Achievements with Antimatter

from the CERN Courier, November 1983
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The discovery of 'heavy light'

Since the 1950s the development of accelerators and storage rings has allowed particles to be collided together at higher and higher energies. The access to increasing energy, which can convert into matter in accordance with Einstein's famous equation, has revealed the existence of several hundreds of shortlived particles in addition to the tiny handful of stable particles, like the proton and electron, of which our Universe is built. If so many particles have already been found, why the jubilation about finding yet two more? Why should the discovery of the W and Z have generated so much excitement in the world of physics?

Discovering the role of light

The story goes back over a hundred years. A good starting point would be 1864 when the Scottish physicist James Clerk Maxwell published a paper called 'The Dynamical Theory of the Electromagnetic Field'. For some time it had been known that electrical and magnetic effects accompany one another - for example, a wire carrying a current is surrounded by a magnetic field - but it needed the genius of Maxwell to construct the equations which link electricity and magnetism. Two types of behaviour, previously considered apart, were brought together in one theory. It was a tremendous advance in understanding - phenomena as different as a high voltage spark, the pattern of iron filings scattered in a magnetic field, electrical currents, the swing of a compass needle, etc., etc. could all be explained from the same source.

Maxwell's famous equations also suggested something else; they very much resembled the equations describing waves, hinting that electromagnetic energy could be transmitted in this way. The existence of such electromagnetic waves was confirmed several years later in experiments by Heinrich Hertz.

In May 1983, the central detector of the UA1 experiment at CERN reveals the tell-tale signature of the long-awaited Z particle as it decays into an electron-positron pair (arrowed).

The human eye is sensitive to electromagnetic radiation of a small band of wavelengths, ranging from those corresponding to red through to violet light. Thus light is just one aspect of electricity and magnetism. Electromagnetic waves play the role of communicator in electrical and magnetic effects and resolve one of the famous philosophical puzzles of action at a distance. How, for example, does a negatively charged electron know that the positively charged proton is sitting in the nucleus (and hence goes into orbit around it)? Answer: electromagnetic waves radiating from each particle establish a communication between the charged particles.

In the course of the 20th century, the picture of electromagnetic waves changed with the advent of quantum theory. At the atomic level the radiation is granular, emitted and absorbed in 'lumps' rather than smooth, continuous waves. These lumps of energy communicating electromagnetism are called photons, the particles of light. They have no mass and can communicate the electromagnetic force over great distances. The theory of 'quantum electrodynamics' is one of the most accurate known to science. Electromagnetic effects can be calculated with seemingly perfect precision.

Our W and Z story is a rerun of the electricity and magnetism story in which another range of phenomena is pulled into the theory and in which the Ws and Zs play the communicator role similar to that of light.

Understanding the weak nuclear force

At the turn of the century, Henri Becquerel, Pierre and Marie Curie and others discovered radioactivity; photographic plates were fogged by some kind of radiation emerging from matter. Later investigations identified one type as 'beta decay', the emission of an electron from a neutron in the nucleus of the atom, converting it into a proton. Here at last the alchemists' dream of converting one chemical element to another was seen to be happening - but so feebly that its origin was named 'the weak nuclear force'. As well as in radioactivity, it plays a vital role in the burning of the sun, the formation of heavy elements, and many other phenomena.

The feebleness of the force, however, is not always the case. As energy (temperature) increases, the weak force gets stronger. In the extreme temperatures of the great primaeval fireball which was the birth of the Universe, we now realize that electromagnetism and the weak force were one and the same thing. But as this fireball cooled down, it eventually reached a temperature where the weak interaction 'froze'. Beta radioactivity and other low energy weak force effects are only the fossil remains of what happened in this early Universe. However modern accelerators and storage rings, like LEP, can recreate those primaeval conditions and we can see again the 'weak' interaction rise to rival electromagnetism in strength.

The realization that the more obscure phenomena due to the weak force could be added to the same theory that covered the long list of 'everyday' electromagnetic effects was slow to come and its tortuous zig-zag progress is described in 'Piecing together a theory'.

The quest for unification - to explain as many things as possible from a minimal number of postulates - is a central theme in basic physics. After developing his momentous theories of relativity and gravitation early in the 20th century, Albert Einstein spent much of his life unsuccessfully trying to weld gravitation and elec-tromagnetism. On a different front, Enrico Fermi and others in the 1930s looked at the possibility of unifying electromagnetism and the weak force. Sheldon Glashow, getting nearer, tried again in the early 1960s. But it needed the development of vital new concepts before Abdus Salam and Steven Weinberg, working independently in the mid 1960s, came up with an innovative solution. These powerful intuitive ideas needed to be checked against ex-periment. Also, the theory could not yet be cast in a form capable of pro-viding precise, unique predictions. But by 1971, the theoretical recipe had been completed, and it was up to experimenters to make the next steps.

