Introduction to the Standard Model

1. Introduction to the Standard Model Quarks and leptons Bosons and forces The Higgs Bill Murray, RAL, March 2002

2. Outline: • An introduction to particle physics • What is the Higgs Boson? • Some unanswered questions

3. From you to the quark Electrons orbiting nucleus d type quark u type quarks

4. The Matter Particles Proton u Mass: 1.7 10-27 kg charge: +1 u d d Neutron Charge: 0 u d Mass: 0.0005 proton mass charge: -1 e Electron Mass: ~<10-9 proton mass? Charge: 0 Neutrino 

5. How do we know about quarks? Rutherford found a nucleus in the atom by firing alpha particles at gold and seeing them bounce back Stanford Linear Accelerator Centre, California Fire electrons at protons: See big deflections! Late 1960’s

6. The particles of Matter Why 3 colours? u u ‘up’ quark u Come in 3 versions, known as colours d ‘down’ quark d d Exercise to check this later  neutrino Electron e e

7. The particles of Matter  u e d All ordinary matter is composed of these  (There is a corresponding antiparticle for each) e See Stefania’s talk later

8. The Matter particles  u  c  t e d m s t b 1st Generation Ordinary matter 2nd Generation Cosmic rays 3rd Generation Accelerators Why 3 generations?

9. The Matter particles 1.7GeV 1.7GeV 175GeV  u  c  t e d m s t b 4.5GeV 1st Generation 2nd Generation 3rd Generation 0.1GeV  E=mc2 1GeV~Proton Mass e Others are lighter

10. The Matter particles 1974 2000 1995  u  c  t e d m s t b 1977 1st Generation 2nd Generation 3rd Generation 1975 1897

11. How do quarks combine? A proton: two ‘u’ quarks and one ‘d’ quark u u d With 6 quark types there are hundreds of combinations A neutron: 2 ‘d’ quarks and 1 ‘u’ quark d u d u Mesons have a quark and an anti-quark Many created, not stable - d

12. Can we see a quark? Probably not The picture shows the result of making a pair of quarks at LEP, CERN The quarks are not seen: A jet of ‘hadrons’ is instead

13. + Forces in Ordinary Physics Classically, forces are described by charges and fields + + + Field

14. Forces in Particle Physics High energies and small distances  quantum mechanics Continuous field  exchange of quanta For Electromagnetism + + The quanta are photons, 

15. The Forces of Nature Higgs may give a link?

16. The Forces of Nature

17. Mediation of the Forces Electron At each ‘vertex’ charge is conserved. Heisenberg Uncertainty allows energy borrowing.  Positron (anti-electron) Feynman Diagram

18. Particles and forces Heavier generations have identical pattern

19. What is the Higgs boson? • The equations describing the forces and matter particles work well. • Unfortunately they demand that they all weigh nothing • We know this is not true • Prof. Higgs proposed an addition which corrects this. The Standard Model Together Known as…

20. The Waldegrave Higgs challenge In 1993, the then UK Science Minister, William Waldegrave, issued a challenge to physicists to answer the questions: 'What is the Higgs boson, and why do we want to find it?’ on one side of a sheet of paper. David Miller of UCL won a bottle of champagne for the following:

21. The Waldegrave Higgs challenge Imagine a room full of political activists

22. The Waldegrave Higgs challenge The Prime Minister walks in

23. The Waldegrave Higgs challenge He is surrounded by a cluster of people Analogous to generation of Mass

24. The Waldegrave Higgs challenge Imagine the same room again

25. The Waldegrave Higgs challenge A interesting rumour is introduced

26. The Waldegrave Higgs challenge Thanks to D. Miller and CERN © PhotoCERN Soon we have a cluster of people discussing it Analogous to Higgs boson

27. What does Higgs theory imply? Higgs’ mechanism gives mass to W and Z bosons, and to the matter particles. Mass of the W predicted We can check it It also predicts one extra particle: The Higgs boson The Higgs Boson mass is not predicted

