the huge electron-positron collider at the European Laboratory for Particle Physics, CERN – will be ready to switch on. Physicists will begin to see the results of the hundreds of millions of pounds and the tens of thousands of work-years already spent in creating the massive tunnels, the gigantic detectors, and the gargantuan control and analysis computer programs.
LEP has been called the perfect tool for studying the Standard Model – the theoretical framework within which the forces and matter of the Universe interact. The Standard Model describes four fundamental forces in terms of so-called carrier particles. Gravity is carried by the graviton, electromagnetism is mediated by the photon, the strong force is carried by the gluon, and the weak force is carried by so-called vector bosons – the W+ the W– and the Z° particles. The particles of matter exist in three generations of quarks and leptons divided between six quarks, called up, down, charmed, strange, top and bottom, and six leptons, the electron, electron neutrino, the muon and its neutrino, and the tau and its neutrino. All these particles also have an antimatter partner.
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The collider will allow physicists to test the theory behind the Standard Model using data from four detectors, Aleph, Delphi, L3 and Opal. These will detect and measure particles produced by colliding beams of electrons and positrons. But some people think that LEP might be able to do more. LEP might allow us to study deeper aspects of nature that at first sight seem beyond its reach. This impetus is partly due to the fact that LEP’s rival, the Stanford Linear Collider in California (This Week, 22 April), is beginning to produce important results. LEP’s great precision could reveal more subtle effects hinting at deeper, underlying truths that go beyond the Standard Model.
Physicists would like to test the predictions that are extensions of the Standard Model because although it can describe many aspects of fundamental physics, it has fatal flaws. It certainly cannot claim to be The Theory of Everything. The model predicts the existence of particles, but it does not explain their properties. There are many important parameters that you simply have to put in by hand because the model will not predict them. It is like glimpsing the peaks of a submerged mountain range breaking through the surf of an ocean. We know that there is something below, but all we can do is measure some of the peaks and note the similarities between them. We have to guess at what lies beneath the waves.
How could LEP explore the deep regions beyond the Standard Model? Machines, such as LEP, accelerate beams of particles, increasing their energies enormously. When the particles collide, their energy and mass is converted completely into energy. The energy then forms new masses according to Einstein’s famous equation, E = mc2, much as water droplets condense from steam. Nature picks out ‘mass structures’, or particles, which for some reason it favours, although they may survive for only a trillion-trillionth of a second. These particles then decay to other particles, which in turn decay until eventually stable particles are formed.
A convenient way of representing what happens during these decays was invented by the physicist Richard Feynman. Using these Feynman diagrams, we can represent an electron annihilating a positron as follows: The axes represent the spatial distance and time. One possibility is that the electron (e-) and the positron (e+) collide and form a photon, which then decays into quarks and antiquarks (q and q¯) or leptons and their antimatter partners (l and l¯). At higher energies, the carrier of the weak force – the Z° boson – forms, which then decays into quarks. These zoom off at speeds close to that of light but then rapidly decay. We only ever see their decay products, the so-called hadrons and leptons. The heaviest leptons can decay into quarks, which again form hadrons.FIG-mg16725001.jpg
The Feynman diagrams show that the particles formed after the collision depend on the energy of the accelerator. If the mass of the particle is greater than the equivalent energy available from the colliding beams, then it will not be formed. For this reason, one fundamental particle important to the Standard Model, the top quark, has not yet been made.
LEP will operate at an energy that will produce an abundance of Z° bosons. We hope to produce about 100 000 Z° s by next Christmas; a million in 1990 and 10 million by 1992. Many of the essential measurements for the Standard Model require between 1000 and 10 000 particle collisions.
Tracing the decays of short-lived particles, such as the Z° , allows us to probe the Standard Model, in particular, the relationship between the four forces and how they came into existence when the Universe was created in the big bang. According to our best theories, these particles last existed in the high-energy conditions just after the big bang. So in effect, the accelerator mimics these early conditions. As we increase the energy of the accelerator, we travel backwards in time to the beginning of the Universe. There is also a relationship between the energy of a particle and the scale at which we are studying the Universe. If you increase the energy of the particles, you can pick out smaller features in the structure of matter, rather like increasing the power of a microscope.
Theorists describe the relationships between the force particles, and between the matter particles in terms of the mathematical concept of symmetry, whereby one group of fundamental particles can be turned into another group of particles by undergoing what is called a symmetry transformation. The properties of symmetry become more apparent at higher energies, or equivalently, at smaller distances, and at a time closer to the big bang. In fact, theorists think that at the first instant after the big bang there was just one force. But then the symmetry of the Universe started to ‘break’, after 10-43 seconds, as gravity separated, the strong force by about 10-34 seconds, and the electroweak force diverged into electromagnetic and weak forces after 10-10 seconds. Now, after 10-17 seconds, the four forces appear very different.
Because LEP will operate in an energy region that will produce directly the particle responsible for the weak force, the ZDegree boson, we will be able to study the symmetry between the electromagnetic force and the weak force. Also, aspects of symmetry between the strong force and the combined electroweak force will be more obvious than at lower energies.
