50 years of quarks PDF
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9249_9789814618090_tp.indd 1 24/2/15 10:18 am 9249_9789814618090_tp.indd 2 24/2/15 10:18 am Published by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. 50 YEARS OF QUARKS Copyright © 2015 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN 978-981-4618-09-0 ISBN 978-981-4618-10-6 (pbk) Printed in Singapore SongYu - 50 Years of Quarks.indd 1 11/2/2015 5:07:26 PM February12,2015 11:52 BC:9249–50YearsofQuarks Preface pagev Preface Harald Fritzsch Physics Department, Ludwig-Maximilians-University, D-80333 Munich, Germany The year 1964 was a remarkable year in physics. In this year CP-violation − was discovered, the Ω -resonance was observed, the cosmic microwave back- ground radiation was discovered and the quark model was introduced. In 1947 the four K-mesons and the Λ-hyperon were discovered, followed by the discovery of the three Σ-hyperons and of the two Ξ-hyperons. In 1961 Murray Gell-Mann and Yuval Ne’eman suggested that these particles are described by a new symmetry, based on the group SU(3). The SU(3)- symmetry is an extension of the isospin symmetry, which was introduced in 1932 by Werner Heisenberg. The observed hadrons are members of specific representations of SU(3). The lowest baryons and mesons are described by octet representations. The baryonoctetcontainsthetwonucleons,theΛ-hyperon,thethreeΣ-hyperons and the two Ξ-hyperons. The members of the meson octet are the three pions, the four K-mesons and the η-meson. The ten baryon resonances are members of a decuplet representation. In the SU(3) scheme the observed hadrons are octets and decuplets. There are no hadrons, described by the triplet or sextet representation. In 1964 Gell-Mann introduced the triplets as constituents of the baryons and mesons, the “quarks”. This name appears in the novel “Finnegans wake” of James Joyce on page 383: “Three quarks for Muster Mark”, as mentioned in the contribution of Gell-Mann to this volume. The triplet model was also considered by the Caltech graduate student George Zweig, who was visiting CERN in 1964 (see the contribution of Zweig). The three quarks are the “up”, “down” and “strange” quarks: (u,d,s). The baryons are bound states of three quarks — the proton consists of two up-quarks and one down-quark: p ∼ (uud). The Λ-hyperon has one strange quark: Λ ∼ (uds). The Ω−-resonance is a bound state of three strange v February12,2015 11:52 BC:9249–50YearsofQuarks Preface pagevi vi H. Fritzsch quarks: Ω− ∼ (sss). Thus the s-quark has the electric charge (−1/3), as the d-quark. The charge of the u-quark is (+2/3). In the quark model it remained unclear, why there are only bound states ofthreequarksorthreeantiquarks,thebaryonsorantibaryons, orquarkand antiquark, the mesons. Why are there no bound states, consisting of two quarks? Why aretherenofreequarks? Thosewould bestable particles with nonintegral electric charges. Thus the quark model had many problems. Gell-Mann considered the quarks as mathematical objects, not as real particles. Zweig, however, assumed that the quarks are constituents of the mesons and baryons. In 1968 the first experiments in the deep inelastic scattering of elec- trons and nuclei were carried out at the Stanford Linear Accelerator Center (SLAC). The measured cross sections depend on two variables, the energy transfer from the electron to the nucleus and the mass of the virtual photon. It was observed that the cross sections show a scaling behavior — they were only functions of the ratio of thetwo variables. Thisindicated that thereare point-like constituents inside the nucleons with non-integral electric charges — the quarks were discovered. They were not only mathematical objects, but constituents of the atomic nuclei. Murray Gell-Mann and I introduced in 1971 a new quantum number for the quarks, which we called “color”. Each quark has three different colors: red,greenorblue. Thetransformationsamongthethreecolorsaredescribed by the group SU(3). The quarks are color triplets, two quarks can either be in a color sextet or in a color anti-triplet, but three quarks can form a color singlet, likewise a quark and an anti-quark. The color symmetry was considered to be an exact symmetry of nature (see the contribution of Fritzsch). The simplest color singlets were the bound states of a quark and an antiquark (meson) or three quarks (baryon). We assumed that all hadrons are color singlets, and all configurations with color, e.g. the color triplet quarks, are permanently confined inside the hadrons. In 1972 Gell-Mann and I discussed a gauge theory for the description of the strong interactions. The color degree of freedom was gauged, as the electric charge in QED. The gauge bosons were mass-less gluons, which transformed as an octet of the color group. Later we called this theory “Quantum Chromodynamics”. The gluons interact with the quarks, but also with themselves. This self- interaction leads to the interesting property of asymptotic freedom. The gauge coupling constant of QCD decreases logarithmically, if the energy is February12,2015 11:52 BC:9249–50YearsofQuarks Preface pagevii Preface vii increased. Experiments at SLAC, at DESY, at the Large