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Review Article STANDARD MODEL of PARTICLE PHYSICS—A Review Article STANDARD MODEL OF PARTICLE PHYSICS—A HEALTH PHYSICS PERSPECTIVE J. J. Bevelacqua* publications describing the operation of the Large Hadron Abstract—The Standard Model of Particle Physics is reviewed Collider and popular books and articles describing Higgs with an emphasis on its relationship to the physics supporting the health physics profession. Concepts important to health bosons, magnetic monopoles, supersymmetry particles, physics are emphasized and specific applications are pre- dark matter, dark energy, and new particle discoveries sented. The capability of the Standard Model to provide health (PDG 2008). Many of the students’ questions arise from physics relevant information is illustrated with application of conservation laws to neutron and muon decay and in the misconceptions regarding the Standard Model and its rela- calculation of the neutron mean lifetime. tionship to the field of health physics (Bevelacqua 2008a). Health Phys. 99(5):613–623; 2010 The increasing number and complexity of these questions Key words: accelerators; beta particles; computer calcula- and misconceptions of the Standard Model motivated the tions; neutrons author to write this review article. The intent of this paper is to present the Standard Model to health physicists in a manner that minimizes the INTRODUCTION mathematical complexity. This is a challenge because the THE THEORETICAL formulation describing the properties Standard Model of Particle Physics is a theory of interacting and interactions of fundamental particles is embodied in fields. It contains the electroweak interaction (Glashow the Standard Model of Particle Physics (Bettini 2008; 1961; Weinberg 1967; Salam 1969) and quantum chromo- Cottingham and Greenwood 2007; Guidry 1999; Grif- dynamics (QCD) (Gross and Wilczek 1973; Politzer 1973). fiths 2008; Halzen and Martin 1984; Klapdor- QCD is a gauge field theory that describes the strong Kleingrothaus and Staudt 1995; PDG 2008; Peskin and interactions of colored quarks and gluons. Schroeder 1995; Quigg 1997). The Standard Model also The construction of the Standard Model has been provides a basis for the unification or consistent descrip- guided by symmetry principles that are supported by tion of the strong, electromagnetic, and weak interac- group theory. Some of these models and underlying tions. However, it does not include gravity. principles may be unfamiliar to health physicists. Ac- During presentation of the author’s health physics cordingly, this paper will address these models and their certification review courses, a steadily increasing interest applications. However, the presentation will omit much in the Standard Model has been noted. Student inquiries of the mathematical rigor required from a theoretical usually occur during presentations of pion, muon, and physics perspective and focus on those elements of the neutron decay characteristics, and discussion of acceler- Standard Model that are important to health physicists. ator radiation types. Questions often involve the quark As part of the discussion, the properties of low-energy model, contemporary theories such as dark matter or particles are introduced and related to the fundamental supersymmetry, and the discovery of new particles. interactions. These properties are governed by the medi- These questions arise principally from recent graduates ators of the fundamental interactions. The low-energy and appear to be influenced by the content of introduc- tory physics and modern physics courses and various particles are also characterized in terms of their under- lying quark content, and the characteristics of the quarks are reviewed. * Bevelacqua Resources, 343 Adair Drive, Richland, WA 99352. For correspondence contact: Joseph Bevelacqua at the above An overview of the four fundamental interactions address, or email at [email protected]. (strong, electromagnetic, weak, and gravitational) and (Manuscript accepted 18 March 2010) 0017-9078/10/0 basic conservation laws governing particle interactions is Copyright © 2010 Health Physics Society also presented in the context of the Standard Model. This DOI: 10.1097/HP.0b013e3181de7127 overview provides a foundation for a discussion of the 613 614 Health Physics November 2010, Volume 99, Number 5 characteristics of particle decays and particle interac- bottom (b), and top (t). These designations are further tions, and the resultant radiation types. These character- defined in subsequent discussion, sometimes desig- istics and associated conservation laws are shown to nated by just the first letter of the quark name; determine which particle decay modes are allowed. ● generation—A grouping of quarks and leptons. The The significance of symmetry is also