Option 212: UNIT 2 Elementary Particles

Transcription

1 Department of Physics and Astronomy Option 212: UNIT 2 Elementary Particles SCHEDULE 26-Jan pm LRB Intro lecture 28-Jan pm LRB Problem solving (2-Feb-15 1.am E Problem Workshop) 4-Feb pm LRB Follow-up

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

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

4 Web Sites Õ Brief introduction to Particle Physics Õ CERN web site Õ 212 Option - Lecture notes in MS Powerpoint & PDF

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6 INTRODUCTION to Elementary Particle Physics * Fundamental building blocks of which all matter is composed: Elementary Particles * Pre-193s it was thought there were just four elementary particles electron proton neutron photon Cosmic Rays 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

7 CLASSIFICATON 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 195 s many new elementary particles were discovered. The question arose whether perhaps there were too many to all be elementary. This has led to the need for classification of particles.

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

10 HADRONS Hadrons interact through strong forces. There are two classes, mesons and baryons. Mesons have zero or integral spin ( 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

11 LEPTONS Leptons interact through weak interactions, but not via the strong force. All leptons have spin of 1/2. There are six kinds of lepton: electron e -, muon µ, and tau t -, and 3 neutrinos ν e, ν µ, ν τ Note that each distinct neutrino is associated with one of the other 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

12 Beta Decay and the discovery of the neutrino (Tipler p.1314) 3 1 H 3 2 He + e H 3 2 He + e - + ν e Energy Distribution - Relative intensity Energy (kev) 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.

13 Spin A particle has an intrinsic spin angular momentum Spin ½ particles: Electrons, protons, neutrons and neutrinos all have an intrinsic spin characterised by the quantum number s = 1/2 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 =, 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

14 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 Here are some examples e - - electron p - proton n - neutron ν - neutrino e + - positron p - antiproton n - antineutron ν - antineutrino

15 Antimatter For each particle there is an associated antiparticle Anti-particles always created in particle-anti particle pairs s Electron Pair Production s γ -> e - + e + e - E γ > 2 x 511 kev e +

16 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 Electron Pair Annihilation s m o c 2 s = 1/2 e - + e + ->2γ e - s Each photon gets e + e γ = m e c 2 p γ = m e c s m o c 2 s = 1/2 * The fate of a positron is annihilation

17 Some Fundamental Particles Particle Symbol Rest energy MeV Charge Spin Antiparticle Mass less boson photon γ 1 γ Leptons Neutrino Electron Muon ν e µ /2 1/2 1/2 ν e + µ + Meson Pion π + π o π π o Baryons Proton neutron p + n o /2 1/2 p - - n

18 Conservation of energy The Conservation Laws Can a conceivable reaction or decay occur? The total rest mass of the decay products must be less than the initial rest mass of the particle before decay 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

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

20 The Conservation Laws Can a conceivable reaction or decay occur? 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 L e, L µ and L τ to the particles e.g. for e and ν e, L e =, for their anti-particles L e =-1, and for all other leptons and other particles L e = 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

21 Some Fundamental Particles Category Photon Particle Symbol Rest energy MeV B L e L µ L τ S Antiparticle photon γ γ Leptons Hadrons Mesons Neutrino Electron Muon Tau Pion Kaon ν e µ τ π + π o K + K o Baryons Proton p p_ - Neutron n o n _ Lambda Λ ο _ Sigma Σ + _ Σ ο Σ See also Tipler Table 41-1 Page 1337 For strangeness, examine Figure 41-3 Page ν e + µ + τ + π π o K_ - K o Λ ο Σ + Σ_ ο Σ

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