dr. bill pezzaglia particle physics updated: 2010may20 modern physics series 1 rough draft
TRANSCRIPT
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Dr. Bill Pezzaglia
Particle Physics
Updated: 2010May20
Modern Physics Series1
ROUGH DRAFT
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Particle Physics(aka “high energy physics”)
A. Fundamental ForcesB. Classical ParticlesC. Nuclear ForceD. LeptonsE. More ParticlesF. Quark ModelG. The Standard Model
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A. Fundamental Forces
1) Gravity
2) Electromagnetism
3) Strong (Nuclear) Force
4) Weak Force (beta decay)
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1. Gravity & EM
• Electrical Force is infinite in range, mediated by massless “photon”. Dominates atomic/molecular
• Gravitational Force is infinite in range, mediated by massless “graviton” (unconfirmed). Dominates in the large (because macroscopic matter is neutral)
• Einstein attempts to unify these two (Unified Field Theory)
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B. Classical Particles
1) Classical period (up to 1930)
2) Spin, Pauli Exclusion Principle
3) Antimatter
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B. Classical Particles
1) Up to 1930, atoms and spectra explained by:
• Electron(1897)
• Photon (1905)
• Proton (1911)
• Neutron (1932)
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B1a. The Electron
• 1891 Stoney proposes “electron” as fundamental electric charge
• 1897 Thomson discovers the electron. Three experiments on “cathode rays”
1) deflected by magnetic field
2) Deflected by electric field
3) Measures e/m
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B1b. The Proton
• 1886 Goldstein discovers “canal rays” which move in opposite direction as “cathode rays”
• 1918 Rutherford’s experiment demonstrates small size of Hydrogen nucleus, which is 1800x more massive than electron
• Rutherford calls it the “proton” (greek word “protos” for “first”)
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B1c. The Neutron
• 1920 Rutherford proposes neutral particle in nucleus (thought it was a proton combined with electron) to explain nuclear masses (e.g. helium is mass of 4, but only has charge of +2 protons)
• 1932 Chadwick discovers neutron (Nobel prize!)
• Slightly heavier than proton; spin ½ like proton, even though it is neutral, it has a significant magnetic moment!
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B2. Spin
• 1922 Stern Gerlach Experiment shows 2 spin states
• 1924 Pauli introduces “spin” quantum number
• Pauli Exclusion principle: “fermions” (half integral spin) obey it, but “bosons” (integral spin) do not.
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B.3. Antimatter
Every particle has an “antiparticle”, which is analogous to the particle moving backwards in time
• 1927 Paul Dirac predicts “anti-electron”
• 1931 Anderson finds it (“positron”)
• 1955 Segre & Chamberlain discoverthe “antiproton” (at UCB !)
• 1956 the “anti-neutron” is discovered at UCB !
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C. Nuclear Force
1) Yukawa Potential
2) Pi Meson Prediction
3) Pion Reactions mediate nuclear force
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1. Nuclear Force and Yukawa Potential
• Electromagnetic force is mediated by the massless “photon” and has infinite range
• “Strong Force” holds the nucleus together, but has range of only about 1.5x10-15 meters.
• 1935 Yukawa proposes mediated by a massive particle, which limits range
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2 The Pion (Pi-Meson)
• Yukawa estimates mass ofparticle (“meson”) fromequating range to itsCompton wavelength
• Mass estimated to be 130 MeV
• 1947 the “pion” is discovered (140 MeV)• Has zero spin (“boson”)• Three types: + 0 -
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3. Pion Reactions
• Can change a proton to neutron, etc
+ + n p
- + p n
p p + 0
n n + 0 So, a neutron can decay to proton, emits a pi-
which is absorbed by a proton, turning into a neutron. This reaction creates an attractive force between the nucleons.
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D. Leptons and Weak Interaction
1. Three “generations” of the electron (muon, tau)
2. Three types of neutrinos
3. 1979 Electroweak Theory (1983 Vector Boson Discovery)
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E. More Particles
1. “Strange” Mesons
2. More Baryons
3. The 8 fold way
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1. More Mesons
• “Strange” Kaon particles
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2. Strange Baryons19
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Murray Gell-Mann
1969 Nobel Prize (for quark model)
1962 The 8 fold way Predicts a particle that had not yet been found
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Even More Baryons21
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F. Quark’s Model (1963)22
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2a. Quark model of Baryons23
• All Baryons made of 3 quarks (one of each “color”, so that they can all three be in the same “1s” orbital and not violate pauli exclusion principle)
• Proton is an “uud”, which adds up to plus charge
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2b. The Neutron24
• The neutron is a “udd” combination, which has net zero charge.
• The “beta” decay of a neutron into a proton is hence due to one of the “d” quarks decaying into an “u” quark
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More Quarks25
• 1974 “c” Charmed Quark
• 1977 “b” Bottom (beauty) quark
• 1995 “t” Top (truth) quark
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Charmed Baryons: Spin 1/2 26
C=+2
C=+1
C=0
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Charmed Baryons: Spin 3/2 27
C=+2
C=+1
C=0
C=+3
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2c. B Baryons (Fermilab) 28
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2c. Mesons in quark model29
• All mesons are made of a quark-antiquark pair.
• Pi plus would be “up” plus “antidown” quark
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Charmed MesonsSpin 0
Hexadecimet
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3. Gluons
1. Gluons mediate the strong force
2. They hold the quarks together
3. i.e. the ‘squiggle” between u and d quarks
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G. The Standard Model
1. Three generations
3 isospin doublets of quarks
Matches 3 generations of lepton doublets
Matches (?) 4 fundamental forces?
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Fundamental Particles and Interactions33
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3. CPT Symmetry
1951 Schwinger suggests that physics is invariant under a CPT transformation
• Parity: The laws of physics would be the same in a “mirror” universe (weak interactions violate this, neutrinos are only left circularly polarized, antineutrinos are right circularly polarized).
• Time Reversal: The laws of physics should be valid if we run the movie backwards (problems with entropy)
• Charge Conjugation: Replace all particles with their antiparticles. Is physics the same?
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Summary
• The standard model is an empirical set of rules.
• There is no theory that yet gives:• Masses of quarks• Why quarks all decay to u & d• the number of generations as being fixed to 3• why there are only 4 forces
why only mesons (qq) and baryons (qqq) are allowed.
• String Theory was our hope to produce this, but so far it has not succeeded. Many feel that this “theory of everything” is actually a “theory of nothing”.
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References/Notes36