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Option 212: UNIT 2 Elementary Particles

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  1. Department of Physics and Astronomy Option 212: UNIT 2Elementary Particles SCHEDULE 29-Jan-14 12.00pm LRA Intro lecture 3-Feb-14 9.00am LRB Problem solving (10-Feb-14 9.00am E Problem Workshop) 12-Feb-14 12.00pm LRA Follow-up

  2. 1st Lecture Introduction Hadrons and LeptonsSpin & Anti-Particles The conservation laws: Lepton Number Baryon number Strangeness 2nd Lecture 3rd Lecture Follow-upFundamental forces and field particles The standard model UNIT 2: OUTLINE SYLLABUS: Problem solving Check a decay for violation of conservation laws Quarks Properties of a particle given quark combination

  3. Recommended Books • Chapter 41, PA Tipler • Quarks Leptons and The Big Bang, J Allday • The Cosmic Onion, F Close

  4. Web Sites • Brief introduction to Particle Physics http://superstringtheory.com/experm/index.html • CERN web site http://home.web.cern.ch/ • 212 Option - Lecture notes in MS Powerpoint & PDF • http://www.star.le.ac.uk/mrb1/lectures.html

  5. INTRODUCTION to Elementary Particle Physics Cosmic Rays • Fundamental building blocks of • which all matter is composed: • Elementary Particles • Pre-1930s it was thought there were just four elementary particles electron proton neutron photon 1932 positron or anti-electron discovered, followed by many other particles (muon, pion etc) We will discover that the electron and photon are indeed fundamental, elementary particles, but protons and neutrons are made of even smaller elementary particles called quarks

  6. CLASSIFICATON OF PARTICLES The question arose whether perhaps there were too many to all be elementary. This has led to the need for classification of particles. An elementary particle is a point particle without structure that is not constructed from more elementary entities With the advent of particle accelerator in the 1950’s many new elementary particles were discovered.

  7. FUNDAMENTAL INTERACTIONS AND THE CLASSIFICATION OF PARTICLES Fundamental interactions Participating particles • gravitation • electromagnetic • strong nuclear force • weak nuclear force • all particles with mass • those carrying charge • Hadrons (and quarks) • Leptons (and quarks)

  8. HADRONS Hadrons interact through strong forces. There are two classes, mesons and baryons. Mesons have zero or integral spin (0 or 1) with masses that lie between the electron and the proton. Baryons have half integral spin (1/2 or 3/2) and have masses that are always greater than or equal to that of the proton. Hadrons are not elementary particles. As we will see later, they are made of quarks

  9. Leptons interact through weak inter- actions, but not via the strong force. All leptons have spin of 1/2. There are six kinds of lepton: electron e-, muon m-, and tau t -, and 3 neutrinos ne, nm, nt LEPTONS Leptons were originally named because they were “Light-particles”, but we now know the Tau is twice as heavy as a proton Neutrinos were originally thought to be massless, but they probably have a small mass Read more in Tipler p. 1336 Note that each distinct neutrino is associated with one of the other leptons

  10. 32He + e- + ne 31H 32He + e-× 31H Beta Decay and the discovery of the neutrino (Tipler p.1314) √ - • In Beta decay a neutron decays into a proton plus an electron • If decay energy shared by proton and emitted electron, energy of electron would be unique • But observed electrons have a range of energies – must be a third particle involved: the neutrino • Third particle must have no charge or mass, as they are accounted for by the He nucleus and electron.

  11. Spin Spin ½ particles: Electrons, protons, neutrons and neutrinos all have an intrinsic spin characterised by the quantum number s = 1/2 A particle has an intrinsic spin angular momentum Particles with half-integer spin (1/2, 3/2, 5/2, …) are called Fermions They obey the Pauli exclusion principle Particles with integer spin (s = 0, 1, 2, …. ), e.g. mesons, are called Bosons They do not need to obey the Pauli exclusion principle, and any number can occupy the same quantum state

  12. Here are some examples e- - electron e+ - positron p - proton p - antiproton n - neutron n - antineutron - antineutrino n - neutrino n Matter & Antimatter Every particle has an antiparticle partner Read Tipler P.1339 to find out how Dirac predicted the existence of anti-particles in 1927

  13. Electron Pair Production g -> e- + e+ s e- E2 x 511 keV e+ Antimatter For each particle there is an associated antiparticle Anti-particles always created in particle-anti particle pairs s

  14. s s e- + e+ ->2 Each photon gets eg = mec2 pg= mec e+ e- moc2 s = 1/2 moc2 s = 1/2 Antimatter • * Antiparticle has the same mass and magnitude of spin as the particle • * Antiparticle has the opposite • charge to the particle • The positron is stable but has a short-term existence because our Universe has a large supply of electrons • The fate of a positron is annihilation Electron Pair Annihilation s

  15. e  Some Fundamental Particles Spin Antiparticle Rest energy MeV Charge Particle Symbol Mass less boson   0 0 1 photon Leptons Neutrino Electron Muon 1/2 1/2 1/2 0 -1 -1  e  0 0.511 105.7 Meson Pion 140 135 0 0 +1 0  o  o Baryons Proton neutron p- 1/2 1/2 p+ no +1 0 938.3 939.6 - n

  16. The Conservation Laws • Conservation of energy • The total rest mass of the decay products must be less than the initial rest mass of the particle before decay Can a conceivable reaction or decay occur? • Conservation of linear momentum • When an electron and a positron at rest annihilate, two photons must be emitted • Angular momentum must be conserved in a decay or reaction • Net electric charge before must equal net charge after a decay or reaction

  17. The Conservation Laws • Conservation of Baryon number • We assign Baryon Number B=+1 to all Baryons, B=-1 to all anti-Baryons, and B=0 to all other particles • Baryon number must be conserved in a reaction Can a conceivable reaction or decay occur? • Conservation of Lepton number • Lepton number must be conserved in a reaction • BUT…..

  18. The Conservation Laws • Conservation of Lepton number contd: • …..because the neutrino associated with an electron is different to a neutrino associated with a muon, we assign separate Lepton numbers Le, Lmand Ltto the particles • e.g. for e and ne, Le=+1, for their anti-particles Le=-1, and for all other leptons and other particles Le=0 Can a conceivable reaction or decay occur? • Conservation of Strangeness • There are other conservation laws which are not universal, e.g. strange particles have a property called strangeness which must be conserved in a decay or reaction

  19. Some Fundamental Particles Rest energy MeV B Le L Category Particle Symbol L S Antiparticle Photon photon   0 0 0 0 0 0 Neutrino Electron Muon Tau 0 0.511 105.7 1784  e  - Leptons                +      e   Hadrons Mesons 140 135 Pion Kaon  o            +1  o         K- Ko 493.7 497.7 K+ Ko Baryons 938.3 939.6 1115.6 1189.4 1192.5 1197.3             Proton Neutron Lambda Sigma p- n L    p+ no L                See also Tipler Table 41-1 Page 1337 For strangeness, examine Figure 41-3 Page 1344 _ _ _ _ +1 +1 +1 +1 +1 +1 _ _