Another key question concerning symmetry in the early Universe that we would like to answer is how the particles obtained their masses. At LEP, we will be searching hard for the so-called Higgs boson, which some people think gives particles mass, but, like the top quark, the chance of it being seen first at LEP is quite small because it also requires extremely high energies to be formed.
Physicists would also like to explore further symmetry relationships that are extensions of the Standard Model. One popular idea, which has no experimental support, is ‘supersymmetry’. The ‘matter’ particles, the quarks and leptons, are related to the ‘force’ particles, such as the photons and the gluons, by an additional symmetry, that invoke the existence of further, supersymmetric particles. These particles have names ending with ‘ino’, so the supersymmetric partner of a photon is a photino, and that of a gluon is a gluino. Again, they cannot be produced directly because their masses are greater than the beam energy at LEP.
Nevertheless, there is a way of detecting particles that exist beyond the energy regime of LEP. It takes advantage of that strange property in quantum mechanics, whereby particles have a small probability of existing where you would not expect to find them. This property is called tunnelling. The probability of quantum mechanics predicts that the fabric of spacetime shimmers with ‘virtual’ particles fluctuating, or tunnelling in and out of existence. According to Heisenberg’s uncertainty principle, a particle mass can wobble out of the vacuum for a period of time such that the mass multiplied by the time is not more than Planck’s constant. Particles that would normally come into existence at very high energies become accessible, therefore, at the lower energies of LEP because of quantum fluctuations causing particles to decay in unusual ways.
For example, the heaviest quark that we have detected is the beauty or bottom quark. It is almost always inside another particle, the B meson, which consists of a bottom quark coupled to a lighter quark, such as an up or a down quark. The Standard Model predicts that it decays predominantly into a charmed quark which is seen in another meson, known as a D meson (consisting of a charmed and the lighter quark). People have looked hard for other, non-charmed decays, but so far nobody has had undisputed success. The usual decay alternative to decaying into a charmed quark has been to the up quark. The proportions of decays of the bottom quark into up and charmed quarks are two examples of the unknown parameters that the Standard Model is saddled with.
These decays occur mostly by what are known as ‘spectator diagrams’. This is easily shown with the Feynman diagrams. In a spectator diagram, the bottom quark decays and the lighter antidown quark just keeps out of the way.This is illustrated here.FIG-mg16725002.jpg
There is no obvious way that bottom quarks can decay by the spectator diagram into any other kind of quark, such as a strange quark. So the decays of bottom quarks into strange quarks would be interesting events to look for. The reason is that they are a window to higher energy conditions. By allowing mass and energy to trade with time in a way governed by the uncertainty principle, nature can find a pretty devious way to pull off these decays. Three important examples of such decays proceed through a so-called quantum chromodynamic (QCD) ‘penguin’ diagram and through two electromagnetic (EM) penguin diagrams,illustrated here.FIG-mg16725003.jpg
Looking at the QCD case first, the loop labelled g represents the production of a supersymmetric particle known as a gluino – one that has appeared (as allowed by the uncertainty principle) as a quantum fluctuation. Its mass could well be much larger than the energy available from the beams at LEP. If supersymmetry is more than just the idle musings of a theorist, this process could increase the probability of the bottom quark decaying into a strange quark by about a factor of a hundred. For various reasons, this decay is hard to detect, so let us consider the second possibility.
In the second penguin diagram, the bottom quark tunnels and changes (again with the constraints imposed by the uncertainty principle) into a top quark and a W-boson. To conserve energy and momentum the top quark shoots off a photon and then recombines with the W to make the strange quark. The strange quark sails happily off with the spectator antidown quark. The photon – the particle of light at the end of the quantum mechanical tunnel – comes out at a gamma-ray. Now, if a fourth family of particles existed, as some theorists have suggested, there would be another quark heavier than the top quark. And this diagram could increase the rate at which bottom quarks decay to strange quarks by a factor of a hundred.
In the second EM diagram, the photon converts to a pair of leptons, for example, a muon and an antimuon. This decay explores the characteristics of possible Higgs particles, the particle claimed to induce mass in other particles. For example, if there were two pairs of Higgs bosons, then the rate at which bottom quarks decay to strange quarks by his diagram could increase also by a factor of a hundred.
In the two EM diagrams, you will see that the loop can be a top quark. The likelihood of this loop occurring depends heavily on the mass of the top quark: it increases as its fifth power. If you can find this decay, you are studying nature at energies way beyond the energy of your accelerator. It is like being able to do nuclear physics just using chemistry.
People have looked for these decays at the Electron Synchrotron (CESR) at Cornell University in New York and at the DORIS accelerator in Hamburg. They have not found one yet, but the upper limits they have imposed on the production rate are getting near what you would expect in the Standard Model. With a million or 10 million collisions from LEP, we will be able to constrain, hopefully reject, or even – just possibly – discover physics from beyond the Standard Model. When that occurs, physicists hope that those troublesome features of the Standard Model will be revealed for what they are: our misinterpretation, on incomplete evidence, of something much deeper and much more beautiful.
John Hassard teaches at Imperial College, London and does particle physics at CERN in Geneva and at DESY in Hamburg.