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Elementary particles

Elementary particles. Spring 2005, Physics 123. Concepts . antimatter leptons quarks fundamental interactions. Mass and energy. Mass and energy are interchangeable Energy can be used to create mass (matter) Mass can be destroyed and energy released. Mass is energy:.

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Elementary particles

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  1. Elementary particles Spring 2005, Physics 123 Lecture XXV

  2. Concepts • antimatter • leptons • quarks • fundamental interactions Lecture XXV

  3. Mass and energy • Mass and energy are interchangeable • Energy can be used to create mass (matter) • Mass can be destroyed and energy released Lecture XXV

  4. Mass is energy: Energy, mass and momentum • vc: • Energy –momentum - mass • v=c if and only if m0=0 • Mg=0 • Units for mass • Units for momentum Lecture XXV

  5. Particle acceleration • Electric field is used to accelerate the elementary particles and thus increase their energy • Energy is conserved, because particles receive their energy from the electric field • Energy of accelerated particles can be used to produce new particles (matters) Lecture XXV

  6. Particle acceleration RF cavities Lecture XXV

  7. Particle accelerators • Fermilab • 40 miles west of Chicago • Tevatron – at the moment world’s highest energy collider • 1 TeV proton beam collides with 1 TeV antiproton beam • 6.28 km circumference • Top quark discovery - 1996 Lecture XXV

  8. Large Hadron Collider (LHC) • Next collider – LHC - is built in Europe, operational 2008 • 27 km; • 14 Tev - LHC will discover Higgs if it exists. • Two high PT experiments _CMS and Atlas Lecture XXV

  9. Magnetic fields are used to separate positive from negative And measure particle velocity “Mustache” = matter – antimatter pairs Bubble chamber picture Lecture XXV

  10. Detecting particles • Tracking charged particle in magnetic field - p • Calorimeter – collect all energy, energy loss  light • The only particle that can survive calorimeter material – muon • Calorimeter is followed by another set of tracking devices – muon chambers Lecture XXV

  11. Tracking: connecting the dots 40 cm Lecture XXV

  12. Nature’s scales Antimatter Lecture XXV

  13. Matter = fermions (s=1/2) • All fermions interact gravitationally and weakly. • All charged particles interact electromagnetically. • Only quarks interact strongly For each fermion there exists an antiparticle with opposite electric charge Lecture XXV

  14. Periodic table of forces S=1 S=2 4 fundamental forces – others combinations of these. Lecture XXV

  15. Fundamental interactions and Feynman diagrams • Gauge bosons (photon, W, Z, gluon, graviton) mediate fundamental interactions • Example: photon – quantum of EM field • Electron knows about the presence of another electron through EM field. In quantum language – through exchange of photons: e- e- g e- e- Lecture XXV

  16. Periodic table of matter and forces Matter: Forces • 1st generation – enough to build the Universe • Why 3 generation? • Mass hierarchy? • Why top is so heavy? Gravity g • EM and weak unified • Why M(g)=0 M(W)=80GeV, M(Z)=90 GeV? –Electro Weak Symmetry Breaking W,Z Lecture XXV

  17. Higgs boson – generator of mass • Theoretical hypothesis: • Space is saturated with bosonic field (Higgs, s=0) with nonzero vev; • W, Z bosons absorb a component of this field and gain mass, while photon does not and remains massless • fermions acquire mass through interaction with Higgs boson. • Analogy – popular person in a party (massive particle) attracts a lot of people (Higgs boson) thus effectively gaining mass. • To test the hypothesis – find Higgs Lecture XXV

  18. Higher generations – heavier replicas of the first generation • Muon discovered in 1930’s • Mass =105 MeV/c2 • Was a big surprise – first hint of extra generations • Particles of higher generations decay into particles of lower generation I.I. Rabi Lecture XXV

  19. Top production • Statistics up to now : • 600 pb-1 3x1013collisions • 4200 top pairs produced Lecture XXV

  20. Top ID in “lepton+jets” channel • Fingerprint of top pair production: • 2 b-jets • Lepton: electron or muon • Neutrino (from energy imbalance) • 2 q’s – transform to jets of particles Lecture XXV

  21. Top event Lecture XXV

  22. Meson = combination of quark and antiquark: Spin s=0 Spin s=1 Baryons = combination of 3 quarks Spin s=1/2 Spin s=3/2 Hadrons = composite quark states Lecture XXV

  23. Conservation laws • Electric charge • Energy and momentum • Number of leptons and baryons (antilepton = -1, antibaryon = -1) Energymass Mass energy Lecture XXV

  24. Decays • Z-boson • e+e- 3.36% • m+m- 3.36% • t+t- 3.36% • uubar 10.1% • ccbar 10.1% • ddbar 16.6% • ssbar 16.6% • bbbar 16.6% • All neutrinos 20% Z Lecture XXV

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