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
+ - + - -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. Another crucial vision was that of Martinus Veltman, who suggested that it would be worthwhile to make the cross-section of the tunnel large enough so that in the future it might accommodate a proton–proton collider at much higher energies, given that proton bremsstrahlung is negligible, so that much higher magnetic fields and therefore, of momentum, are possible with protons. This made possible what is now the LHC. In the summer of 1980, perhaps a dozen of physicists, among them Jacques Lefrançois, René Turley, Heiner Wahl, Friedrich Dydak and Steinberger himself, began discussing what one might try to do at LEP, if LEP would be built (LEP was approved in 1982 [Picasso 2012] ). The focus was on the idea to study a particular reaction, rather than build an all-purpose detector, but in the end they could not find a suitable reaction, and decided to design a general-purpose detector. It was clear that LEP physics would be dominated by the study of the newly discovered, massive vector bosons, W+, W- and Z0, and that therefore the detector should be comparably sensitive in all directions. In consequence Steinberger had the idea to make a spherical detector, incorporating a spherical magnet, no small challenge, but which Guido Petrucci did manage to invent. Luckily, the rest of the group, which grew in number with time (in the end up to almost 400), knew better and chose a superconducting solenoid instead (it seems all collider detectors now have such solenoidal geometries). They called their detector ALEPH [footnote3]. All important decisions on ALEPH design were taken at open collaboration meetings, and we enjoyed working together, technicians, engineers, physicists, at many laboratories in Europe. One of the most important decisions, following the insight of Jacques Lefrançois, was that in order to identify π0’s, the electromagnetic calorimeter should focus on angular rather than on energy resolution. They received a great deal of help by Pierre Lazeyras, who, early on became technical coordinator of the collaboration, and so took on the job of keeping them together in the construction and later on, in the operation of the detector. By winter 1981-1982 they had decided on the basic design: length 11.5 m, diameter 10 m, superconducting coil i.e. 5 m, length 7 m, 1.5 Tesla, TPC tracking chamber (time projection chamber), the electromagnetic calorimeter made with lead sheet, and proportional wire chamber sandwiches, read out in 4 layers of 70,000 towers pointing to the collision point, the return yoke and hadron calorimeter composed of 25 sandwiches of 4 cm iron sheets and Iarocci tube detectors, serving also as muon detector (Fig.9).
Fig 9: The ALEPH detector, as proposed in 1982.
Fig 10: ALEPH artist, Wolfgang Richter, sketches the mounting of the TPC. The memory of two related incidents gives Steinberger special pleasure. In 1982 the LEP committee was created, with Guenther Wolff chairman, and they submitted their proposal. One day Wolff came to Steinberger’s office to point out that the precision which they claimed for the TPC tracker was substantially better than that of a smaller one, operating at the TRIUMF laboratory in Vancouver. Could this be wrong? Steinberger was very impressed by this visit; he had not known about the TRIUMF TPC, and especially also, because it is very unusual that the member of an experimental committee understands something better than the people who propose the experiment. The question was immediately taken up by the then young collaborators, Luigi (Gigi) Rolandi and Francesco Ragusa. Before, none of them had tried to thoroughly understand the basic physics of the TPC. But, within a month, Gigi and Francesco had managed to understand the physics of the electron drift and how the final precision depends quantitatively on the gas, the magnetic field, the electric field and the wire chamber detection at the ends [footnote4]. At the time a TPC test module was already under construction at the Max Planck Institute in Munich (Fig.10). Within a few months it was in a test beam at CERN, and permitted the test and verification of the calculations of Rolandi and Ragusa. The report of these measurements, at one of the weekly meetings, by Julia Sedgebeer, was the moment of greatest pleasure of Steinberger’s LEP experience. During the period of ALEPH construction, one of his fears was that they would suffer from lack of computer capacity in the analysis, given their financial limitations, but this turned out not to be a problem, it was solved by the appearance, during those years, of computer work stations. One of the biggest challenges in ALEPH construction was the readout electronics, but when LEP started during the fall of 1989, ALEPH was operational (Figs. 11/12).
Fig 11: Some days before LEP startup, a cosmic ray event is recorded 150 m underground - its energy is of the order of ~ 107 GeV, 100.000 times the LEP energy.
+ - 0 + - + + - - + Fig 12: Two ALEPH events. Top: e + e → Z * → τ + τ , τ → e + νe + ντ, τ →2π + π . Bottom: e+ + e- → Z0* → quark + quark + gluon.
