Jack Steinberger: Memories of the PS and Of
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Jack Steinberger: Memories of the PS and of LEP [footnote1] Emmanuel Tsesmelis, CERN 1.The creation of the PS The CERN PS (Proton Synchrotron), which started in 1959, and the Brookhaven AGS (Alternating Gradient Synchrotron) in 1960, represented an advance by a factor of more than five in the energy of proton accelerators, from the 5 GeV of the Berkeley Bevatron to about 30 GeV. These accelerators made possible the large progress in our understanding of particles and their interactions over the next two decades, culminating in the electroweak and QCD gauge theories. As a start, one should remember the origin of this development, the theoretical advance in accelerator design, the invention, in 1952 by Ernst Courant, M. Stanley Livingston and Hartland Snyder of strong focusing [Plass 2012], where the idea is nicely summarized in the abstract of their original paper [Courant 1952]: Fig. 1: Abstract of the pioneering paper on strong focussing [Courant 1952] Next, one might remember how fortunate CERN was, in its beginning years shortly after a war which severely damaged European science, to have already an accelerator team of outstanding capabilities, which made the design of this new accelerator possible. Steinberger mentioned four of these: John Adams, Mervyn Hine, Simon van der Meer, and Kjell Johnsen. 0 + - 2. Interference of KS → and KL in the decay K → K + K Steinberger’s first attempt at the PS did not meet with success: in the summer of 1960, Roberto Salmeron and he looked in the propane bubble chamber [footnote2], about a meter cube, for events due to neutrinos from the decay of charged pions and kaons produced at an internal target in the PS, but nothing was seen. The next attempt at the PS came four years later. In the spring of 1964, James Christenson, James Cronin, Val Fitch and René Turley made the important discovery of CP violation in 0 + - long lived K L decay into π and π , [Cronin 2012] and some months later, Steinberger had the 0 idea that the phase of this decay relative to K S could be measured by studying the interference 0 between coherently produced short and long lived K ’s. Already before this event he had planned to spend the second half of his sabbatical, 1964–65, at CERN. Carlo Rubbia agreed to join in this endeavour, and together they designed an experiment at the PS. It used a 0 0 K L beam, and the coherent K S’s were regenerated in a plate of copper. The interference between short and long lived neutral kaons could be clearly seen, the phase difference, unfortunately complicated by the regeneration phase, was measured, and as a by-product, the interesting, very small, mass difference between the two neutral kaons could be seen and measured (Fig. 2a and 2b). The experiment was interesting enough for Steinberger to stay an extra year at CERN to continue the experiment. Fig 2a: Observed K → π+ π- decay rate as a function of proper time. The best fit solutions for the cases of interference and no interference are shown, as well as the calculated efficiencies [Steinberger 1966]. 0 0 + - Fig 2b: First look at the interference between K L and K S in the π π decay [Steinberger 1966] 3.Gargamelle Discovery of the neutral current and quark charges Probably the experiment at CERN with the all-time greatest impact on the field of elementary particles was the discovery of neutral currents, in 1973, at the PS, using the Gargamelle bubble chamber (Fig.3), then the largest in the world, 4 m long and 2 m in diameter, filled with Freon, in a neutrino beam, focused using a van der Meer horn. Fig 3: Left, the Gargamelle bubble chamber. Right, the van der Meer horn used in the Gargamelle neutrino beam. (courtesy CERN) The years 1968 to 1972 had seen the development of a very complex Yang–Mills gauge theory, which unified the weak and electromagnetic interactions and which could solve the problem of the Fermi weak interaction at higher energies. The theory predicted a “neutral current” weak interaction and the W+, W- and Z0 heavy bosons, which nobody had seen. The theory was so complex and hypothetical that not many physicists tried to understand it, but Jacques Prentki managed to convince the Gargamelle constructor, André Lagarrigue, and his team, to look for the neutral current, that is, neutrino interactions in which the neutrino is not converted into a muon, but is re-emitted as a neutrino. Some hundred muonless, and therefore neutral current events, were observed (Fig.4), the strength of the neutral current interaction was measured, and the validity of this theory, now a cornerstone of the theory of elementary particle physics, was shown to be highly probable [Hasert 1973], [Cashmore 2003]. Fig 4: