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Phys 174

Phys 174. High Energy Physics. Discovery of New Particles. New particles Beginning in 1945, many new particles were discovered in experiments involving high-energy collisions Characteristically unstable with short lifetimes Over 300 have been cataloged

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Phys 174

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  1. Phys 174 High Energy Physics

  2. Discovery of New Particles • New particles • Beginning in 1945, many new particles were discovered in experiments involving high-energy collisions • Characteristically unstable with short lifetimes • Over 300 have been cataloged • A pattern was needed to understand all these new particles

  3. Elementary Particles – Quarks • Physicists recognize that most particles are made up of quarks • Exceptions include photons, electrons and a few others • The quark model has reduced the array of particles to a manageable few • Protons and neutrons are not truly elementary, but are systems of tightly bound quarks

  4. Fundamental Forces • All particles in nature are subject to four fundamental forces • Strong force • Electromagnetic force • Weak force • Gravitational force • This list is in order of decreasing strength

  5. Forces and Mediating Particles

  6. Antiparticles • Every particle has a corresponding antiparticle • From Dirac’s version of quantum mechanics that incorporated special relativity • An antiparticle has the same mass as the particle, but the opposite charge • The positron (electron’s antiparticle) was discovered by Anderson in 1932 • Since then, it has been observed in numerous experiments • Practically every known elementary particle has a distinct antiparticle • Among the exceptions are the photon and the neutral pi particles

  7. Pair Production, cont • A photograph of pair production produced by 300 MeV gamma rays striking a lead sheet • All particles have antiparticles

  8. Annihilation • The reverse of pair production can also occur • Under the proper conditions, an electron and a positron can annihilate each other to produce two gamma ray photons e- + e+ 2γ

  9. Richard Feynman • 1918 – 1988 • Developed quantum electrodynamics • Shared the Noble Prize in 1965 • Worked on Challenger investigation and demonstrated the effects of cold temperatures on the rubber O-rings used

  10. Feynman Diagram –Weak Interaction • An electron and a neutrino are interacting via the weak force • The Z0 is the mediating particle • The weak force can also be mediated by the W± • The W± and Z0 were discovered in 1983 at CERN

  11. Classification of Particles • Two broad categories • Classified by interactions • Hadrons – interact through strong force • Leptons – interact through weak force

  12. Hadrons • Interact through the strong force • Two subclasses distinguished by masses and spins • Mesons • Decay finally into electrons, positrons, neutrinos and photons • Integer spins (0 or 1) • Baryons • Masses equal to or greater than a proton • Half integer spin values (1/2 or 3/2) • Decay into end products that include a proton (except for the proton) • Not elementary, but composed of quarks

  13. Leptons • Do not interact through strong force • Do participate in electromagnetic (if charged) and weak interactions • All have spin of 1/2 • Leptons appear truly elementary • No substructure • Point-like particles

  14. Leptons, cont • Scientists currently believe only six leptons exist, along with their antiparticles • Electron and electron neutrino • Muon and its neutrino • Tau and its neutrino • Neutrinos may have a small, but nonzero, mass

  15. Bubble ChamberExample of Strange Particles • The dashed lines represent neutral particles

  16. Creating Particles • Most elementary particles are unstable and are created in nature only rarely, in cosmic ray showers • In the laboratory, great numbers of particles can be created in controlled collisions between high-energy particles and a suitable target

  17. Murray Gell-Mann • 1929 – • Studies dealing with subatomic particles • Named quarks • Developed pattern known as eightfold way • Nobel Prize in 1969

  18. The Eightfold Way • Many classification schemes have been proposed to group particles into families • These schemes are based on spin, baryon number, strangeness, etc. • The eightfold way is a symmetric pattern proposed by Gell-Mann and Ne’eman • There are many symmetrical patterns that can be developed • The patterns of the eightfold way have much in common with the periodic table • Including predicting missing particles

  19. An Eightfold Way for Baryons • A hexagonal pattern for the eight spin 1/2 baryons • Stangeness vs. charge is plotted on a sloping coordinate system • Six of the baryons form a hexagon with the other two particles at its center

  20. An Eightfold Way for Mesons • The mesons with spins of 0 can be plotted • Strangeness vs. charge on a sloping coordinate system is plotted • A hexagonal pattern emerges • The particles and their antiparticles are on opposite sides on the perimeter of the hexagon • The remaining three mesons are at the center

  21. Eightfold Way for Spin 3/2 Baryons • The nine particles known at the time were arranged as shown • An empty spot occurred • Gell-Mann predicted the missing particle and its properties • About three years later, the particle was found and all its predicted properties were confirmed

  22. Quarks • Hadrons are complex particles with size and structure • Hadrons decay into other hadrons • There are many different hadrons • Quarks are proposed as the elementary particles that constitute the hadrons • Originally proposed independently by Gell-Mann and Zweig

