Physics 32: Elementary Particles

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Physics 32 Elementary Particles

• Christopher Chui

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High-Energy Particles-1

• Matter-wave de Broglie wavelength, l h/(mv)
• Van de Graaff generator and accelerator produce
  30 MeV to accelerate particles. A source of H or
  He ions is accelerated towards the target
• Cyclotron uses a magnetic field to maintain
  charged ions, usually protons, in circular paths
• Force due to magnetic field qvB ma mv2/r
• Period T distance/speed 2 pr/(qBr/m) 2
  pm/qB
• A modified cyclotron (synchrocyclotron) allows
  complex electronics to decrease the frequency as
  the protons increases in speed to reach larger
  orbits

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High-Energy Particles-2

• Synchrotron increases the magnetic field B as the
  particles speed up. Machines in Fermilab and CERN
  accelerate protons to energies of 500 GeV
• Tevatron at Fermilab uses superconducting magnets
  to accelerate protons to 1000 GeV 1 TeV
• Synchrotron radiation causes energy loss
• Linear accelerators accelerate electrons to 50
  GeV
• Colliding beams allow two beams of particles to
  collide head-on using storage rings up to 186 GeV

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Yukawa Particle

• In 1935 Yukawa predicted the existence of a new
  particle (meson) to mediate the strong nuclear
  force
• For EM force between two particles, photons are
  exchanged to give rise to a force
• Feynman diagram shows a photon acting as the
  carrier of the EM force between two electrons
• Strong nuclear force mc2 hc/(2pd)
• In 1947 meson was discovered in p, p-, and p0
• Reactions pp?pp p0 and pp?pn p
• Particles presumed to transmit weak force W,
  W-, and Z0 were detected in 1983
• The quantum of gravitational force, graviton, has
  not been found

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Four Forces in Nature

• TYPE Relative Strength Field Particle
• Strong nuclear 1 Gluons
• Electromagnetic 10-2 Photon
• Weak nuclear 10-6 W, W-, Z0
• Gravitational 10-38 Graviton (?)

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Stable Particles under Strong Decay

• Category Name Symbol Antiparticle Spin Lifetime
  
• Leptons electrons e- e ½ stable
• neutrino e ne ne ½ stable
• muons m- m ½ 2.2x10-6
• neutrino m nm nm ½ stable
• tau t- t ½ 2.9X10-13
• neutrino t nt nt ½ stable
• Hadrons(mesons) pion p p- 0 2.6x10-8
• kaon p 0 self 0 0.84x10-16
• eta h0 self 0 5x10-19
• Baryons proton p p ½ stable
• neutron n n ½ 887
• G-bosons Photon g Self 1 stable
• W W W- 1 3x10-25
• Z Z0 self 1 3x10-25

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Stability, Resonance, and Strange

• Lifetime depends on which force is acting
• Very short-lived particles are inferred from
  their decay products
• Resonance refers to a large peak in the KE of
  particle bombardment, such as the D particle
• Strange particles refer to reactions having twice
  as long half-lives as would be expected
• Strangeness is conserved in strong interactions
  but not in weak

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Quarks and Antiquarks

• Proton 2 ups 1 down 2/3 e 2/3 e 1/3 e
• Neutron 1 up 2 downs 2/3 e 1/3 e 1/3 e
• Meson pion 1 u 1 antidown 2/3 e 1/3 e
• Antipion 1 antiup 1 down -2/3 e 1/3 e
• Antikaon 1 antiup 1 strange -2/3e 1/3e
• Charmed quark has 2/3 e and C1
• Top and bottom quarks are 3rd generation quarks
• Electrons, protons, and neutrons are fermions
  (1/2 spin) for structure of matter all others
  are called bosons (integer spin) for carriers of
  forces

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QCD, and Electroweak Theory

• Quarks have 6 flavors u, d, s, c, b, t
• Quarks might have color charge
• Strong force between quarks is color force
• QCD is the force that acts between color charges
• Strong force between 2 hadrons is a force between
  the quarks are called gluons
• Color force increases with increasing distance!
• When 2 quarks approach each other, the force
  between them becomes very smallasymptotic
  freedom
• Gauge theory attempts to unify the weak and
  electromagnetic interactions, which are seen as 2
  diff manifestations of a single electroweak
  interaction

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GUT

• GUT attempts to unify electroweak theory into QCD
• On a scale of 10-30 m, the EM, weak, and strong
  forces appear to work on 1 class of
  particleleptons and quarks baryons and lepton
  numbers would not be conserved
• Lepton could become a quark through X bosons with
  a mass of 1014 GeV/c2 or 1014 times heavier than
  a proton
• String theory attempts to unify all 4 forces of
  nature into a single theory
• When supersymmetry is applied?superstring theory
• Superstring theory predicts fermions would change
  into bosons, and all quarks have squark partners,
  and all leptons have slepton partners
  bosons?photinos and gluions. All these have not
  been detected yet