PARTICLE physicists, in their study of the ultimate constituents of matter and how they stick together, find themselves in a remarkable position. They can interpret all established observations within one encompassing theoretical framework known as the ‘Standard Model’. This provides a coherent description of all the fundamental particles and forces. Yet the theory leaves a number of fundamental questions unanswered. Why, for example, are the masses of the particles what they are? Why do the electric charges of the electron and proton balance so precisely? In other words, there must be a deeper theory, so what is it?
To answer this last question, particle physics requires experiments that can lead it beyond the orthodoxy of the Standard Model. Experiments in particle physics usually involve studying the high-energy reactions between fundamental particles when they collide. The challenge lies in designing the right experiments with appropriate particles and energies. These will probably evolve in two complementary directions. On the one hand, some experiments will follow the traditional route of exploring matter at the highest possible energies, using beams from particle accelerators. On the other hand, there will be a need for precise measurements, which require particles of modest energy but in large quantities. The next high-energy particle machines will be complemented by new particle ‘factories’, whose task will be to produce high intensities of selected particles.
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In the past, the major advances in experimental particle physics have usually occurred at the high-energy frontier. In the 1930s, for example, physicists studying cosmic rays – the high-energy particles that originate in space – found the first examples of an antiparticle (the positron, or antielectron), an exotic particle called a pion, and a ‘heavy’ electron (the muon). More recently, in 1984, two teams studying high-energy collisions of protons and antiprotons at CERN, the European centre for particle physics in Geneva, produced the first examples of W and Z particles. These are the particles that, according to the Standard Model, carry the weak force, one of the four fundamental forces of Nature .
Not all discoveries have come from exploring at the high-energy frontier, however. Since the 1950s, physicists have measured the properties of muons, muon-neutrinos, pions and kaons in so-called ‘hadron factories’, low-energy accelerators that mass-produce these particles. Other discoveries have come through precise measurements revealing either effects that were previously obscured or processes that are important but rare. One example is the discovery in 1974 of the J/psi particle at the Brookhaven National Laboratory, New York, and the Stanford Linear Accelerator Center (SLAC) in California. The J/psi weighs a little more than three times the mass of the proton. Accelerators with enough energy to create particles of this mass had existed for some time. But only when the experiments at Brookhaven and SLAC made it possible to pinpoint the masses of particles precisely enough, did the J/psi reveal its existence. This discovery was a watershed in establishing the Standard Model, because the J/psi proved to contain a new type of elementary constituent of matter – the charmed quark .
Thus, history provides a guide for striking out beyond the Standard Model. One way will be to look for new particles that are both expected in the Standard Model – the top quark and Higgs particle – and unexpected. This will be a major goal of experiments at new high-energy machines. Another line of attack will be to investigate the Standard Model in detail – to look for subtle violations of old ‘conservation’ laws; to measure accurately those quantities that can be calculated precisely; and to search for small deviations that may point the way to new discoveries. And this is where particle factories will come into their own.
There are also several technical reasons supporting this two-fold evolution of experimental particle physics. At higher energies, pinpointing one specific process will become more and more like looking for needles in the proverbial haystack. This may be because the important reaction occurs along with many uninteresting processes, or because it occurs rarely, or both. The problem will be particularly true of any new process, such as the birth of new massive particles, which will have to compete against many other reactions occurring more readily. Moreover, especially in the case of experiments at the next generation of proton accelerators, a great deal of work will be necessary to develop suitable detectors.
At lower energies, meanwhile, several decades of experience have opened up the possibility of building accelerators with beams of extremely high intensities. In certain cases, it should be feasible to make beams that produce reactions between particles at up to 1000 times the rates obtained in current machines. Detectors, today, perform much better than their predecessors, so these high rates of reaction could lead to important advances. Moreover, because physicists already know where to find the particles that exist at present energies, they will be able to optimise their machines and detectors to focus on the important physics.
What will a particle factory look like? How will it evolve from the present types of particle accelerator, which have their roots in the pioneering work of the 1930s? Ernest Lawrence and his student, Stanley Livingston, built the first circular particle accelerator at the University of California in Berkeley in 1930. Lawrence had been inspired by the work of Rolf Wideroe who had used electric fields varying at radio frequencies to accelerate charged particles in a straight line. Lawrence’s breakthrough was to use a magnetic field to bend the track of the particles into a circle so that they passed repeatedly though a modest accelerating field, until they reached a relatively high energy.