The theory, now usually called the electroweak theory (a name adopted by Salam), predicted the existence of heavy particles to communicate the weak force. It also indicated that they would come in two types, now called W (which is the communicator when the particles involved exchange electrical charges) and Z (when no charge exchange takes place).

But when the theory took shape, this second type of weak interaction, where the particles involved do not swap charges, had never been observed. The first convincing clue that the theory was on the right track was the discovery of such 'weak neutral currents' in neutrino experiments, first at CERN and then at Fermilab, in 1973.

In the meantime, a nagging problem had been solved. In the first formulation of the theory, other neutral current interactions (in addition to those eventually seen in the neutrino experiments) could exist. An example is the decay of a neutral kaon into two muons. But such a decay had never been seen. Glashow, John lliopoulos and Luciano Maiani predicted a new constituent of matter, the charmed quark, which prevented this type of kaon decay in a natural way. This prediction was dramatically vindicated in 1974 in the experiments of Burt Richter at SLAC and Sam Ting at Brookhaven which found charmed particles.

Confidence was boosted further in 1978 when a remarkable experiment at the big linear electron accelerator at Stanford succeeded in measuring tiny asymmetries due to the delicate interference between electromagnetism and the weak neutral current. At just one part in ten thousand, these were at the level predicted by the new theory. The stage was set for the final 'coup de théatre' - the discovery of the W and Z particles, the predicted communicators of the weak force.

The electroweak theory, combined with results from new experiments, made precise predictions of the properties of these carrier particles. At some 80 and 90 times the mass of the proton respectively, the W and Z would be the heaviest particles ever seen - about as heavy as a nucleus of strontium. For the first time, physicists knew where they would have to look to find the longsought carriers of the weak force.

The discovery of 'weak neutral currents' at CERN in 1973 showed that the new electroweak theory was on the right track. In the photograph, a high energy neutrino has passed through the Gargamelle bubble chamber, itself undetected, but in its wake setting other particles in motion.

However in the late 1970s such heavy particles were beyond the energy range of any existing machine. Our story now swings to the brilliant achievements in accelerator physics that provided enough energy to make W and Z particles.

The invention of 'beam cooling'

The biggest jump in available energies at accelerator Laboratories came with the mastery of storage rings which enabled particles to be collided head-on into one another, so that no 'knock-on' energy is lost. The ability to collide electron and positron beams was developed at Frascati in collaboration with Orsay, inspired by Bruno Touschek, and at Stanford in collaboration with Princeton, led by Burt Richter. The ability to collide two proton beams was developed with the Intersecting Storage Rings (ISR) at CERN built by the team led by Kjell Johnsen.

Colliding electron and positron beams has the advantage of requiring only one magnet ring in which the particles of opposite sign circulate in mutually opposite directions. It would seem an obvious parallel step to collide protons and antiprotons using a single ring but this runs into the difficulty of producing sufficiently intense beams of antiprotons. Without these intense beams, the number of collisions when the particle beams cross is too low for the physicists to see anything of interest. The invention of so-called 'cooling' techniques made intense antiproton beams feasible and opened the door to the W and Z.

The technique of cooling gets its name from the relationship between temperature and particle energy. However beam cooling refers not to a process whereby all the particle energies in a beam are reduced (so that the beam is made cold) but to a process which reduces (or 'cools') the spread of particle energies around a desired value. The significance of such a cooling is that a storage ring will accept and retain only a small span of energies around its design value. If antiprotons with an initially wide span of energies can be cooled into the small span of the storage ring, intense antiproton beams become feasible.

In 1972 a report entitled 'Stochastic damping of betatron oscillations in the ISR' was published. Its author is a brilliant accelerator physicist, Simon van der Meer, who concluded his paper with the following note: 'This work was done in 1968. The idea seemed too far-fetched at the time to justify publication. However the fluctuations upon which the system is based were experimentally observed recently. Although it may still be unlikely that useful damping could be achieved in practice, it seems useful now to present at least some quantitative estimation of the effect.' In this modest way the idea which has been crucial to the CERN experiments was launched.

