Carlo Rubbia The Nobel Prize in Physics 1985

autobiography and work

Standard Model of Particle Physics - nobel lecture

The behavior of all known subatomic particles can be described within a single theoretical framework called the Standard Model. This model incorporates the quarks and leptons as well as their interactions through the strong, weak and electromagnetic forces. Only gravity remains outside the Standard Model. The force-carrying particles are called gauge bosons, and they differ fundamentally from the quarks and leptons. The fundamental forces appear to behave very differently in ordinary matter, but the Standard Model indicates that they are basically very similar when matter is in a high-energy environment. 6 Although the Standard Model does a credible job in explaining the interactions among quarks, leptons, and bosons, the theory does not include an important property of elementary particles, their mass. The lightest particle is the electron and the heaviest particle is believed to be the top quark, which weighs at least 200,000 times as much as an electron. In 1964 Scottish physicist Peter W. Higgs of Edinburgh University proposed a mechanism that provided a way to explain how the fundamental particles could have mass. Higgs theorized that the whole of space is permeated by a field, now called the Higgs field, similar in some ways to the electromagnetic field. As particles move through space they travel through this field, and if they interact with it they acquire what appears to be mass. A basic part of quantum theory is wave-particle duality--all fields have particles associated with them. The particle associated with the Higgs field is the Higgs boson, a particle with no intrinsic spin or electrical charge. Although it is called a boson, it does not mediate force as do the other bosons (see below). The Higgs boson has not yet been observed. Finding it is the key to discovering whether the Higgs field exists, whether Higgs’s hypothesis for the origin of mass is indeed correct, and whether the Standard Model will survive. 7

Classification of Elementary Particles

Two types of statistics are used to describe elementary particles, and the particles are classified on the basis of which statistics they obey. Fermi-Dirac statistics apply to those particles restricted by the Pauli exclusion principle; particles obeying the Fermi-Dirac statistics are known as fermions. Leptons and quarks are fermions. Two fermions are not allowed to occupy the same quantum state. Bose-Einstein statistics apply to all particles not covered by the exclusion principle, and such particles are known as bosons. The number of bosons in a given quantum state is not restricted. In general, fermions compose nuclear and atomic structure, while bosons act to transmit forces between fermions; the photon, gluon, and the W and Z particles are bosons. 8 Basic categories of particles have also been distinguished according to other particle behavior. The strongly interacting particles were classified as either mesons or baryons; it is now known that mesons consist of quark-antiquark pairs and that baryons consist of quark triplets. The meson class members are more massive than the leptons but generally less massive than the proton and neutron, although some mesons are heavier than these particles. The lightest members of the baryon class are the proton and neutron, and the heavier members are known as hyperons. In the meson and baryon classes are included a number of particles that cannot be detected directly because their lifetimes are so short that they leave no tracks in a cloud chamber or bubble chamber. These particles are known as resonances, or resonance states, because of an analogy between their manner of creation and the resonance of an electrical circuit. 9 See table entitled Elementary Particles. 10

Conservation Laws and Symmetry

Some conservation laws apply both to elementary particles and to microscopic objects, such as the laws governing the conservation of mass-energy, linear momentum, angular momentum, and charge. Other conservation laws have meaning only on the level of particle physics, including the three conservation laws for leptons, which govern members of the electron, muon, and tau families respectively, and the law governing members of the baryon class. 11 New quantities have been invented to explain certain aspects of particle behavior. For example, the relatively slow decay of kaons, lambda hyperons, and some other particles led physicists to the conclusion that some conservation law prevented these particles from decaying rapidly through the strong interaction; instead they decayed through the weak interaction. This new quantity was named “strangeness” and is conserved in both strong and electromagnetic interactions, but not in weak interactions. Thus, the decay of a “strange” particle into nonstrange particles, e.g., the lambda baryon into a proton and pion, can proceed only by the slow weak interaction and not by the strong interaction. 12 Another quantity explaining particle behavior is related to the fact that many particles occur in groups, called multiplets, in which the particles are of almost the same mass but differ in charge. The proton and neutron form such a multiplet. The new quantity describes mathematically the effect of changing a proton into a neutron, or vice versa, and was given the name isotopic spin. This name was chosen because the total number of protons and neutrons in a nucleus determines what isotope the atom represents and because the mathematics describing this quantity are identical to those used to describe ordinary spin (the intrinsic angular momentum of elementary particles). Isotopic spin actually has nothing to do with spin, but is represented by a vector that can have various orientations in an imaginary space known as isotopic spin space. Isotopic spin is conserved only in the strong interactions. 13 Closely related to conservation laws are three symmetry principles that apply to changing the total circumstances of an event rather than changing a particular quantity. The three symmetry operations associated with these principles are: charge conjugation (C), which is equivalent to exchanging particles and antiparticles; parity (P), which is a kind of mirror-image symmetry involving the exchange of left and right; and time-reversal (T), which reverses the order in which events occur. According to the symmetry principles (or invariance principles), performing one of these symmetry operations on a possible particle reaction should result in a second reaction that is also possible. However, it was found in 1956 that parity is not conserved in the weak interactions, i.e., there are some possible particle decays whose mirror-image counterparts do not occur. Although not conserved individually, the combination of all three operations performed successively is conserved; this law is known as the CPT theorem. 14

