by Roberto Lalli
Richard E. Taylor
Nobel Prize in Physics 1990 together with Jerome I. Friedman and Henry W. Kendall
"for their pioneering investigations concerning deep inelastic scattering of electrons on protons and bound neutrons, which have been of essential importance for the development of the quark model in particle physics".
Education in experimental particle physics
Richard E. Taylor was born on November 2, 1929, in Medicine Hat—a small town in the Canadian province of Alberta. According to his recollections, Taylor became aware of the extraordinary impact of science on human life during World War II. He was impressed by the way in which military installations radically modified the town in which he was living and, even more, by the construction and use of the first nuclear weapons. After the war, Taylor pursued his interest in science, majoring in mathematics and physics at the University of Alberta (Edmonton) in 1950. Taylor was particularly attracted by experimental physics and, in 1952, earned the M.Sc. degree with a thesis on the measurement of beta-decay with a Wilson cloud chamber at the same institution.
For his PhD studies, Taylor moved to Stanford, where he met outstanding physicists such as Felix Bloch, Leonard Schiff, Willis Lamb, Robert Hofstadter, and W.K.H. Panofsky. In 1954, Taylor began working at Stanford’s High Energy Physics Laboratory, where he had the opportunity to employ the newly built electron linear accelerator. Taylor’s doctoral research focused on the production of polarized gamma rays and on the employment of such rays to investigate the properties of π-meson production. Taylor was supervised by the experimental physicist Robert F. Mozley—an expert on high-energy physics who had collaborated with L. Alvarez and Panofsky on the construction of the first linear proton accelerator at Berkeley, before moving to Stanford in 1953. Taylor’s stay at Stanford was interrupted in 1958, when he received an invitation from the École Normale Supérieuere in Paris to work in the construction of a new linear accelerator at the nearby town of Orsay. It was the first time that Taylor actively participated in the construction phase of an accelerator. He became aware of the numerous technical difficulties related to the design and realization of novel linear accelerators and acquired knowledge and skills that would prove useful in his subsequent achievements. Taylor was involved in the project until 1961, also participating in the early experiments on electron scattering performed at the accelerator.
After this three-year experience, Taylor returned to Stanford to complete his dissertation, earning his PhD in 1962. As soon as he had his PhD in hand, Taylor joined the Lawrence Berkeley Laboratory (LBL) group, but he stayed at Berkeley only one year because his skills were required at Stanford. The Stanford Linear Accelerator Center (SLAC) had just been established to house a two-mile long linear accelerator, which was planned to become the most powerful and largest electron accelerator of the World.
Linear accelerators at work
Since the late 1940s, most physicists understood that laboratory particle accelerators would rapidly replace cosmic-ray observations as the preferred means to investigate particle properties. The early high-energy machines employed circular paths because in such paths charged particles could be accelerated to enormous energies in a relatively small space—or, in any case, smaller than in straight paths. The projectiles of such machines were usually protons, and by 1952 the first proton synchrotron reaching the GeV range began operations at the Brookhaven National Laboratory (BNL). Being much lighter than protons, electrons were not efficient as projectiles of circular accelerators, because in circular paths electrons rapidly loose their energy. To produce high-energy electron beams, it was therefore necessary to employ linear accelerators. With respect to proton synchrotrons, there was a considerable delay in the construction of linear accelerators, mainly because it was thought that proton-induced reactions were more advantageous as a mean to gain information about the fundamental interactions of elementary particles.
Stanford was one of the few settings where research on electron acceleration was successfully carried on. The driving force was R. Hofstadter, who had been consolidating a strong tradition in electron accelerator physics since 1953. That year, Hofstadter began a project aimed at investigating the elastic and inelastic scatterings between high-energy electrons and nuclei at the Stanford High Energy Physics Laboratory. Hofstadter employed the Mark III 1 GeV linear accelerator that by 1955 could accelerate electrons up to 550 MeV. This was the same accelerator Taylor employed to conduct his dissertation research.
