DOCSLIB.ORG

Top-Quark and Top-Squark Production at Hadron Colliders at Electroweak NLO

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Top-Quark and Top-Squark Production at Hadron Colliders at Electroweak NLO Technische Universitat Munchen Max-Planck-Institut fur Physik (Werner-Heisenb erg-Institut ) Top-Quark and Top-Squark Production at Hadron Colliders at Electroweak NLO Monika Kollar Vollstandiger Abdruck der von der Fakultat fur Physik der Technischen Universitat MUnchen zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigten Dissertation. Vorsitzender : Univ.-Prof. Dr. L. Oberauer Prufer der Dissertation : 1. Univ.-Prof. Dr. W. F. L. Hollik 2. Univ.-Prof. Dr. A. J. Buras Die Dissertation wurde am 28.03.2007 bei der Technischen Universitat Munchen eingereicht und durch die Fakultat fur Physik am 31.05.2007 angenommen. To Dano Abstract In this work, the impact of O(a) contributions on the cross sections for the top- quark pair production within the SM and for the top-squark pair production within the MSSM is investigated. For these processes, the EW-QCD interference leads to additional contributions at level which are not present at Born-level. In addi­ tion, parton densities at NLO QED give rise to non-zero photon density in the proton. It is shown that the size of photon-induced production rates is comparable to other EW NLO contributions. The cross sections differential in V! and pT are studied and discussed in kinematic ranges accessible at the LHC and at the Tevatron. The NLO EW contributions become significant at high p T and high VI and should be included in the numerical analysis. iii Contents 1 Introduction 1 2 Standard Model 5 2.1 Basics of the Standard Model .............................................................................. 5 2.2 Open questions and the role of quantum corrections .................................... 8 3 Supersymmetry 11 3.1 Motivation for supersymmetry .............................................................................. 11 3.2 Properties of supersymmetric theories ................................................................ 12 3.2.1 Supersymmetry breaking ....................................................................... 13 3.2.2 R-parity ........................................................................................................... 14 3.2.3 Low-energy supersymmetry .................................................................... 15 3.3 The Minimal Supersymmetric Standard Model .............................................. 17 3.4 Particle content of the MSSM.............................................................................. 20 3.4.1 Quarks and Leptons .................................................................................. 20 3.4.2 Squarks and Sleptons .................................................................................. 21 3.4.3 Higgs and Gauge bosons ........................................................................... 22 3.4.4 Higgsinos and Gauginos ........................................................................... 24 4 Regularization and Renormalization 27 4.1 Regularization .............................................................................................................. 27 4.2 Renormalization ........................................................................................................... 28 4.2.1 Renormalization schemes ........................................................................... 29 5 Hadronic cross sections 33 5.1 Parton model .............................................................................................................. 33 5.2 Factorization .............................................................................................................. 35 5.2.1 Factorization schemes .................................................................................. 37 5.2.2 Splitting functions ..................................................................................... 38 v vi CONTENTS 5.3 Parton distributions with QED contributions .................................................. 39 6 Top pair production at NLO QED 41 6.1 Top pair production ................................................................................................ 41 6.1.1 tt cross section at the partonic level..................................................... 44 6.2 Structure of the NLO QED contributions ......................................................... 45 6.2.1 Virtual corrections ..................................................................................... 45 6.2.2 Real corrections ............................................................................................ 50 6.3 Photon-induced tt production .............................................................................. 53 6.4 Soft and collinear photon/gluon emission ......................................................... 55 6.4.1 Phase space slicing ..................................................................................... 55 6.4.2 Dipole subtraction method ....................................................................... 58 6.5 Hadronic cross sections for pp/pp ^ ttX......................................................... 59 6.5.1 Integrated hadronic cross sections ......................................................... 60 6.5.2 Differential hadronic cross sections ..................................................... 60 6.5.3 Mass factorization ..................................................................................... 62 6.6 Numerical results....................................................................................................... 