The word 'stochastic' means random, and stochastic cooling works by reducing the random motion of particles in the beam so that they become concentrated around the desired value. It does this by observing the 'centre of gravity'of a slice of the beam using pick-up electrodes at one point of the ring. Signals are then sent across the ring to apply an electric field to the same slice of the beam, when it has travelled around, so as to nudge the centre of gravity towards the desired position. Because of the random motion of the particles, this nudge acts unfavourably on some particles but it does what is wanted to most of them, so that the process is convergent. However it has to be repeated millions of times, progressively cooling the beam.

Simon van der Meer, architect of the 'beam cooling' techniques which opened the door to the CERN antiproton project.

First tests were carried out in the ISR in 1974 and the results, though not startling, were enough to show that the idea worked. (It is entertaining to note that one Carlo Rubbia, of whom much more later, was not at all keen on giving time to this machine physics since he was then busy with an experiment at the ISR!)

To test the cooling technique, a small storage ring was rapidly converted at CERN in 1976-77. The ring was renamed ICE - Initial Cooling Experiment - and the results that it achieved in 1977-78 for stochastic cooling of a beam in all three dimensions were extremely encouraging. Antiproton beams of sufficient intensity to do colliding beam physics in the CERN Super Proton Synchrotron looked to be just about achievable.

The proton-antiproton project

The physicist who picked up the antiproton baton in 1976 and has run with it, through to the discoveries of the W and Z, is Carlo Rubbia. It was his breadth of interest which enabled him to appreciate the physics potential, to understand the accelerator possibilities and to conceive an overall scenario, including a major detector. Hundreds of people have contributed to the successes but there is no doubt that, throughout the story, Rubbia has been a constant driving force. He pulled together a large team to put forward an experiment proposal which was code-named UA1, after 'Underground Area' since its location on the SPS required a large cavern to be excavated. This team grew to involve some 130 physicists from 13 research centres - Aachen, Annecy LAPP, Birmingham, CERN, Helsinki, Queen Mary College London, Collège de France Paris, Riverside, Rome, Rutherford, Saclay, Vienna and Wisconsin. Organizing the work of such a large collaboration has been an exercise in sociology in itself. The proposal for a huge 'general purpose' detection system to look at 540 GeV proton-antiproton collisions was accepted at the 27th Meeting of the CERN Research Board on 29 June 1978.

With the results from ICE, the proton-antiproton project necessary to perform the experiment could be sketched out (see 'From AA to Z'). Its main new component was an Antiproton Accumulator (AA) - a storage ring where stochastic cooling would produce the intense antiproton beams. It took only two years from construction authorization of this intricate machine to the announcement of first operation by Roy Billinge (who led the construction team) at the International Accelerator Conference at CERN in July 1980. For the experiments over the past two years the AA has performed with unbelievable reliability. While the AA was being built, the Proton Synchrotron and the Super Proton Synchrotron also needed massive attention to prepare them for the new gymnastics with antiproton beams. The aim was to collide protons and antiprotons, with adequate beam intensities, in the Super Proton Synchrotron with an energy of 270 GeV per beam (an energy that the SPS could sustain with its magnets operating in a d.c. rather than pulsed mode).

The CERN management, and particularly the Research Director-General Leon Van Hove, showed considerable courage and determination in backing the project in mid-1978. CERN has a heavy responsibility to provide appropriate research facilities for a community of some 3000 high energy physicists. There are understandable reasons, with so many physicists breathing down your neck, to back 'safe' experiments. The proton-antiproton project seemed in 1978 only just technologically feasible and there was no guarantee, even if all went well technically, that it would be possible to extract clean physics. Throwing three quarks and three antiquarks, plus their gluon companions, at one another at the high energy of 540 GeV, even at modest beam intensities, would produce a lot of confused debris. Also the project ate considerably into the money, time and manpower resources available for many other experiments. The Research Board's decision was therefore not an easy one and at the time it was not universally acclaimed.

Carlo Rubbia at his UA1 detector. He was a constant driving force in the CERN antiproton project, from its inception through to the scientific discoveries.

However, if the gods were kind, it was clear that dramatic physics was within CERN's grasp. A minor goad to CERN at that time was a series of media comments implying that the Laboratory, though furnishing a huge quantity of thorough physics, consistently missed the headline discoveries. This was not really fair since discoveries like the neutral currents rank with any, but it had enough truth to be an irritant. Our American colleagues had more often shown a flair and imagination in experiments that produced Nobel Prizes. The proton-antiproton project demanded flair and imagination and, happily, the courage was there to pick up the challenge.

