Classifying particles

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1. Particles Classification of Particles Hadrons and Quarks Leptons Friday, 25 November 2011

2. The Stanford linear accelerator In 1968 the Stanford accelerator shot a beam of 20 GeV electrons on a target. The results showed clearly that the electrons were strongly scattered by stationary protons and sometimes even bounced backward. q q q

3. The Stanford linear accelerator Does that remind you of another famous experiment? What can be deduced about the internal structure of the proton? The outcomes of the experiment resemble Rutherford’s experiment that led to the discovery of the nucleus. The proton must be made up of sub-nuclear particles, some of which carry negative charge. This provides proof for the existence of quarks.

4. The Structure of a Proton These results pointed at a structure of nucleons like the proton as being made up of three sub-nuclear particles called Quarks . q q q

5. Classifying Particles All particles can be classified in three main categories: Hadrons  made up of quarks. They are affected by strong forces Leptons  fundamental particles, i.e. they don’t have an internal structure. In other words, they are not made up of smaller particles and are not affected by strong forces. Quarks  smaller particles that combine to form hadrons. They carry fractional charges (fractions of the charge of the electron). So, what is everything made of? Quarks and Leptons

6. Quarks Increasing mass

7. Leptons Increasing mass

8. Hadrons Increasing mass

9. Lego Particles Up quark (+2/3) Down quark (-1/3) Strange quark (-1/3) Anti-up quark (+2/3) Anti- Down quark (-1/3) Anti- Strange quark (-1/3) Electron (-1) Positron (+1) Neutrino (0) Anti-Neutrino (0) Muon (-1) Anti- moun (+1)

10. Fundamental particles A proton is made of two Up quarks and one down. Show that the sum of the three charges gives +1. +2/3 + 2/3 – 1/3 = +3/3 = +1 A neutron is made of two down quarks and one up quark. Show the neutrality of this distribution. -1/3 – 1/3 + 2/3 = -2/3 + 2/3 = 0 u u d d u d

11. Lepton Number All leptons have an additional property called Lepton Number . The lepton number is always conserved . Particle Lepton Number All leptons +1 All anti-leptons -1 Hadrons (baryons and mesons)

12. b - Decay and lepton number Explain why both an electron and an anti-neutrino must be formed in a b - decay. The lepton number must be conserved. e - lepton no = +1 n e lepton no = -1 +1 – 1 = 0  an electron and an anti-neutrino must be created for lepton number to be conserved

13. Properties of quarks Quarks and anti-quarks have some properties that you might not have encountered before: Relative Charge  all quarks and anti-quarks carry a charge which is a fraction of the charge of the electron, i.e. 1.6 x 10 -19 C. In all interactions charge must be conserved . Baryon Number  all quarks and anti-quarks have a baryon number. The baryon number is +1/3 for quarks and -1/3 for all anti-quarks. In all interactions baryon number must be conserved . Strangeness  all quarks and anti-quarks have strangeness = 0 apart from the strange quark (strangeness = -1 ) and the anti-strange quark (strangeness = +1) . In all interactions involving the STRONG FORCE strangeness must be conserved , but in weak interactions strangeness can be conserved, or change by ±1 .

14. Properties of quarks Name Symbol Charge Baryon Number Strangeness Quarks up u +2/3 +1/3 down d -1/3 +1/3 strange s -1/3 +1/3 -1 Anti-quarks anti-up u -2/3 -1/3 anti-down d +1/3 -1/3 anti-strange s +1/3 -1/3 +1

15. Mesons and Quarks A K + meson is made up of an up quark and an anti-strange quark. Work out the relative charge, baryon number and strangeness of this particle. Charge +2/3 + 1/3 = +3/3 = + 1 Baryon no  +1/3 – 1/3 = 0 Strangeness  0 + 1 = +1 s u

16. Mesons and Quarks A p - meson is made up of an anti-up quark and a down quark. Work out the relative charge, baryon number and strangeness of this particle. Charge -2/3 - 1/3 = -3/3 = -1 Baryon no  +1/3 – 1/3 = 0 Strangeness  0 – 0 = 0 u d

17. Change of Quarks in b - Decay In a b - decay one quark in the neutron changes character (flavour) to form a proton . Complete the diagram with the correct quarks in the proton. proton neutron e - W - Before After n e A down quark in the neutron changes into an up quark, emitting an electron and an anti-neutrino. u d d u d u

18. Stable and Unstable Baryons In a b - decay one quark in the neutron changes character (flavour) to form a proton . But why does that happen? The proton is the only stable Baryon. All other Baryons eventually decay into a proton . So , it is not surprising that in “nature” the neutron decays into a proton releasing an electron and an anti-neutrino.

19. Change of Quarks in b + Decay In a b - decay one quark in the neutron changes character (flavour) to form a proton . Complete the diagram with the correct quarks in the proton and the neutron. neutron proton e + W + Before After n e An up quark in the neutron changes into an down quark, emitting a positron and a neutrino. u d u u d d

20. Conservation Laws In all particle interactions these conservation laws apply and must be fulfilled for the interaction to happen: Conservation of Charge  In all interactions charge must be conserved . So, Sum of Charges before = Sum of Charges after Conservation of Baryon Number  In all interactions baryon number must be conserved . Conservation of Strangeness  In all interactions involving the STRONG FORCE strangeness must be conserved , but in weak interactions strangeness can be conserved, or change by ±1 . Conservation of Lepton Number  In all interactions the lepton number must be conserved .

21. Conservation Laws Applying the conservation laws, show whether the following interactions are possible or not . Answer Answer Answer Answer

22. Answer This reaction can occur because: Charge is conserved  Before: After: +1 – 1 + 0 = 0 Baryon no is conserved  Before: +1 After: +1 + 0 + 0 = +1 Lepton no is conserved  Before: After: 0 + 1 - 1 = 0 Strangeness is conserved  Before: After:

23. Answer This reaction can occur because: Charge is conserved  Before: After: -1 + 1 = 0 Baryon no is conserved  Before: After: 0 + 0 = 0 Lepton no is conserved  Before: After: +1 - 1 = 0 Strangeness is conserved  Before: After:

24. Answer This reaction cannot occur because: Charge is conserved  Before: After: +1 - 1 + 0 = 0 Baryon no is conserved  Before: After: 0 + 0 + 0 = 0 Strangeness is conserved  Before: After: But, Lepton no is not conserved  Before: After: +1 + 1 = 2

25. Answer This reaction cannot occur because: Charge is conserved  Before: After: -1 + 2/3 + 1/3 = 0 But, Baryon not is conserved  Before: 1 After: 0 + 0 = 0 Strangeness is changed by +1 conserved  Before: After: +1 But, Lepton no is not conserved  Before: After: +1 + 0 = +1