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4735 results in Particle Physics and Nuclear Physics

Page 99 of 237

  • 17 - Quarks and Gluons, Confinement and Jets

  • Gordon Kane, University of Michigan, Ann Arbor
  • Book:
    Modern Elementary Particle Physics
    Published online:
    28 May 2018
    Print publication:
    09 February 2017, pp 132-138

  • Summary

    Electrons and protons have electric charge. Opposite charges attract, and they bind into hydrogen atoms held together by the electromagnetic force. The hydrogen atom has a ground state, an infinite tower of radial excited states for each angular momentum, and an infinite number of orbital angular momenta. The energy levels are modified by spin-orbit forces, all of which are spin-dependent electromagnetic forces. In particle language, the bound state potential is generated from an infinite set of photon exchange diagrams. No finite set of perturbation theory diagrams will give a bound state, so actual calculation of the binding effects is a complicated problem; the fully relativistic bound state problem is not yet solved, even in quantum electrodynamics.

    Since quarks have electric charge, they will form “atoms” as well. But quarks of course feel another force, the QCD or color force mediated by gluons. Since the gluon force is considerably stronger than the electromagnetic force, its properties will determine what spectroscopic patterns are expected. From a space-time point of view, a single gluon is like a single photon, but, because of the self-interaction of gluons, multigluon diagrams can lead to quite different properties, and indeed they do.

    From the point of view of the spectroscopic rules, the electromagnetic force is very simple. Atoms only form from opposite charges. That a very different result will hold for QCD is suggested by observing that the electromagnetic force is characterized by a U(1) symmetry, while the QCD interaction is characterized by an SU(3) symmetry. The difference leads to the existence of baryons!

    Since the QCD force is very complicated, and perturbation theory arguments based on a few diagrams are not expected to give the dominant effects for most bound state questions, we shall describe the situation by writing a few rules, showing that the observed states obey the rules, and motivating the rules from QCD-based arguments.

    Confinement of Color, and Color-Singlet Hadrons

    The essential point to make is that it is thought that QCD has the property that the potential energy of two colored particles increases approximately linearly with the distance between them.

  • 1 - Introduction

  • Gordon Kane, University of Michigan, Ann Arbor
  • Book:
    Modern Elementary Particle Physics
    Published online:
    28 May 2018
    Print publication:
    09 February 2017, pp 1-9

  • Summary

    The Standard Model of particle physics is an awesome theory, providing a description and explanation of the world we see, in a full relativistic quantum field theory. It leaves no puzzles in its domain. It achieves the goals of four centuries of physics. This book describes and explains the Standard Model to anyone who has had an introductory course in quantum theory.

    Remarkably, the field of particle physics is completely different today from what it was before the 1970s. That's because there was huge progress in a few years around 1970, when some major experimental and theoretical developments occurred and merged together. Physicists learned that quarks and leptons are the fundamental objects of which all matter is composed; they interact via the exchange of “gauge bosons.” The forces that significantly affect them are the unified electroweak force, whose gauge bosons are the photon and the W± and Z0 bosons, and the strong force. The theory of the strong force is called quantum chromodynamics (QCD); the gauge bosons of the strong force are the (eight) gluons. (All the new terms here will be defined as the new physics appears in context.)

    In another sense there is great continuity. The theory fully incorporates special relativity and quantum field theory. And there has been a continuous development of relativistic quantum field theory from its inception before 1930. Theorists have learned how to deal with difficult problems such as mass and renormalization. There has been steady and extraordinary progress in particle physics, both in understanding quantum field theory and in learning what to include in the Lagrangian; no revolution has occurred. The theories which describe the particles and their interactions seem to be “gauge theories,” a special class of quantum theories where there is an invariance principle that necessarily implies the existence of interactions mediated by gauge bosons. In gauge theories the interaction Lagrangian is, in a sense, inevitable rather than being introduced in an ad hoc way as in quantum theory.

    The theory is formulated in terms of a function called “the Lagrangian.” In practice, “theory” and “Lagrangian” mean the same thing.

    The basic formulation of the theory is accessible to anyone having a simple undergraduate introduction to quantum mechanics including spin.

  • 13 - Experiments and Detectors

  • Gordon Kane, University of Michigan, Ann Arbor
  • Book:
    Modern Elementary Particle Physics
    Published online:
    28 May 2018
    Print publication:
    09 February 2017, pp 111-116

  • Summary

    This chapter provides a brief description of how information is obtained about the particles that emerge from a collision. In order to understand fully how the theory and its predictions are tested, it is necessary to have some understanding of how the experiments function.

    A further goal is to identify the main detectors from which results could emerge during the next decade. At colliders the products of an interaction can emerge in any direction, so detectors must cover essentially all of the 4π solid angle – they are called “4π” detectors. The cost of a detector that can identify hadrons, photons, electrons, and muons, and also know when something has escaped, which is crucial to find (or not) new physics, has become so large that only a few detectors at each machine can be funded or find manpower for construction and operation and analysis, and they generally will have a rather long lifetime, being upgraded once or twice to do more physics as the machine improves or the physics goals change because of new knowledge. Consequently the detectors have taken on a life of their own, and results will often be quoted as “CMS (or ATLAS) reports …” or “MARK II discovered ….” We list the main detectors which have been important for the Standard Model, and those which will be the most common sources of results over the next decade. The size and complexity of detectors implies that they are very costly and take several years to build, so it is now possible to specify most or all of the detectors which will provide results over the next decade.