  23. Original Quark Model • Three types or flavors • u – up • d – down • s – strange • Associated with each quark is an antiquark • The antiquark has opposite charge, baryon number and strangeness • Quarks have fractional electrical charges • -1/3 e and +2/3 e • Quarks are fermions • Half-integral spins

  24. Original Quark Model – Rules • All the hadrons at the time of the original proposal were explained by three rules • Mesons consist of one quark and one antiquark • This gives them a baryon number of 0 • Baryons consist of three quarks • Antibaryons consist of three antiquarks

  25. Quark Composition of Particles – Examples • Mesons are quark-antiquark pairs • Baryons are quark triplets

  26. Additions to the Original Quark Model – Charm • Another quark was needed to account for some discrepancies between predictions of the model and experimental results • A new quantum number, C, was assigned to the property of charm • Charm would be conserved in strong and electromagnetic interactions, but not in weak interactions • In 1974, a new meson, the J/Ψwas discovered that was shown to be a charm quark and charm antiquark pair

  27. More Additions – Top and Bottom • Discovery led to the need for a more elaborate quark model • This need led to the proposal of two new quarks • t – top (or truth) • b – bottom (or beauty) • Added quantum numbers of topness and bottomness • Verification • b quark was found in a Y- meson in 1977 • t quark was found in 1995 at Fermilab

  28. Numbers of Particles • At the present, physicists believe the “building blocks” of matter are complete • Six quarks with their antiparticles • Six leptons with their antiparticles

  29. Particle Properties

  30. LHC at CERN 27 km circumference 14 years to build $8 billions E =5TeV

  31. Old Days.

  32. More About Quarks • No isolated quark has ever been observed • It is believed that at ordinary temperatures, quarks are permanently confined inside ordinary particles due to the strong force

  33. Color • There is an additional property called the color charge • The color has nothing to do with the visual sensation from light, it is simply a name

  34. Colored Quarks • Color “charge” occurs in red, blue, or green • Antiquarks have colors of antired, antiblue, or antigreen • These are the quantum “numbers” of color charge • A combination of quarks of each color produces white (or colorless) • Baryons and mesons are always colorless

  35. Quantum Chromodynamics (QCD) • QCD gave a new theory of how quarks interact with each other by means of color charge • The strong force between quarks is often called the color force • The strong force between quarks is mediated by gluons • Gluons are massless particles • When a quark emits or absorbs a gluon, its color may change

  36. Quark Structure of a Meson • A green quark is attracted to an antigreen quark • The quark – antiquark pair forms a meson • The resulting meson is colorless

  37. Quark Structure of a Baryon • Quarks of different colors attract each other • The quark triplet forms a baryon • Each baryon contains three quarks with three different colors • The baryon is colorless

  38. QCD Explanation of a Neutron-Proton Interaction • Each quark within the proton and neutron is continually emitting and absorbing gluons

  39. Elementary Particles – A Current View • Scientists now believe there are three classifications of truly elementary particles • Leptons • Quarks • Field particles • These three particles are further classified as fermions or bosons • Quarks and leptons are fermions • Field particles are bosons

  40. Weak Force • The weak force is believed to be mediated by the W+, W-, and Z0 bosons • These particles are said to have weak charge • Therefore, each elementary particle can have • Mass • Electric charge • Color charge • Weak charge • One or more of these charges may be zero

  41. Electroweak Theory • The electroweak theory unifies electromagnetic and weak interactions • The theory postulates that the weak and electromagnetic interactions have the same strength when the particles involved have very high energies • Viewed as two different manifestations of a single unifying electroweak interaction

  42. The Standard Model • A combination of the electroweak theory and QCD for the strong interaction form the standard model • Essential ingredients of the standard model • The strong force, mediated by gluons, holds the quarks together to form composite particles • Leptons participate only in electromagnetic and weak interactions • The electromagnetic force is mediated by photons • The weak force is mediated by W and Z bosons • The standard model does not yet include the gravitational force

  43. The Standard Model – Chart

  44. Particle Paths After a Collision

  45. The Big Bang • This theory states that the universe had a beginning, and that it was so cataclysmic that it is impossible to look back beyond it • Also, during the first few minutes after the creation of the universe all four interactions were unified • All matter was contained in a quark-gluon plasma • As time increased and temperature decreased, the forces broke apart

  46. A Brief History of the Universe

  47. Hubble’s Law • The Big Bang theory predicts that the universe is expanding • Hubble claimed the whole universe is expanding • Furthermore, the speeds at which galaxies are receding from the earth is directly proportional to their distance from us • This is called Hubble’s Law

  48. Hubble’s Law, cont • Hubble’s Law can be written as v = H R • H is called Hubble’s constant • H » 17 x 10-3 m / s ly

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