ÐÓ°ÉÔ­´´

One of the cornerstones of modern particle physics theory is the notion that particles such as protons and neutrons are composites, each made up from fundamental entities known as quarks. Both the proton and the neutron are thought to contain three of these quarks. Each quark carries an electric charge one-third or two-thirds of the size of the charge on an electron, which is usually thought of as the fundamental unit of charge. Particles made out of quarks are collectively known as hadrons.

Quark theory has been extremely successful in describing the properties of particles at the subatomic level. It is now part of what is known as the Standard Model of particle physics. But there is a snag: no experiment using particle accelerators has ever produced unequivocal traces of particles with fractional charge, and evidence for the presence of fractionally charged particles in ordinary matter has proved equally ambiguous.

Now it seems that experimenters may have been looking in the wrong place. There is an increasing weight of evidence that unusual forms of matter made from quarks may be arriving at Earth in the form of cosmic rays.

Quark theory was developed in the 1960s following suggestions made independently by Murray Gell-Mann and George Zweig (both at the California Institute of Technology, but working separately) that properties of what had previously been regarded as indivisible fundamental particles could best be explained if they were made of quarks (Zweig suggested the term ‘aces’, but Gell-Mann’s suggested name ‘quarks’, taken from a line in James Joyce’s novel Finnegan’s Wake, is the one that has stuck).

This proposal resolved a problem that had worried physicists: since the Second World War, the number of so-called ‘fundamental’ particles identified had risen dramatically. In the 1930s, physicists knew of only three fundamental particles, the electron, proton and neutron. Using just these three particles, they could explain the properties of all the known chemical elements. But by the early 1960s there were more than 100 varieties of known particles. Many were discovered in cosmic rays while others were detected using particle accelerators. The quark hypothesis showed how all of these (except the electrons) could be built up from a handful of genuinely fundamental entities. This simplified the picture, echoing the way the elements are described in the periodic table in terms of protons, neutrons and electrons. Everyday matter can be completely described in terms of two varieties of quark, called ‘up’ and ‘down’, together with the electron and its associated neutrino. The proton contains two up quarks, each with charge +2/3 (in units where the charge on the electron is -1), and one down quark, with a charge of -1/3; the neutron contains two down quarks and one up quark. In order to explain all the properties of other particles known at the time, it was also necessary to invoke the existence of a third variety, dubbed the ‘strange’ quark, which has more mass than the other two.

Physicists now believe that there are, in fact, six quarks which come in pairs – up/down, strange/charm, and top/bottom. These match up with three generations of electron-like particles, each with its associated neutrino, collectively dubbed leptons – the electron itself and the mu and tau particles. These quarks and leptons are thought by many to be the truly fundamental building blocks of the Universe.

In order to test the theory, physicists tried to look for free quarks. At first, experimenters expected that the fractional charge on the quarks would make them easy to detect; but things soon turned out otherwise, leading theorists to suggest various reasons why individual free quarks might find it difficult to lead an independent existence. The search went three ways. One was to look for quarks in the ordinary matter around us. Another was to look for quarks produced by collisions between particles in large accelerators, like those at the European particle physics laboratory in Geneva (CERN). The third approach was to look for quarks in relatively low energy cosmic radiation, among particles from space with energies in the range from 1010 to 1012 electronvolts.

The first technique involves variations on the classic experiment used by Robert Millikan in 1911 (and subsequently by almost every physics undergraduate) to measure the size of the charge on an electron. Millikan, who worked at the University of Chicago, invented the technique of spraying a fine mist of oil droplets into the space between the horizontal plates of a capacitor, and watching the movement of individual droplets through a microscope. Some of the droplets pick up electric charge from their surroundings, and by adjusting the charge on the capacitor it is possible to make such a droplet hover, with the upward electric force it experiences exactly balancing the downward tug of gravity. This provides a way to measure the amount of charge on the droplet, which is almost always found to be a multiple of -1.6 times 10 ^-19 coulombs – the fundamental unit of charge on a single electron.

In fact, Millikan did report one observation of a charge with two-thirds of the usual value, though this was dismissed for decades as experimental error. And a group of researchers at Stanford University has consistently claimed, since the development of the quark hypothesis, to have found charges of 1/3 or 2/3 on small niobium spheres. But these claims have not been widely accepted as evidence for the existence of free quarks.