    What Emerges from a Collider

    The beams available to collide quarks, leptons, and gauge bosons are electrons and positrons (e±), quarks (q), and gluons (g), where the quarks and gluons are carried in hadrons. When the accelerated hadrons are p±, the quarks are mainly u and d quarks. In addition, quarks and electrons emit photons and W± and Z0 particles at sufficient rates effectively to make photon and W and Z beams under appropriate conditions. We do not distinguish quarks from antiquarks for our purposes. Various particles will emerge from the collisions. By detecting what emerges, with what momentum, at what angle, and how often, we hope to test the present understanding of particles and their interactions and to find any new physics effects.

  • 14 - Low Energy and Non-Accelerator Experiments

  • Gordon Kane, University of Michigan, Ann Arbor
  • Book:
    Modern Elementary Particle Physics
    Published online:
    28 May 2018
    Print publication:
    09 February 2017, pp 117-118

  • Summary

    The previous two chapters emphasized high energy colliders and their detectors. In order that the reader does not get the impression that the only frontier is the high energy one, here we briefly mention other directions from which major discoveries might come. It turns out that our basic subject in this book, the Standard Model and its tests, is indeed more naturally the domain of high energy, high luminosity machines. That is because the natural scale of the Standard Model is of the order of MW or MZ, that is, of the order of 100 GeV. Testing the high energy predictions of the Standard Model requires TeV energies in the quark, lepton, or gluon collisions.

    Nevertheless, our purpose is also to prepare the reader to understand any future developments in particle physics, both by providing an explanation of the Standard Model as the foundation on which anything new will stand, and by knowing whether the Standard Model is conceptually incomplete. Several kinds of experiments are in progress or planned which could extend the Standard Model in new directions. Some use low energy secondary beams at accelerators, and others are truly non-accelerator experiments. They include:

  • (1) searches for neutrino mass effects from neutrino oscillations, from detection of solar neuutrinos or atmospheric neutrinos, or from nuclear β decays;

  • (2) searches for neutrinoless double beta decay, which is sensitive to neutrino masses, right-handed currents, and any new light particles which might couple to neutrinos;

  • (3) searches for rare or forbidden decays of mesons or leptons or quarks;

  • (4) searches for dark matter;

  • (5) searches for nucleon decay;

  • (6) searches for magnetic moments. In particular, the reported magnetic moment of the muon currently deviates from the Standard Model by about three standard deviations, based on an important measurement at Brookhaven National Laboratory. The experiment has been rebuilt at Fermilab, and should take data in 2017. The experiment is based on a muon storage ring and measuring the correlation of the muon momenta and spins. In addition, the theory has subtleties that are being better understood.

  • (7) Electric dipole moments violate CP invariance. The CP invariance described in Chapter 21 implies a very tiny effect for electrons and neutrons, but some physics beyond the Standard Model could give larger observable effects, and searching for them is an active and important field since nearly all extensions of the Standard Model give effects, sometimes significant ones.

  • Preface

  • Gordon Kane, University of Michigan, Ann Arbor
  • Book:
    Modern Elementary Particle Physics
    Published online:
    28 May 2018
    Print publication:
    09 February 2017, pp x-xiv

  • Summary

    All life is a struggle in the dark… This dread and darkness of the mind cannot be dispelled by the sunbeams, the shining shafts of day, but only by an understanding of the outward form and inner workings of nature.

    And now to business. I will explain…

    Lucretius, On the Nature of the Universe (translation by R. E. Latham, Penguin Books)

    In the early 1980s I was asked to teach our University of Michigan undergraduate course in particle physics. Soon after agreeing, I found that there was no book available at the undergraduate level which presented particle physics as the successful theory of quarks, leptons, and their interactions that it has become. In 1970 there was no theory of weak or strong interactions; no confidence that a fundamental set of constituents had been identified; no way to calculate or explain a variety of results. Today there is a theory of strong, weak, and electromagnetic interactions, with the latter two unified. There have been several extraordinary experimental discoveries, and there are no experimental results that appear to fall outside of the framework of that theory. The so-called Standard Model of particle physics that accomplishes all of that is now widely tested in a variety of ways, and it is here to stay. It is expected that deviations from the Standard Model will someday be found, as clues to improved understanding and to new physics, but the Standard Model will describe physics on the scale where strong, weak, and electromagnetic interactions are important.

    Given that development, it seemed essential to have a presentation of the Standard Model that could be used for advanced undergraduate teaching or broadly accessible first year grad courses. In addition, it seemed even more important to have a book that any scientist who understood the necessary background material could read in order to learn about the developments in particle physics. Those developments should become a part of the education of anyone interested in what mankind has learned about the basic constituents of matter and the forces of nature, but there was no place that many people who had the required background could go to learn them. After teaching the course once, I was convinced that an introductory course in quantum theory was the essential background.

Page 99 of 237