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 (see previous article) for the long awaited sixth ('top') quark puts another rivet in the already firm structure of today's Standard Model of . 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 . 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, , 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

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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

CERN Courier, June 1994 5 The Standard Model

4 tracks '- 5 tracks 4.1 GeV 4.3 GeV

Evidence for the , 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. The weak \.... -" \\ TASSO and strong forces, on the other hand, appear to have ranges limited to sub- nuclear dimensions. It might seem natural, therefore, to place the weak indeed include both weak and elec­ remains massless, the photon, and and strong forces within the same tromagnetic interactions. Such an one that has a similar mass to the theory, as indeed Hideki Yukawa "electroweak" theory requires four charged messengers. This massive attempted in his meson theory in the spin-1 massless messenger parti­ neutral messenger should give rise to 1930s. However, the big step forward cles, or gauge bosons - two charged a weak "neutral current" - a weak with the weak force proved eventu­ (+,-) and two neutral. The two interaction with no change of charge. ally to come through linking it inti­ charged messengers explain weak The particle is now well known as the mately with the wide-ranging electro­ reactions such as beta-decay where Z°, although by the end of the 1960s magnetic force. Surprisingly, the charge changes hands (a neutron there was still no evidence for the phenomena of electrostatics and decays into a proton, for example). weak neutral currents that it should magnetism have a common origin However, the messengers must be mediate. with the weak processes of radioac­ heavy to explain the short range of The introduction of the higgs field tive beta-decay and proton-proton the weak force and introducing into electroweak theory solves the fusion in the Sun. masses for these at first seemed to problem with the gauge symmetry The link between the electromag­ wreck the gauge symmetry! As for because in the basic theory the netic and weak forces seems remark­ the neutral messengers, while one messenger particles have no mass, able when you consider that the short could be the massless photon, what and so the symmetry remains unbro­ range of the weak force implies that of the other? ken. It is only at the relatively low the messenger particle must be The solution to the difficulty with the energies of our experiments that the heavy, unlike the photon which is massive messenger particles came underlying symmetry appears bro­ massless. The time for which the with the introduction of a new field, ken, because at low energies the energy of the massive particle can be which we now call the higgs field, higgs field is not zero. At higher borrowed is short so that it cannot with its own messenger particle, the energies, the higgs field goes to zero move far, even at the speed of light; . The charged messen­ (rather as the field in a. magnet the massless photon, on the other gers of the electroweak theory - the disappears above the Curie tempera­ hand, can take an infinitely small particles known as W+ and W- - ture) and the symmetry is there for all amount of energy and travel an appear massive in our experiments to see! infinite distance. as a result of their interactions with One question that remained by the However, during the 1960s, at­ the higgs field. The neutral states of end of the 1960s was whether the tempts to find a theory of the weak the underlying theory behave in a unified electroweak theory was force with gauge symmetry sug­ slightly more complicated manner to renormalizable - in other words, gested that the correct theory should produce one neutral messenger that would awkward infinities that natu-

6 CERN Courier, June 1994 You Do Have a Choice

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CERN Courier, June 1994 7 The Standard Model

Just ten years after the discovery of the Z° at CERN, the LEP electron-positron collider now mass-produces them. In 1993 the four experiments together saw 3 million Zs, making a total of some eight million since LEP began operating in 1989. This hefty slice of precision data has played a vital role in refining the Standard Model parameters.