By 1956, the Mark III allowed Hofstadter and his collaborators to generate beams of electrons with energy sufficient to penetrate the nucleus. After analysing the angular distribution of the elastically scattered high-energy electrons, Hofstadter and his collaborators could infer some properties of the nucleus and its constituents. Among the major discoveries, Hofstadter showed that protons and neutrons did not have a point-like structure. Rather, they possess a finite size with radius about 10-13 cm and also have a smooth charge distribution. This research earned Hofstadter one half of the Physics Nobel Prize in 1961.
Hofstadter’s achievements were a strong impulse to pursue research on the scattering of electrons and positrons at higher energies. In 1957, Stanford physicists made a proposal for a two-mile electron linear accelerator called Project M to be built at Stanford. After the proposal was accepted in 1961, W. K. H. Panofsky and R. B. Neal began directing the construction of the facility, re-baptised Stanford Linear Accelerator Center (SLAC). The SLAC was one of the most costly and ambitious physics projects carried out in the 1960s, and was planned to reach energies around 22 GeV.
The construction of the SLAC was a novelty in the landscape of particle accelerators. Since proton accelerators were considered the main instrument to investigate particle interactions, only few high-energy electron machines had been constructed. Although in the 1960s other large linear electron accelerators had been, and were being, built, the SLAC would become by far the most powerful of these machines. The amount of energy that the SLAC would reach allowed, in principle, for the investigation of the structure of the nucleons by means of both elastic and inelastic scattering experiments.
In 1964, Taylor became responsible for the design and engineering of the End Station A—the larger of the two experimental areas for the detection of the scattered particles. Various groups of experimenters were planning to conduct research at the SLAC, and the detectors should have been employed for the different kinds of experiments. There were several discussions about the number and the maximum energies of the spectrometers to be employed in the End Station A, and Taylor had a leading role in the final resolutions. At the end, it was decided that the End Station A would contain three different spectrometers, which had as maximum energies 1.6 GeV, 8 GeV, and 20 GeV. The apparatus was also equipped with an on-line computer, which allowed a reduction of the efforts to perform every single experiment at SLAC. By the late 1966, the construction phase of the SLAC was completed and operations began to make the preliminary measurements on the scattering of electrons.
From elastic scattering to inelastic scattering
During the construction of the SLAC several proposals were put forward to make experiments with the newly built accelerator. A collaboration of physicists coming from SLAC, MIT, and Caltech proposed to employ the linear accelerator to: 1) measure the elastic electron-proton scattering with the 8 GeV spectrometer, 2) measure the inelastic electron-proton scattering with the 20 GeV spectrometer, 3) compare the cross section of the scattering of electrons and positrons. Taylor was the team leader of the SLAC group and worked on the design of the abovementioned experiments while the SLAC was being constructed. The focus of the three-group proposal was the prosecution of Hofstadter’s work to improve the knowledge about the structure and the form factor of the nucleons and to explore the properties of hadronic resonances.
By the beginning of 1967, the 8 GeV spectrometer was completed and the experiment on the elastic scattering could begin in May 1967, leading to the conclusion that the cross sections of the scattered electrons measured at SLAC were very similar to those measured at lower energies: The electron-proton cross section rapidly decreased as the scattering angle increased. At higher energies the proton showed the same extended structure that was observed in previous experiments. The second part of the experimental proposal—the inelastic scattering experiments with the 20 GeV spectrometer with a liquid hydrogen target—began in August 1967, after the Caltech team had quitted the collaboration because they believed that there would be nothing new to be discovered in such experiments.
Taylor led the SLAC team, while Henry W. Kendall and Jerome I. Friedman headed the MIT group. At the beginning, the investigators considered the experiment a little more than a preliminary survey of inelastic scattering of electrons on protons. Much to the experimenters’ surprise, they observed that at large angles the cross section of the scattered electrons was much greater than expected. By the end of 1968, it was clear that the experimental results did not fit with the image that the proton was a diffusely extended body.
Within the nucleons
A posteriori, the SLAC-MIT deep inelastic scattering experiment was considered the first empirical proof that hadrons are composed of smaller constituents: the fractionally charged quarks and the vector bosons mediating the strong interaction called gluons. The path that led to this interpretation of the experimental data was, however, long and tortuous and was due to series of theoretical developments partially related to the SLAC-MIT experiments.