One of the hundred-odd Gargamelle muonless, and therefore neutral current events. The neutrino beam arrives from the top (of course the neutrinos cannot be seen). Seen in the chamber are the particles produced in the interaction, a positively charged track and a neutral particle, a Λ hyperon, which is seen to decay into a π- and proton. The charged track is a K+, which, after scattering, stops and decays. (courtesy CERN) The year following, Gargamelle made another very important discovery, the first clear, quantitative sign of the quark nature of the partons, the constituents of hadronic matter. In comparing the deep inelastic scattering cross-sections of neutrinos with those observed in electron scattering at SLAC (Stanford Linear Accelerator), the quark hypothesis predicts the ratio of 5/18, due to the non integral electric charges of the quarks, and this was observed (Fig. 5). Fig. 5: The Gargamelle experiment which showed that baryonic deep inelastic scattering is related to electron deep inelastic scattering, which had been previously measured at SLAC, by the factor of ((2/3)2 +(1/3)2)/2 = 5/18, due to the 1/3 integral charges of the quarks, and so was a first confirmation of the Zweig–Gell-Mann proposal that quarks are the constituents of nucleons [Deden 1975]. 4.Multiwire proportional chambers and CP violation parameters with higher precision In 1968 Steinberger came back to CERN, and became a member of the staff. It is also the year in which Georges Charpak discovered the proportional wire chamber. It was clear that wire chambers would permit much higher counting rates for spectrometers than the spark chambers which had been used, and Steinberger and some of his group focused on an experiment which, with the help of this new technique, would permit a more precise study of the CP violating 0 0 interference between K S and K L, this time in a beam in which short and long lived kaons were produced at the same target, so that the uncertainties in the regeneration amplitude and phase could also be avoided. The technology of the chambers proved to be a challenge. Charpak had been working with chambers 10 cm in size; for this experiment they needed chambers 2 meters by 1 meter. The first time some 50 cm long wires were tried, they immediately sparked and broke. It took us a while to understand the cause: the electro-static repulsion of adjacent wires under tension, which could be solved by intermediate mechanical supports. As soon as a bigger chamber was tested in a beam, it turned out that after short use, it stopped to function. Here, Charpak helped by suggesting some gases which could be added to stop the formation of the material which caused the degradation. Another very interesting challenge at the time was the electronics; here rescue came beautifully from Bill Sippach, an electronics engineer at Steinberger’s previous laboratory, the Nevis laboratory of Columbia University. Finally, the construction proved to be nontrivial, and here they were very lucky to have access to the excellent facilities at CERN at the time, in particular the help of Giovanni Muratori. The geometry of this first detector using Charpak chambers was similar to the + - previous one, which used spark chambers. The result for the KS – KL interference in the π π decay, now free of the complication of the regeneration phase, are also shown in Fig. 6. Fig 6. First experiment to use proportional wire chambers: left, detector and right, KS – KL interference in π+ π- decay as function of time [Geweniger 1974a]. The same detector permitted high precision measurements of the CP violating charge asymmetry in semi-leptonic decay, as well as the KS – KL mass difference, using the two regenerator method (Fig.7) [Geweniger 1974b, Geweniger 1974c]. 0 0 Fig. 7. Measurement of the K L – K S mass difference using the two-regenerator method. + - + - +- +- +- With <KL→π π >/<KS→π π > = η = |η |exp(iφ )= ε + ε’ , where ε and ε’ are respectively the contributions of mixing and direct transition to CP violation in KL decay, the main results were |η+-| = (2.30 ± 0.035) x 10-3, and φ+- = (45.9 ± 1.6)º. Charge asymmetry in semi-leptonic decay: + - + - -3 δL = (N - N )/(N + N ) = 2Re ε = (3.41 ± 0.18) x 10 and KS – KL mass difference: -10 -1 mL – mS = (0.533 ± 0.004) x 10 s The same detector permitted also the first measurements of hyperon nuclear interactions, in this case the lambda and anti-lambda hyperons. 5. LEP According to Steinberger’s recollections, the first suggestion for an e+ e- storage ring at LEP energies came from Burton Richter [Richter 2012], who realized that the bremsstrahlung of electrons and positrons limits the practically attainable energy of e+ e- storage ring colliders, which could be envisioned at that time, the late 1970s.