63 7 SUSY-EW corrections to stop pair production 77 7.1 Top-squark pair production .................................................................................. 77 7.1.1 Partonic cross sections at lowest order .............................................. 79 7.2 General aspects of NLO SUSY-EW corrections .............................................. 80 7.2.1 Quark and squark self-energies at one-loop level............................. 81 7.2.2 On-shell renormalization conditions ..................................................... 84 7.3 Classification of NLO SUSY-EW corrections .................................................. 85 7.3.1 Virtual corrections ..................................................................................... 85 7.3.2 Real corrections ............................................................................................ 90 7.4 Photon-induced top-squark production ............................................................ 92 7.5 Numerical results ....................................................................................................... 92 7.6 Analysis of parameter dependence ........................................................................... 101 8 Conclusions 107 Zusammenfassung 111 A Choice of parameters 113 A.1 Standard Model Parameters...................................................................................... 113 A.2 SPS 1a parameter set of the MSSM................................................................... 114 CONTENTS vii B Basic principles of supersymmetry 117 B.1 Conventions ..................................................................................................................... 117 B.2 Spinors ................................................................................................................................118 B.2.1 Weyl spinors .................................................................................................. 118 B.2.2 Dirac and Majorana spinors .......................................................................119 B.3 Superfields .................................................................................................................... 120 B.3.1 Grassmann variables ..................................................................................... 120 B.3.2 Chiral and vector superfields .......................................................................121 B. 4 Supersymmetric Lagrangian ..................................................................................... 122 C Parton densities 125 C. 1 LO splitting functions ................................................................................................125 C.2 QED-modified DGLAP evolution equations .........................................................126 D Loop integrals 127 E Analytical expressions for the NLO QED corrections 131 Bibliography 135 Acknowledgements 153 viii CONTENTS Chapter 1 Introduction Over the twentieth century, unceasing theoretical and experimental efforts in particle physics were concentrated on building a model that would describe the basic elements of nature in a consistent way. It was found that the protons and neutrons in the nuclei are not fundamental objects, but they are composed of quarks, which together with the leptons are considered to be the elementary particles. The fundamental forces of strong, weak and electromagnetic interactions are widely believed to be described by quantum field theories based on local gauge symmetries. The elementary particles can then interact through exchange of gauge field quanta (gluons, weak bosons and photons). All of this knowledge is embedded into the currently established model of elementary particle physics, the Standard Model (SM) [1-4]. Only the fourth fundamental force, gravity, is not yet included in the quantum theory picture. After many years of testing, which proved that the Standard Model is an impres ­ sively consistent theory for the energy ranges
Load more

Recommended publications
  • Arxiv:Astro-Ph/0010112 V2 10 Apr 2001
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server NYU-TH-00/09/05 Preprint typeset using LATEX style emulateapj v. 14/09/00 NEUTRON STARS WITH A STABLE, LIGHT SUPERSYMMETRIC BARYON SHMUEL BALBERG1,GLENNYS R. FARRAR2 AND TSVI PIRAN3 NYU-TH-00/09/05 ABSTRACT ¼ If a light gluino exists, the lightest gluino-containing baryon, the Ë , is a possible candidate for self-interacting ¼ ª =ª 6 ½¼ Ñ dÑ b dark matter. In this scenario, the simplest explanation for the observed ratio is that Ë ¼ ¾ Ë 9¼¼ MeVc ; this is not at present excluded by particle physics. Such an could be present in neutron stars, with hyperon formation serving as an intermediate stage. We calculate equilibrium compositions and equation of ¼ state for high density matter with the Ë , and find that for a wide range of parameters the properties of neutron ¼ stars with the Ë are consistent with observations. In particular, the maximum mass of a nonrotating star is ¼ ½:7 ½:8 Å Ë ¬ , and the presence of the is helpful in reconciling observed cooling rates with hyperon formation. Subject headings: dark matter–dense matter–elementary particles–equation of state–stars: neutron 1. INTRODUCTION also Farrar & Gabadadze (2000) and references therein. The lightest supersymmetric “Ê-hadrons” are the spin-1/2 The interiors of neutron stars offer a unique meeting point ¼ gg Ê ~ bound state ( or glueballino) and the spin-0 baryon between astrophysics on one hand and nuclear and particle ¼ ¼ g~ Ë Ë Ùd× bound state, the .The is thus a boson with baryon physics on the other.
