Option 212: UNIT 2 Elementary Particles

1. Department of Physics and Astronomy Option 212: UNIT 2Elementary Particles SCHEDULE 29-Jan-14 12.00pm LRA Intro lecture 3-Feb-14 9.00am LRB Problem solving (10-Feb-14 9.00am E Problem Workshop) 12-Feb-14 12.00pm LRA Follow-up

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

3. Checking Baryon Numbers a) p+ + n p+ + n 2p+ + p + n p+ + p + p _ _ _ Answer: a) B = 1+1 on left hand side B = 2 on right hand side too! Allowed reaction! b) B = 2 on left hand side B = -1 on right hand side Forbidden reaction

4. Checking Lepton Numbers a) µ- b) π+ e- + ne + n µ+ + n + ne _ Answer: a) Before decay Le = 0 and Lm = +1 After decay Le = 0 and Lm = +1 Allowed reaction! b) Before decay Lm = 0 and Le = 0 After decay Lm = 0 and Le = 1 Forbidden reaction!

5. Is Strangeness Conserved? a) π+ + n b) π- + p K+ +  -+  Answer: a) Initial state has S = 0 Final state has S = +1 - 1 = 0 Allowed reaction! b) Initial state has S = 0 Final state has S = -1 Forbidden reaction!

6. Some Fundamental Particles Rest energy MeV B Le L Category Particle Symbol L S Antiparticle Photon photon   0 0 0 0 0 0 Neutrino Electron Muon Tau 0 0.511 105.7 1784  e  - Leptons                +      e   Hadrons Mesons 140 135 Pion Kaon  o            +1  o         K- Ko 493.7 497.7 K+ Ko Baryons 938.3 939.6 1115.6 1189.4 1192.5 1197.3             Proton Neutron Lambda Sigma p- n L    p+ no L                See also Tipler Table 41-1 Page 1337 For strangeness, examine Figure 41-3 Page 1344 _ _ _ _ +1 +1 +1 +1 +1 +1 _ _

7. Conservation Laws • Test the following decays for violation of the conservation of electric charge, baryon number and lepton number. • (a) n -> p+ + p- + m+ + m- • (b) p0 -> e+ + e- + g

8. Conservation LawsSolution • Method: Use Table 41-1 and the conservation laws for Baryon number and Lepton number • (a) n -> p+ + p- + m+ + m- • Total charge on both sides = 0 : conserved • Baryon number changes from +1 to 0: violated • Lm = 0 on both sides : conserved • Process not allowed • (b) p0 -> e+ + e- + g • Total charge on both sides = 0 : conserved • Baryon number on both sides = 0 : conserved • Le = 0 on both sides: conserved • Process is allowed

9. Quarks - The Smallest Building Blocks of Matter Gell–Mann & Zweig 1963

10. π+ Meson u Name Spin Charge Baryon Strangeness Up u 1/2 +2/3 1/3 0 Down d 1/2 -1/3 1/3 0 Strange s 1/2 -1/3 1/3 -1 Anti-quarks maintain spin, but change sign of S and B! d p u u Baryon d Three Different Types of QUARKS There are three elementary quarks (flavors) That make up the fundamental particles: Up u Down d Strange s

11. Different types of quarks contd. • Mesons – quark + anti-quark ( q q ) • Baryons – three quarks ( q q q ) • Anti-baryons – three anti-quarks ( q q q) By 1967 it was realised that new kinds of quarks were required to explain discrepancies between the model and experiment Charm (c) Bottom (b) – discovered 1977 Top (t) – discovered 1995

12. Quark combinations • Find the baryon number, charge & strangeness of the following quark combinations and identify the hadron: • (a) uud • (b) udd • (c) uus • (d) dds

13. Quark combinations Solution Method: for each quark combination determine the baryon number B, the charge q and the strangeness S; then use Tipler Table 41-2 to find a match. • (a) uud • B = 1/3 + 1/3 + 1/3 = 1 • q = 2/3 + 2/3 – 1/3 = 1 • S = 0 • It is a proton • (b) udd • B = 1/3 + 1/3 + 1/3 = 1 • q = 2/3 – 1 /3 – 1/ 3 = 0 • S = 0 • It is a neutron • (c) uus • Ditto, B=1, q=1, S= -1 and it is a S+ • (d) dds • Ditto, B=1, q=-1, S= -1 and it is a S-

14. Some Fundamental Particles Rest energy MeV B Le L Category Particle Symbol L S Antiparticle Photon photon   0 0 0 0 0 0 Neutrino Electron Muon Tau 0 0.511 105.7 1784  e  - Leptons                +      e   Hadrons Mesons 140 135 Pion Kaon  o            +1  o         K- Ko 493.7 497.7 K+ Ko Baryons 938.3 939.6 1115.6 1189.4 1192.5 1197.3             Proton Neutron Lambda Sigma p- n L    p+ no L                See also Tipler Table 41-1 Page 1337 For strangeness, examine Figure 41-3 Page 1344 _ _ _ _ +1 +1 +1 +1 +1 +1 _ _

15. Quark spin • The angular momentum vector of a spin ½ quark can have one of two settings up or down • So a meson can have its two quark spins parallel with each other or anti-parallel: Spin 1 Spin 0

16. Quark spin contd. Spin 3/2 Spin 1/2 • Baryons e.g. uud: The spin ½ particle is a proton, spin 3/2 particle is a D+ Note that is also spin ½ (parallel, parallel, anti-parallel)

17. EIGHT FOLD WAY PATTERNS (ddu) (uud) n S = 0 p S = -1     S = -2 0  The Baryon Octet - Eight Spin 1/2 Baryons Q = +1 Q = 0 Q = -1