The Omega-Minus Experiment An account of the experiment, performed with Brookhaven National Laboratory's 33-billion-electron-volt accelerator, that confirmed the existence of a new particle predicted by the "eightfold way" by William B. Fowler and Nicholas P. Sarnios ong the landmarks of physics in properties the missing particle must delta-plus and delta-double-plus (with the 20th century are the success­ possess. two units of positive charge). The mem­ K ful predictions of theoretical The situation was somewhat like the bers of this quartet have a spin angular physics. The Davisson-Cermer experi­ one presented by Dmitri Mendeleev momentum of 3/2 units, positive parity ment showing that electrons behave like when he devised the periodic table of and an effective mass of 1,238 million waves (predicted by Louis de Broglie), the elements in 1869. Mendeleev saw electron volts. (In particle physics mass the discovery of the massless neutrino that there were three vacancies at cer­ is now commonly expressed in terms of (predicted by Wolfgang Pauli), the dis­ tain positions in his table, and he in­ energy.) Now, if one looks around for coveries of the positron and other anti­ sisted that elements corresponding to other known baryons that may belong to particles (predicted by P. A. M. Dirac) those vacancies, whose properties he the same family, the sigma particle of -these are some of the successes of described, must exist and would some­ mass 1,385 mev (million electron volts) hypotheses that to many seemed hardly day be found. They were discovered not immediately appears to be a likely rela­ more than speculations when they were long afterward. Similarly, the missing tive. Like the deltas, the three charge first proposed. Each was a concept in­ particle described by Cell-Mann, which versions of the sigma particle (sigma­ vented by abstract, mathematical rea­ he named omega-minus, was found minus, sigma-zero and sigma-plus) have soning based on earlier experimental when it was searched for, and it proved a spin angular momentum of 3/2 and results, and each was eventually veri­ to have exactly the properties specified positive parity. The quantum number fied by physical detection of the pre­ by the theory. describing the strangeness of the delta dicted phenomenon. The latest in this particles is zero; that of the sigmas is series of remarkable "inventions" is the This article will relate how the omega- minus one. The deltas have an isotopic omega-minus particle, whose existence minus was produced and identified. spin of 3/2; the sigmas, an isotopic spin was predicted only two years ago and The details of the theory need not con­ of I-a half-unit difference. Thus the has now been confirmed by actual pro­ cern us here; they were thoroughly dis­ sigma triplet, at a higher mass level than duction of the particle. cussed in the February issue by Cell­ the delta quartet, begins to form a sym­ There are special reasons why the Mann and his coauthors. It classifies metrical pyramid [see illustration on discovery of this particle has aroused the nuclear particles (baryons and mesons) page 42]. interest of physicists. It supports a by a system based on the branch of theory that shows promise of introduc­ mathematics called group theory. The ing some kind of order into the puz­ system is known as SU(3) symmetry and zling assortment of particles that physi­ has also been named the "eightfold cists have detected in their dissection of way," because it involves eight mathe­ the atomic nucleus. The theory was pro­ matical operators related to the quan­ BROOKHAVEN ACCELERATOR with posed in 1961 by Murray Cell-Mann of tum numbers of particles. which the omega·minus experiment was the California Institute of Technology According to this scheme the particles performed appears in the aerial photograph and independently by Yuval Ne'eman constituting a family must all have the on the opposite page. The ring.shaped con· of Tel Aviv University in Israel [see same spin angular momentum (speed of figuration of ground to the right of the "Strongly Interacting Particles," by intrinsic spin) and the same parity large building at left center is a mound cov­ Ceoffrey F. Chew, Murray Cell-Mann (left-handedness or right-handedness). ering the tunnel that houses the magnet ring in which protons are accelerated to 33 bil. and Arthur H. Rosenfeld; SCIENTIFIC They may differ in mass, electric charge lion electron volts. The beam of protons is AMERICAN, February]. It groups the and the properties known as "strange­ injected into the ring by a linear accelerator particles into families according to their ness" and "isotopic spin," but in these that is housed in the small building at lower quantum properties. Cell-Mann noted quantities they must be related to one right center. The main target area is in the that one of the families apparently was another by certain rules. building at left center. The accelerating incomplete. Nine members of the family The family with which we are con­ ring is some 800 feet in diameter. The accel· were known; the theory indicated there cerned starts with the baryons called erator, known as an alternating gradient syn­ should be a tenth, and it specified the delta particles-delta-minus, delta-zero, chrotron, has been in operation since 1960. 