    [Show full text]
  • Introduction to Supersymmetry
    Introduction to Supersymmetry Pre-SUSY Summer School Corpus Christi, Texas May 15-18, 2019 Stephen P. Martin Northern Illinois University [email protected] 1 Topics: Why: Motivation for supersymmetry (SUSY) • What: SUSY Lagrangians, SUSY breaking and the Minimal • Supersymmetric Standard Model, superpartner decays Who: Sorry, not covered. • For some more details and a slightly better attempt at proper referencing: A supersymmetry primer, hep-ph/9709356, version 7, January 2016 • TASI 2011 lectures notes: two-component fermion notation and • supersymmetry, arXiv:1205.4076. If you find corrections, please do let me know! 2 Lecture 1: Motivation and Introduction to Supersymmetry Motivation: The Hierarchy Problem • Supermultiplets • Particle content of the Minimal Supersymmetric Standard Model • (MSSM) Need for “soft” breaking of supersymmetry • The Wess-Zumino Model • 3 People have cited many reasons why extensions of the Standard Model might involve supersymmetry (SUSY). Some of them are: A possible cold dark matter particle • A light Higgs boson, M = 125 GeV • h Unification of gauge couplings • Mathematical elegance, beauty • ⋆ “What does that even mean? No such thing!” – Some modern pundits ⋆ “We beg to differ.” – Einstein, Dirac, . However, for me, the single compelling reason is: The Hierarchy Problem • 4 An analogy: Coulomb self-energy correction to the electron’s mass A point-like electron would have an infinite classical electrostatic energy. Instead, suppose the electron is a solid sphere of uniform charge density and radius R. An undergraduate problem gives: 3e2 ∆ECoulomb = 20πǫ0R 2 Interpreting this as a correction ∆me = ∆ECoulomb/c to the electron mass: 15 0.86 10− meters m = m + (1 MeV/c2) × .
    [Show full text]
  • Beyond the Standard Model
    Beyond the Standard Model Mihoko M. Nojiri Theory Center, IPNS, KEK, Tsukuba, Japan, and Kavli IPMU, The University of Tokyo, Kashiwa, Japan Abstract A Brief review on the physics beyond the Standard Model. 1 Quest of BSM Although the standard model of elementary particles(SM) describes the high energy phenomena very well, particle physicists have been attracted by the physics beyond the Standard Model (BSM). There are very good reasons about this; 1. The SM Higgs sector is not natural. 2. There is no dark matter candidate in the SM. 3. Origin of three gauge interactions is not understood in the SM. 4. Cosmological observations suggest an inflation period in the early universe. The non-zero baryon number of our universe is not consistent with the inflation picture unless a new interaction is introduced. The Higgs boson candidate was discovered recently. The study of the Higgs boson nature is extremely important for the BSM study. The Higgs boson is a spin 0 particle, and the structure of the radiative correction is quite different from those of fermions and gauge bosons. The correction of the Higgs boson mass is proportional to the cut-off scale, called “quadratic divergence". If the cut-off scale is high, the correction becomes unacceptably large compared with the on-shell mass of the Higgs boson. This is often called a “fine turning problem". Note that such quadratic divergence does not appear in the radiative correction to the fermion and gauge boson masses. They are protected by the chiral and gauge symmetries, respectively. The problem can be solved if there are an intermediate scale where new particles appears, and the radiative correction from the new particles compensates the SM radiative correction.
    [Show full text]
  • Supersymmetry: What? Why? When?
    Contemporary Physics, 2000, volume41, number6, pages359± 367 Supersymmetry:what? why? when? GORDON L. KANE This article is acolloquium-level review of the idea of supersymmetry and why so many physicists expect it to soon be amajor discovery in particle physics. Supersymmetry is the hypothesis, for which there is indirect evidence, that the underlying laws of nature are symmetric between matter particles (fermions) such as electrons and quarks, and force particles (bosons) such as photons and gluons. 1. Introduction (B) In addition, there are anumber of questions we The Standard Model of particle physics [1] is aremarkably hope will be answered: successful description of the basic constituents of matter (i) Can the forces of nature be uni® ed and (quarks and leptons), and of the interactions (weak, simpli® ed so wedo not have four indepen- electromagnetic, and strong) that lead to the structure dent ones? and complexity of our world (when combined with gravity). (ii) Why is the symmetry group of the Standard It is afull relativistic quantum ®eld theory. It is now very Model SU(3) ´SU(2) ´U(1)? well tested and established. Many experiments con® rmits (iii) Why are there three families of quarks and predictions and none disagree with them. leptons? Nevertheless, weexpect the Standard Model to be (iv) Why do the quarks and leptons have the extendedÐ not wrong, but extended, much as Maxwell’s masses they do? equation are extended to be apart of the Standard Model. (v) Can wehave aquantum theory of gravity? There are two sorts of reasons why weexpect the Standard (vi) Why is the cosmological constant much Model to be extended.