28. The W, the top quark and Higgs • We can calculate the mass of the W boson • Need the mass of the Zand the strength of the forces; these are well known • It is also affected by: • Top quark mass: Weak effect • Higgs mass: Tiny effect

29. The W, the top quark and Higgs The W mass is about 80GeV/c2

30. The W, the top quark and Higgs Expanded scale The W mass depends upon the top and Higgs masses

31. CERN’s Collider ring 2 LHC experiments 4 LEP experiments: LHC : pp, Ecms~ 14000 GeV LEP : e+e-, Ecms~ 210 GeV DELPHI CERN © PhotoCERN

32. What are LEP and LHC? Work started 1980’s Complementary machines – but needing the same tunnel

33. In the LEP tunnel 27km of vacuum pipe and bending magnets © PhotoCERN

34. Now to be the LHC tunnel 27km of vacuum pipe 8.3Tesla bending magnets, 3o above absolute zero © PhotoCERN

35. One experiment: ‘OPAL’ One of four rather similar detectors © PhotoCERN Assembly in 1989 Note the people

36. An LHC experiment: ATLAS © PhotoCERN Note the people In construction

37. Z studies at LEP From 1989 to 1995 LEP created 20,000,000 Z bosons Rate n These were used for detailed studies of its properties Here you see the analysis which established the number of neutrinos as 3 Peak at Z mass, 91GeV They can say something about the Higgs too.

38. Feynman Diagram for Z production Electron Z Cannot see the Z Only its decay products Positron

39. Indirect Search for the Higgs Boson Properties of the Z boson changed by ‘loop’ effects: top Basic Feynman diagram Higgs What is affected? Z decay rate to b’s - sensitive to top mass Angular distributions - sensitive to W & H mass

40. Indirect Search for the Higgs Boson 0.1% Precision needed! top • By carefully studying Z’s we: • Predict mtop and mW; • Compare with measurements; • Predict mH; • Compare with measurements. Higgs

41. How to recognize Z decays: - e) Z->nn - Z  qq:Two jets, many particles Z  e+e-, m+m-:Two charged particles (e or .) Z  t+t-: Each t gives 1 or 3 tracks

42. How to recognize Z decays: - e) Z->nn (weak and slow decays to lighter quarks) Z  nn: Not detectable. - - - Z  bb:Like qq events, with detached vertices, measured in accurate vertex detectors

43. One example Z distribution: Zm+m- Angular distributions • This distribution depends on the W mass DELPHI 1993-1995 46GeV Rate of occurrence Many things are used: Z mass, Several angular distributions, Z decay fraction to bb 47GeV 45GeV Cos(q) of outgoing m-

44. The W, the top quark and Higgs The W and top masses from Z studies agree with theory i.e. they lie on the curves Result from Z studies They can be checked by direct measurement

45. The W, the top quark and Higgs Completely consistent! W mass from LEP and Tevatron This suggested the top quark had a mass of 175GeV before it had been discovered

46. The W, the top quark and Higgs Again, incredible consistency Top mass from Tevatron The top mass directly measured agrees completely with the predicted one

47. The W, the top quark and Higgs The data (especially if they are averaged) suggest a Higgs mass around 100GeV 10GeV 100GeV Scale has been expanded further 1000GeV This procedure worked for the top quark. Will it work again?

48. Summary of model • W mass agrees with Higgs theory • to 1 part in 1000 • Electro-weak corrections verified: • W mass agrees with prediction • Top mass agrees with prediction • Higgs mass should be:

49. The Search for the Higgs • In the late 1990’s ‘LEP’ at CERN ran with enough energy to make W pairs • There was also hopes it might make a Higgs.

50. The W pairs: W’s produced by reactions like this one Each W decays in ~10-26 seconds Into leptons (3 sorts) e-, m-, t- Or quarks (2 sorts) nenm nt