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Antimatter of fact Mirrors and transformations Antiparticle collision course
What is antimatter?
Elementary particles
Cool antiprotons
Antimatter fact and fiction The designer equation Low energy antiparticles
The anti-particle emerges Atomic antimatter
The antiproton Continuing the tradition

Antimatter of fact

A unique machine – the Antiproton Decelerator (AD) – has opened up a new phase in CERN's tradition of milestone scientific discoveries using beams of antiparticles – the smallest constituents of antimatter. Physics experiments at the AD will embark on the first precision study of the behaviour of atoms of antimatter.

In 1927 a British physicist called Paul Dirac wrote down a new equation for the electron which led him to predict that the electron had a counterpart particle, the 'anti-electron'. This particle was later discovered (and renamed the positron). Knowing that electrons actually had antiparticles led Dirac to go one step further and predict that every one of the fundamental particles that make up matter could also have an antiparticle. In the same way that ordinary particles combine to form atoms, he pointed out that these antiparticles could combine to make atoms of antimatter.

Initially an intellectual curiosity, antiparticles have become a major tool of modern research, as CERN’s antimatter factory, the Antiproton Decelerator (or AD for short) delivers antiprotons to experiments.

Antiprotons are the antimatter counterparts of the ordinary protons of atomic nuclei. Two of the AD experiments are using these antiprotons to synthesize atoms of antihydrogen, and for the first time study the behaviour of this, the simplest possible atom of antimatter. By comparing their antihydrogen measurements with those of ordinary hydrogen, these experiments will search for systematic differences between the behaviour of matter and antimatter.

The Big Bang presumably created a Universe containing equal amounts of matter and antimatter. However the Universe today appears to contain only matter. Where has the Big Bang's antimatter gone? Any disparity between the behaviour of hydrogen and antihydrogen could shed new light on the fate of primordial antimatter.

What is antimatter?

Like positive and negative, like debt and credit, matter and antimatter are equal and opposite. They mutually destroy each other, disappearing in a burst of energy. Conversely, a blast of energy, such as the Big Bang that marked the beginning of the Universe, creates matter and antimatter in equal amounts. Antimatter is a vital component of any creation scenario. The realization that antimatter had to exist was one of the major intellectual achievements of the 20th century.

Antimatter fact and fiction

Antiparticles are an essential tool of 21st century physics. But antimatter is frequently cited as the ultimate science fiction. Antimatter would be the ideal fuel a few grams of antimatter could power a rocketship, or an entire city for a short time. But there is no antimatter 'mine' where it can just be dug out of the ground. All antimatter first has to be manufactured. The resultant fuel cannot supply more energy than was used to make it. In addition, antimatter synthesis is itself highly inefficient. It is also very difficult - the maximum yield of antiprotons ever produced at CERN during a year would supply enough electricity to light a bulb for a few seconds!

Apart from their role in scientific research, positrons are used in radiography for positron emission tomography, PET. Injecting a positron-emitting tracer into a patient's body or a piece of machinery, the positrons soon annihilate with the surrounding atoms. In doing so, they emit a characteristic radiation signature which quickly pinpoints where the annihilations took place. These PET radiographs reveal inner structure quickly and with low doses of positrons. They are widely used in brain scans, both for medical diagnosis and for revealing how the brain itself works.

Mirrors and transformations

When we look into an ordinary mirror, our left hand appears to have been transformed into a right hand, and vice versa. But apart from this left-right reversal, the mirror image appears to behave in a predictable way. An ordinary looking-glass mirror is a simple example of what is known in mathematics as a transformation. Progress in understanding the physics of the Universe comes by writing down equations that are valid under wider and wider transformations.

A example came early in the 20th century with Einstein's Special Theory of Relativity, which addressed the puzzle of why light always appears to travel at the same speed. The light emitted from a star arcing across the sky at thousands of kilometres per second appears to have the same velocity as light emitted from a stationary source on Earth. Why is the speed of light not boosted by the speed of its source?

Einstein argued that images of objects travelling at high speeds become distorted, and this has to be taken into account whenever physics describes objects moving at very high velocities. Rulers appear to shrink, and time intervals appear to get longer.