    [Show full text]
  • Supersymmetry Min Raj Lamsal Department of Physics, Prithvi Narayan Campus, Pokhara Min [email protected]
    Supersymmetry Min Raj Lamsal Department of Physics, Prithvi Narayan Campus, Pokhara [email protected] Abstract : This article deals with the introduction of supersymmetry as the latest and most emerging burning issue for the explanation of nature including elementary particles as well as the universe. Supersymmetry is a conjectured symmetry of space and time. It has been a very popular idea among theoretical physicists. It is nearly an article of faith among elementary-particle physicists that the four fundamental physical forces in nature ultimately derive from a single force. For years scientists have tried to construct a Grand Unified Theory showing this basic unity. Physicists have already unified the electron-magnetic and weak forces in an 'electroweak' theory, and recent work has focused on trying to include the strong force. Gravity is much harder to handle, but work continues on that, as well. In the world of everyday experience, the strengths of the forces are very different, leading physicists to conclude that their convergence could occur only at very high energies, such as those existing in the earliest moments of the universe, just after the Big Bang. Keywords: standard model, grand unified theories, theory of everything, superpartner, higgs boson, neutrino oscillation. 1. INTRODUCTION unifies the weak and electromagnetic forces. The What is the world made of? What are the most basic idea is that the mass difference between photons fundamental constituents of matter? We still do not having zero mass and the weak bosons makes the have anything that could be a final answer, but we electromagnetic and weak interactions behave quite have come a long way.
    [Show full text]
  • Super Symmetry
    MILESTONES DOI: 10.1038/nphys868 M iles Tone 1 3 Super symmetry The way that spin is woven into the in 1015. However, a form of symmetry very fabric of the Universe is writ between fermions and bosons called large in the standard model of supersymmetry offers a much more particle physics. In this model, which elegant solution because the took shape in the 1970s and can quantum fluctuations caused by explain the results of all particle- bosons are naturally cancelled physics experiments to date, matter out by those caused by fermions and (and antimatter) is made of three vice versa. families of quarks and leptons, which Symmetry plays a central role in are all fermions, whereas the physics. The fact that the laws of electromagnetic, strong physics are, for instance, symmetric in and weak forces that act on these time (that is, they do not change with particles are carried by other time) leads to the conservation of particles, such as photons and gluons, energy. These laws are also symmetric The ATLAS experiment under construction at the which are all bosons. with respect to space, rotation and Large Hadron Collider. Image courtesy of CERN. Despite its success, the standard relative motion. Initially explored in model is unsatisfactory for a number the early 1970s, supersymmetry is a of reasons. First, although the less obvious kind of symmetry, which, graviton. Searching for electromagnetic and weak forces if it exists in nature, would mean that supersymmetric particles will be a have been unified into a single force, the laws of physics do not change priority when the Large Hadron a ‘grand unified theory’ that brings when bosons are replaced by Collider comes into operation at the strong interaction into the fold fermions, and fermions are replaced CERN, the European particle-physics remains elusive.
    [Show full text]
  • Quantum Universe
