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Lecture 12 – Asymptotic freedom and the electrodynamics of quarks

Lecture 12 – Asymptotic freedom and the electrodynamics of quarks. Asymptotic freedom and the running of coupling ”constants”. Testing the theory of the quarks and strong force in e - e + reactions. The theory of the strong force QCD. q. q. q. q. What is the charge of a particle ? .

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Lecture 12 – Asymptotic freedom and the electrodynamics of quarks

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  1. FK7003 Lecture 12 – Asymptotic freedom and the electrodynamics of quarks • Asymptotic freedom and the running of coupling ”constants”. • Testing the theory of the quarks and strong force in e- e+reactions.

  2. FK7003 The theory of the strong force QCD q q q q

  3. FK7003 What is the charge of a particle ?

  4. FK7003 Screening in a dielectric qeff q/er r Intermolecular separation The effective charge increases at small distances

  5. FK7003 Vacuum polarisation e- e- e- e- e+,e- + e+,e- e+,e- e- e- e- e-

  6. FK7003 Screened charges

  7. FK7003 The electromagnetic coupling Barely changes (Momentum transfer=Q)2 /GeV2 Interaction distance

  8. FK7003 What about the strong force ? q q q q q q q q q,q + other higher order diagrams q,q q,q q q q q q q q q

  9. FK7003 Asymptotic freedom as Q Momentum transfer, Q (GeV) Interaction distance

  10. FK7003 Running of the coupling ”constants” Similar story for weak force. If interactions occur over distance scales which suppress screening The three forces would be of (roughly) the same size Predicted behaviour (tested with measurements at lower energies) Measurements The coupling strengths of the strong,electromagnetic, and Weak forces converge at higher energies. Is this evidence that they are part of a single force ? Later lecture.

  11. FK7003 Consequences

  12. FK7003 Scaling violations Described by QCD over several orders of magnitude in Q2.

  13. FK7003 OZI suppression In OZI-suppressed processes, the gluons carry all of the momenta of some or all of the final particles and there is consequently a lower probability of emitting such ”hard” gluons in comparison with ”soft” gluons in non-OZI suppressed reactions.

  14. FK7003 Testing what we know about the quarks and their interactions

  15. FK7003 Ratio of hadronic to muonic production R R Centre-of-mass energy ECM (GeV)

  16. FK7003 Taking a closer look with correction w/o correction

  17. FK7003 ”Observing” gluon emission jet jet jet jet jet jet

  18. FK7003 Summary • Vacuum polarisation makes the couplings of the fundamental forces ”run” with energy • Asymptotically free quarks! • At high energies QCD makes precise calculations • Scaling violations • 3 jet events • Tested our picture of quarks, gluons and the strong force on e- e+interactions.

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