-Caveat Lector- from: http://www.economist.com/science/displayStory.cfm?Story_ID=387866 Click Here: <A HREF="http://www.economist.com/science/displayStory.cfm?Story_ID=387866"> Economist.com: New realities?</A> ----- New realities? Oct 5th 2000 >From The Economist print edition The “standard model” of the way the universe works is just about complete. Time to start looking for a new one A HUNDRED years is a suspiciously round number. But if researchers at CERN, the European particle-physics laboratory near Geneva, turn out to be correct, it is exactly the period needed to build a model of how the universe works. Construction began in 1900 with Max Planck’s publication of the first incarnation of quantum theory. Since then, and particularly with the development of high-energy particle accelerators in the 1930s and 1940s, the structure of matter has been probed in greater and greater detail while theorists have sought to impose order on what has been discovered. The result of their labours, now known as the standard model, will be complete—bar the odd dotting of “i”s and crossing of “t”s—with the discovery of a particle called the Higgs boson. This would round off the 18-strong menagerie of fundamental, irreducible particles required by the model. (The 17th, known as the tau neutrino, was announced two months ago by researchers at CERN’s American rival, Fermilab.) And over the past few weeks indications have emerged from CERN that the Higgs is indeed a reality. Discovering the Higgs would be an impressive piece of work. Historians of science may, however, pause at this point. For it sounds suspiciously like the consensus that prevailed at the end of the 19th century, just before the publication of Planck’s paper. Then, too, physicists had a description of the universe that had few apparent flaws. Some of the more hubristic thought the job was done and that the science they had created was, in effect, a “theory of everything”. But it wasn’t. Classical physics, as it is now called, turned out to be a mere engineer’s approximation to reality—good enough for everyday working, but actually explaining nothing fundamental. Today, physicists are more cautious. Few believe that the standard model is really a theory of everything, but none knows for certain what the next step—the equivalent of Planck’s paper—will actually be. That, however, makes the future of physics much more exciting. For the first time in several decades (assuming that the world’s taxpayers will continue to fork out for the necessary equipment) fundamental physics will become a voyage into the unknown. Brave new world At first sight, the standard model looks relatively simple. The 18 particles are divided into fermions (the actual constituents of matter) and bosons (which carry the forces that allow fermions to interact). The fermions themselves are divided into two groups, the quarks and the leptons. Each of these comes in three generations, or “flavours”, of successively heavier species. The first-generation consists of the ingredients of everyday matter. Its quarks, the up and the down, are the bits inside protons and neutrons, the constituents of atomic nuclei. The first generation of leptons consists of the electron (the third constituent of atoms) and its corresponding neutrino—a particle so small that people are still arguing about whether it has any mass at all. Particles of the second and third generations are mostly short-lived and so rarely found outside particle-physics laboratories, though they were common at the beginning of the universe. The second generation’s quarks are known as charm and strange; its leptons are the electron-like muon and the muon neutrino. The quarks of the third generation are called top and bottom; its leptons are the tau and the tau neutrino. The most familiar of the bosons is the photon—the particle of light. Photons carry the electromagnetic force, the weakest in the standard model, and have no mass. The second force in the model, known as the weak nuclear force, controls radioactive decay. It is stronger than the electromagnetic force, but operates at shorter range, and is carried by three bosons, the W+, the W- and the Z. Weak bosons, unlike photons, are massive. The model’s third force, known as the strong nuclear force, is the most powerful of the three. It holds quarks, and thus atomic nuclei, together. This force is carried by particles known as gluons. To confuse matters further, the strong-nuclear “charge” comes in three varieties, known to physicists as red, green and blue. Because of their different “colours” each quark comes in three varieties, and there are eight distinguishable gluons. Gluons, like photons, are massless. All very neat. But then there is—or at least there ought to be—the Higgs. The Higgs boson was actually the standard model’s first “kludge”. The model’s original mathematics had the inconvenient consequence that they failed to predict the existence of mass. Introducing the Higgs boson solved this, and also explained why, among all the other bosons, weak-nuclear ones are the only ones with mass: the Higgs has no electric or colour charge, and so it conveniently affects only the weak force. But the original calculations about the Higgs gave it, and in turn the Z and the two W particles, a near-infinite mass. Physicists had to rid themselves of this annoying problem (called “unnaturalness”) by using a bit of mathematical trickery to add appropriate fiddle factors to their equations. Its failure to solve the unnaturalness problem in a more convincing way is one of the main outstanding defects in the standard model. It is not, however, the only one. Another defect is that the model fails to account for the most ubiquitous force of nature, gravity. A third, called the flavour problem, has two parts. One is the puzzle of why there are three and only three generations of fermion, given that the second and third generations seem almost redundantly similar to the first, except for their higher masses and shorter lifetimes. The other is why the particles in each generation have the masses that they do. (These range from the imperceptible for neutrinos, to something larger than a gold atom for the top quark.) The fourth mystery, the hierarchy problem, is why the different forces operate at such different energies, whether they are actually all