J.4.1State the three-family structure of quarks and leptons in the standard model. J.4.2State the lepton number of the leptons in each family. J.4.3Solve problems involving conservation laws in particle reactions. J.4.4Evaluate the significance of the Higgs particle (or Higgs boson). Option J: Particle physics J4 Leptons and the standard model

State the three-family structure of quarks and leptons in the standard model. ●This graphic shows part of an organizational structure for particles called the standard model. ●These are the quarks from which mesons and hadrons are formed. They are also fermions. Option J: Particle physics J4 Leptons and the standard model FYI  Particles are divided into “generations” or “families” of increasing mass.

State the three-family structure of quarks and leptons in the standard model. ●This graphic shows part of an organizational structure for particles called the standard model. ●These are the leptons, the most common of which is the electron. They are also fermions. Option J: Particle physics J4 Leptons and the standard model FYI  Muons - created in upper atmosphere by cosmic rays. Tau particles – created in the laboratory.

State the lepton number of the leptons in each family. ●Quarks have baryon numbers B = +1/3. Antiquarks have B = -1/3. ●Since hadrons are made of three quarks, they have a baryon number of B = +1. ●Since mesons are made of a quark and an antiquark, they have a baryon number of B = 0. Option J: Particle physics J4 Leptons and the standard model Quarks (antiparticles have opposite Q and B) Q B1 st Generation2 nd Generation3 rd Generation +2/3 +1/3Up (u)Charm (c)Top (t) -1/3 +1/3Down (d)Strange (s)Bottom (b)

State the lepton number of the leptons in each family. ●Leptons have lepton numbers L = +1. Antileptons have L = -1. Option J: Particle physics J4 Leptons and the standard model Leptons (antiparticles have opposite Q and L) Q L1 st Generation2 nd Generation3 rd Generation 0 1Electron neutrino ( e ) Muon neutrino (  ) Tau neutrino (  ) -1 1Electron (e) Muon (  )Tau (  ) FYI  Lepton number must be conserved by generation.  In any reaction involving leptons the total number in each generation must remain the same.  Leptons, like quarks, are fermions, which means that their spin is in the set {  1/2,  3/2}, and that they obey Pauli’s exclusion principle.

Solve problems involving conservation laws in particle reactions. Option J: Particle physics J4 Leptons and the standard model EXAMPLE: Consider the following reactions. Assign lepton numbers and baryon numbers to each particle to determine the feasibility of each. p → n + e + + e n → p + e - +  n + p →  + +  Baryon number: Lepton number: Charge: Baryon number: Lepton number: Charge: Baryon number: Lepton number: Charge: I I = = = FEASIBLE I II =  = II + 1 II  = = NOT FEASIBLE L must be conserved by family. B must be conserved.

Solve problems involving conservation laws in particle reactions. Option J: Particle physics J4 Leptons and the standard model ●L must be conserved by family. ●Thus L II and L I are not conserved. ●A pion is a meson and has B = 0. ●p and n each have B = 1. ●Baryon number not conserved. ●Baryon number not conserved. ●Charge not conserved.

Solve problems involving conservation laws in particle reactions. Option J: Particle physics J4 Leptons and the standard model ●Gluons.

Solve problems involving conservation laws in particle reactions. Option J: Particle physics J4 Leptons and the standard model ●Conservation of charge. ●Conservation of baryon number. ●Conservation of lepton number (by family). ●Also strangeness, parity, isotopic spin, angular momentum.

Solve problems involving conservation laws in particle reactions. Option J: Particle physics J4 Leptons and the standard model ●Family I lepton number is not conserved. ●Equation needs Family I lepton with no charge and L = -1.  e fits the bill. ●  n  p + e - + e.

Evaluate the significance of the Higgs particle (or Higgs boson). ●The charge, lepton number, baryon number, etc., are conserved in particle interactions, but mass seems to come and go as it pleases. ●Scientists have postulated that perhaps mass is not an inherent property of a particle so much as it is a property of the space the particle is in. ●Recall that mass is really that property of matter which makes it resist acceleration. ●Consider the following analogy: A popular person is at a party. Being popular, she has a lot of partygoers gathered around her. For her to walk to the bar to get another drink is more difficult (her acceleration is less) than for a less popular person to do so, because of the resistance of the surrounding people. Option J: Particle physics J4 Leptons and the standard model

Evaluate the significance of the Higgs particle (or Higgs boson). ●Another party analogy is this: A partygoer has some interesting gossip to disseminate, that not only causes people to gather round, but also causes that gossip to spread, as a disturbance through the crowd. ●We call the process whereby mass is not the property of the particle, but part of space itself the Higgs mechanism. ●For the Higgs mechanism to work, all of space has to be covered by some sort of field called the Higgs field. ●Just as the electromagnetic field has a particle associated with it (a photon) so too does the Higgs field—in this case the particle associated with the Higgs field is the Higgs boson. Option J: Particle physics J4 Leptons and the standard model

Evaluate the significance of the Higgs particle (or Higgs boson). ●In the party gossip analogy, the Higgs boson is represented by the cluster of people passing on the gossip. ●One of the design criteria for CERN was the capability of discovering the Higgs boson (sometimes called the “god” particle). ●The expected structure of the Higgs field is shown here: Option J: Particle physics J4 Leptons and the standard model

Evaluate the significance of the Higgs particle (or Higgs boson). ●To date the Higgs particle has yet to be observed, although there is increasing evidence of its existence at CERN. ●Discovery of the Higgs particle would be evidence that the standard model is correct. ●Without the Higgs particle, the standard model will not extend into the realm of general relativity. ●String theory, which needs more than 3 spatial dimensions to work, could be an alternative to the standard model. String theory will be covered in Option J6. ●The next slide shows the framework for the standard model, and includes the not only the force carriers, but the Higgs field. Option J: Particle physics J4 Leptons and the standard model