1. Particle Physics J4 Leptons and the standard model

2. Particle Physics J4 Objectives State the three family structure of quarks and leptons in the standard model State the lepton number of the leptons in each family Solve problems involving conservation laws in particle reactions Evaluate the significance of the Higgs particle (boson)

3. Probing deep into matter Quarks Leptons and standard model Quarks + Leptons = The standard model It consists of three families where each family appears to be a copy of the one before it and heavier. A number of decays which appear possible do not occur Energy, charge, momentum and angular momentum… fine It is for this reason that conservation of lepton number was introduced where each family has it’s own Lepton number and each antiparticle the opposite.

4. Leptons e e Quarks d u     s c b t Family 1 (light) Family 2 (heavy) Family 3 (v. heavy) Leptons interact via the weak and electromagnetic force not the strong force!

5. Particle Physics Leptons In interactions Family lepton number is conserved Which of these reactions conserve lepton number ? e +  -  e - +  e + n  p +  -

6. Particle Physics A muon decays in 2.2 x 10 -6 seconds (weak force decay) Muons can replace electrons in atoms! Tau particle (1974) decays in 3.3 x 10-13 seconds Tau mass 3.5 million x electron mass and has only been seen in labs! We don’t know why we have these particles, their discovery came as a surprise!

7. Probing deep into matter Conservation laws In all reactions the following quantities are ALWAYS conserved! 1.Energy 2.Momentum 3.Angular momentum 4.Electric Charge 5.Baryon Number 6.Colour 7.Lepton Number

8. Probing deep into matter The Higgs Particle – The easily confused should look away now Neutral, spin zero key part of the standard model. Estimated mass approximately 200 GeV/c2 Both fermilab and the LHC are looking for it, nothing yet It is linked to mass Photons, W and Z (electroweak bosons) should have zero mass according to the mathematically (symmetry) theory. The Higgs mechanism allows the W and Z to have mass (many others reached a similar conclusion)

9. Probing deep into matter The Higgs Particle It required the introduction of a new particle which, when it interacts with the particles in the standard model, leads to mass It is the quantum of a Higgs field (like a photon and an electromagnetic field) In the later case zero field leads to zero energy, in the former there is no zero value in the lowest energy state! Think of a ball being pulled through a liquid, effectively the mass increases