His first machine was 13 centimetres in diameter and it accelerated protons to an energy of 80000 electronvolts, or 80 kiloelectronvolts. It was also the direct ancestor of the Superconducting Super Collider (SSC), which particle physicists are planning to build in Texas. The SSC will be 200 000 times the diameter of Lawrence’s first machine, and it will accelerate protons to 20 million million electronvolts, or 20 teraelectronvolts.
For the first 25 years or so following Lawrence’s work, physicists fired the beams from particle accelerators at fixed targets. But in the quest for higher energies, there has been a major shift in the past two decades towards ‘colliding-beam’ machines where moving particles meet head-on. Again, Wideroe made a vital observation. In 1943, he realised that not only would colliding beams make more energy available, but also that you could store the beams and collide them many times each second. Another advantage is that you could store separate beams of particles and antiparticles in the same vacuum tube in a single ring of guiding magnets. The magnetic fields would steer particles and antiparticles on the same path, provided they travelled in opposite directions – which is just what you want with a colliding beam machine.
The earliest particle colliders worked with electrons and positrons, beginning with a machine called ADA (Annelo d’Accumulazione) at Frascati. In these machines, an electron and a positron can come close enough to annihilate, a process whereby the two particles are converted to pure energy. Almost instantly, the energy rematerialises as matter, which takes the form of new particles if the energy is high enough.
SPEAR, the electron-positron machine at SLAC, provided the first dramatic demonstration of the superiority of colliders, in the mid-1970s. In the space of a few years, physicists working at SPEAR discovered several new particles, including another ‘heavy’ electron, known as the tau, and particles containing the charmed quark. More recently, in 1981, the conversion of the Super Proton Synchrotron at CERN in Switzerland into a proton-antiproton collider led to the discoveries of the W and Z particles two years later.
Electron-positron colliders
Almost all future accelerators for particle physics will now be colliders. This is as true for particle factories as it is for the high-energy machines, such as the SSC. A particle factory must satisfy two criteria. It must produce abundantly the particles to be studied, while the ‘background’ of unwanted particles must be as low as possible. In other words, there must be more needles than hay in the stack. With few exceptions, this implies that electron-positron colliders provide the best factories. In these machines, the total energy of the two beams can be ‘tuned’ to produce specific particles against conditions of very low background.
Indeed, the best background conditions exist close to the minimum energy required to produce the required particles – the ‘threshold’ for the particles. This is because, in contrast with collisions at higher energies, heavier particles are absent. Also, in electron-positron annihilations, the region of energy around threshold provides the largest probability for producing a particle. So the overall ratio of ‘signal’ (the desired process) to background is unrivalled.
Figure 1 illustrates the performance of present and future electron-positron colliders, indicating thresholds for various particles – the phi, the tau lepton and particles with charmed quarks, particles with bottom quarks, and Z0 particles. Physicists are considering each of these threshold regions as the basis for operation of an electron-positron factory. One such factory, the tau-charm factory, would investigate the region of energy between 3.0 and 4.5 gigaelectronvolts.FIG-mg16685601.jpg
The original idea for the tau-charm factory emerged during 1987, from countless discussions between two physicists at CERN, while waiting for their children at the stop for the school bus. One of the physicists, Jasper Kirkby, knew what physics he wanted to measure and what the detector should look like. He also knew the basic performance that a tau-charm factory should provide if designed correctly.
But it was a friend, John Jowett, a specialist in the theory of particle accelerators, who first considered the design of the machine. He found that, ‘surprisingly’, to use his own words, a machine with the required performance was feasible. Since then, other groups have begun more detailed design studies for a tau-charm factory. Among these are a team at SLAC that includes Martin Perl, who discovered the tau lepton in 1975, and a team of Spanish physicists led by Juan Antonio Rubio of CIEMAT, the institute for particle physics in Madrid.
The energy region that the tau-charm factory will cover has already proved particularly rich, yielding several discoveries, and it promises a continuing potential for important physics. Chinese physicists realised this several years ago, when they began to build an electron-positron collider in Beijing. This machine, the BEPC, is currently being commissioned; it should produce particles at 10 times the rate at SPEAR.