    QUANTUM UNIVERSE THE REVOLUTION IN 21ST CENTURY PARTICLE PHYSICS DOE / NSF HIGH ENERGY PHYSICS ADVISORY PANEL QUANTUM UNIVERSE COMMITTEE QUANTUM UNIVERSE THE REVOLUTION IN 21ST CENTURY PARTICLE PHYSICS What does “Quantum Universe” mean? To discover what the universe is made of and how it works is the challenge of particle physics. Quantum Universe presents the quest to explain the universe in terms of quantum physics, which governs the behavior of the microscopic, subatomic world. It describes a revolution in particle physics and a quantum leap in our understanding of the mystery and beauty of the universe. DOE / NSF HIGH ENERGY PHYSICS ADVISORY PANEL QUANTUM UNIVERSE COMMITTEE QUANTUM UNIVERSE CONTENTS COMMITTEE MEMBERS ANDREAS ALBRECHT EDWARD KOLB CONTENTS University of California at Davis Fermilab University of Chicago SAMUEL ARONSON Brookhaven National Laboratory JOSEPH LYKKEN Fermilab iii EXECUTIVE SUMMARY KEITH BAKER Hampton University HITOSHI MURAYAMA 1 I INTRODUCTION Thomas Jefferson National Accelerator Facility Institute for Advanced Study, Princeton University of California, Berkeley 2 II THE FUNDAMENTAL NATURE OF JONATHAN BAGGER MATTER, ENERGY, SPACE AND TIME Johns Hopkins University HAMISH ROBERTSON University of Washington 4 EINSTEIN’S DREAM OF UNIFIED FORCES NEIL CALDER Stanford Linear Accelerator Center JAMES SIEGRIST 10 THE PARTICLE WORLD Stanford University Lawrence Berkeley National Laboratory University of California, Berkeley 15 THE BIRTH OF THE UNIVERSE PERSIS DRELL, CHAIR Stanford Linear Accelerator Center SIMON
    [Show full text]
  • The Quantum Vacuum and the Cosmological Constant Problem
    The Quantum Vacuum and the Cosmological Constant Problem S.E. Rugh∗and H. Zinkernagely To appear in Studies in History and Philosophy of Modern Physics Abstract - The cosmological constant problem arises at the intersection be- tween general relativity and quantum field theory, and is regarded as a fun- damental problem in modern physics. In this paper we describe the historical and conceptual origin of the cosmological constant problem which is intimately connected to the vacuum concept in quantum field theory. We critically dis- cuss how the problem rests on the notion of physically real vacuum energy, and which relations between general relativity and quantum field theory are assumed in order to make the problem well-defined. 1. Introduction Is empty space really empty? In the quantum field theories (QFT’s) which underlie modern particle physics, the notion of empty space has been replaced with that of a vacuum state, defined to be the ground (lowest energy density) state of a collection of quantum fields. A peculiar and truly quantum mechanical feature of the quantum fields is that they exhibit zero-point fluctuations everywhere in space, even in regions which are otherwise ‘empty’ (i.e. devoid of matter and radiation). These zero-point fluctuations of the quantum fields, as well as other ‘vacuum phenomena’ of quantum field theory, give rise to an enormous vacuum energy density ρvac. As we shall see, this vacuum energy density is believed to act as a contribution to the cosmological constant Λ appearing in Einstein’s field equations from 1917, 1 8πG R g R Λg = T (1) µν − 2 µν − µν c4 µν where Rµν and R refer to the curvature of spacetime, gµν is the metric, Tµν the energy-momentum tensor, G the gravitational constant, and c the speed of light.
    [Show full text]
  • Supersymmetry
    Supersymmetry Physics Colloquium University of Virginia January 28, 2011 Stephen P. Martin Northern Illinois University 1 The Standard Model of particle physics • The “Hierarchy Problem”: why is the Higgs mass so • small? Supersymmetry as a solution • New particles predicted by supersymmetry • Supersymmetry is spontaneously broken • How to find supersymmetry • 2 The Standard Model of Particle Physics Quarks (spin=1/2): Name: down up strange charm bottom top − 1 2 − 1 2 − 1 2 Charge: 3 3 3 3 3 3 Mass: 0.005 0.002 0.1 1.5 5 173.1 Leptons (spin=1/2): − − − Name: e νe µ νµ τ ντ Charge: −1 0 −1 0 −1 0 Mass: 0.000511 ∼ 0 0.106 ∼ 0 1.777 ∼ 0 Gauge bosons (spin=1): ± 0 Name: photon (γ) W Z gluon (g) Charge: 0 ±1 0 0 Mass: 0 80.4 91.2 0 All masses in GeV. (Proton mass = 0.938 GeV.) Not shown: antiparticles of quarks, leptons. 3 There is a last remaining undiscovered fundamental particle in the Standard Model: the Higgs boson. What we know about it: Charge: 0 Spin: 0 Mass: Greaterthan 114 GeV, and not between 158 and 175 GeV ( in most simple versions) Less than about 215 GeV ( indirect, very fuzzy, simplest model only) Fine print: There might be more than one Higgs boson. Or, it might be a composite particle, made of other more basic objects. Or, it might be an “effective” phenomenon, described more fundamentally by other unknown physics. But it must exist in some form, because... 4 The Higgs boson is the source of all mass.