Elementary particles

Atoms (from the Greek 'uncuttable') were supposed to be the smallest constituents of matter. However experimental research at the end of the 19th and the beginning of the 20th century showed that atoms are not indivisible. Atoms are like miniature solar systems, with lightweight negatively-charged electrons orbiting around heavy positively-charged nuclei. The latter contain more than 99.9 per cent of the mass of the atom, but the lightweight electrons nevertheless account for the chemical and electrical behaviour of the atoms. Further research showed that nuclei are composed of electrically charged protons, and electrically neutral neutrons.

Trying to explain this remote world deep inside the atom, physicists soon found that everyday experience was not a reliable guide. Subatomic particles behave in very unexpected ways. The transformations at work deep inside the atom are very special. This 'quantum mechanics' emerged in the 1920s as a new branch of physics, requiring innovative thought and revolutionary ideas.

The quantum electron appeared to behave like a on-off electrical switch it has to be in one of two configurations. This was attributed to the angular momentum, or spin, of the electron. The electron's spin can either point up or down. This duality apparently required the electron to have two mathematical components.

The high velocities of Einsteinian relativity are moreover not confined to the astronomical fireworks of outer space. An electron also travels at these speeds, and the equation of an electron ultimately has to take into account the effects due to quantum mechanics and to relativity.

The designer equation

Nobody had been able to write down an equation for the electron that obeyed both quantum mechanics and the laws of special relativity. Then in 1927 Paul Dirac wrote down his new equation for the electron. But this equation needed electrons with four, not two, components. These extra components moreover carried positive, rather than negative, electric charge. What did these extra labels mean?

For several years, as if afraid of what his equation was telling him, Dirac tried to explain the extra labels as the proton, indeed positively charged, but 2000 times heavier than the electron. However the beautiful symmetry of Dirac's equation was not compatible with such imbalance, and in 1931 he predicted what his equation had said all along. The extra two electron components were the spin up and down states of an 'antimatter' counterpart of the electron, having the same mass as the electron but opposite electric charge. Dirac initially called it the 'anti-electron'.

If they met, Dirac's anti-electron and an ordinary electron should mutually annihilate, producing a burst of radiation. With the anti-electron having the same mass, m, as the electron, the total mass that disappears in this annihilation is 2m. According to Einstein's relativity, the energy would be 2mc2, where c is the speed of light. Conversely, a burst of radiation, if it has more than 2mc2 of energy, can materialize as an electron and a positron.

The anti-particle emerges

Dirac worked at Cambridge, near the Cavendish Laboratory led by Ernest Rutherford, where many famous subnuclear discoveries were made in the 1920s and 1930s. Despite the fact that the two scientists were working at the same university, Dirac and Rutherford did not communicate much. Rutherford worked closely with his experimental physics colleagues. The sophisticated theory of Dirac was at the time an unfamiliar language, and Dirac rarely spoke at all.

At the same time, in Pasadena, California, a young researcher called Carl Anderson was preparing to study 'cosmic rays' – the rain of subatomic particles which arrive at the Earth's surface from outer space. He was using a 'cloud chamber', an instrument which revealed the tracks left by subatomic particles, analogous to the vapour trails produced in the sky by high-flying aircraft. The cloud chamber tracks were not the particles themselves, but showed where the particles had passed. The instrument had been perfected as a physics research tool at Rutherford's laboratory.

Anderson's cloud chamber was fitted with a powerful magnet, which bent the tracks of electrically charged particles. The slower the particles, the more the tracks were bent. Examining the tracks in his chamber, Anderson was intrigued to see electron-like paths which veered the wrong way. At first he assumed these were rare examples of cosmic electrons bouncing up from collisions beneath the cloud chamber, but by analysing carefully the tracks, was able to show that they were coming down from above. Unaware of Dirac's theory and prediction of antimatter, in 1932 Anderson announced he had found the 'positron', an electron carrying positive electric charge.