The tau-charm factory will aim for rates 100 times higher still. It will focus on four fundamental particles: the charmed quark, the tau lepton, the tau-neutrino, which is the tau’s neutral partner, and the gluon – the particle carrying the strong nuclear force. The factory will produce the charmed quark in the guise of charmed mesons (D+, D–, D0, D–S and D+S), which contain a charmed quark bound with an antiquark of another variety. And experiments will study gluons through the production of ‘glueballs’, which should appear in the decays of J/psi particles. Glueballs are particles consisting purely of gluons, with no quarks. The Standard Model predicts the existence of such particles, but so far no one has observed them convincingly.
The tau, the J/psi and the charmed mesons are not new – they have been studied at various machines for the past 15 years. What is new is that the tau-charm factory will generate them in unprecedented quantities. One year of operation of the machine will be equivalent to operating the collider SPEAR for 1000 years. SPEAR is currently running in the same region of energy.
The tau-charm factory will produce different particles according to the total energy of the beams, rather as the tuning knob on a radio selects different stations. For instance, it will produce J/psi particles at 3.1 gigaelectronvolts, pairs of taus and anti-taus at 3.67 gigaelectronvolts, and pairs of charmed mesons at 3.77 gigaelectronvolts. In one year, the tau-charm factory could produce 20 million pairs of taus and antitaus, 50 million pairs of charmed mesons and their antiparticles, and 10 thousand million J/psi particles.
Each of these particles has only a short lifetime (at most, 10-12 seconds) before it decays into lighter particles that live long enough, however, to leave tracks in a detector. Experiments record ‘events’ – the patterns of tracks produced by the decays of the particles formed in individual electron-positron annihilations. It is the subtle characteristics of these events that could reveal new physics.
One specific example of an important experiment for the tau-charm factory is the measurement of the mass of the tau-neutrino. This example also illustrates the complementary roles of factories and colliders. LEP, the Large Electron-Positron collider at CERN, due to start up in July, should tell us how many kinds of light neutrino there are. This, in turn, should reveal the number of ‘generations’ of particles there are. The tau-charm factory, on the other hand, will be able to make the most precise measurements specifically on the tau-neutrino.
The possibility that neutrinos have mass, rather than zero mass, is of central importance to physics. Neutrinos with mass would profoundly alter the Standard Model; they would give rise to a ‘mixing’ between the generations of leptons that would echo the mixing already observed between generations of quarks. Massive neutrinos could also solve two of the major puzzles of astrophysics – the deficiency in the number of solar neutrinos and the composition of the ‘dark’ or ‘missing’ matter of the Universe (‘The search for dark matter in the laboratory’, ¿ìè¶ÌÊÓƵ, 15 April 1989).
The best way to measure the mass of the tau-neutrino is to study the decays of the tau lepton into five charged particles (pions) and a neutrino. There are many ways in which the pions and the tau-neutrino can share the energy released when the tau decays. Sometimes the pions will take as much energy as they can, leaving the tau-neutrino with no energy other than that corresponding to its mass – if it has any mass, that is. By measuring precisely the maximum energy carried away by the pions, an experiment can either determine the mass of the tau-neutrino or establish an upper limit to it. This limit depends on how sensitive the experiment is.
At present, all we can say is that the mass of the tau-neutrino is less than 35 megaelectronvolts. The tau-charm factory, however, will be able to measure it to as low as 1 megaelectronvolt. To reach this sensitivity, physicists will first have to measure the mass of the tau itself, which the tau-charm factory will achieve to an accuracy of 0.2 megaelectronvolts. One megaelectronvolt may not seem very accurate compared with the mass of the electron-neutrino, which is less than 18 electronvolts. But in the same way that the tau lepton is far more massive than the electron, the tau-neutrino should be much heavier than the electron-neutrino.
Taken together, cosmology and particle physics indicate that 1 megaelectronvolt is the lowest mass that an unstable tau-neutrino can have if it is responsible for the dark matter of the Universe. If its mass is below 1 megaelectronvolt, however, then the combined results of cosmology and particle physics indicate that this neutrino must be stable and its mass must be much smaller – below 65 electronvolts.
What would a tau-charm factory look like? The original conception is of an integrated machine consisting of an electron/positron injector, two rings to store and collide the beams, and a detector. By designing the apparatus as an entity we hope to optimise the various parts in a coordinated way, to give the highest possible performance. An important measure of the efficiency of a particle accelerator is the luminosity. This relates the rate at which a specific reaction is produced to the cross-section, or probability, of the reaction occurring. The aim with the tau-charm factory will be to yield as high a luminosity as possible. In the ring of a particle collider, the radio-frequency electromagnetic fields and focusing magnets confine the particles into narrow needle-like bunches. If we consider one set of bunches as the ‘beam’, and the other as the ‘target’, then the number of reactions per second between two trains of counter-rotating bunches is the product of three factors: the number of particles per ‘beam’ bunch, the ‘opacity’ of the ‘target’ bunches, and the number of times the pairs of bunches cross per second.
