Particle physics - Standard Model

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Particle physics - Standard Model

  1. 1. Particle Physics
  2. 2. Elementary ParticleA particle with no internal structure.
  3. 3. Three types of elementary particlesQuarksLeptonsExchange Particles (Gauge Bosons)
  4. 4. elementary Gauge Bosonsparticlesthat feelstrong force FERMIONS – follow Pauli exclusion principleelementaryparticles DO NOT followthat do not Pauli exclusionfeel strong principleforce
  5. 5. FERMIONSTwo types of fundamental particles are classifiedas FERMIONS (they follow Pauli’s exclusionprinciple and have ½ spin numbers)Present theory states that these particles cannotbe broken down into even “smaller” particles.These two classes of fundamental particles are. Leptons – do not feel the strong force Quarks – feel the strong force
  6. 6. LeptonsThere are six types of lepton and each has anantiparticle (opposite charge).Family -1 charge zero charge 1 electron (e) electron-neutrino ( e) 2 muon ( ) muon-neutrino ( ) 3 tau ( ) tau-neutrino ( )Each lepton has a designated lepton number of +1. Theantiparticles of each lepton are -1. For any interaction, thesum of all the lepton numbers must remain constant. Thisis the lepton number conservation law.
  7. 7. Quarks (isolated quarks have never been detected)There are six types of quarks and consequently sixtypes of anti-quarks (with opposite charge).Family +2/3 charge -1/3 charge 1 up (u) down (d) 2 charm (c) strange (s) 3 top (t) bottom (b)Quarks and anti-quarks combine to form compositeparticles called HADRONS: two families of hadrons3 quarks = baryon (ex. protons and neutrons)2 quarks = meson (ex. pions)
  8. 8. Fermions Bosonselementary particles elementary particles gauge bosons HADRONScomposite particles composite particlesbaryons mesons(made of 3 quarks) (one quark + one anti quark)
  9. 9. Elementary Particles
  10. 10. Exchange Particles – Mediate Fundamental Forces gauge bosons graviton gluon (gravity)(strong) photon w+, w -, z0 (electromagnetic) (weak) electroweakRange: gravity, electromagnetic >> strong > weakStrength: strong > electromagnetic >> weak >> gravityMass: weak >>>> strong, gravity, electromagnetic
  11. 11. The Higgs BosonNot discovered yet, only theorizedAn exchange particle that gains masswhen it interacts with other particles.The existence of Higgs is importantbecause it is fundamental to theoriesabout how particles have mass. If itdoesn’t exist, much of the current theorywill need to be revised.
  12. 12. Classifying Particles There are many different properties used to classify a particle. These intrinsic properties are expressed as quantum numbers. Quantum numbers tell us about -electric charge - spin - strangeness -.charm - color (not actual color) - lepton number - baryon number
  13. 13. Pauli’s Exclusion PrincipleNo two particles in a closed system (such as anatom) can have the same set of quantum numbers.All fermions follow the PEPBosons do not follow the PEP
  14. 14. Quantum Number – electrical chargeFundamental particles can have positive, negativeor no charge.An ANTIPARTICLE has the identical mass to aparticle but opposite charge (if charged) andopposite spin (if there is spin).
  15. 15. Classifying Particles There are many different properties used to classify a particle. These intrinsic properties are expressed as quantum numbers. Quantum numbers tell us about -electric charge - spin - strangeness -.charm - color (not actual color) - lepton number - baryon number
  16. 16. Quantum Number - SPINAll fermions have non-integer spinexample electrons +½ (or – ½ )All bosons have integer (or zero) spin
  17. 17. Classifying Particles There are many different properties used to classify a particle. These intrinsic properties are expressed as quantum numbers. Quantum numbers tell us about -electric charge - spin - strangeness -.charm - color (not actual color) - lepton number - baryon number
  18. 18. Particles - Summary All observed particles fermions bosons ½ integral spin zero or integral spin obey Pauli exclusion do not obey Pauli exclusion mesons gaugeleptons quarks (2 quarks) bosons Hadrons baryons (3 quarks)
  19. 19. Fundamental InteractionsThe four fundamental interactions of nature are: electromagnetic, strong, weak, and gravityThe electromagnetic and the weak interactions are twoaspects of the same interaction, the electroweak interaction
  20. 20. Mediation of Fundamental ForcesThe fundamental forces are mediated by theexchange of particles. These particles are calledexchange bosons.A Feynman diagram can be used to show howinteractions between particles are mediated bybosons.The electromagnetic force ismediated by photons. Thesephotons are unobservableand are termed virtualphotons to distinguish themfrom real ones.
  21. 21. Exchange Particles : the nature of forceAll four of the fundamental forces involve thecontinuous exchange of “virtual” particlesThe creation of “virtual” particles is a breach ofconservation laws (as they are created from nothing) sothey can only exist for a short period of time.The maximum range of an exchange force is dictatedby the Heisenberg uncertainty principle.
  22. 22. The Heisenberg Uncertainty Principle (HUP)It is impossible to make precise measurements of both theposition and momentum (velocity) of electrons or any otherparticles.The very act of measuring changes these quantities. Themore precise one measurement is, the less precise the otherone becomes..
  23. 23. Implications of the Uncertainty PrincipleHUP can be applied to the hrelationship between energy E tand time. 4Here, the uncertainty principle implies that the life timeof a virtual particle is inversely proportional to itsmass (energy)The more massive the exchange particle, the shorter its life.Why is the range of the strong and weak nuclear force verysmall compared to the infinite range of the electromagneticand gravitational force?
