Omega-Minus Plus 25 Years

Omega-minus plus 25 years

Authors of the 'Eightfold Way' - Murray Gell-Mann (right) and Yuvai Ne'eman.

(Photos Kathleen Blumenfeld)

A quarter of a century ago, in February 1964, the world learned that particle physicists had found the omega-minus particle, setting the stage for a new era in our understanding of the components of the atomic nucleus. Particle physics had hit the head­ lines before and has done so many times since, but the dramatic di­ scovery by a Brookhaven bubble chamber team of a particle whose exact properties had been pre­ dicted by seemingly simple chalk- and-blackboard arguments was es­ pecially romantic. In the 1920s, physicists had strived to explain all the underlying structure of matter in terms of just two particles, the proton - the hea­ vy constituent of the atomic nu­ cleus - and the light electrons or­ biting round the nucleus or given The underlying mathematical the proton, the neutron and the off in beta decay. When the neu­ ideas were further developed in the strange lambda. No real physics tron - the electrically neutral com­ mid-50s by C.N. Yang and R. Mills followed, but the seeds of an idea ponent of the nucleus - was dis­ (and independently by R. Shaw) had been sown. covered by James Chadwick at who explored how the formalism In 1961 Murray Gell-Mann at Cambridge in 1932, understanding of abstract isospaces depended Caltech and Yuval Ne'eman at Lon­ <*6ok a step backwards. also on space and time (gauge the­ don's Imperial College realized that Then Werner Heisenberg ory). many particles could be grouped pointed out that the proton and the At around this time, it was be­ into octets - 'the eightfold way' - neutron, apart from their electric coming clear that the subnuclear characteristic of a special form of charge, were otherwise quite simi­ world had many other denizens - symmetry under three-dimensional lar - both lived inside nuclei and particles created in high energy col­ rotations (SU3). The symmetry both weighed about 900 MeV. lisions but which did not normally also had other multiplets but at the Perhaps, suggested Heisenberg, live inside the nucleus. Why were time these could not be filled up they were the two possible quan­ there so many particles? To make with particles. tum states of a single entity - the some order out of the chaos, Mur­ The usefulness of the SU3 octet nucleon - inhabiting some abstract ray Gell-Mann and Kazuhiko Nishiji- pattern had been noticed before in two-dimensional 'isospace'. A new ma proposed a new quantum num­ attempts to find write down the al­ quantity - 'isospin' - was intro­ ber - 'strangeness'. Some of the gebra of carrier particles transmit­ duced to describe quantum sym­ particles with approximately equal ting the nuclear forces, but Gell- metries in this space, analogous to masses appeared to fall into neat Mann and Ne'eman saw that the spin, the label that emerged from multiplets. These could be grouped neat octets fitted other particles quantum physics in ordinary space in turn into larger families, hinting too. and time. at some deeper symmetry hidden (By this time Gell-Mann was well The nucleon is an isospin doub­ underneath. known in the field, having devel­ let, while the pion, with three elec­ Many attempts were made. In oped with Nishijima the idea of trical charge options (positive, ne­ the late 50s Shoichi Sakata toyed strangeness, and with Richard gative and neutral), is classified as with the idea of explaining the rash Feynman some important links in an isospin triplet. of particles in terms of just three - our understanding of the weak