High energy bursts of cosmic radiation can transform into electron-anti-electron pairs. The anti-electrons from these pairs were the positrons of Anderson. Other experiments, this time in Cambridge, soon showed characteristic V-shaped pairs of tracks, curling off in mutually opposite directions. This sighting of particle-antiparticle pairs was vital evidence for Dirac's new theory. In 1933, Dirac was awarded the Nobel Physics prize. In 1936 Anderson received the prize for his discovery of the positron, as the anti-electron is now universally known.

The antiproton

Having hesitated for several years before predicting the anti-electron, Dirac, emboldened by the anti-electron/positron discovery, said that a similar duality of electric charge had to exist for the other charged subatomic particle known at the time, the proton. 'There is a complete and perfect symmetry between positive and negative charge, and if this symmetry is really fundamental in nature, it must be possible to reverse the charge on any kind of particle,' he said.

Thus was the stage set for the antiproton. However providing enough energy to make proton-antiproton pairs demanded a lot more energy than most cosmic rays supplied, so a special machine was built to supply this energy. The 'Bevatron', the world's largest particle accelerator at the time, came into operation at the Lawrence Berkeley Laboratory in 1955. A team led by Owen Chamberlain and Emilio Segrè soon discovered the negatively-charged antiproton. Chamberlain and Segrè shared the 1959 Nobel Prize for the discovery. The antineutron, too, soon turned up at the Bevatron.

Antiparticle collision course

In a particle accelerator like the Bevatron, a beam of electrically charged particles is held in orbit by a powerful magnetic field. The energy of the beam is increased by supplying radiofrequency power. In 1960, Bruno Touschek, a flamboyant Austrian physicist working in Italy, had a brainwave that was to transform the face of this experimental physics.

Inject particles and antiparticles into a particle accelerator, but in different directions, suggested Touschek. Keep the two counter-rotating beams carefully apart, and then bring them together. Bang! - the particles and antiparticles annihilate. The energy liberated in these annihilations would not be just the mass of the particles, but would also include the extra energy supplied by the accelerator. It was a new scenario for transforming energy and matter.

Soon several such electron-positron colliders were operating, in Europe, in the US, and in Russia. In 1974, the whole physics community was shaken by the discovery by a team working at the small (80m diameter) SPEAR collider at Stanford of a new particle, the J/psi, which heralded whole new generations of subatomic particles. (This particle was also discovered simultaneously at Brookhaven without using beams of antimatter.) Many more particle discoveries have followed via this electron-positron collision route.

Cool antiprotons

If electrons and positrons could be made to collide, why not protons and antiprotons? Here it was not so easy. Antiprotons are much more difficult to manufacture than positrons, and emerge with a wide spread of velocities. Filling a particle accelerator with these antiprotons has been likened to trying to feed a high-pressure hose through a shower attachment. To make antiprotons work for physics, a scheme was needed to control the unruly particles.

In the 1970s, Simon van der Meer, a Dutch machine physicist at CERN, showed how charged particles could be controlled by a sophisticated fast feedback system called 'stochastic cooling' (as the unwanted velocity components of the beam are reduced, it becomes smoother, or 'cooler'.)

A few years later, using this technique, CERN began to build an ambitious proton-antiproton collider in its 7km SPS synchrotron ring. The scheme was masterminded by Carlo Rubbia, and in 1983 experiments at the collider discovered the W and Z carriers of the weak force. For this achievement, Carlo Rubbia and Simon van der Meer shared the 1984 Nobel Prize.

In 1989, the world's largest particle-antiparticle collider, CERN's 27-kilometre LEP electron-positron machine, came into operation. LEP closed down at the end of 2000 after more than a decade of successful research. It is being replaced by the Large Hadron Collider, due to start up in 2007.

Low energy antiparticles

These large particle-antiparticle colliders set out to exploit the sheer energy liberated by the annihilations. But there were other research goals. How does annihilation itself proceed? Can antiparticles be studied in other ways?

In parallel with its big high energy proton-antiproton collider, CERN commissioned a small (80m circumference) low energy antiproton ring, LEAR, which came into operation in 1982, supplying low energy antiprotons to experiments.

Like any other electrically charged particle, an antiproton can be made to orbit in a magnetic field – the principle of the cyclotron. Comparing the frequencies of this rotation for an antiproton and a proton gives a direct comparison of the masses of the particle and its antiparticle.