Electron–Positron Collider in CERN and at the Tevatron in the Fermi National Laboratory have measured this decrease of the coupling-constant. InQCDthescalingpropertyofthecrosssections,observedindeepinelas- ticscattering, isnotanexactproperty,butitisviolatedbysmalllogarithmic terms. The scaling violations were observed and are in good agreement with the theoretical predictions. Today the theory of QCD is considered to be the true theory of the stronginteractionsandisanessentialpartoftheStandardTheoryofparticle physics(seealsothecontributionsofHeinrichLeutwyler andFinnRavndal). The birth of the quark model is described in the contributions of David Horn, Sidney Meshkov and Lev Okun. Details of the quark model are dis- cussed in the contribution of Willy Plessas. The color quantum number can be observed in electron–positron annihi- lation at high energy, but also in the electromagnetic decay of the neutral pion,asdescribedinthecontributionofRodneyCrewther. Althoughquarks are permanently confined, they can be observed indirectly as “quark jets”, as discussed in the contribution of Rick Field. Also gluons can be observed as “gluon jets” — this is the main topic of the contributions of John Ellis and Sau Lan Wu. In QCD one can also make predictions about exclusive processes, as discussed in the contribution of Stan Brodsky. Although quarks are permanently confined inside the hadrons, they do have masses, which determine in particular the symmetry breaking. The determinationofthequarkmassesisdescribedinthecontributionofCesareo Dominguez. If hadrons are compressed to high density, e.g. in heavy ion collisions or in a big neutron star, the quarks and gluons form a “quark–gluon plasma”, as discussed in the contribution of Ulrich Heinz. In reality there are not only three quarks, but today we know six quarks: u,d,c,s,t,b (see also the contribution of Shelley Glashow and of Marek Kar- liner). The six quarks had been predicted in 1972, since with six quarks one can understand CP-violation, as discussed by Makoto Kobayashi. The six quarks form three electroweak doublets: u-d, c-s and t-b. These quarks are not mass eigenstates, but mixtures, and one obtains the flavor mixing, as discussed by Zhi-zhong Xing. Also the leptons are forming three electroweak doublets, involving the electron and its neutrino, the muon and its neutrino and the tauon and its neutrino. Since neutrinos have masses, flavor mixing happens also for the February12,2015 11:52 BC:9249–50YearsofQuarks Preface pageviii viii H. Fritzsch leptons. This can be observed by the neutrino oscillations, as discussed by Rabindra Mohapatra. The Standard Theory of particle physics is the theory of quantum chro- modynamics and the gauge theory of the electroweak interactions. Many physicists speculate about a unified theory beyond the Standard Theory. There might be a unification of QCD and the electroweak theory at very high energy — the QCD coupling constant and the two coupling constants of the electroweak theory come together. An interesting possibility is the gauge group SO(10). A unified theory might have new kinds of symmetries, e.g. supersymme- try (see the contributions of Stephen Adler, of Gordon Kane and Malcolm Perry, and of Mikhail Shifman and Alexei Yung). Some physicists speculate that the leptons, quarks and the gauge bosons are manifestations of small one-dimensional objects, the superstrings. A theory, based on superstrings, might also be a theory of quantum gravity. February13,2015 9:31 BC:9249–50YearsofQuarks content pageix Contents Preface v H. Fritzsch A Schematic Model of Baryons and Mesons 1 M. Gell-Mann Quarks 5 M. Gell-Mann Concrete Quarks 25 G. Zweig On the Way from Sakatons to Quarks 57 L. B. Okun My Life with Quarks 95 S. L. Glashow Quarks and the Bootstrap Era 105 D. Horn From Symmetries to Quarks and Beyond 115 S. Meshkov How I Got to Work with Feynman on the Covariant Quark Model 127 F. Ravndal What is a Quark? 149 G. L. Kane & M. J. Perry Insights and Puzzles in Particle Physics 163 H. Leutwyler Quarks and QCD 181 H. Fritzsch The Discovery of Gluon 189 J. Ellis ix February13,2015 9:31 BC:9249–50YearsofQuarks content pagex x Contents Discovery of the Gluon 199 S. L. Wu The Parton Model and Its Applications 227 T. M. Yan & S. D. Drell From Old Symmetries to New Symmetries: Quark, Leptons and B −L 245 R. N. Mohapatra Quark Mass Hierarchy and Flavor Mixing Puzzles 265 Z.-Z. Xing Analytical Determination of the QCD Quark Masses 287 C. Dominquez CP Violation in Six Quark Scheme — Legacy of Sakata Model 315 M. Kobayashi The Constituent Quark Model — Nowadays 325 W. Plessas − From Ω to Ωb, Doubly Heavy Baryons and Exotics 345 M. Karliner Quark Elastic Scattering as a Source of High Transverse Momentum Mesons 367 R. Field Exclusive Processes and the Fundamental Structure of Hadrons 381 S. J. Brodsky Quark-Gluon Soup — The Perfectly Liquid Phase of QCD 413 U. Heinz Quarks and Anomalies 435 R. J. Crewther Lessons from Supersymmetry: “Instead-of-Confinement” Mechanism 453 M. Shifman & A. Yung Quarks and a Unified Theory of Nature Fundamental Forces 473 I. Antoniadis SU(8) Family Unification with Boson–Fermion Balance 487 S. L. Adler
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