by Frank Close
ν + n → e– + p
In this sense we see that the neutrino has an affinity for the electron. An antineutrino has an analogous affinity for the positron. The conservation of electric charge then prevents an antineutrino interacting with a neutron to make an analogoue of the above, but if it hits a proton, it can reveal itself:
We have seen how three quarks unite to make particles such as the proton and neutron (generically these three-quark composites are known as baryons). The clusters of three antiquarks are then known collectively as antibaryons. It is possible to cluster quarks and antiquarks; one of each is sufficient. So if we use q to denote either of u or d, and to denote the antiquarks, it is possible to make four combinations of clusters . As a three-quark cluster is called a baryon, so this combination of quark and antiquark is known as a meson. As was the case for the proton and neutron, there are higher-energy ‘resonant’ states for these mesons too.
One of the most famous properties of antimatter is that when it meets with matter, the two mutually annihilate in a flash of radiation, such as photons of light. It is no surprise then that mesons do not live very long. A quark and an antiquark, restricted to the femtouniverse of 10–15 m, mutually annihilate within a billionth of a second, or less. Even so, such ephemeral mesons play a role in building our universe. The most familiar, and the lightest, configuration are the pions, such as the and which were predicted by the Japanese theorist Yukawa in 1935 as ephemeral entities within atomic nuclei that provided the strong attractive force that holds nuclei together. Their subsequent discovery in 1947 brilliantly confirmed this theory. Today we know of their deeper structure, and also have a more profound understanding of the forces at work on quarks, and antiquarks, which build up the mesons and baryons and ultimately atomic nuclei (see Chapter 7).
There are two neutral combinations that we can form: and . These make the electrically neutral pion, πo, and seed another meson, the electrically neutral eta, η. Why it is that a single quark can grip a single antiquark like this, but that three quarks or three antiquarks are attracted to form baryons or antibaryons, will be described in the next chapter.
Chapter 5
Accelerators: cosmic and man-made
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Cosmic rays come free of charge but are random; the need for controlled experiments leads to particle accelerators. This chapter looks at smashing beams of particles into targets in the lab and colliding beams head on, and the advantages of each. Also, beams of matter and antimatter – electrons and positrons at the Large Electron Positron (LEP) collider, protons and antiprotons, particle factories.
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For a century beams of particles have been used to reveal the inner structure of atoms. These have progressed from naturally occurring alpha and beta particles, courtesy of natural radioactivity, through cosmic rays to intense beams of electrons, protons, and other particles at modern accelerators. By smashing the primary beams into a target, some of the energy can be converted into new particles, which can themselves be accumulated and made into secondary beams. Thus beams of pions and neutrinos, as well as other particles called kaons and muons, have been made, along with antiparticles such as positrons and antiprotons. There are even beams of heavy ions – atoms stripped of their electrons – which enable violent collisions between heavy nuclei to be investigated.
Different particles probe matter in complementary ways. It has been by combining the information from these various approaches that our present rich picture has emerged.
Sometimes the beams are directed at static targets. In recent years there has been an increasing strategy of making counter-rotating beams of particles and antiparticles, such as electrons and positrons, or protons and antiprotons, and colliding them head on. Such techniques enable questions to be investigated that would otherwise be impossible, as we shall see later.
There has also been a renewed interest in cosmic rays, where nature provides particles at energies far beyond anything that we can contemplate achieving on Earth. The problem is that such rays come at random, and are much less intense than beams made at accelerators. It was the desire to replicate the cosmic rays under controlled conditions that led to modern high-energy physics at accelerators. Today we are recognizing that the Big Bang may have made exotic particles, far more massive than we can ever make on earth, but which might arrive in cosmic rays occasionally. We discovered strange particles (see Chapter 8) in cosmic rays, and later made them to order at accelerator experiments; there is hope that similar fortunes might await us.
Stars and supernovae emit neutrinos; special laboratories have been constructed underground to obstruct the arrival of all but the most penetrating particles, such as neutrinos. Neutrino astronomy is a new area of science that is expected to flower in the early decades of the 21st century. There are also attempts to find evidence of extremely rare events, such as the possibility that protons are not stable and decay, even with a half life that exceeds 1032 years. The technique is to have huge samples, such as swimming pool volumes of pure water. Although protons on average have such an immense life expectancy, quantum theory implies that an individual proton might live far longer, or shorter, than this. So in a large sample of order 1033 protons, such as could be found in a vast pool, one or two might decay in a year. Wait long enough, and you might be lucky enough to witness it.
These are examples of what is known as non-accelerator physics, where natural processes have produced the particles and we detect their effects. Here on Earth we can make intense beams of high-energy particles in laboratories with particle accelerators. In this chapter I shall focus on how accelerators have developed and what is involved in making them. This will also give an insight into the plans for the immediate future in high-energy particle physics.