The tau-charm factory aims to have a much greater luminosity than SPEAR, by increasing these factors as follows. One of the biggest gains comes by increasing the rate at which the bunches cross. SPEAR has a single ring that stores one electron bunch and one positron bunch. These collide at two points. The design for the tau-charm factory has two separate rings for electrons and positrons, each with 24 bunches that collide at a single point (see Figure 2). This increases the luminosity 15-fold.FIG-mg16685602.jpg
There are two additional techniques to increase the chances of annihilations between electrons and positrons in the two sets of colliding bunches. First, strongly focusing magnets, placed a small distance from the collision point, can squeeze the bunches down to the smallest possible cross-section. At SPEAR, such magnets reduce the vertical spread of the beam to 40 micrometres; at the tau-charm factory, we should be able to go down to 8 micrometres, which would increase the luminosity five-fold.
Secondly, we can increase the number of particles in each bunch. The difficulty here is that the bunches exert long-distance elecromagnetic forces on each other when they collide, and these forces disrupt the trajectories of the particles around the ring. Above a certain point, the machine can no longer maintain the particles along their orbits. However, it should be possible to make the bunches in the tau-charm factory contain four times as many particles as in the bunches in SPEAR when it is running at 4 gigaelectronvolts, leading to a gain in luminosity of 16.
These factors combine together to give the tau-charm factory a thousand-fold increase in luminosity compared with SPEAR. With many bunches, and many particles per bunch, the total current circulating in each beam will be around 500 milliamps; in SPEAR running at 4 gigaelectronvolts, the current is 10 milliamps. Such a high beam current may lead to instabilities due to the electromagnetic interactions between the beams and the metallic surroundings. But 20 years of experience with electron-positron colliders provides invaluable guidance on how to handle these problems.
The increase in the precision of measurements at the tau-charm factory will come not only from the collider’s luminosity, but also from the performance of the detector. This will consist of various ‘onion layers’ in a format that is not standard at colliding-beam machines (see Figure 3). The layers of the detector wrap around the TOPm pipe at the point where the collisions occur. Each layer is designed to make different, but complementary, measurements on the particles flying out from annihilations at the heart of the detector. The aim will be to identify directly all the particles produced. That is, all except the elusive neutrinos which interact with matter so weakly that they will escape the detector without trace.FIG-mg16685603.jpg
Nevertheless, for the first time in this energy region, the detector at the tau-charm factory will be able to identify neutrinos indirectly. This is because it will measure precisely the energies of all other particles produced. The difference between this total energy and the original energy of the two colliding particles will then reveal the missing energy spirited away by neutrinos.
Where are the major discoveries in particle physics going to be made? No one knows. Perhaps there is a top quark with a mass of 100 gigaelectronvolts, or perhaps a tau-neutrino with a mass of 5 megaelectronvolts. There is a democracy at work in this field of research whereby new physics appears not only at the highest energies. It can spring up unexpectedly anywhere. With machines such as the tau-charm factory to complement the high-energy super colliders, particle physicists will be able to cover all possibilities.
Jasper Kirkby is an experimental physicist working at CERN in Geneva in Switzerland.
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The particles and forces of the Standard Model
ACCORDING to the Standard Model of particle physics, all matter consists of two types of basic building block – quarks and leptons. These seem to exist in (at least) three ‘generations’ of increasing mass, with a pair of quarks (charges 2/3 and -1/3) and leptons (charges -1 and 0) in each generation, although as yet no experiment has discovered the top quark. The building blocks are held together by four fundamental forces, each carried by a particle called a ‘gauge boson’. The graviton is responsible for gravity; the photon for the electromagnetic force; the W and Z particles for the weak force; and the gluon for the strong force. (The weak and strong forces act only over the dimensions of the atomic nucleus). One additional particle is required by the Standard Model – the Higgs Boson. This particle figures in the mechanism that makes the W and Z particles heavy, in the part of the model that unifies the electromagnetic and weak forces.