The Discovery of Elementary Particles

The first subatomic particle to be discovered was the electron, identified in 1897 by J. J. Thomson. After the nucleus of the atom was discovered in 1911 by Ernest Rutherford, the nucleus of ordinary hydrogen was recognized to be a single proton. In 1932 the neutron was discovered. An atom was seen to consist of a central nucleus—containing protons and, except for ordinary hydrogen, neutrons—surrounded by orbiting electrons. However, other elementary particles not found in ordinary atoms immediately began to appear. 15

In 1928 the relativistic quantum theory of P. A. M. Dirac hypothesized the existence of a positively charged electron, or positron, which is the antiparticle of the electron; it was first detected in 1932. Difficulties in explaining beta decay (see radioactivity) led to the prediction of the neutrino in 1930, and by 1934 the existence of the neutrino was firmly established in theory (although it was not actually detected until 1956). Another particle was also added to the list: the photon, which had been first suggested by Einstein in 1905 as part of his quantum theory of the photoelectric effect. 16

The next particles discovered were related to attempts to explain the strong interactions, or strong nuclear force, binding nucleons (protons and neutrons) together in an atomic nucleus. In 1935 Hideki Yukawa suggested that a meson (a charged particle with a mass intermediate between those of the electron and the proton) might be exchanged between nucleons. The meson emitted by one nucleon would be absorbed by another nucleon; this would produce a strong force between the nucleons, analogous to the force produced by the exchange of photons between charged particles interacting through the electromagnetic force. (It is now known, of course, that the strong force is mediated by the gluon.) The following year a particle of approximately the required mass (about 200 times that of the electron) was discovered and named the mu meson, or muon. However, its behavior did not conform to that of the theoretical particle. In 1947 the particle predicted by Yukawa was finally discovered and named the pi meson, or pion. 17

Both the muon and the pion were first observed in cosmic rays. Further studies of cosmic rays turned up more particles. By the 1950s these elementary particles were also being observed in the laboratory as a result of particle collisions produced by a particle accelerator. 18

One of the current frontiers in the study of elementary particles concerns the interface between that discipline and cosmology. The known quarks and leptons, for instance, are typically grouped in three families (where each family contains two quarks and two leptons); investigators have wondered whether additional families of elementary particles might be found. Recent work in cosmology pertaining to the evolution of the universe has suggested that there could be no more families than four, and the cosmological theory has been substantiated by experimental work at the Stanford Linear Accelerator and at CERN, which indicates that there are no families of elementary particles other than the three that are known today.

autobiography

I was born in the small town of Gorizia, Italy, on 31 March, 1934. My father was an electrical engineer at the local telephone company and my mother an elementary school teacher. At the end of the World War II most of the province of Gorizia was overtaken by Yugoslavia and my family fled to Venice first and then to Udine. As a boy, I was deeply interested in scientific ideas, electrical and mechanical, and I read almost everything I could find on the subject. I was attracted more by the hardware and construction aspects than by the scientific issues. At that time I could not decide if science or technology were more relevant for me. After completing High School, I applied to the Faculty of Physics at the rather exclusive Scuola Normale in Pisa. My previous education had been seriously affected by the disasters of the war and the subsequent unrest. I badly failed the admission tests and my application was turned down. I forgot about physics and I started engineering at the University of Milan (Politecnico). To my great surprise and joy a few months later I was offered the possibility of entering the Scuola Normale. One of the people who had won the admission contest had resigned! I am recollecting this apparently insignificant fact since it has determined and almost completely by accident my career of physicist. I moved to Pisa, where I completed the University education with a thesis on cosmic ray experiments. They have been very tough years, since I had to greatly improve my education, which was very deficient in a number of fundamental disciplines. At that time I also participated under my thesis advisor Marcello Conversi to new instrumentation developments and to the realization of the first pulsed gas particle detectors.

