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Particle Physics

Particle Physics. James Stirling Institute for Particle Physics Phenomenology University of Durham. What Is Particle Physics?. We aim to answer the question: “ What is the world made of? ” Want to know the basic particles Want to know the forces that hold them together.

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Particle Physics

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  1. Particle Physics James Stirling Institute for Particle Physics Phenomenology University of Durham

  2. What Is Particle Physics? • We aim to answer the question: “What is the world made of?” • Want to know the basic particles • Want to know the forces that hold them together

  3. What Are the Basic Particles? • How far can we keep breaking matter into smaller pieces? • …forever?

  4. Periodic Table • Soon get to Elements • In 1860’s Mendeleev arranged the elements by property into Periodic Table • Surely too many to all be fundamental? • Gaps in the table – predicted new elements • Deduce an underlying structure - Atoms

  5. Are Atoms Fundamental? • Around 100 years ago atoms thought to be “indivisible balls” • Rutherford took large gold atoms and fired radioactive particles at them • He found that most of the particles went straight through • BUT occasionally some did bounce back…

  6. A New Picture of the Atom • Atom is dense nucleus surrounded by cloud of electrons • Now understand experiment • Most of atom is empty space !

  7. Is the Nucleus Fundamental? • Do similar experiment to Rutherford • Fire nuclei at each other… • Find nucleus made of smaller particles, protons and neutrons

  8. In the 1960s physicists began to collide protons together To their horror, found LOTS of particles –S, L, X ,… Is there a pattern? Can we deduce some underlying structure? Is the Proton Fundamental?

  9. Finding Patterns • Like Mendeleev, group particles with similar properties together • Patterns  Substructure • In 1964 Murray Gell-Mann suggested that the hundreds of particles found could all be made of just three quarks* • He called them up, down and strange *the word is from Finnegan’s Wake by James Joyce

  10. Quarks and electrons are fundamental As far as we can tell no further substructure Model of Atom Today

  11. For very large or very small numbers it is easier to use powers of ten, e.g. 100,000,000 = 108 0.000000000000001 = 10-15 Note: 1 = 100 Powers of Ten

  12. Molecule 10-9 m Atom 10-10 m Nucleus 10-14 m Proton 10-15 m Quark < 10-18 m How Small Is A Particle?

  13. How Small Is A Particle?

  14. We’re Not Finished Yet! • In 1930s beta decay was not understood – there was missing energy • Pauli grudgingly introduced a new particle to account for this – the neutrino • The electron and neutrino are collectively called leptons

  15. Antimatter Too! • In 1932 Anderson discovered the positron • For every particle there is also an antiparticle • Antimatter just like ordinary matter but with opposite charge • electron negative, positron positive!

  16. muon (-) electron (e-) positron (e+) E=mc2 ! antimuon (+) Creating New Particles

  17. The Fundamental Particles • There are only 12 fundamental particles of matter (also the antiparticles)

  18. Discovery Timeline • 1897 electron • 1933 neutrino • 1937 muon • 1964 up, down, strange • 1974 charm • 1975 tau • 1977 bottom • 1996 top

  19. What About Forces? • Particles interact with each other via forces • There are four types –gravity, electromagnetism and the strong and weak nuclear forces • Each is described by the exchange of a force-carrying particle between the matter particles

  20. The Four Forces of Nature

  21. How Do We Know All This? • All our experiments involve colliding particles • Matter and antimatter are accelerated to near light-speed by an accelerator • They are brought together inside a detector and collide • They annihilate and their energy is released to create new particles according to E=mc2

  22. muon (-) electron (e-) positron (e+) antimuon (+) Creating New Particles

  23. The LEP Accelerator at CERN

  24. The LEP Accelerator at CERN

  25. How Detectors Work A detector does various jobs… • Track the positions of particles • Measure energy of particles • Detector layered like an onion • Each layer measures different things

  26. The OPAL Detector at CERN

  27. A View Inside

  28. In the inner layers magnetic fields are used to find the charge and momentum of particles In the outer layers the energy of the particles are measured Finding Information

  29. A Picture From a Detector

  30. Summary • There are 12 fundamental matter particles, the Quarks and Leptons • There are 4 forces transmitted by particles • Accelerators are used to collide and study particles

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