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2 CERN Courier, March 1989 nuclear force. Ne'eman, a promi­ triangle, he wrote down its mass, gammas pointed back to a com­ nent Israeli army officer, had come heavier than any particle ever seen mon origin. By careful measure­ to London in a diplomatic role, and before. ment and calculation, the team un­ put his multifarious talents to addi­ At Brookhaven, a new 80-inch ravelled the complex series of de­ tional use in Abdus Salam's school, bubble chamber was being readied cays, finally backtracking to a parti­ where Salam, with J.C. Ward, was at the Alternating Gradient Synch­ cle carrying minus three units of working on particle classification rotron (AGS). This was the highest strangeness, and with a mass of schemes.) energy accelerator in the world at 1686 MeV, almost right on top of At an international conference at the time and Leon Lederman, Jack Gell-Mann's blackboard prediction! •1ERN in 1962, Bernard Gregory of Steinberger and Mel Schwartz had The commissioning of the bub­ Trance's Ecole Polytechnique and just used it to provide the world's ble chamber had not been plain George Snow of Maryland summar­ first beam of neutrinos. However sailing. As it had only one window, ized the status of the physics of when it came to finding new parti­ special reflectors had to be used the new strange particles. After cles, the physicists using Luis Al­ so that the tracks could be both il­ Snow's talk, Murray Gell-Mann varez' bubble chambers at Berkeley luminated and photographed from went to the blackboard and were still the acknowledged ex­ the same side. Early in the run, pointed out the appealing regulari­ perts. some of these reflective 'coat han­ ties inside a then hypothetical In December 1963, Brookhav- gers' had come adrift and fallen on decuplet of heavier cousins of the en's new bubble chamber came the window. As well as obscuring nucleon. into action, and a team of more some of the field of view, these The decuplet was in the form of than thirty physicists took up Gell- could have damaged the window, a triangle drawn in a grid of isospin Mann's challenge. In January 1964, and with 1,000 explosive litres of and strangeness quantum num­ after looking at 50,000 photo­ liquid hydrogen inside, the future of bers. Nine of the grid points could graphs, they stumbled on a shot the experiment hung in the balance. be filled by known particles - a showing a pion carrying a lot of After gingerly testing the chamber quartet at the base, with a triplet transverse momentum, with a with a few expansions, team leader and a doublet above, but the apex strange particle produced some di­ Ralph Shutt decided to continue. of the triangle was empty. Gell- stance away from the primary colli­ The omega-minus showed that Mann predicted a new negatively- sion. Looking closer, they spotted SU3 symmetry was behind the charged particle carrying three units the spirals of not one, but two of neat particle families. But why of strangeness, and even gave it a the characteristic electron-positron SU3? What parentage was behind name, the omega-minus. Extrapo­ pairs of a high energy gamma ray such large families of eight and lating from the mass patterns in the (photon). Moreover these two ten? Even before the omega-minus

CERN Courier, March 1989 3 The complete Brookhaven 1964 bubble chamber picture. About a quarter of the chamber was invisible due to an illumination problem.

appeared, Gell-Mann, and indepen­ chanism, at least on paper. It took another ten years of pain­ dently George Zweig, working at (As often happens in physics, staking work to find the tiny grains CERN, showed how the eights and ideas born in one context can lead of matter hidden deep inside the tens followed naturally from triplets to useful spinoff in another. The nucleon and show that they be­ of more basic entities - termed gauge theory groundwork devel­ haved like quarks. However they 'quarks' by Gell-Mann. oped in parallel with isospin and steadfastly refuse to come out into In the same way that the Per­ the eightfold way went on to pay the open, staying locked inside iodic Table of elements paved the handsome dividends in the formula­ their nucleon prisons. To see free way for our present picture of the tion of the 'electroweak' unification quarks, physicists probably need tqi atom, so Gell-Mann and Ne'eman's of the 1960s, showing how the recreate the conditions of the Big octets had hinted that subnuclear weak nuclear force and electro- Bang which started it all off. particles had some internal me­ magnetism have a common origin.)

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The World Laboratory and the free circulation of information and cal (seismology, volcanology, ) Third World Academy of Sciences researchers. The bottom line is an and environmental (climate, pollu­ are examples of ambitious new glo­ impressive list of ongoing multidis- tion, ) monitoring and modelling, bal ventures using the established ciplinary projects, with a wide geo­ together with computer projects in broad base of science and technol­ graphical spread. the education and health areas. The ogy in the industrialized nations as Under its president Antonino Eloisatron basic physics pro­ a springboard for important pro­ Zichichi, and with its main coord­ gramme includes the Eloisatron jects in and among developing ination centre in Geneva and with project for a multithousand GeV countries. regional coordination centres in proton collider, ongoing plans at Established in 1986, the World Moscow and Beijing, World Lab's CERN's LEP electron-positron Laboratory aims to promote truly ongoing programme is grouped collider, and the establishment in global, open cooperation in techni­ into four main areas. The Archi­ China of Advanced Centres for cal and scientific research, with medes programme covers geologi­ Science and Technology and for

4 CERN Courier, March 1989