Soon after my degree, in 1958 I went to the United States to enlarge my experience and to familiarize myself with particle accelerators. I spent about one and a half years at Columbia University. Together with W. Baker, we measured at the Nevis Syncro-cyclotron the angular asymmetry in the capture of polarized muons, demonstrating the presence of parity violation in this fundamental process. This was his first of a long series of experiments on Weak Interactions, which ever since has become my main field of interest. Of course at that time it would have been quite unthinkable for me to imagine to be one day amongst the people discovering the quanta of the weak field! Around 1960 I moved back to Europe, attracted by the newly founded European Organization for Nuclear Research, where for the first time the idea of a joint European effort in a field of pure Science was to be tried in practice. The Syncro-cyclotron at CERN had a performance significantly superior to the one of the machine in Nevis and we succeeded in a number of very exciting experiments on the structure of weak interactions, amongst which I would like to mention the discovery of the beta decay process of the positive pion, p+ = p0 + e + v and the first observation of the muon capture by free hydrogen, µ-+ p = n + v.

In the early sixties John Adams brought to operation the CERN Proton Syncrotron. I moved to the larger machine where I continued to do some weak interaction experiments, like for instance the determination of the parity violation in the beta decay of the lambda hyperon. During the summer of 1964 Fitch and Cronin announced the discovery of CP violation. This has been for me a tremendously important result and I abandoned all current work to start a long series of observations on CP violation in K0 decay and on the KL-KS mass difference. Unfortunately the subject did not turn out to be as prolific as in the case of the previous discovery of parity violation and even today, some thirty years afterwards we do not know much more about the origin of CP-violation than right after the announcement of the discovery. I returned again to more orthodox weak interactions a few years later, when together with David Cline and Alfred Mann we proposed a major neutrino experiment at the newly started US laboratory of Fermilab. The operational problems associated with a limping accelerator and a new laboratory made very difficult, albeit impossible for us during the Summer of 1973 to settle definitively the question of the existence of neutral currents in neutrino interactions, when competing with the much more advanced instrumentation of Gargamelle at CERN. Instead, about one year later we could cleanly observe the presence of all-muons events in neutrino interactions and to confirm in this way one of the crucial predictions of the GIM mechanism, hinting at the existence of charm, glamorously settled only few months later with the observation of the Y/J particle.

In the meantime and under the impulse of Vicky Weisskopf a new, fascinating adventure had just started at CERN with a new type of colliding beams machine, the Intersecting Storage Rings, in which counter-rotating beams of protons collide against each other. This novel technique offered a much more efficient use of the accelerator energy than the traditional method of collisions against a fixed target. From the very first operation of this new type of accelerator, I have participated to a long series of experiments. They have been crucial to perfect the detection techniques with colliding beams of protons and antiprotons needed later on for the discovery of the Intermediate Bosons. By that time it was quite clear that Unified Theories of the type SU(2) x U(1) had a very good chance of predicting the existence and the masses of the triplet of intermediate vector bosons. The problem of course was the one of finding a practical way of discovering them. To achieve energies high enough to create the intermediate vector bosons (roughly 100 times as heavy as the proton) together with David Cline and Peter Mc Intyre we proposed in 1976 a radically new approach. Along the lines discussed about ten years earlier by the Russian physicist Budker, we suggested to transform an existing high energy accelerator in a colliding beam device in which a beam of protons and of antiprotons, their antimatter twins, are counter-rotating and colliding head-on. To this effect we had to develop a number of techniques for creating antiprotons, confining them in a concentrated beam and colliding them with an intense proton beam. These techniques were developed at CERN with the help of many people and in particular of Guido Petrucci, Jacques Gareyte and Simon van der Meer.

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