Dr. Bill Pezzaglia

Particle Physics

Updated: 2010May20

Modern Physics Series1

ROUGH DRAFT

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”)

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

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

F. Quark’s Model (1963)22

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

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

More Quarks25

• 1974 “c” Charmed Quark

• 1977 “b” Bottom (beauty) quark

• 1995 “t” Top (truth) quark

Charmed Baryons: Spin 1/2 26

C=+2

C=+1

C=0

Charmed Baryons: Spin 3/2 27

C=+2

C=+1

C=0

C=+3

2c. B Baryons (Fermilab) 28

2c. Mesons in quark model29

• All mesons are made of a quark-antiquark pair.

• Pi plus would be “up” plus “antidown” quark

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

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