The TRAP experiment working at LEAR was able to isolate individual antiprotons and ascertain that the proton and antiproton masses are equal with increasing precision, eventually to just one part in ten billion. Making a measurement to such an astonishing accuracy is equivalent to fixing the position of an object on the surface of the Earth to within a few millimetres!

This is by far (a factor of a million) the most incisive comparison yet of proton and antiproton properties. According to the fundamental theorems of physics (the CPT theorem), a particle and an antiparticle should be exactly equal and opposite, so that their scalar quantities like mass are the same, but additive quantum numbers like electron charge should have opposite signs.

Atomic antimatter

In the same way that a negatively-charged electron respectfully orbits around a positive proton to make a stable atom of neutral hydrogen, so a positive positron and a negative antiproton should be able to pair together and form an atom of antihydrogen. However, early attempts to do so were foiled by the antiparticles quickly annihilating with whatever matter particles they met.

In 1994, physicists from one of the existing LEAR experiments set out to synthesize antihydrogen by first using the LEAR antiproton beams to make electron-positron pairs. Hopefully, some of the positrons emerging along the direction of the parent antiprotons might stick to them. In 1995, the experiment discovered 9 examples of synthetic atoms of antihydrogen. Atomic antimatter had arrived, see the press release.

Continuing the tradition

Even though LEAR reduced the energies of the supplied antiprotons by a factor of ten, the particles still emerged at energies equivalent to temperatures of billions of degrees. Its planned research career complete, LEAR was closed in 1996. But at LEAR new methods were developed to supercool antiprotons to within a few degrees of absolute zero. This, and the fascination of antimatter physics, led to a modest new CERN machine, the 188m circumference Antiproton Decelerator, AD, for an antiproton encore. This would make use of equipment used for the antiproton source built at CERN in the 1980s for the first generation of antimatter experiments. (Other antiproton equipment from CERN was given to the Japanese KEK physics laboratory in Tsukuba.)

Two experiments at the AD ('ATHENA' and 'ATRAP' - the latter having grown out of the earlier TRAP experiment at LEAR) capture the decelerated antiprotons and then cool them even further to extremely low temperatures, about 10 million times lower than when they leave the decelerator. This is done by capturing and locking antiprotons in a trap using electric and magnetic fields. The precious antiparticles are then brought in contact with a dense electron gas, which is 'self-cooling' by the emission of synchrotron radiation in the strong magnetic field. Antiprotons can peacefully co-exist with electrons, since they are not particle and antiparticle of each other. This cooling technique was first demonstrated by the TRAP experiment at LEAR (see above).

ATHENA and ATRAP aim to study the simplest anti-atom, antihydrogen. Synthesizing atomic antimatter at LEAR was a major achievement, but no measurements were made the antihydrogen was too hot and dissociated quickly into its component positrons and antiprotons. ATRAP and ATHENA aim to collect supercold antihydrogen that can be stored for further study.

Do atoms of antimatter behave in exactly the same way as those of ordinary matter? Any difference, however small, could help explain how the Universe around us appears to be entirely composed of matter. If the Original Big Bang produced equal amounts of matter and antimatter, where has all the primordial antimatter gone? Physicists think they know the reason why, but are not yet sure. Some species of antimatter (notably the neutral kaon particle) do behave differently to their matter counterparts. But are these obscure and very delicate differences, just a few parts in a thousand, enough to explain the eradication of all antimatter from the entire Universe?

Comparing the properties of ultracold antihydrogen with hydrogen under the same conditions will provide a much more stringent test of whether matter and antimatter do behave in exactly the same way.

Strictly speaking the trap experiments at LEAR did not measure the antiproton's mass explicitly, but rather the ratio of its electric charge to its mass. Another LEAR forerunner of the AD experimental programme resolved this question by irradiating antiprotons trapped in helium atoms with laser beams, and measuring the laser frequency that caused them to jump from one orbit to another. A third AD experiment, ASACUSA, now continues this line of investigation. It sets out to study in detail the properties of the antiproton itself by the way it interacts with ordinary matter, in particular via the study of exotic atoms, in which the negatively-charged antiproton replaces the electron of everyday atoms.


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