model as gravitational effects on radiation, such as visible light, the built from four "elements" - earth, fire, particles are far smaller than the photons are "real"; in other words, air and water - with which they tried effects of the other forces.) energy is conserved when they are to explain various phenomena in the The Standard Model is a synthesis emitted or absorbed. However world about them. Nowadays, of our present understanding of the photons that do not conserve energy astrophysicists talk of a "standard quarks and leptons and the forces can also exist, albeit only temporarily. solar model", which follows the that act upon them. The key word Their "borrowed" energy must be evolution of the Sun from an initial here is "synthesis", for the model is repaid according to the dictates of prescription to its present state. not an elegantly hewn theory from Heisenberg's Uncertainty Principle, In particle physics, the Standard which the quarks and leptons and which limits the time for the "loan" - 4 CERN Courier, June 1994 The Standard Model mm 1/1 The matter particles - quarks and leptons - of the Standard Model. (On the right are the 'messenger' or field particles which carry the different forces of Nature.) the time between emission and absorption. In this way, the imbal­ ance is not observable, but is masked by the uncertainty inherent in processes that occur at a quantum level. Such photons living on bor­ rowed energy are known as "virtual" photons, and they are the messenger particles of electromagnetism. It is v <m<* the p&rt&tes eanyaag forees fegfwseen them they that carry the electromagnetic force between electrically charged particles. Quarks and leptons Quantum electrodynamics (QED), of everyday matter. Nature how­ the quantum field theory of electro- Research in particle physics has ever apparently requires the magnetism, is arguably the best revealed a wide variety of suba­ existence of two additional, theory we have, the effects of the tomic particles, from the long-lived heavier "generations" of quark electromagnetic force being predict­ proton and electron to the pairs - charm and strange, top and able to better than 1 part in 100 resonances that live for only ICt23 bottom. million. One of the theory's most seconds, no longer than it takes The lepton family also contains important features is its "gauge for light to cross them. Beneath six members, but in this case they symmetry", which means that when this rich world, however, lies a can be grouped in pairs with independent changes to local field deeper simplicity, for we have charges -1 and 0. The most values are made at different points in found that it is built from only two familiar lepton is also the lightest space the equations of QED are not types of building block - the quarks charged lepton, the electron. This changed. However, this symmetry is and the leptons. occurs in stable matter, while its ensured only if the quantum descrip­ The quarks are distinguished by neutral partner, the electron- tion of a charged particle contains an the fact that they feel the strong neutrino, is emitted naturally in electromagnetic field with its messen­ force, and that they carry "frac­ beta-decay when radioactive ger particle; in other words, gauge tional" electric charge, either 2/3 or nuclei change to more stable symmetry demands the existence of -1/3 the size of the electron's forms. As with the quarks, this pair the electromagnetic force and the charge. The strong force binds the of leptons is repeated in two more photon! Moreover, the symmetry is quarks within the larger clusters generations, in which the charged intimately linked to the ability to we observe as particles. Three member increases in mass. The "renormalize" QED, so that it yields quarks form a baryon, such as the neutral members - the neutrinos - sensible, finite results. familiar proton, while a quark have little mass by comparison, The messenger particle must have coupled with an antiquark together and may indeed all be massless. an intrinsic angular momentum, or form a meson, for example the In 1989, some of the first results spin, of one unit, just as the photon pions and kaons common in from the new SLD and LEP does. Particles that have whole units cosmic rays and in accelerator electron-positron colliders at of spin are known as "bosons", so the experiments. Stanford (SLAC) and CERN photon, which ensures the gauge To make all the known baryons respectively showed that there are symmetry of QED, is referred to as a and mesons, five quarks are no further types of lightweight "gauge boson". This property of spin needed - up, down, charm, neutrino like these to be discov­ distinguishes the photon (and other strange, and bottom - with the ered, and this in turn implies that messengers) from the matter parti­ sixth quark - top - completing a there are probably no more gen­ cles (quarks and leptons) which have pattern of six pairs. Only the erations of quarks and leptons like half a unit of spin, and are known as lightest quarks of each charge, up these. Why there are three gen­ "fermions". (2/3) and down (-1/3), are needed erations, but no more, is a mystery QED appears to be much more to build the protons and neutrons that remains to be solved. CERN Courier, June 1994 5 The Standard Model 4 tracks '- 5 tracks 4.1 GeV 4.3 GeV Evidence for the gluon, the carrier of the strong inter-quark force, emerged in 1979, when the TASSO experiment at the DESY Laboratory, Hamburg, saw three clear sprays, or 'jets', of particles coming from electron- positron collisions in the PETRA collider. Two • / <^ of these jets come directly from the produced quark and antiquark, while the third is from a gluon radiated by the quark or the antiquark. than an arbitrarily successful theory. It has therefore rightly served as a blueprint for theories of the other forces that act upon matter particles - the strong force and the weak force. \ \ Although QED deals specifically with the interactions of charged particles, its underlying structure provides a \ -\\\' guide to the essential nature of a \ x\\ workable theory for any force be­ \ \\ 4 tracks tween particles. The range of the electromagnetic \ \\\ 7.8.GeV field is infinite, and so gives rise to large-scale phenomena.