Option 212: UNIT 2 Elementary Particles Department of Physics and Astronomy SCHEDULE 3-Feb pm Physics LRA Dr Matt Burleigh Intro lecture 7-Feb am Physics LRA Dr Matt Burleigh Problem solving (10-Feb am Physics F2Problem Workshop) 14-Feb am Physics LRA Dr Ted Thomas Follow-up

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

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

Web Sites  Brief introduction to Particle Physics  Introductions to Particle Physics  CERN web site  212 Option - Lecture notes in MS Powerpoint

INTRODUCTION to Elementary Particle Physics *Fundamental building blocks of which all matter is composed: Elementary Particles electron proton neutron photon Cosmic Rays * Pre-1930s it was thought there were just four elementary particles 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 1932 positron or anti-electron discovered, followed by many other particles (muon, pion etc)

CLASSIFICATON OF PARTICLES With the advent of particle accelerator in the 1950’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. An elementary particle is a point particle without structure that is not constructed from more elementary entities

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

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

LEPTONS 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    and tau t -, and 3 neutrinos e     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 Note that each distinct neutrino is associated with one of the other leptons

Beta Decay and the discovery of the neutrino 31H31H 3 2 He + e - × Electrons have a range of energies – must be a third particle involved! Most probable energy < max KE Third particle must have no charge or mass, as they are accounted for by the He nucleus and electron. 31H31H 3 2 He + e - + √

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 Fermions Particles with half-integer spin (1/2, 3/2, 5/2, …) are called Fermions They obey the Pauli exclusion principle (Tipler p.833) Bosons 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

Matter & Antimatter Every particle has an antiparticle partner e - - electron e + - positron p - proton p - antiproton Here are some examples n - neutron - neutrino n- antineutron - antineutrino Read Tipler P.1317 to find out how Dirac predicted the existence of anti-particles in 1927

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

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 e - + e + ->2  s s m o c 2 s = 1/2 e-e- e+e+ Each photon gets e  = m e c 2 p   = m e c m o c 2 s = 1/2 s

Some Fundamental Particles ParticleSymbol Rest energy MeVCharge SpinAntiparticle Mass less boson  00 1  photon Leptons Neutrino Electron Muon Meson Pion Baryons Proton neutron e    /2 oo oo p+nop+no /2 pnpn e   

The Conservation Laws Can a conceivable reaction or decay occur? Conservation of energy 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

The Conservation Laws Can a conceivable reaction or decay occur? 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 Conservation of Lepton number Lepton number must be conserved in a reaction BUT…..

The Conservation Laws 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 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  and L   to the particles e.g. for e and e, L e =+1, for their anti-particles L e =-1, and for all other leptons and other particles L e =0

Some Fundamental Particles ParticleSymbol Rest energy MeVB LeLe Antiparticle Neutrino Electron Muon Tau Pion Kaon Proton Neutron Lambda Sigma e      oo oo p+nop+no e      LL LL S  00  photon Leptons Photon       Hadrons Mesons K+KoK+Ko     K-KoK-Ko    Baryons pnpn      Category See also Tipler Table 41-1 Page 1315 For strangeness, examine Figure 41-2 Page 1322 _ _ _ _ _ _