manifestations of the same underlying phenomenon, and how they can be united mathematically if they are. Kludging all of these problems in the way that was done for the Higgs would be hard. It would also be unsatisfactory. Most physicists have a deeply held belief that universal laws should be elegant, so what they are looking for is not a kludge, but a better theory. Such a theory would reduce the standard model, too, to an engineer’s approximation. But it might prove to be a stepping-stone to a real theory of everything. Strange, but true? There are, so far, three main contenders for the next great theory of physics. They are known as technicolour, supersymmetry and extra dimensions. All have the full complement of weirdness and wonderfulness that the man in the street might expect. And all depend on gaining a deeper understanding of the symmetries that are believed to underpin the structure of reality. Symmetry, in mathematical terms, means more than just the commonplace idea of mirror images. A process is mathematically symmetrical if it conserves something. Different forces have different symmetries; indeed, they are defined by them. When an electron radiates a photon, electric charge is conserved. When it exchanges a W boson with a neighbour, it is the weak charge that does not vary. And when quarks interact via gluons, the conserved property is colour. However, if the energy with which particles are slammed together in accelerators is increased, there is reason to believe that the symmetries (and hence the forces) will merge—bringing the unity of nature a step closer. That has happened once already. At a suitably high energy, the electromagnetic and weak nuclear forces unite. They are, in other words, two manifestations of an underlying “electroweak” force. All the contenders to replace the standard model come with their own unifying symmetries built into them. It is these that define them. In the case of technicolour, this symmetry is connected with a postulated “technicolour force” that is a scaled-up version of the strong force. This force would be powerful at high energies, but weak at those achieved in existing accelerators, which is why it has not yet been noticed. Technicolour theory suggests that the Higgs boson, instead of being an elementary particle, is in fact a mushy complex made up of new particles called “techniquarks”. That eliminates the problem of unnaturalness because the techniquark complex does not need the fine-tuning that the Higgs mechanism requires. And, conveniently, the complex would fall apart, breaking electroweak symmetry, at exactly the energy level at which the transition occurs in nature, giving this phenomenon a rational explanation missing in the standard model. Unlike any of its rival theories, technicolour can also dispose of the flavour problem by explaining why there are three generations of fermions, and where their strange pattern of masses comes from. At high energies, it suggests, quarks, leptons and techniquarks are all unified into a single extended “technifermion”. At lower energies, the symmetry is broken, and the three quark and lepton generations will split off in the way that is seen in nature. Technicolour, according to its proponents, can also explain the hierarchy problem. This is because the force between techniquarks becomes weaker at higher energies. Go high enough and it has about the same strength as the electroweak force and can be unified with it. Technicolour should also be eminently testable. It predicts, in the form of the techniquark complexes, a whole new complement of massive particles. These have not yet been observed, but when new experiments at Fermilab get under way it should become clear whether technicolour is real or not. Guido Altarelli, a theoretical physicist at CERN believes that it will turn out not to be. He reckons that supersymmetry is a stronger contender for the throne. This theory argues that for every fermion in the standard model there is a corresponding “supersymmetric” boson, and vice versa. The electron, for example, would have a new bosonic partner called the selectron, and the Ws would have fermionic twins called Winos. Similarly, there will be such objects as gluinos, sneutrinos, photinos and squarks. Supersymmetry elegantly solves the problem of unnaturalness. The fiddle factors required by the extended set of bosons cancel those required by the extended set of fermions, so no fine-tuning is required. And the masses of the standard-model particles will conveniently come out right, too. Another neat aspect of supersymmetry is that it points towards the unification of the strong, electromagnetic and weak forces—part of the hierarchy problem. Taking the interactions of supersymmetric particles into account gives a prediction for the strengths of the various forces which agrees with the values measured in the laboratory better even than the standard model does. That, given the accuracy with which the standard model no rmally matches reality, is encouraging for supersymmetry’s proponents. The most compelling reason to prefer supersymmetry to technicolour, however, is that it can accommodate gravity. This is because the equations describing its symmetries match those of Einstein’s general theory of relativity, the best explanation so far of how gravity works. Testing supersymmetry could be tricky. The theory does not really specify the masses of the new particles that it predicts—apart from suggesting that they will be large. But this means that a substantial amount of missing energy should show up (or, rather, not show up) in the detector of a particle accelerator that was set up to look for them. Noticing that missing energy would allow new sorts of particle to be tracked down. A proton-antiproton collision, for example, should produce a pair of squarks in the right circumstances—if supersymmetry theory is correct, that is. That would carry away a lot of energy that should otherwise be detected. Franco Bedeschi, physics co-ordinator of CDF, one of the experiments at Fermilab, says that assuming a squark mass of around 300 billion electron volts (an electron volt is a measure of a particle’s energy and thus, according to Einstein’s special theory of relativity, of its mass) around 1,000 such events should be detected in the first two years of operation of Fermilab’s