The other two approaches rely on attempting to measure the ionisation produced by quarks moving at high speeds. This is proportional to the square of the charge on the fast-moving particle, and so is expected to be one-ninth or four-ninths of that for fast-moving electrons. The searches for quarks in accelerator laboratories have proved fruitless. No one, as far as I know, has claimed to have found a quark track among the debris from particle collisions there. This is no surprise; Zweig had indeed calculated that the probability of producing free quarks under the conditions attainable in accelerators (essentially limited by the energy available) is ‘prohibitively small’. But nature can still produce bigger ‘bangs’ than anything possible on Earth, and one product of those bangs is high energy cosmic rays.

The searches in low energy cosmic radiation have produced results rather like those in the Millikan-type experiments. A few research groups have found what they believe to be evidence for fractional charges, but none of these claims has been generally accepted as direct observations of free quarks. So some experimenters, including the group I work with at the University of Sydney, have turned their attention to searches in high energy cosmic radiation. There is now another powerful motivation for this work – the suggestion that cosmic rays may contain bits of matter in a state known as ‘strange matter’ or ‘quark nuggets’.

The standard theory suggests that strange matter that is composed of roughly equal numbers of up, down and strange quarks would be stable, but that such matter could be formed only under conditions of extremely high pressure or energy, far beyond anything attainable on Earth. This was regarded as just a quirk of the equations describing the theory, until astrophysicists got in on the act. They have suggested that strange matter might be produced in the Universe in two ways. First, Ed Witten, a theorist at Princeton University, pointed out in 1984 that strange matter may have formed under the extreme conditions of the big bang in which the Universe was born, producing quark nuggets which are spread throughout the Universe today. Secondly, under the extreme conditions of pressure at the centres of some neutron stars (stars as dense as an atomic nucleus, with more mass than that of our Sun packed into a sphere only a few kilometres across), everyday matter (in the form of neutrons) may be squeezed hard enough to form a ‘quark soup’. In the most extreme versions of this scenario, chunks of strange matter may be ejected from the rapidly spinning neutron star and travel far across space.

A primordial quark nugget, left over from the big bang, might have a mass of about a tonne, packed within a sphere only about 0.001 centimetres across. Chunks of strange matter ejected from neutron stars to form cosmic rays would probably be considerably less massive. But either way, if such an object hit the top of the atmosphere of the Earth it would break up to produce a shower of particles, some of which would be very likely to carry the characteristic signature of the fractional charge of the quarks. Some of the ‘particles’ reaching the Earth from space might contain dozens or hundreds of quarks. It was just such a claim that strange particles have been found in cosmic rays that turned the spotlight back on to the search for quarks last year (New ÐÓ°ÉÔ­´´, Science, 10 November 1990). The claim came from Japanese researchers, headed by Takeshi Saito of Tokyo University’s Institute for Cosmic Ray Research and including physicists from Kobe University. They used a large balloon to fly a particle detector close to the top of the atmosphere, intending to measure the flux of nuclei of elements heavier than boron in cosmic radiation. In 1981, in a flight lasting 28 hours, they found 127 000 events attributable to ordinary atomic nuclei. In addition, they found two events with very unusual characteristics. The events both corresponded to particles with a charge of 14 (the same as the charge on a silicon nucleus) but a mass of 370 – that is, 370 times heavier than a proton, and far heavier than any known nucleus, stable or unstable. The discovery was made three years before the suggestion that strange matter might exist. At first, the anomalous events were ignored while the researchers concentrated on their analysis of the 127 000 events that were the raison d’etre for the balloon experiment. But since then the Japanese team has spent years trying to find a conventional explanation for their two anomalous observations, without success.

The researchers were left with only one explanation for these events – that they correspond to the arrival of fragments of strange matter in the detector. Indeed, two quark theorists, Edward Farhi and R. L. Jaffe, of Massachusetts Institute of Technology, had even predicted in 1984 that agglomerations of strange matter with charges of about 12 and masses of about 316 could be produced in neutron stars and survive the journey across interstellar space. A study of cosmic radiation by a Brazilian-Japanese collaboration has also found several events with unusual characteristics that might be explained in terms of strange quark matter.

Our research group at Sydney University, which studies cosmic rays, was particularly excited by these discoveries, because they lend weight to claims we made more than 20 years ago that our detectors had observed effects due to quarks. Indeed, one of the Sydney University cosmic ray experiments may have detected quark effects in 1963 – the year before Zweig and Gell-Mann startled the physics community with their new ideas about what protons and neutrons are made of.