Electrically charged particles are accelerated by electric forces. Apply enough electric force to an electron, say, and it will go faster and faster in a straight line, as in the linear accelerator at Stanford in California, which can accelerate electrons to energies of 50 GeV.
Under the influence of a magnetic field, the path of a charged particle will curve. By using electric fields to speed them, and magnetic fields to bend their trajectory, we can steer particles round circles over and over again. This is the basic idea behind huge rings, such as the 27-km-long accelerator at CERN in Geneva.
From cyclotrons to synchrotrons
Exploration of the atom had begun with beams of alpha and beta particles from radioactive bodies. But the individual particles had small energies and restricted ability to get inside the nuclear environment. Beams of high-energy particles changed all that.
The original idea had been to accelerate particles to high energy through a series of small pushes from relatively low accelerating voltages. Particles travel through a series of separate metal cylinders in an evacuated tube. Within the cylinders there is no electric field and the particles simply coast along. But across the gaps between the cylinders electric fields are set up by means of alternating voltages, which switch between positive and negative values. The frequency of the alternating voltage is matched with the length of the cylinders, so that the particles always feel a kick, not a brake, as they emerge into a gap. In this way, the particles are accelerated every time they cross between one cylinder and the next. This is the basis of the operation of the modern linear accelerators. Usually such ‘linacs’ are short, low-energy machines, but they can be high energy and lengthy, as at the Stanford Linear Accelerator in California. They are most commonly used in the preliminary stages of acceleration at today’s big rings.
The idea of creating a ring-shaped accelerator originated with Ernest Lawrence, who used a magnetic field to bend the particles into a circular orbit. Two hollow semi-circular metal cavities, or ‘Ds’, were placed facing one another to form a circle, with a small gap between the two flat faces of the Ds. The whole construction was only about 20 cm across, and Lawrence placed it between the circular north and south poles of an electromagnet, to
swing the particles round the curve, while an electric field in the gap speeds them. After being accelerated by the electric field in the gap, they curved round in a circular path until they met the gap half an orbit later. By this device they could pass across the same accelerating gap many times, rather than travel through a succession of gaps. They spiral outwards as their speed increases, but the time intervals between successive crossings of the gaps remain constant.
To accelerate the particles continuously, the electric field in the gap must switch back and forth at the same frequency with which the particles complete the circuit. Then particles issuing from a source at the centre of the whirling device would spiral out to the edge and emerge with a greatly increased energy.
This device was known as a ‘cyclotron’, and worked on the principle that the particles always take the same time to complete a circuit.
This is, however, only approximately true in practice. As the energy of the particles increases, the effects of special relativity play an ever more important role. In particular, there is an increasing resistance to acceleration, where more force is required to obtain the same acceleration as the speed approaches that of light. The accelerated particles take longer to complete a circuit, eventually arriving too late at the gap to catch the alternating voltage during the accelerating part of its cycle.
The solution was to adjust the frequency of the applied voltage so that it remains in step with the particles as they take longer to circulate. However, there is a catch: a machine operating at variable frequency can no longer accelerate a continuous stream of particles, as the cyclotron had done. Changing the frequency to keep in time with higher-energy particles would mean that any particles still at lower energies would become out of step. Instead the ‘synchrocyclotron’ takes particles from the source a bunch at a time, and accelerates these bunches out to the edge of the magnet.
The synchrocyclotron was able to accelerate protons to sufficient energies that collisions with nuclei produce pions, the lightest particles that, we now know, are made from a single quark and an antiquark. However, the machine was nearly 5 m in diameter and to go to higher energies, such as those needed to produce the more massive strange particles, was impractical.
11. Lawrence’s original cyclotron was only 13 cm diameter. The magnetic field that steers the particles on a circular path is supplied by two electromagnets. These generate a vertical north–south field through the path of the particles, which are contained in a horizontal plane. They are accelerated by an electric field, which is provided across a gap between the two hollow D-shaped metal vacuum chambers. A radioactive source at the centre provides the particles. The particles curl round in the cyclotron’s magnetic field, but as they increase in energy they curl less and so spiral outwards until they emerge from the machine.
The solution was to increase the strength of the magnetic field continuously as the circling particles gain energy, thereby keeping them on the same orbit instead of spiralling outwards. Moreover, the enormous single magnet of the cyclotron can be replaced by a doughnut-like ring of smaller magnets, which is the shape familiar to modern accelerator rings. The particles travel through a circular evacuated pipe held in the embrace of the magnets; they are accelerated during each circuit by an alternating voltage of varying frequency, which is applied at one or more places around the ring; and they are held on their circular course through the pipe by the steadily increasing strength of the magnetic field. Such a machine is called a synchrotron, and it is still the basis of large modern accelerators. The first major synchrotrons were at Brookhaven in the USA and CERN in Geneva, with energies up to 30 GeV by 1960.