A fundamental step was the close collaboration of the experimental teams with the theorists working at SLAC. The field theorist James Bjorken had been focusing on the theoretical foundation of electron-hadron scattering since he joined the SLAC in 1963. When the SLAC-MIT experiment began, the theoretical basis was provided principally by a work by S.D. Drell and J.D. Walecka (1964), who had derived the formula for the differential cross section of the deep inelastic scattering within the single-photon-exchange approximation. The formula depended on two structure functions, W1 and W2, which, in turn, were related to the structure of the target particles. In particular, the two quantities were function of two independent kinematic variables: the energy lost by the electron (symbolised as v), and the squared four-momentum transfer q2. By 1968, employing sum rules of the current algebra, Bjorken hypothesised that the two structure functions (more precisely, W1 and the product v W2) would depend on a single variable (ω=2 Mv/q2, where M is the mass of the target hadron) for large q2 and v—a phenomenon that was called Bjorken scaling. Following Bjorken’s advice, the SLAC-MIT experimenters plotted the data against the variable ω and verified that the observations were consistent with the Bjorken scaling for value of q2 and v greater that a few GeV.
Richard Feynman interpreted the two unexpected results of the deep inelastic scattering experiments—namely, the rather weak dependence of the cross sections on q2 and the Bjorken scaling—as a confirmation of his theoretical model of hadrons as clouds of constituent particles, which Feynman had christened partons. In the Feynman model, high-energy interactions involving the hadrons take place as if the partons were independent quasi-free particles—namely, Feynman supposed that the partons did not interact with one another while the virtual boson mediating the interaction was being exchanged. While Feynman had developed the model in the mid-1960s to explain the strong interaction in a field theoretic fashion, the model worked very well in providing an explanation of the scaling behaviour of the deep inelastic scattering. SLAC physicists became aware of Feynman’s interpretation of the SLAC-MIT experiment during Feynman’s visits in August and October 1968, which convinced Panofsky that the data demonstrated that the proton was made out of point-like constituents.
As soon as the results were announced, various alternative theoretical models were proposed that did not assume the parton structure of hadrons. While consistent with the Bjorken scaling, the comparison between the predictions of these non-constituent models with the data led to significant discrepancies. The parton model had other problems, though. It was rather common to consider the partons as the spin ½ fractionally charged quarks—introduced by M. Gell-Mann and, independently, by G. Zweig in 1964 to explain the symmetries of the hadrons that were being discovered. The quarks, however, had never been observed and were commonly regarded as mathematical constructs, helpful as heuristic tools. The success of the parton model in the interpretation of the deep inelastic scattering revived the interest towards the quarks, but the observational inference that they were permanently confined within the hadrons was hardly consistent with the free-like properties of the partons. Some theorists supposed that partons were other kinds of particles with different charge and spin, e.g. pions and bare nucleons.
After 1969, Friedman, Kendall and Taylor continued to lead the SLAC-MIT collaboration to put into test the various theoretical models that had been proposed. Experiments on deep inelastic scattering over a large value of angles and energies definitely ruled out the non-constituent models. In the meantime, theoretical investigations had clarified the possible connection of the properties of the partons with experimental observations. It was calculated that a combination of the structure functions W1 and W2, denoted R, depended in a predictable way from the partons’ spin. In the limit of large v and q2, R should be zero if partons had spin 1/2, while non-zero for values of spins 0 or 1. In 1971, after and extensive series of observations over an ample range of energies and four-momentum transfers, the experimenters concluded that R was very small and, in view of the experimental error, consistent with the hypothesis of partons as spin ½ particles.
