Elementary Particles A basic physics Contents 1 Fermion 1 1.1 Elementary fermions ......................................... 1 1.2 Composite fermions .......................................... 2 1.2.1 Skyrmions ........................................... 2 1.3 See also ................................................ 2 1.4 Notes ................................................. 2 2 Quark 3 2.1 Classification ............................................. 3 2.2 History ................................................ 4 2.3 Etymology .............................................. 5 2.4 Properties ............................................... 6 2.4.1 Electric charge ........................................ 6 2.4.2 Spin ............................................. 6 2.4.3 Weak interaction ....................................... 6 2.4.4 Strong interaction and color charge .............................. 7 2.4.5 Mass ............................................. 8 2.4.6 Table of properties ...................................... 8 2.5 Interacting quarks ........................................... 9 2.5.1 Sea quarks .......................................... 9 2.5.2 Other phases of quark matter ................................. 9 2.6 See also ................................................ 10 2.7 Notes ................................................. 10 2.8 References ............................................... 10 2.9 Further reading ............................................ 13 2.10 External links ............................................. 13 3 Hadron 14 3.1 Etymology ............................................... 14 3.2 Properties ............................................... 14 3.3 Baryons ................................................ 15 3.4 Mesons ................................................ 16 3.5 See also ................................................ 16 i ii CONTENTS 3.6 References ............................................... 16 4 Boson 17 4.1 Types ................................................. 17 4.2 Properties ............................................... 18 4.3 Elementary bosons ........................................... 18 4.4 Composite bosons ........................................... 18 4.5 To which states can bosons crowd? .................................. 19 4.6 See also ................................................ 19 4.7 Notes ................................................. 19 4.8 References .............................................. 19 5 Lepton 20 5.1 Etymology ............................................... 20 5.2 History ................................................ 21 5.3 Properties ............................................... 21 5.3.1 Spin and chirality ....................................... 21 5.3.2 Electromagnetic interaction ................................. 22 5.3.3 Weak Interaction ....................................... 22 5.3.4 Mass ............................................. 22 5.3.5 Leptonic numbers ...................................... 23 5.4 Universality .............................................. 23 5.5 Table of leptons ............................................ 24 5.6 See also ................................................ 24 5.7 Notes ................................................. 24 5.8 References .............................................. 25 5.9 External links ............................................. 25 6 Meson 26 6.1 History ................................................. 26 6.2 Overview ............................................... 27 6.2.1 Spin, orbital angular momentum, and total angular momentum . 27 6.2.2 Parity ............................................. 27 6.2.3 C-parity ............................................ 28 6.2.4 G-parity ............................................ 28 6.2.5 Isospin and charge ...................................... 28 6.2.6 Flavour quantum numbers .................................. 29 6.3 Classification ............................................. 29 6.3.1 Types of meson ........................................ 30 6.3.2 Nomenclature ........................................ 30 6.4 Exotic mesons ............................................. 30 6.5 List .................................................. 30 CONTENTS iii 6.6 See also ................................................ 30 6.7 Notes ................................................. 30 6.8 References .............................................. 31 6.9 External links ............................................. 32 6.9.1 Recent findings ........................................ 32 7 Photon 33 7.1 Nomenclature ............................................. 33 7.2 Physical properties ........................................... 34 7.2.1 Experimental checks on photon mass ............................. 35 7.3 Historical development ........................................ 35 7.4 Einstein’s light quantum ........................................ 36 7.5 Early objections ............................................ 36 7.6 Wave–particle duality and uncertainty principles ........................... 37 7.7 Bose–Einstein model of a photon gas ................................. 38 7.8 Stimulated and spontaneous emission ................................. 39 7.9 Second quantization and high energy photon interactions ....................... 40 7.10 The hadronic properties of the photon ................................. 41 7.11 The photon as a gauge boson ..................................... 41 7.12 Contributions to the mass of a system ................................. 41 7.13 Photons in matter ........................................... 42 7.14 Technological applications ....................................... 42 7.15 Recent research ............................................ 43 7.16 See also ................................................ 43 7.17 Notes ................................................. 43 7.18 References ............................................... 44 7.19 Additional references ......................................... 48 7.20 External links ............................................. 48 8 Gluon 49 8.1 Properties ............................................... 49 8.2 Numerology of gluons ........................................ 49 8.2.1 Color charge and superposition ................................ 49 8.2.2 Color singlet states ...................................... 49 8.2.3 Eight gluon colors ...................................... 50 8.2.4 Group theory details ..................................... 50 8.3 Confinement ............................................. 50 8.4 Experimental observations ...................................... 50 8.5 See also ................................................ 51 8.6 References .............................................. 51 8.7 Further reading ............................................ 52 iv CONTENTS 9 Higgs boson 53 9.1 A non-technical summary ....................................... 53 9.1.1 “Higgs” terminology ..................................... 53 9.1.2 Overview ........................................... 54 9.2 Significance .............................................. 54 9.2.1 Scientific impact ....................................... 54 9.2.2 “Real world” impact ..................................... 54 9.3 History ................................................ 55 9.3.1 Summary and impact of the PRL papers ........................... 56 9.4 Theoretical properties ......................................... 56 9.4.1 Theoretical need for the Higgs ................................ 56 9.4.2 Properties of the Higgs field ................................. 57 9.4.3 Properties of the Higgs boson ................................. 58 9.4.4 Production .......................................... 58 9.4.5 Decay ............................................. 59 9.4.6 Alternative models ...................................... 59 9.4.7 Further theoretical issues and hierarchy problem ....................... 60 9.5 Experimental search ......................................... 60 9.5.1 Search prior to 4 July 2012 .................................. 61 9.5.2 Discovery of candidate boson at CERN ........................... 62 9.5.3 The new particle tested as a possible Higgs boson ...................... 62 9.5.4 Preliminary confirmation of existence and current status ................... 63 9.6 Public discussion ........................................... 63 9.6.1 Naming ............................................ 63 9.6.2 Media explanations and analogies .............................. 64 9.6.3 Recognition and awards ................................... 64 9.7 Technical aspects and mathematical formulation ........................... 65 9.8 See also ................................................ 66 9.9 Notes ................................................. 66 9.10 References .............................................. 67 9.11 Further reading ............................................ 75 9.12 External links ............................................. 76 9.12.1 Popular science, mass media, and general coverage ..................... 76 9.12.2 Significant papers and other ................................. 76 9.12.3 Introductions to the field ................................... 77 9.13 Text and image sources, contributors, and licenses .......................... 78 9.13.1 Text .............................................. 78 9.13.2 Images ............................................ 82 9.13.3