When a high energy cosmic ray (one with energy above about 5 times 10 ¹⁴ electronvolts) arrives at the top of the atmosphere, it usually collides with an atomic nucleus in the atmosphere to produce a jet of secondary particles. These are particles created, like those created in collisions in accelerators, out of pure energy, according to Einstein’s equation E = mc2. These particles, together with fragments of the original cosmic-ray particle, in turn make further interactions with atmospheric particles to produce what is known as an extensive cosmic-ray shower. By the time it reaches sea level, such a shower may contain many hundreds of thousands of particles; the total number depends on the energy of the primary particle. The shower will be spread over an area of several square kilometres, and needs comparably large detectors for its investigation.

One of the experiments carried out by the Sydney University cosmic ray group had a collecting area of around 100 square kilometres. Using the information about the ground level showers it provided, we could infer properties of primary cosmic rays with energies up to 1020 electronvolts. In a different version of the experiment we looked at the central regions (cores) of showers which had a primary energy of around 1015 electronvolts. This array was not looking for quarks. It was planned and built before quarks were postulated. But what we had found, by 1963, was solid evidence for secondary particles in the showers moving sideways at high speed, travelling with high transverse momenta.

The accelerators of those days were using comparatively low energies, and they had established that the secondary particles produced in their collisions had limited transverse momenta, no more than about 0.5 GeV/c (1 gigaelectronvolt is 109 electronvolts; c is the speed of light). We were finding values at least 100 times greater than that. It was difficult to convince the accelerator physicists that our results were real. They remained sceptical until 1972, when the Intersecting Storage Rings at CERN came into action at energies equivalent to some of those we had been observing in cosmic radiation – 1000 gigaelectronvolts (1012 electronvolts). Then, they too observed particles with high transverse momenta.

Even before this confirmation from the accelerator experiments, I had become intrigued by these high transverse momenta. The interesting thing about them was that they provide clear evidence of effects due to forces much stronger than the forces we had been used to dealing with between nuclear particles at lower energies. In the middle of the 1960s, I went shopping around the world for some possible explanations.

By that time, the quark hypothesis had been put forward, and one suggestion was that the ‘new’ force was the force that binds quarks together. This is now known as the ‘glue’ force, and is said to be carried by ‘gluons’, in the same way that the electromagnetic force is carried by photons.

If the high transverse momenta we were observing were related to forces operating between quarks, maybe we could detect the quarks themselves. I returned to Sydney and added four instruments known as Wilson cloud chambers to our array. These provide a standard way of measuring the ionisation produced by fast particles, and hence the charge that they carry. The chambers can be traversed by many particles simultaneously, and still give a good ionisation measurement for each of these particles.

After two years operating the modified system, we obtained the event shown at the beginning of this article. One track, labelled QQ, is caused by a much more lightly ionising particle than the other parallel tracks, labelled 11, 22 and so on. The specific ionisation for the particle causing a track is measured by counting the number of droplets per centimetre along the track. QQ gave 16.2 drops per centimetre, with a standard deviation of 2.5. The mean for many ‘normal’ tracks is 40.2 drops per centimetre. The expected value for a quark with charge 2/3 is 40.2 times 4/9, or 17.9. Track QQ is within one standard deviation of this value, and more than nine standard deviations away from the value for a particle with charge 1. In other words, such a result might arise by chance for one track with a probability of less than 10 ^-15.

We published this result in Physical Review Letters in 1969 (vol 23 p 658). Various objections to our interpretation of the data were put forward, some of them using extremely ingenious suggestions to account for the aberrant track without invoking a free quark. Dealing with these objections involved more experiments and more theoretical work over the following 10 years, but by 1979 none of the objections still stood, and the final rebuttal of the non-quark ‘explanations’ appeared in the Australian Journal of Physics in 1983 (vol 36 p 717). By then, there was more evidence for quark events in cosmic ray showers.

One of the new experiments was a straightforward search for quarks where we had found them, in the cores of air showers. Fred Ashton and colleagues at the University of Durham used a device called a neon hodoscope, which has a much larger collecting area than our cloud chambers, but pays for this with poorer ionisation discrimination. After running the experiment for 15 000 hours, they had found two tracks with all the characteristics of particles with charge 1/3.

A very large air shower array has been operated high in the Tien Shan mountains by physicists from the P. N. Lebedev Institute in Moscow. This has many types of detector, but the one that is interesting for quark hunters is called an ionisation calorimeter. Although designed to investigate normal hadrons, this too has found many events consistent with the expected behaviour of quarks.