36 © 1964 SCIENTIFIC AMERICAN, INC © 1964 SCIENTIFIC AMERICAN, INC © 1964 SCIENTIFIC AMERICAN, INC What doublet might constitute the The new xi particles qualiRed for mem­ that was to engage a large number of next level of the pyramid? The dif­ bership in the family with the delta and physicists (including the authors of this ference in mass between the delta and sigma particles by their mass (very close article), engineers and technicians. the sigma particles, 147 mev, suggested to the predicted 1,532 mev). What was that the mass of the next pair of patti­ more, their strangeness was minus two, What interaction might produce the cles should be about 1,532 mev, 147 further conRrming their position in the omega particle? There was avail­ mev higher than that of the sigmas. No pyramid, because the strangeness quan­ able a beam of high-energy K mesons, such particle, bearing the right quantum tum numbers now ran: delta quartet, generated as secondary products by the numbers, had been detected. At the zero; sigma triplet, minus one; xi dou­ 33-bev alternating-gradient synchrotron. biennial International Conference on blet, minus two. Calculations suggested that sufficiently High Energy Physics in Ceneva in July, It was now possible to predict with energetic K-minus particles, on inter­ 1962, however, experimenters reported reasonable conRdence that the pyramid acting with target protons, would pro­ the discovery of a pair of xi particles must be crowned by a singlet at the duce the omega-minus baryon and K­ (xi-minus and xi-zero) of mass 1,530 apex-a particle of strangeness minus plus and K-zero mesons by the follow­ mev. Cell-Mann, who was present at three, spin 3/2, positive parity, nega­ ing reaction: K-+]J � n- + K+ + KO. the conference, immediately saw the tive charge and mass about 1,676 mev How much energy would the K-minus connection between this almost routine (146 mev higher than that of the projectiles need to generate this re­ announcement and the eightfold way. xi particles). A determined, full-scale action? The masses of the products search for this particle, already named (omega-minus, K-plus and K-zero) re­ omega-minus, was therefore launched spectively represented energies of 1,676 almost immediately. mev, 494 mev and 498 mev, or a total Many of the nearly 100 known nu­ of 2,668 mev. Because part of the en­ clear particles had already been placed ergy of the projectiles in such a reac­ in families (consisting of eight members tion goes into the energy of motion, or each) on the basis of the new theory of momentum, of the products emerging classiRcation. They seemed to Rt into from the reaction, it was calculated that the theoretical system of organization the K-minus mesons would need to hit very satisfactorily. It is one thing, how­ the target protons with an energy of at ever, to devise a scheme describing a least 3,200 mev. To heighten the prob­ set of known facts and quite another ability of producing the reaction, it to create a generalization that will bring would be desirable to use a K-minus to light new phenomena previously un­ beam with substantially more energy dreamed of. The test of any grand­ than that minimum. Fortunately the C L---_ scale theory is its ability to predict what accelerating and beam-separating sys­ e was previously unpredictable and to tem was capable of creating such beams lead to new knowledge. If the eight­ with energies up to 5,000 mev. fold way could produce, out of pure The experiment was therefore de­ theory, the exact properties of a new signed as follows. A 5,000-mev beam of particle, it might be an important step K-minus mesons would be shot into the TT toward unraveling the obscurities sur­ bubble chamber. To avoid producing a rounding the fundamental nature of confusion of tracks in the chamber the matter and energy. The quest far this K-minus injections would be limited to particle was worth a large effort. bursts of 10 to 20 particles during the It required, Rrst of all, powerful exposure time of each picture. It was BUBBLE·CHAMBER PHOTOGRAPH on and sophisticated tools, and fortunately hoped that an occasional K-minus meson the opposite page was the first to show the tools adequate for the job were already would interact with a proton in the existence of the omega·minus particle.