  24. 24. The uncertainty in the energy of a virtual photonis 7.1 × 10-19 J. Determine the uncertainty in the time forthe electromagnetic interaction between two electronsexchanging the virtual photon.. 34 h 6.6 10 17 t 19 7.4 10 s 4 E 4 (7.1 10 )
  25. 25. Range of Interactions of Exchange Particles. The range of a virtual particle (and hence the force it mediates) is governed by the equation below (from HUP) h h is Planck’s constantR c is the speed of light m is the REST MASS of the virtual particle 4 mcWe see here again that range is inversely proportional to the rest mass
  26. 26. The strong force has a range of about 10-15 m. Calculate the restmass of the related exchange particle. What type of particle isthis? 34 h 6.6 10 28R 15 8 2 10 kg 4 mc 4 (10 )(3.0 10 ) this is a gluon
  27. 27. FEYNMAN DIAGRAMSExchange forces are often pictured with Feynman diagrams.At each vertex in a Feynman diagram, conservation lawssuch as charge, lepton number and baryon number must beobeyed
  28. 28. Different lines are drawn for different particles. There aresome variations in the conventions that are applied. or W and Z bosons sometimes gluons
  29. 29. InteractionsInteractions are illustrated using Feynmandiagrams. Here are two examples:Gluon exchange holds A meson interactionquarks together. (which at the quark level involves gluons) holds nucleus together
  30. 30. Practice : Draw Feynman diagrams to illustrate the followinga) an electron absorbing a photon of energyb) a positron (anti-electron) emitting a photon of energyc) an electron-positron pair annihilation to form a photond) Formation of an electron and positron from a photon
  31. 31. Review Problem
  32. 32. Review Problem
  33. 33. Review Problem
  34. 34. Review Problem
  35. 35. Review Problem
  36. 36. Review Problem
  37. 37. Quarks (isolated quarks have never been detected)There are six types of quarks and consequently sixtypes of anti-quarks (with opposite charge).Generation +2/3 charge -1/3 charge 1 up (u) down (d) 2 charm (c) strange (s) 3 top (t) bottom (b)Quarks and anti-quarks combine to form hadrons.There are two classes of hadrons3 quarks = baryon (ex. protons and neutrons)2 quarks = meson (ex. pions)
  38. 38. Here are some examples of baryons and mesons.
  39. 39. Baryons (three quarks)Baryon numbers are examples of quantumnumbers.Baryon numbers are +1 and -1 (anti-particles)respectively. The baryon number is conservedin any interaction.All other particles have a baryon number of zero.(only a Baryon can be +1 or -1)
  40. 40. Individual quarks have baryon numbers of 1/3 (or -1/3) Protons consist of two up quarks and one down. This is written as uud and referred to as up, up, down.Note that the overall baryon number is1/3 + 1/3 + 1/3 = 1And the overall electrical charge would be equal to+ 2/3 + 2/3 + (-1/3) = +1
  41. 41. Charges in quarksEZ to rememberProton UUDNeutron UDDmake sense?
  42. 42. Quarks and SpinRecallAll fermions have non-integer spin ex. electrons have spin number ½ ex. protons have spin number ½ ex. quarks have spin number ½All bosons have integer (or zero) spin
  43. 43. There are two spin states referred to as UP andDOWNSo spin number +½ UP spin number - ½ DOWNIn a proton, the two up quarks cannot have thesame spin number.
  44. 44. Quarks and QCDQuarks also have different “colors”.The color force between quarks is mediated by gluons.quarks come in three colors: red, blue, greenanti-quarks are : anti-red (cyan), anti-blue (yellow) andanti-green (magenta)
  45. 45. The “colorless” property of bound quarks is calledconfinement.Only combinations of color-neutral (add to white) quarkshave been found. Baryons R + G + B = white Mesons color + anti-color = whiteThe combination though must always be color neutral(white or colorless). This is why particles consisting of 4quarks have never been found.
  46. 46. Strangeness – yet another quantum numberDepends on number ofstrange (-1) and anti-strange(+1) quarks in a compositeparticle.Only conserved in interactionsinvolving gluons and photons. (not the WEAK force)
  47. 47. InteractionsYou do not need to worry about the composition ofbaryons (other than protons and neutrons) ormesons. You should however be able to applyconservation laws to interactions. They are:Conservation of mass-energy.Conservation of baryon and lepton numbers.Conservation of electrical chargeConservation of angular momentum. Each particlehas a spin number. The total spin before and afterthe interaction remains the same.
  48. 48. Practice ProblemA common process examined is beta decay.neutron  proton + electron + anti-neutrinoThe anti-neutrino is required to conserve thelepton number : zero = zero + 1 – 1 uud To convert a neutron to a ? proton a down quark must change its flavor. udd
  49. 49. Beta decay continued:For udd  uud conversionAll quarks have baryon number of 1/3 so baryonnumber is conserved. Charge however is notconserved. A negative charge must be removed. uud Beta decay is mediated by the weak force. The weak w- force boson w – changes the flavor of the up quark in the neutron. udd
  50. 50. Interactions and Other Processes e- uud w- Arrows pointing down in a Feynman diagram indicate anti-particles, udd NOT direction.The electron and anti-neutrino leptonnumbers are + 1 and -1 so lepton number isconserved, as is electrical charge.
  51. 51. Elementary Particles Composite Particles Do not feel strong force Color combinations = white Lepton # = 1Obey PEP (anti leptons = -1) Baryons Baryon # = 1 Feel strong force Baryon # = 1/3 (anti quarks = -1/3) Hadrons Gauge Bosons MesonsObey PEP graviton &Do Not Higgs (undetected) Strong EM Weak

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