During July and August of 1976 a group of 90 physicists from 56 laboratories in 21 countries met in Erice for the 14th Course of the International School of Subnuclear Physics. The countries represented were Argentina, Australia, Austria, Belgium, Denmark, the Federal Republic of Germany, France, the German Democratic Republic, Greece, Israel, Italy, Japan, Mexico, Nigeria, Norway, Sweden, the United Kingdom, the United States of America, Vietnam, and Yugoslavia. The School was sponsored by the Italian Ministry of Public Education (MPI), the Italian Ministry of Scientific and Technological Research (MRST), the North Atlantic Treaty Organi zation (NATO), the Regional Sicilian Government (ERS), and the Weizmann Institute of Science. The program of the School was mainly devoted to the elucida tion and discussion of the progress achieved in the theoretical and experimental understanding of the fundamental constituents of matter. On the theoretical front we had a series of remarkable lecturers (C. N. Yang, S. Weinberg, G. C. Wick) attempting a description of finite size particles. Another group of lecturers covered such topics as the understanding of the new particles (H. J. Lipkin), whether or not jets really exist (E. Lillethun), and the unexpected A-dependence of massive dileptons produced in high-energy proton- nucleus collisions (J. W. Cronin). Two other outstanding questions were covered by E. Leader and G. Preparata respectively: whether strong interactions are still within the Regge framework, and if it is really possible to master strong interactions. A. J. S.

A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons, such as baryons (of which protons and neutrons are examples), and mesons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves. Quarks have various intrinsic properties, including electric charge, mass, color charge and spin. Quarks are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces(electromagnetism, gravitation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge. There are six types of quarks, known as flavors: up, down, strange, charm, top, and bottom. Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process of particle decay: the transformation from a higher mass state to a lower mass state. Because of this, up and down quarks are generally stable and the most common in the universe, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators). For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties have equal magnitude but opposite sign. This book gives a comprehensive overview of the quark.

Dramatic progress has been made in all branches of physics since the National Research Council's 1986 decadal survey of the field. The Physics in a New Era series explores these advances and looks ahead to future goals. The series includes assessments of the major subfields and reports on several smaller subfields, and preparation has begun on an overview volume on the unity of physics, its relationships to other fields, and its contributions to national needs. Nuclear Physics is the latest volume of the series. The book describes current activity in understanding nuclear structure and symmetries, the behavior of matter at extreme densities, the role of nuclear physics in astrophysics and cosmology, and the instrumentation and facilities used by the field. It makes recommendations on the resources needed for experimental and theoretical advances in the coming decade.

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