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Contents Main topic Sub topic Page 1a. Elementary particles 1b. Three classes of observed particles 1c. Identifying elementary particles 1d. Mass and quantum numbers of particles 1e. Spin 1f. Antiparticles 1g. Pauli’s Exclusion Principle 1h. The four fundamental interactions 1i. Exchange particles 1j. Heisenberg’s Uncertainty Principle 1k. Feynman diagrams 1l. Virtual particles 1m. Interaction range 1n. Pair production and annihilation 3 3 4 4 4 4 5 6 7 7 8 9 11 11 12 2a. High energy for mass 2b. High energy for size 2c. Linear accelerator 2d. Cyclotron 2e. Synchrotron 2f. Bremsstrahlung braking radiation 2g. Pros and cons of accelerators 2h. Photomultiplier 2i. Bubble chamber 2j. Wire chamber 13 13 13 14 14 15 16 17 17 18 19 3a. Structure in terms of quarks 3b. The need for colour 3c. Strangeness 3d. Quark confinement 20 20 21 22 23 4a. The three classes of fundamental particles 4b. Conservation laws 4c. The Higgs boson 24 24 24 25 5a. The eight-fold way 5b. Deep inelastic scattering 5c. Asymptotic freedom 5d. Neutral current 5e. Evidence for the standard model 26 26 26 28 29 30 6a. Temperature and the Big Bang 6b. Early particle interactions 6c. Matter vs antimatter 6d. String theory 31 31 31 33 34 1. Particles and interactions 2. Particle accelerators and detectors 3. Quarks 4. Leptons and the standard model 5. Experimental evidence 6. Cosmology and strings 7. Common exam mistakes 35 8. Questions 36 9. Answers 39 2 Option J Particle physics J1 Particles and interactions 1a. Elementary particles -an elementary or fundamental particle is one that is not composed of anything smaller. -there are three types of elementary particles, leptons, quarks and exchange bosons. -leptons are particles that feel the weak force but do not feel the strong (colour) force. -the electron is the most well known lepton. -there are six types of leptons: electrons, muons and tauons and each has its own neutrino, the electron-neutrino, mu-neutrino and tau-neutrino. -each lepton has its antiparticle (see below), the positron, anti-muon, anti-tauon and the antiparticle of each neutrino (anti-neutrino). -quarks feel the strong force, or now more correctly named the colour force. -there are six types of quarks; up, down, strange, charm, top and bottom. -each quark has an antiparticle. -leptons and quarks split into three generations, electron, muon and tauon. -each generation contains two quarks and two leptons (the lepton and its neutrino). -particles in each generation have a counterpart in the other generations. -each counterpart has the same charge. -the masses of counterpart particles increase down the generations. -all fundamental particles are shown in the table below: name of particle electron electron neutrino down up muon muon neutrino strange charm tauon tauon neutrino bottom top type of particle lepton lepton quark quark lepton lepton quark quark lepton lepton quark quark symbol charge e νe d u µ νµ s c τ ντ b t -1 0 -1/3 +2/3 -1 0 -1/3 +2/3 -1 0 -1/3 +2/3 Lepton number Le = 1 Le = 1 0 0 Lµ = 1 Lµ = 1 0 0 Lτ = 1 Lτ = 1 0 0 generation electron electron electron electron muon muon muon muon tauon tauon tauon tauon Mass (approx) 511 keV 0.03 eV ? 0.35GeV 0.35GeV 106 MeV < .27MeV 100 MeV 1.2 GeV 1.78GeV < 31MeV 4 GeV 170 GeV -note masses for quarks are approximate. -note the antiparticles have the opposite charges and lepton numbers. -however they will all have the same masses as their matter counterparts. -gauge bosons or exchange particles (or force carriers) carry the fundamental forces. -they include the photon in the Coulomb force, gravitons in gravity, the W and Z particles in the weak force and gluons in the strong force. -note the graviton is predicted by theory, but has yet to be discovered, though there is indirect evidence (eg the binary pulsar). -note there is another particle called the Higgs boson that might also be fundamental. -for clarity these particles will be referred to mainly as gauge bosons. Task 1 : Okay let’s see if you’re sober enough to read from the above table. Give the name for each of these fundamental particles: 1. it has a charge of -1/3 and a mass of 4GeV …………………… 2. it is from the muon generation and has no charge ……………… 3. it has a charge of +1 and mass of 511keV ……………………….. 4. it is a quark with a mass 1.2GeV……………………………….. answers shown below 3 ms-1 (but ignoring relativity), it would not travel far before disappearing: about 2x10-13m, which is barely a fraction the distance across an atom! -note the Uncertainty Principle can also be applied to the momentum and position of a particle. If the uncertainty in the momentum is ∆mv and the uncertainty in position is ∆r then h 4π ∆mv. ∆r ≥ -this variation of the Principle is useful when finding how far a short half life particle will travel before it decays, ∆r being the range and the velocity is taken as the speed of light c. (See later). Key points -two non-commuting operators of a particle cannot be both known with perfect accuracy. -this is Heisenberg’s Uncertainty Principle. -this is a consequence of quantum mechanics. 1k. Feynman Diagrams -particle interactions are shown in Feynman diagrams. -time is taken as the horizontal axis, increasing left to the right. -leptons, quarks or hadrons are shown as arrows to the right. -antimatter particles are shown as arrows pointing to the left -note: not all books show the backward pointing arrow. -exchange particles (gauge bosons) are shown as wavy lines between them. -e.g. take two electrons approaching each other. -they are first shown as two arrows converging. -a photon is exchanged; the two arrows are then shown diverging. -suppose a proton and electron are considered. -a photon is passed between them causing attraction. -both situations are shown in the diagrams below. charge repulsion charge attraction proton electron photon photon electron electron time time -gravity is attractive so a Feynman diagram resembles charge attraction. -the graviton is the exchange particle between masses (the gauge boson). -this is shown in the diagram below left. Note the quantum theory of gravity is still incomplete, but the reaction is expected to be like this. gravitational attraction the weak nuclear interaction p+ graviton eW- two masses time n time -for the weak interaction, there are three exchange particles: 9 ⎯ν Task 3 answers 1. elementary/ fundamental. 2. 3. quarks, leptons 4.5. exchange particles or gauge bosons 6. fundamental 7.8.9.10. gravity, electromagnetic, weak, strong. 11.12. weak, electromagnetic. 13. electroweak. 14. gravitons. 15. photons. 16. W and Z. 17. meson but at a deeper level gluons.