The first physics focus at LEP was to measure the Z0 resonance line shape with sufficient accuracy to determine the contribution of the neutrinos to its decay, and so to determine the number of neutrino families. Here ALEPH did better than the other LEP collaborations, because they had realized that this could be done with higher statistical precision using the height of the resonance curve rather than its width. This required an adequate measurement of the luminosity, which they managed to provide for. Within a few weeks of LEP start-up, they achieved a precision which showed convincingly, that the number of families is just the number of families which had already been discovered, that is, three (Fig.13), so that one does not have to worry about any more families! This first LEP result was arguably also its most important.
0 Fig 13: Left: Z line shape and number of neutrino families. Top: SLAC, Oct. 11, 1989. Nν = 2.8 ± 0.6. 0 Bottom: ALEPH, Oct. 12, 1989, Nν = 3.27 ± 0.30. Right: The Z lineshape marches on: Top: Dec. 1989, ALEPH, Nν = 2.99 ± 0.14. Bottom, 1992, LEP combined, Nν = 2.98 ± 0.01.
LEP, with its four detectors, ALEPH, DELPHI, L3 and OPAL, each with some three to four hundred collaborators, made beautiful, outstanding contributions to our understanding of particle physics: precision measurements of the basic parameters of the electroweak (E-W) theory such as the masses of the heavy vector bosons and the Weinberg angle, precision tests of the E-W theory, in particular also incorporating radiative corrections, predictions of the Higgs and top masses using radiative corrections, checks on the validity of perturbative QCD and measurement of the QCD interaction strength as function of the relativistic four- momentum transfer squared Q2 (Fig.14), searches for the Higgs and SUSY particles, all negative, but providing lower limits on their possible masses, progress in our understanding of the tau lepton and B meson, and more (Fig. 15).
Fig 14: A LEP check of QCD: The running of the strong interaction coupling strength with momentum transfer.
Fig 15 LEP and flavor physics. Lifetimes of Bs (left) and Λb, both of which were discovered at LEP.
[footnote1] Transcription of talk given by Jack Steinberger in December 2009 at CERN and adapted for this special issue by Emmanuel Tsesmelis. An author version of the transcribed talk will be published in [Tsesmelis 2012].
[Courant 1952] E.D. Courant, M.S. Livingston, H.S. Synder, The Strong Focussing Synchrotron – A New High Energy Accelerator Phys. Rev 88 (1952) 1190 [Plass 2012] G Plass – title – EPH 36/4 etc.
[Tsesmelis2012] L. Alvarez-Gaumé, M. Mangano and E. Tsesmelis (Eds.), From the Proton Synchrotron to the Large Hadron Collider - 50 Years of Nobel Memories in High-Energy Physics, Springer Berlin-Heidelberg, 2012.
[Cronin 2012] J Cronin - title EPH 36/4 etc.
[Cashmore 2003] R Cashmore, L. Maiani, and JP Revol (Eds.), Prestigious Discoveries at CERN 1973 & 1983, Springer Berlin-Heidelberg 2003.
[Richter 2012] B Richter - title EPH 36/4 etc.
[Picasso 2012] E Picasso - title EPH 36/4 etc.
[footnote3] http://aleph.web.cern.ch/aleph/aleph/Public.html
[footnote2] See e.g. Colin A. Ramm, The Principle of the Design of the CERN Propane Bubble Chamber (Ramm and Resegotti) in: Instrumentation for High-Energy Physics, Proceedings of an International Conference held 12-13 September, 1960 in Berkeley, CA. Interscience Publications, 1961., p.127 . [Steinberger 1966] C. Alff-Steinberger etal. , KS and KL interference in the decay mode, CP invariance and the KS – KL mass difference, Phys. Lett. 20 (1966) 207
[Hasert 1973] F J Hasert et al. 1973b Phys. Lett. 46 138.
[Deden 1975] H Deden etal. , EXPERIMENTAL STUDY OF STRUCTURE FUNCTIONS AND SUM-RULES IN CHARGE-CHANGING INTERACTIONS OF NEUTRINOS AND ANTINEUTRINOS ON NUCLEONS, Nucl. Phys. B 85 (1975) 269 – 288
[Geweniger 1974a] C Geweniger et al., A new determination of the K0 --> π+π- decay parameters, Physics Letters B 48 (1974) 487–491
[Geweniger 1974b] C Geweniger et al.
[Geweniger 1974c] C Geweniger et al.
[footnote4] F Ragusa and L Rolandi, E x B Effect in ALEPH TPC, ALEPH-LEP-Note 77, 10 June 1982