In the 1960s the idea of quarks emerged and with this came the challenge to reach energies above 100 GeV in the vain hope of knocking quarks out of protons. Improvements in technology led to more powerful magnets, and by placing them in a ring with a diameter of over a kilometre, by the middle of the 1970s Fermilab near Chicago in the USA and CERN had achieved proton energies of some 500 GeV. By 1982 Fermilab had achieved 1,000 GeV, or ‘1 TeV’, and became known as the ‘Tevatron’.
Today superconducting magnets enable even more powerful magnetic fields to be achieved. At Fermilab, alongside the Tevatron, is a smaller ring known as the Main Injector. One of the Main Injector’s tasks is to direct protons at 120 GeV onto targets to create secondary beams of particles for experiments. The extracted protons strike special targets of carbon or beryllium to produce showers of pions and kaons. The pions are allowed to decay to produce a neutrino beam, while the kaons can be separated out to form a kaon beam for experiments. Different particles with different properties can probe different features of the target and help to build a richer picture of its make-up.
12. The Cosmotron at the Brookhaven National Laboratory was the first proton synchrotron to come into operation, in 1952, accelerating protons to an energy of 3 GeV. The magnet ring was divided into four sections (the nearest is clearly visible here) each consisting of 72 steel blocks, about 2.5 m × 2.5 m, with an aperture of 15 cm × 35 cm for the beam to pass through. The machine ceased operation in 1966.
The Main Injector also directs 120 GeV protons onto a special nickel target at energies sufficient to produce further protons and antiprotons at a rate of up to 200 billion antiprotons in an hour. Antiprotons, the antimatter versions of protons, have negative rather than positive electric charge, and this means that they can travel round the Tevatron’s ring of superconducting magnets at the same time and at the same velocity as the protons, but in the opposite direction. Once the particles are at 1,000 GeV, or 1 TeV, the two beams are allowed to collide head on – and the Tevatron has reached its final goal: collisions of protons and antiprotons at energies that recreate the conditions of the universe when it was less than a trillionth of a second old.
At CERN a 27-km ring of such magnets will guide protons at energies up to 7 TeV at the Large Hadron Collider (LHC) due to start experiments in 2007. Special magnets can steer two counter-rotating beams of protons, or of atomic nuclei, to meet head on. This will be the pinnacle of colliding beam technology, which became a major strategy in high-energy physics in the final years of the 20th century.
Linear accelerators
The Stanford Linear Accelerator is the longest linac in the world. It accelerated electrons to 50 GeV energy in just 3 km, whereas at LEP, a circular accelerator, they reached 100 GeV but required 27 km circumference to the ring. Why this difference and what decides whether to make a linear or circular accelerator?
Electron synchrotrons work perfectly well apart from one fundamental problem: high-energy electrons radiate away energy when they travel on a circular path. The radiation – known as synchrotron radiation – is greater the tighter the radius of the orbit and the higher the energy of the particle. Protons also emit synchrotron radiation, but because they are 2,000 times as massive as electrons, they can reach much higher energies before the
13. A view inside the 27-km (17-mile) circular tunnel of CERN’s Large Electron Positron (LEP) collider, which ran from 1989 to 2000. The electrons and positrons travelled in opposite directions in the beam pipe through hundreds of brown and white bending magnets (dipoles) and blue focusing magnets (quadrupoles). Originally LEP accelerated the beams to a total collision energy of around 90 GeV, but by the time of its final shutdown in October 2000 it reached more than 200 GeV.
14. The 3-km- (2-mile-) long linear accelerator at the Stanford Linear Accelerator Center (SLAC). The electrons start off from an accelerator ‘gun’ where they are released from a heated filament, at the end of the machine at the bottom left of the picture. The electrons in effect surf along radio waves set up in a chain of 100,000 cylindrical copper ‘cavities’, about 12 cm in diameter. The machine is aligned to 0.5 mm along its complete length and situated in a tunnel 8 m below ground. The surface buildings that mark out the line of linac contain the klystrons which provide the radio waves.
amount of energy lost becomes significant. But even at only
a few GeV, electrons circulating in a synchrotron radiate a great deal of energy, which must be paid for by pumping in more energy through the radio waves in the accelerating cavities. It was for these reasons that until recently high-energy electron accelerators have been linear. Indeed, electrons have only been used in circular machines for the special advantages that can arise. Specifically, the head-on collisions make more efficient use of the energy than when a static target is hit. The second major advantage is the ability to probe in ways that would otherwise prove impossible, as for instance at LEP where electrons annihilate with positrons and the counter-rotating beams are the only effective way to achieve the required high intensity.
LEP was a circular machine in a tunnel 27 km long. This is testament to the problems with lightweight electrons and positrons travelling round circles, that such a distance is needed to enable them to reach 100 GeV without wasting too much energy in radiation. To reach energies of several hundred GeV in circular orbits would require distances of hundreds of kilometres, which are out of the question. This is why linear colliders are planned for the longer-term future.