upgraded Tevatron machine, which should see its first collisions in March 2001. The third candidate to replace the standard model is, perhaps, the most bizarre of all. The standard model more or less agrees with common, everyday experience to this degree: it works in four dimensions (three of space and one of time). Theories of extra dimensions, by contrast, say that there are, in fact, at least five of them. The reason the extra dimensions are not experienced in everyday life is that they are so tightly curled up that they cannot be seen. This picture has the virtue that no new particles are required to make it work except for a gravitational boson called a graviton. Not only is gravity an integral part of extra-dimension models, but the gravitational part of the hierarchy problem—why gravity is the weakest force of nature—is easily solved by adding a fifth dimension. This extra dimension is gravity’s true habitat, and there it is a strong force. The weak effect that it exerts on familiar objects is because only part of its strength trickles through to the world of human experience. Testing the theory of extra dimensions poses different difficulties from those associated with the other two. At least with them, there are a lot of new particles to look for. But Graham Ross, a physicist at Oxford University, argues that it should be possible to make gravitons in particle accelerators. They would, according to the theory, show up as “towers” of many different excited states. These so-called “Kaluza-Klein states” would each be separated by a few electron volts and would be detectable because they would result in a large number of characteristic photon or electron pairs. There is no hard evidence yet as to which, if any, of the standard model’s rivals is correct. But a tantalising glimpse of a world beyond came from CDF earlier this year. Its researchers saw two electrons and two photons fly off in a collision, but some of the energy that should have resulted from the collision was missing. This could have been disappearing in the form of a pair of selectrons, which would be evidence in favour of supersymmetry. Experimenters at another of Fermilab’s particle-physics experiments, known as D0, have also seen something odd happening. They have found events that should have corresponded to the production of top quarks. A closer look at the properties of these collisions, however, suggested they were not typical of top quarks at all. The researchers could not be more definite than that, but the possibility remained that they had seen the production of some supersymmetric particles. Testing times The main problem, however, in testing technicolour and supersymmetry is that they generally predict particles with masses too large to be created at the energies probed by today’s particle accelerators. So finding out which, if any, of the new theories is right will probably be the job of new machines now under construction—the improved Tevatron, and the new Large Hadron Collider at CERN which is due for completion by 2005 and will smash protons together at 14 trillion electron volts, the highest energy ever achieved in a laboratory. Finding gems among the dross of familiar particles that these machines will inevitably generate will, however, be hard. Big accelerators create around 10m collisions a second. Physicists are able to track and record only 50 or so of these. This means they have to choose those that look interesting. That, in turn, requires a system of computers which can judge what is interesting within microseconds—filtering out the numerous “standard” events and recording only those with distinctive features that make them stand out from the crowd. Existing systems designed to do this have expectations of what to look for built into them. That has worked for the standard model because its predictions are relatively clear. But the new physics requires a new approach, and that is provided by some physicists at D0. They have invented a new computer program called “Sleuth”. John Womersley, the co-ordinator of D0, believes that Sleuth is the first truly model-independent way of analysing data that has become available in the search for new physics. It works by comparing the range of standard-model predictions for the momentum and missing energy of decay products with those that are observed experimentally. If an intruder is hiding among those products, Sleuth will spot that there is something wrong and alert its masters. Although this will never be as sensitive as a targeted search, Dr Womersley argues that, unlike present analytical methods, it is open to any kind of new particle—even those accounted for by theories that have yet to be dreamed up. All in all, then, it looks as though the standard model has had its day—or soon will have had. The question is, when will its successor be clear? According to Dr Ross, the lightest supersymmetric particles should already be accessible to accelerators, so fans of that theory should be getting nervous that nothing definite has yet turned up. Dr Womersley agrees that physicists must see something soon. He believes that the standard model will be overthrown within the next five years. But which of the contenders will actually make it into the textbooks that physics students 100 years hence will peruse remains to be seen. Watch this space—however many dimensions it turns out to have. Websites Fermilab in Illinois gives an introduction to particle physics, including an explanation of Max Planck’s “Standard Model”. An article in FermiNews discusses the impact of the Higgs boson on the “Standard Model”. The Large Electron-Positron collider (LEP) and Large Hadron Collider (LHC) are described in a nutshell by CERN. Fermilab’s “CDF” and D0 experiments have fuelled the debate. Discover more about the capabilities of the “Sleuth” program. Articles by Graham Ross and Joseph Lykken are available here. BUBL provides extensive links for particle physics resources. Buy Economist books and goods online. OPINION | WORLD | BUSINESS | FINANCE & ECONOMICS | SCIENCE & TECHNOLOGY PEOPLE | BOOKS & ARTS | MARKETS & DATA | DIVERSIONS | PRINT EDITION © Copyright 2000 The Economist Newspaper Limited. All rights reserved. ----- Aloha, He'Ping, Om, Shalom, Salaam. Em Hotep, Peace Be, All My Relations. 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