    [Show full text]
  • Arxiv:1302.6587V2 [Hep-Ph] 13 May 2013
    UCI-TR-2013-01 Naturalness and the Status of Supersymmetry Jonathan L. Feng Department of Physics and Astronomy University of California, Irvine, CA 92697, USA Abstract For decades, the unnaturalness of the weak scale has been the dominant problem motivating new particle physics, and weak-scale supersymmetry has been the dominant proposed solution. This paradigm is now being challenged by a wealth of experimental data. In this review, we begin by recalling the theoretical motivations for weak-scale supersymmetry, including the gauge hierar- chy problem, grand unification, and WIMP dark matter, and their implications for superpartner masses. These are set against the leading constraints on supersymmetry from collider searches, the Higgs boson mass, and low-energy constraints on flavor and CP violation. We then critically examine attempts to quantify naturalness in supersymmetry, stressing the many subjective choices that impact the results both quantitatively and qualitatively. Finally, we survey various proposals for natural supersymmetric models, including effective supersymmetry, focus point supersymme- try, compressed supersymmetry, and R-parity-violating supersymmetry, and summarize their key features, current status, and implications for future experiments. Keywords: gauge hierarchy problem, grand unification, dark matter, Higgs boson, particle colliders arXiv:1302.6587v2 [hep-ph] 13 May 2013 1 Contents I. INTRODUCTION 3 II. THEORETICAL MOTIVATIONS 4 A. The Gauge Hierarchy Problem 4 1. The Basic Idea 4 2. First Implications 5 B. Grand Unification 6 C. Dark Matter 7 III. EXPERIMENTAL CONSTRAINTS 8 A. Superpartner Searches at Colliders 8 1. Gluinos and Squarks 8 2. Top and Bottom Squarks 9 3. R-Parity Violation 9 4. Sleptons, Charginos, and Neutralinos 10 B.
    [Show full text]
  • Supersymmetry and Its Breaking
    Supersymmetry and its breaking Nathan Seiberg IAS The LHC is around the corner 2 What will the LHC find? • We do not know. • Perhaps nothing Is the standard model wrong? • Only the Higgs particle Most boring. Unnatural. Is the Universe Anthropic? • Additional particles without new concepts Unnatural. Is the Universe Anthropic? • Natural Universe – Technicolor (extra dimensions) – Supersymmetry (SUSY) – new fermionic dimensions • Something we have not thought of 3 I view supersymmetry as the most conservative and most conventional possibility. In the rest of this talk we will describe supersymmetry, will motivate this claim, and will discuss some of the recent developments in this field. 4 Three presentations of supersymmetry • Supersymmetry pairs bosons and fermions – integer spin particles and half integer spin particles. • Supersymmetry is an extension of the Poincare symmetry. • Supersymmetry is an extension of space and time. It describes additional dimensions which are intrinsically quantum mechanical (fermionic). 5 Supersymmetry as an extension of the Poincare symmetry • The Poincare symmetry includes four translations . • One way to present supersymmetry is through adding fermionic symmetries which satisfy Note, these are anti-commutation relations – no obvious classical analog. 6 The spectrum • Normally, translations relate a particle at one point to a particle at a nearby point. • Because of the larger symmetry there must be more particles. relates one particle to another. Every particle has a superpartner. • The symmetry pairs bosons and fermions – integer spin particles and half integer spin particles: 7 Supersymmetry as new quantum fermionic dimensions (more abstract) • In addition to the four classical (bosonic) coordinates , we introduce four fermionic coordinates with spin 1/2.
    [Show full text]
  • ELEMENTARY PARTICLES in PHYSICS 1 Elementary Particles in Physics S
    ELEMENTARY PARTICLES IN PHYSICS 1 Elementary Particles in Physics S. Gasiorowicz and P. Langacker Elementary-particle physics deals with the fundamental constituents of mat- ter and their interactions. In the past several decades an enormous amount of experimental information has been accumulated, and many patterns and sys- tematic features have been observed. Highly successful mathematical theories of the electromagnetic, weak, and strong interactions have been devised and tested. These theories, which are collectively known as the standard model, are almost certainly the correct description of Nature, to first approximation, down to a distance scale 1/1000th the size of the atomic nucleus. There are also spec- ulative but encouraging developments in the attempt to unify these interactions into a simple underlying framework, and even to incorporate quantum gravity in a parameter-free “theory of everything.” In this article we shall attempt to highlight the ways in which information has been organized, and to sketch the outlines of the standard model and its possible extensions. Classification of Particles The particles that have been identified in high-energy experiments fall into dis- tinct classes. There are the leptons (see Electron, Leptons, Neutrino, Muonium), 1 all of which have spin 2 . They may be charged or neutral. The charged lep- tons have electromagnetic as well as weak interactions; the neutral ones only interact weakly. There are three well-defined lepton pairs, the electron (e−) and − the electron neutrino (νe), the muon (µ ) and the muon neutrino (νµ), and the (much heavier) charged lepton, the tau (τ), and its tau neutrino (ντ ). These particles all have antiparticles, in accordance with the predictions of relativistic quantum mechanics (see CPT Theorem).
    [Show full text]