The decisive experiments in the clarification of the nature of partons were those on the inelastic scattering of electron on neutrons. While the earlier experiment employed liquid hydrogen as target, since 1970 the SLAC-MIT teams began using liquid deuterium as well. This allowed for a comparison between the form structures of two different hadrons. The investigators derived various conclusions from the extensive sets of data. First, the neutron and protons structure functions had the same approximate scaling behaviour. Second, the values of the various Rs were similar. Moreover, the measured ratio of the neutron and the proton cross sections was a decisive tool to discriminate between the different theoretical models of the partons. QED predicted that individual partons give a contribution to the total structure function proportional to the square of their electric charge. In 1971, the MIT theorists V. Weisskopf and J. Kuti proposed a quark-parton model based on the view that the proton and the neutron were made of three quarks (respectively uud and ddu) and neutral gluons, which do not interact with electrons and carried one half of the nucleons’ momentum. This model fitted fairly well with the data registered during the SLAC-MIT experiments. However, in the early 1970s the status of quarks was still uncertain. The majority of particle physicists were not field theorists and many of them considered the hypotheses added to the quark model to explain the data of the SLAC-MIT experiments as unnecessary complications.
In 1973, theoretical developments led to the demonstration that the gauge field theory of quarks and gluons had the properties of asymptotic freedom. This momentous discovery made by D. Gross and his doctoral student F. Wilczek convinced many that the quarks possessed the necessary features to explain the Bjorken scaling phenomenon recently observed. The gauge field theory of strong interactions based on coloured quarks and vector bosons mediating strong interactions—the gluons—soon gained momentum with the name of Quantum Chromodynamics (QCD).
From the experimental perspective, the observations made by Taylor, Friedman, and Kendall on inelastic electron-nucleon scattering led to the development of a different kind of experiment on neutrino-nucleon scattering. Experiments performed at Fermilab and, especially, CERN on deep inelastic neutrino scattering on nucleons confirmed the scaling behaviour. Moreover, since the cross sections of neutrinos did not depend on the charge of the partons, the ratio of the electron and neutrino deep inelastic scattering cross sections was predicted to depend on the quark charges. The experimental results were consistent with the picture that nucleons were made of the fractionally charged up and down quarks and gluons carrying fifty per cent of the nucleon’s momentum. When the CERN team made the preliminary announcement of their findings in the summer of 1973, the physics community was by and large convinced that the quark picture had enough experimental evidence to be considered as valid. In addition to theoretical developments, the deep inelastic scattering experiments led to the rapid acceptance of the quark hypothesis and its field theoretical description. As leader of the SLAC team who performed the experiments, Taylor was awarded the 1990 Nobel Prize in Physics, along with Friedman and Kendall, “for their pioneering investigations concerning deep inelastic scattering of electrons on protons and bound neutrons, which have been of essential importance for the development of the quark model in particle physics.”
Testing the Standard Model
In QCD the scaling behaviour was valid only as a first approximation. QCD calculations predicted a logarithmic deviation from scaling that was consistent with the results of the experiments Taylor and his co-workers were continuing to pursue at SLAC. In the 1970s, the SLAC-MIT experimenters investigated the deep inelastic scattering employing different targets to explore the dependence of the inelastic scattering on atomic weight.
Taylor also worked on some tests of the invariant properties of the electromagnetic interactions also performed at SLAC. The theoretical framework for the calculation was the electroweak theory first proposed by Glashow and elaborated by Weinberg and Salam in 1967. In the early 1970s, the proof that the Glashow-Weinberg-Salam gauge theory was renormalisable made the theory acceptable from a theoretical perspective. At the same time, several proposals were made to test its empirical predictions. Taylor was involved in an experiment to test the non-conservation of parity in the scattering between electrons and nucleons following the preliminary calculations made by C. Prescott. In 1978, thanks to a new method to produce polarized electrons, Taylor and his colleagues were able to verify a violation of parity conservation in good agreement with the prediction of the electro-weak theory. By that time, the discovery of neutral currents in weak interactions had led the community of physicists to consider the Glashow-Weinberg-Salam unified theory of electromagnetic and weak interactions as an accurate description of elementary particles’ behaviour. The experiment made at SLAC was a further corroboration of this successful theory, for the elaboration of which Glashow, Weinberg, and Salam received the Physics Nobel Prize in 1979. From the 1980s onward, Taylor has worked on the design of various detectors employed in linear accelerators at the SLAC as well as in Europe, also serving as Associate Director for Research at SLAC from 1982 to 1986.
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