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PARTICLE PHYSICS

PARTICLE PHYSICS. Exploring the world of the small Dr Emily Nurse. The structure of atoms. electron. ‘up’ and ‘down’ quarks. nucleus (10 -14 m). proton/neutron (10 -15 m). atom (10 -10 m). Matter Particles :. increasing mass.

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PARTICLE PHYSICS

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  1. PARTICLE PHYSICS Exploring the world of the small Dr Emily Nurse

  2. The structure of atoms electron ‘up’ and ‘down’ quarks nucleus (10-14 m) proton/neutron (10-15 m) atom (10-10 m)

  3. Matter Particles: increasing mass interact via forces, carried by more particles:

  4. For every matter particle there exists an antimatter particle with the same mass But opposite charge. electron positron quark  antiquark proton antiproton

  5. Quarks and jets Quarks carry a colourcharge{cf. electric charge} that the strong force acts on. The further apart two colour charges are the greater the attraction so that quarks are confined into colourless objects known as hadrons. Attempting to separate quarks creates a jet of hadrons.

  6. A mathematical theory called the Standard Model describes these particles and their interactions. The Standard Model is very successful. It has predicted particles before they are discovered! The top quark was predicted in 1977 but not discovered until 1995! This late discovery is due to its HUGE mass… …50,000 times heavier than the up quark! The hunt is now on for the remaining particle in the Standard Model… The Higgs boson.

  7. How do we research particle physics? Particles are accelerated and smashed together at huge energies in particle accelerator machines. Many different particles are created. The higher the energy the more massive the particles can be. The particles are detected via their interactions with matter in a particle detector. The aim is to search for new particles and measure the properties (e.g. masses) of existing particles to test Standard Model predictions.

  8. Situated near Chicago, USA Home to the Tevatron collider

  9. The Tevatron(the world’s highest energy collider) p p 6 km circumference collides protons with antiprotons (accelerates them through 1, 000,000,000,000 volts!)

  10. _ + bending magnet p _p _p p

  11. Many different processes occur when the proton and antiproton collide surround the interaction point with a particle detector.

  12. _ p charged particle detector p e- magnet coil  + calorimeter muon detector

  13. The DØ detector

  14. Magnet Calorimeter Tracking Volume South Endcap

  15. The DØ Detector in Collision Hall, January 2001

  16. The protons and antiprotons collide with each other and produce more particles. P P e.g. Z Heavy particles tend to be unstable … they decay to lighter particles. e.g. e+ Z e-

  17. This process can be represented in a Feynman diagram:

  18. Unstableparticlescannot be directly detected if their lifetime is much less than the time it takes to traverse the detector. • Their existence is inferred by detecting theirdecay products. • Themassof anunstable particleis found from theenergyandmomenta of its decayproducts.

  19. Relativity • The velocity of an object can only be measured relativeto another object (e.g. a car travels at 30 mph relativeto the Earth). • A particles momentum and energy depends on its frame of reference. • A property called the invariant mass is the same whatever the frame of reference: M 2 = ( ∑ E )2 - ( ∑ p )2 • In the particles rest frame: M ≡ mass Note: units of c=1

  20. Your Task You will be looking at the production and decay of: (1) the Z boson (2) the W boson (3) the top quark You will measure the mass of these particles using the energy and momenta of their decay products.

  21. Z production and decay: The Z can also decay to other pairs of particles: e+e-µ+µ-τ+τ- νeνeνµνµντντ uu dd cc ss bb

  22. Z  +-candidate

  23. Z  e+e- candidates DØ

  24. W± bosons are also produced: The W+ can also decay to other pairs of particles: e+νeµ+νµτ+ντ ud us ub cd cs cb

  25. After a collision the sum of the momentum in the direction transverse to the beam must add up to zero. Added complication…. neutrinos are not detected! before collision after collision beam into wall

  26. We can find the transverse momentum and energy of the neutrino by measuring the total missing transverse energy! Define the transverse invariant mass: MT2 = ( ∑ ET )2 - ( ∑ pT )2 (MT is less than or equal to M)

  27. e ET W e candidates DØ EM cluster with track

  28. Top quarks are also produced: The W+ and W- can also decay to other pairs of particles.

  29. The mass of the top (t) quark can be found from the e+, ν and the bottom (b) jet. The mass of the antitop (t) quark can be found from the antibottom (b) jet and the two remaining jets.

  30. Task recap You will be looking at the production and decay of: (1) the Z boson (2) the W boson (3) the top quark. You will measure the mass of these particles using the energy and momenta of their decay products.

  31. Physics beyond the Standard Model… Supersymmetry? The answer to the Universe’s missing matter?

  32. More dimensions than meet the Is the World 11 dimensional ?? YOU can help to find out !

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