A different kind of detector has been operated at even higher altitude on Mount Chacaltaya, in Bolivia. This is the Brazilian-Japanese collaboration that I have already mentioned. The experiment consists of so-called emulsion chambers, each consisting of two multi-layer ‘sandwiches’ separated vertically by an air gap 1.5 metres across (see foot of opposite page). Each sandwich has a surface area of 40 square metres. The top sandwich has seven layers of photographic emulsion, most of it in the form of X-ray film, with layers of lead 1 centimetre thick between each layer of film.

The lower sandwich has six layers. A high energy particle such as an electron or a gamma ray produced by nuclear interactions in the atmosphere above the device will interact with the topmost layer of lead and produce a cascade of particles that shows up as a black spot on the X-ray film. Such gamma rays are produced by the decay of particles known as neutral pions, which are copiously produced in high energy nuclear interactions. The size of the spot gives an indication of the energy of the incident particle. Hadrons in the shower will interact with lead in the top sandwich, and produce more particles (such as pions) which are then detected by the spots they cause in the film in the lower sandwich.

In one event, observed in 1971, a total energy of 220 000 gigaelectronvolts was observed in the lower sandwich, but only 27 000 gigaelectronvolts in the upper sandwich. Because the event has a small ‘head’ on a large ‘body’, it was dubbed Centauro, after the mythical half-man, half-horse. It seems that there had been an interaction in the air above the detector which produced many high energy hadrons but no pions. This was unprecedented, but several similar events have since been detected by the same team and by other groups. These are the events that can best be explained in terms of quark nuggets or ‘globs’.

Centauro events can be explained by the break-up of pieces of quark matter in the air above the detectors. In the debris from the break-up, many quarks would group themselves together in threes to make up normal hadrons, but there would be no interactions of the kind that produce pions until those hadrons hit the lead in the upper sandwich. One variation on this theme is the possibility that a single quark might surround itself with a cloud of protons and neutrons.

Other researchers have investigated the theoretical properties of both strange matter, consisting of up and down and strange quarks, and ‘non-strange quark matter’, which would contain only up and down quarks. The most striking outcome of this theoretical work is the prediction by Farhi and Jaffe in 1984 that a small ‘glob’ of strange matter with charge 12 and mass 316 would be stable.

Overall, it seems to me that there is now strong evidence for free quarks in at least three experiments utilising high energy cosmic radiation, even though free quarks have not been detected in accelerators. In addition, quark matter (most probably strange quark matter) has been detected in several cosmic ray experiments at high altitudes.

All of this fits in well with quark theory. Working down from the top of the atmosphere, the events observed by Saito’s team are due to strange globs arriving in the balloon detectors intact; the Centauros are the immediate by-products of a quark glob break-up in the atmosphere; and the lightly ionising particles observed at sea level are free quarks left over from such a break-up. It is no surprise that free quarks are not found in accelerator experiments, because the energies involved are still too low to produce them.

The cosmic ray evidence does, however, suggest a way in which the accelerator physicists could get in on the act. Combining evidence from all the experiments, it is possible to calculate the flux of free quarks arriving at the surface of the Earth. It works out at 2.5 per square metre per year. This suggests that at any time there may be as many as 48 free quarks in every kilogram of ordinary matter at the Earth’s surface.

Large particle research centres such as CERN may not have the energy to produce free quarks, but they do have the expertise to detect them if they exist – and cosmic radiation reaches the surface all over the globe. Each year, according to my calculations, the area within the perimeter of CERN’s LEP accelerator is traversed by 150 million free quarks. It seems a pity to waste such an abundant natural resource. I suggest that one or more of the large particle research laboratories should undertake a scaled up, high-tech version of the Sydney, Durham or Moscow experiments. For example, scaling up our experiment might involve using more than 100 cloud chambers, each with more than twice the area of our 1960s models, spread out over a correspondingly large area. This would still be a very cheap experiment by CERN standards.

Another possibility is to develop Millikan-type experiments to examine hundreds of grams of material, instead of just a few drops of oil. Many physicists still express doubts about the quark interpretations of cosmic ray events; but if these interpretations are correct, then such comparatively cheap and simple experiments, rather than huge accelerators smashing beams of particles together, are all that we need for final confirmation of the validity of the quark model of particle physics. It seems well worth making the effort.

Brian McCusker is Emeitus Professor of High Energy Physics at the University of Sydney, Australia

Further reading: The Cosmic Onion, by Frank Close, Heinemann, 1983.