Jack Steinberger Learning About Particles 50 Privileged Years

Transcription

1 Jack Steinberger Learning About Particles 50 Privileged Years

2 Jack Steinberger LearningAboutParticles 50 Privileged Years With 117 Figures 123

3 Dr. Jack Steinberger CERN LEP 1211 Geneve Switzerland Library of Congress Control Number: ISBN Springer Berlin Heidelberg New York This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable for prosecution under the German Copyright Law. Springer is a part of Springer Science+Business Media springeronline.com c Springer-Verlag Berlin Heidelberg 2005 Printed in Germany The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Author s data,final processingby LE-TEX Jelonek, Schmidt & Vöckler GbR, Leipzig Cover design: Erich Kirchner, Heidelberg Printed on acid-free paper 57/3141/YL

4 Prelude One of the great cultural achievements of the twentieth century was the progress in our understanding of particles and their interactions. Everything in the universe is determined by these particles and their interactions with each other, whether it is the formation of galaxies or the behavior of our cat. The first particle discovered was the electron, by J. J. Thompson in The photon was recognized as a particle by Einstein in 1904, the proton was discovered by Rutherford in 1914, and the neutron by Chadwick in The same year saw the discovery, by Anderson, in a cloud chamber exposed to cosmic rays, of the positron, the antiparticle of the electron (one of the fundamental symmetries in nature is that of particles and antiparticles, which have the same mass and angular momentum but the opposite charge). In 1933, to conserve energy and angular momentum in the β decay of radioactive nuclei, Wolfgang Pauli proposed the neutrino (now the electron neutrino), followed within a year by Enrico Fermi with a very successful theory of the new weak interaction, the interaction that underlies radioactivity. The first unstable elementary particle, the mesotron (now called muon) with a mean life of two millionth of a second, was discovered in 1937, again in cloud chambers exposed to cosmic rays, by Anderson and Neddermeyer and by Street and Livingston. These were the particles that existed when I entered particle physics in 1947 as a graduate student at the University of Chicago. Now it is known that the proton and neutron are not elementary particles, but are composed of quarks. In addition to these very important beginnings of particle physics, the first half of the twentieth century was marked by two most fundamental vital breakthroughs in physics: the discovery of the theory of general relativity in 1914 and of quantum mechanics and quantum field theory in the twenties and thirties, the latter was the basis for any understanding of the dynamics of particles. The second half of the century, with the advent of particle accelerators of rapidly increasing energies and intensities, saw beautiful progress in our understanding of the particles and their relationship to one another. We now know that the elementary fermions (fermions are particles with angular mo-

5 VI Prelude mentum 1/2 the Planck constant divided by 2π) consist of three families of four members each, each family with increasingly higher masses. The first family consists of the electron neutrino, the electron, the up and the down quarks; the second family of the muon neutrino, the muon, the charmed and the strange quark; and the third family of the tau neutrino, the tau, the top and the bottom quark. In going from one family to another, the masses change rapidly; for instance the tau, third family counterpart of the electron, has more than 3,000 times the electron mass, but the electric charges and the interactions are the same. We know that there are three interactions, each propagated by vector particles (vector particles are particles of angular momentum equal to 1 x the Plank constant divided by 2π): the electromagnetic interaction propagated by the mass-less photons, the weak interaction propagated by the very massive Z 0, W +,andw mesons, about 200,000 times as massive as the electron, and the strong interaction by 8 mass-less, colored gluons. The last of these particles to be seen, the tau neutrino, was found only just at the turn of the century, in the year 2000, although no one could doubt its existence since the seventies. The late sixties, early seventies saw the emergence of a beautiful, unified theory of the electromagnetic and weak interactions, the electro-weak theory, which has been verified in succeeding years with impressive precision. Nineteen seventy-three saw the consolidation of a theory of the strong interaction, named quantum-chromo-dynamics, very similar in its mathematical structure to the electro-weak theory, and which has also been verified in succeeding years, although, for technical reasons, not as precisely. Together, the electro-weak and quantum-chromo-dynamics theories form what we call our standard model. Following the discovery of the first hadronic meson in Bristol in 1947, the pi meson, now known to be composed of a quark and an anti-quark, several dozens of such hadronic particles, both mesons consisting of a quark and an anti-quark, and barions, such as the neutron and proton, composed of three quarks, as well as their antiparticles, the antibarions, such as the antiproton, composed of three anti-quarks, have been discovered and intensively studied, with consequent improvement in our understanding of the strong interaction. All presently observed phenomena can be understood in terms of the known particles and their interactions within the theoretical frame of the standard model. 1 1 This is almost true, but not quite, in more than one way. To understand gravitational forces in the universe, it is necessary to postulate matter of an unknown type, the so-called dark matter, which dominates the total matter content of the present universe. However, extensions of the standard model have been proposed over the last several decades, motivated by difficulties of the standard model at much higher energies. These predict particles which may very well account for the cosmic dark matter, and may even be discovered at the next, very much higher energy accelerator now under construction at CERN in Geneva. Another cosmological need is that of a particle field necessary to produce the inflationary beginnings of the Big Bang. A third deficiency is the missing Higgs particle.

6 Prelude VII It has been my privilege to be immersed in this beautiful progress and to contribute to it during a long professional career of fifty-odd years. It began with my graduate studies at the University of Chicago, immediately following World War II, where I had the immense luck to find Fermi, one of the outstanding founders of this progress, as my master. Again by chance, I found myself in 1949 at Berkeley, at that time the only place in the world with accelerators that could study the physics of the newly discovered mesons. And so on, in the years that followed, I was always at or near laboratories which permitted this research at the most advanced levels, and with very fine colleagues. This has been my professional life, and now that I am more than eighty, I have the need to write this down. The story is not likely to be interesting to many. It is a small part of the history of the evolution of our knowledge of particle physics, but physics goes on, and today s problems are not those of yesterday. I hope that nevertheless there might be a few with the leisure and interest to read a bit of what happened then. Perhaps, to the extent that my story is also connected with some rather grotesque events of those years, such as Nazi Germany and McCarthy U.S.A., this may add some interest. This is a spin zero (so-called scalar) particle necessary for the consistency of the electro-weak theory.

7 Contents Prelude... V 1 Origins and Education FranconianBeginnings TheNewWorld HigherEducationandWar GraduateSchoolandFermi Institute for Advanced Study, , Theory Berkeley, , Accelerators Properties of Pi Mesons Strange Particles and Bubble Chambers Neutrinos I CP Violation Neutrinos, II QuarksandGluons CDHS NucleonStructure Quantitative Confirmation of Scaling Violations Predicted byqcdtheory BeamDumpandTauNeutrino Neutrino Masses, Neutrino Mixing, Neutrino Oscillations Experiments with the LEP e + e Collider...143

8 X Contents A Postlude References...177