In December 1978, the commitment went further. A second big experiment, UA2, was approved for another collision region on the SPS. This was a collaboration of some fifty physicists from Bern, CERN, Copenhagen, Orsay, Pavia and Saclay.

Finding the W and Z

In February 1981, the Proton Synchrotron received and accelerated antiprotons from the AA, thus becoming the world's first Antiproton Synchrotron. On 7 July transfer to the SPS, acceleration and brief storage at 270 GeV were achieved. Carlo Rubbia delayed his departure to the Lisbon High Energy Physics Conference by a day so that on 10 July he was able to announce that the UAl detector had seen its first proton-antiproton collisions. There were runs at modest intensities in the second half of the year and the first visual records of the collisions came from another experiment (UA5) using large streamer chambers. UA5 was then moved out to make way for UA2, which took its first data in December.

One of the first pictures of a 540 Ge V proton-antiproton collision, as recorded in the big streamer chambers of the UA5 experiment at the CERN SPS.

In 1982 an accident to UA1 forced a concentration of the scheduled proton-antiproton running into a single two-month period at the end of the year (October to December). In terms of operating efficiency, it proved a blessing in disguise and Research Director Erwin Gabathuler happily sacrificed a crate of champagne to the machine operating crews as the collision rate was taken to ten times that of the year before. This was the historic run in which the Ws were first observed.

It was astonishing how fast physics results were pulled from the data accumulated up to 6 December 1982. At a 'Topical Workshop on Proton-Antiproton Collider Physics' held in Rome from 12-14 January 1983, the first tentative evidence for observation of the W particle by the UAl and UA2 collaborations was there. Out of the several thousand million collisions which had been seen, a tiny handful gave signals which could correspond to the production of a W in the high energy collision and its subsequent decay into an electron (or positron if the W was positively charged) and a neutrino. The detectors were programmed to look for high energy electrons coming out at a relatively large angle to the beam direction. Also energy imbalance of the particles around a decay indicated the emergence of a neutrino, which itself cannot be detected in the experimental apparatus.
Pierre Darriulat, spokesman of the UA2 experiment.

The tension at CERN became electric, culminating in two brilliant seminars, from Carlo Rubbia (for UA1) on 20 January 1983 and Luigi Di Lella (for UA2) the following afternoon, both with the CERN auditorium packed to the roof. UA1 announced six candidate W events; UA2 announced four. The presentations were still tentative and qualified. However over the weekend of 22-23 January, Rubbia became more and more convinced. As he put it, 'They look like Ws, they feel like Ws, they smell like Ws, they must be Ws'. And on 25 January a Press Conference was called to announce the discovery of the W. The UA2 team reserved judgement at this stage but further analysis convinced them also. What was even more impressive was that both teams could already give estimates of mass in excellent agreement with the predictions (about 80 GeV) of the electroweak theory.

It was always clear that the Z would take longer to find. The theory estimated its production rate to be some ten times lower than that of the Ws. It implied that the machine physicists had to push their collision rates still higher, and this they did in style in the second historic proton-antiproton run from April to July 1983. They exceeded by 50% the challenging goal that had been set and this time it was Director-General Herwig Schopper who forfeited a crate of champagne.

Again there was tension as the run began because the Z did not seem keen to show itself. Although more difficult to produce than the W, its signature is easier to spot because it can decay into an electron-positron pair or a muon pair. Two such high energy particles flying out in opposite directions were no problem for detectors and data handling systems that had so cleverly unearthed the W.

On 4 May, when analysing the collisions recorded in the UA1 detector a few days earlier, on 30 April, the characteristic signal of two opposite high energy tracks was seen. Herwig Schopper reported the event at the Science for Peace meeting in San Remo on 5 May. However the event was not a clean example of a particle-antiparticle pair and it was only after three more events had turned up in the course of the month that CERN 'went public', announcing the discovery of the Z to the Press on 1 June 1983. Again the mass (near 90 GeV) looked bang in line with theory. Just after the run, Pierre Darriulat was able to announce in July that UA2 had also seen at least four good Z decays.
On 25 January 1983, CERN called a press conference to announce the discovery of the W particles.

In addition to the Ws and Zs, the observed behaviour was everything the electroweak theory predicted. Two independent experiments had confirmed a theory of breathtaking imagination and insight. This is one of the great milestones in man's quest to understand the Universe around him.

Next: Piecing together a theory

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