reviewed. Standard Model classifies quarks and leptons into 3 gener- Symmetry principles are illustrated by focusing upon the ations. The first generation includes the u and d quarks and Ϫ electromagnetic interaction and the Maxwell equations. the e and ve and their antiparticles. Second generation Ϫ Symmetry properties are also reviewed in terms of the particles include the s and c quarks and the ␮ and v␮ and field equations and the calculation of particle decay their antiparticles. The third generation includes the b and t Ϫ properties. A specific application of the Standard Model quarks and the ␶ and v␶ and their antiparticles; is illustrated by considering weak interaction induced ● hadron—A particle that interacts primarily through the neutron decay and by calculating the mean lifetime of the strong interaction. Mesons and baryons are hadrons. neutron. Hadrons are complex particles having an internal quark structure; PARTICLE PROPERTIES AND ● lepton—A fundamental particle that interacts primar- SUPPORTING TERMINOLOGY ily through the weak interaction. The electron and the electron neutrino are examples of leptons. Leptons can In order to ensure the reader is familiar with the be electrically charged or uncharged. Standard model background of the Standard Model, basic particle prop- neutrinos are massless. Leptons have no internal struc- erties are reviewed. Terminology that supports particle ture and include 3 generations; characterization and facilitates the discussion is also ● meson—A middleweight particle normally composed presented. The properties of particles that are less famil- of a quark and an antiquark. The charged and neutral iar to some applied health physicists (e.g., muons and pions are examples of mesons. Standard Model me- kaons) are compared to particles of more familiarity sons are composed of quark-antiquark pairs; and (e.g., neutrons, protons, and electrons). ● quark—A particle having a fractional charge that interacts through the strong, electromagnetic, and Terminology. Specific terminology is introduced to weak interactions. Quarks were initially inferred from facilitate presentation of the basic physics associated high-energy electron-proton (e-p) scattering. The e-p with the Standard Model. These terms, as utilized in this scattering cross-section indicates the presence of review, are specific to the Standard Model and include: point-like structures inside the proton that have been ● baryon—A heavy particle normally composed of three interpreted as quarks. The Standard Model incorpo- quarks. Protons and neutrons are baryons. Baryons can rates 6 quark flavors and 3 generations. be electrically charged or uncharged. Standard Model baryons are composed of three quarks; Basic particle properties. Table 1 provides a sum- ● boson—A particle having integer spin. The mediators mary of the properties of selected low-energy particles or carriers of each of the four fundamental interactions having a mass below 2,000 MeV-cϪ2 (PDG 2008). These are bosons. The photon, WϮ and Z0, and gluons are properties include the particle mass, mean lifetime, and mediators of the electromagnetic, weak, and strong dominant decay mode, and are provided for neutrinos interactions, respectively. Pions are also bosons. (electron, muon, and tau), the electron (eϪ) and its Bosons can be electrically charged or uncharged; antiparticle (eϩ), the muon (␮Ϫ) and its antiparticle ● charge—A general term used to assign a particular (␮ϩ), the tau (␶Ϫ) and its antiparticle (␶ϩ), three pions property to a particle or field quanta. Health physicists are (␲ϩ, ␲0, and ␲Ϫ), three kaons (Kϩ,K0, and KϪ), the most familiar with electric charge that influences pro- proton (p) and its antiparticle (p៮), and the neutron (n) cesses such as ionization and governs the electromagnetic and its antiparticle (n៮). force. Other types of charge exist including color charge Neutrinos are neutral leptons, assumed to be mass- that governs the strong interaction, and weak charge that less in the Standard Model, but experimental data sup- manifests itself either as a charged or neutral weak ports a small, but certainly non-zero mass (PDG 2008). current, and these currents govern the weak interaction; There are three known generations of neutrinos (␯) and ● fermion—A particle having half-integer spin. Neu- their corresponding antiparticles (antineutrinos, v៮). Spe- trons, protons, and electrons are examples of fermions. cifically included are the electron neutrino (ve) and its ៮ Fermions can be electrically charged or uncharged; antiparticle (ve), the muon neutrino (v␮) and its antipar- ● flavor—A designation for the type of quark. The ticle (v៮␮), and the tau neutrino (v␶) and its antiparticle (v៮␶). flavors are down (d), up (u), strange (s), charm (c), The electron and muon neutrinos
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