a particle's energy, the shorter its

[Integrated luminosrties seen by experiments 1990 -> 1993g| associated wavelength.) At these small distances, the force between pb-1 quarks is weaker than it is across the dimensions of a proton. This behav­ iour contrasts sharply with that of the electromagnetic force, which dimin­ ishes as the distance between charges increases. Surprisingly enough, a theory very similar to QED has been constructed to explain the strong force. Instead of being built on the property of electric charge, the new theory is built on a "strong charge" that occurs on quarks (but not on leptons, which do not feel the strong force). This strong charge differs markedly from electric charge in that it must occur in three forms, with three opposite "anticharges". (By rally occur in the theory cancel out, mainstays of the Standard Model, contrast, positive and negative so that it could, like QED, make was here to stay. electric charges are just one form of sensible predictions? In 1971, the The second important structural charge and its anticharge.) Because young Gerard't Hooft made the element of the Standard Model deals it occurs in three varieties, the strong breakthrough that proved this was with the third of the forces, the strong charge has been given the name of indeed the case. Two years later a force. Like electroweak theory (and "colour", in analogy with the three last missing piece of experimental QED contained therein), the theory primary colours of light. The quarks evidence fell into place. The for the strong force is a quantum field occur in each of the three colours, Gargamelle team at CERN found the theory with gauge symmetry. In this but it seems that they can together first neutral current events: neutrino case the theory has again to deal form only particles that are neutral, or interactions that could be explained with a force that acts over a short "white", in colour. Baryons, with three only by the existence of the predicted range but this time only on quarks, quarks, contain a quark of each massive neutral messenger particle, and in such a way that quarks are colour, while mesons contain a the Z°. unable to exist on their own. Quarks quark-antiquark pair with opposite By 1979 the accumulated mass of always occur bound within larger colour and anticolour. evidence for electroweak theory had particles: clusters of three quarks The theory, in which colour charge become unavoidable, and the Nobel form baryons, such as the proton and replaces electric charge, is known as committee duly awarded the main neutron, while quark and antiquark , or QCD, protagonists, Sheldon Glashow, pairs form the various kinds of emphasising the structural similarities Abdus Salam and Steven Weinberg meson. If you try to knock a quark with QED, although the more com­ with the Nobel Prize for physics. out of a proton by hitting it with plex origin for the strong force leads Then in 1983, CERN's proton- another high-energy proton, say, you naturally to a more complex theory. antiproton collider yielded sufficient succeed only in creating new quarks Instead of one messenger particle as energy and intensity to shake loose a and antiquarks, that is, new mesons. in QED, the theory of QCD contains handful of real (as opposed to virtual) However, a clue to understanding eight messengers - the gluons - W and Z particles. The Nobel Prize the strong force comes from studying which are massless particles, like the followed swiftly for CERN's Carlo it at high energies, which in the photon, with one unit of spin. At first Rubbia and Simon van der Meer, and quantum world is equivalent to sight, this might seem unlikely, as the electroweak theory, one of the probing small distances. (The higher strong force is short range, acting

8 CERN Courier, June 1994 The development of science this century 2 - from 1946 to 1970 by Victor F. Weisskopf

only at sub-nuclear distances. How­ design of the proximity fuse. Scien­ ever, a crucial difference between the tists who previously were mainly strong force and the electromagnetic This is the second in a series of interested in basic physics, con­ force lies with the fact that while the three articles which together are a ceived and constructed the nuclear photon itself has no electric charge, slightly revised version of a talk bomb under the leadership of one of the gluons carry colour. This means delivered at the meeting of the the most 'esoteric' personalities, J.R. that gluons can interact among American Association for the Oppenheimer. E. Fermi constructed themselves, with fascinating conse­ Advancement of Science, in the first nuclear pile, E. Wigner was quences for the strong force. Boston, on 14 February 1993, and instrumental in designing the reactors A virtual photon emitted by an at a CERN Colloquium, on 5 that produced plutonium, and J. electron can in principle travel off August 1993, entitled 'Science - Schwinger developed a theory of towards infinity, unaffected by other yesterday, today and tomorrow'. waveguides, essential for radar. It nearby photons. A gluon, however, Together they describe the tre­ was more than that: some of these feels the influence of any other mendous growth of scientific people were excellent organizers of gluons, and their interaction can knowledge and insights acquired large-scale research and develop­ indeed lead to more gluons. So a since the beginning of this century. ment projects having good relations virtual gluon emitted by a quark, say, In a highly abridged form, some of with industry, such as the aforemen­ cannot proceed far before it is in these ideas were used in an tioned military projects. effect caught in a net created by its earlier CERN Courier article When World War II was over, the interactions with other gluons. The ('Crisis - the Weisskopf view'; public was under the impression that result is that the strong force has a October 1993, page 22). Because the physicists had won it. Of course, short range. of the restrictions of a single issue this was a vast exaggeration, but it is In summary, the Standard Model of the CERN Courier, the text has a fact that radar saved the United sees the world as built from two sets been repackaged as three articles, Kingdom and reduced the submarine of particles, the quarks and leptons, each covering an identifiable threat to transatlantic convoys, and whose interactions are described by historical epoch. The first, covering that the atomic bomb led to an two similar theories, electroweak the period from 1900 to World War immediate end of the war with Japan. theory and QCD. In these theories, II, was published in the May issue. Physics and science in general forces are transmitted by messenger The third article will cover the earned a high reputation. This led to particles - the gauge bosons - some period from 1970 to the end of the of which acquire mass through their century. interactions with an additional field, the higgs field. The model is a curious amalgam, evolved over years he time from 1946 to about 1970 of wrong turns and dead ends. So T was a most remarkable period far, it has worked much better than for all sciences. The happenings of we have any right to expect. World War II had a great influence, especially on physics. To the aston­ ishment of government officials, physicists became successful engi­ neers in some large military research and development enterprises, such as the Radiation Laboratory at MIT, the Manhattan Project, and the

Eugene Wigner - instrumental in designing reactors that produced plutonium. (Photo Kathleen Blumenfeld)

CERN Courier, June 1994 9