a brief history of modern physics
TRANSCRIPT
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A Brief History of Modern PhysicsA Brief History of Modern PhysicsModern Physics rests on two pillars:
1. Theory of Relativity (Einstein)
Special Relativity 1905
General Relativity 1915→ nature of space and time (phenomena at high speed)
→ gravity as a result of curved spacetime
2. Quantum Mechanics (Bohr, Heisenberg, Schrödinger, …)
~1900 - 1925
phenomena at very short distance scales→ structure of the atom
→ behavior of light, subatomic particles
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What is Particle Physics About?What is Particle Physics About?
Experiments can address long standing puzzles / questions:
• What are the fundamental constituents of matter?
• What are the fundamental forces between elementary particles?
• Can the forces of nature be unified? Including gravity?
• What is the origin of mass?
• What is the origin of the matter-antimatter asymmetry of the Universe?
• What is dark matter?
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Constituents of Matter (I)Constituents of Matter (I)
Thomson Model of the Atom (early 1900’s)
• electrons are embedded in homogeneous positively charged mass“raisins in plum pudding”
diffuse positive charge
Note: protons not yet discovered
in early 1900’s
Problems:
Emission lines cannot be explained
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Constituents of Matter (II)Constituents of Matter (II)• How can we probe the structure of the atom?→ Perform scattering experiments with high energy particles
with de Broglie wavelength
• Rutherford Scattering Expts (1910)
Projectiles:
α particles (He nucleus)produced in radioactive decays
e.g. 232Th → 228Ra + α
Kinetic energy of α particle
K = 4 MeV ⇒ λ ≈ 10-14 m
Observations: ~1 in 104 α particles is back scattered
⇒ Large angle deflections are due to nearly head-on
collisions between the α particles and a very small and dense nucleus
hp
λ =
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Constituents of Matter (III)Constituents of Matter (III)• Late 1960’s: repeat of Rutherford expt at huge particle accelerators like
the 2-mile long linac at the Stanford Linear Accelerator Center (SLAC)
Projectiles: linear accelerator takes electrons from rest to K = 50 GeV⇒ de Broglie wavelength = 2.5 x 10-17 m
⇒ moving close to the speed of light: v = 0.999 999 999 95 c
⇒ Electrons do not see 2 mile-long linac but a contracted length of only 1 in.!
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Constituents of Matter (IV)Constituents of Matter (IV)Scattering experiments at SLAC established the existence ofquarks as fundamental constituents of protons and neutrons→ What do we currently know about the structure of matter?
Atom = bound system of positive nucleus + orbiting electrons ~ 10-10 m
Nucleus = bound system of protons + neutrons (nucleons) ~ 10-15 m
Nucleons = bound system of up and down quarksQuarks = ? no known structure down to < 10-18 m
Name Spin Charge
up (u) ½ +⅔ e
down (d) ½ −⅓ e mass
strange (s) ½ −⅓ e
charmed (c) ½ +⅔ e
bottom (b) ½ −⅓ e
top (t) ½ +⅔ e
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ForcesForcesHow do these fundamental constituents interact with one another?• Four different forces are known:Interaction Rel. strength Range
Strong 1 ~2 fm
Electromagnetic 10-2 ∞
Weak 10-5 ~10-3 fm
Gravitational 10-39 ∞
• Forces mediated by particles:
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4 Forces4 Forces
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Probing short distance scales (high energy) uncovers deep regularities,
symmetries and can lead to unified descriptions of different phenomena
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Particle acceleratorsallow us to peer intothe earliest momentsof the Universe
Forces believed tobe unified atextreme energies(or tiny distancescales)
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News from the CosmosNews from the Cosmos•• Quarks and leptons make up only 5% of the Universe!Quarks and leptons make up only 5% of the Universe!
•• Deep mystery: what is dark energy and dark matter? Deep mystery: what is dark energy and dark matter?
Antimatter: 0%
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Matter-antimatter Asymmetry (I)►Baryogenesis Puzzle
Early Universe: Universe Today:
matter and antimatter no antimatter!
created in equal amounts
• Mystery: Where did the antimatter go?
Why is there any matter left today?
Big Bang
time
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Matter-antimatter Asymmetry (II)• A. Sakharov (1967) proposes a mechanism that
requires three ingredients to explain the asymmetry:
1. Baryon number violating reactions occur
2. C and CP violation (CPV) take place
in these reactions3. Reactions occur out of thermal equilibrium
(Big Bang)
Sakharov’s paper summary in verse:From S. Okubo’s effectAt high temperatureA coat is tailored for the UniverseTo fit its skewed shape
Violation of CP-invariance, C-asymmetryand baryon asymmetry of the Universe
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Matter-antimatter Asymmetry (III)• What is CP violation?
Observation that the Laws of Physics are not exactly the same under
the combined transformation:
Charge conjugation C particle ↔ antiparticle
Parity P left-handed helicity ↔ right-handed helicity
CP symmetry is preserved in strong and electromagnetic interactions
BUT weak interactions violate CP symmetry – Cronin, Fitch (1964)
• Manifestation: different decay rates in K and B meson decays
For example, the decay rate for K0L → π− µ+ νµ is slightly higher than
that for K0L → π+ µ− νµ (rate asymmetry = 0.3%)
(mirror symmetry)
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Matter-antimatter Asymmetry (IV)• Does the Standard Model provide Sakharov’s
three ingredients?
YES!
• How much asymmetry do we need?
1 in 109 baryons must survive annihilation to
generate the baryon asymmetry observed today:
(nB – nB) / nγ = 6 x 10-10 (WMAP)
• Can the Standard Model do that?
NO!
Amount of CP violation is too smallby ~10 orders of magnitude…
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ee++ ee-- →→ ϒϒ(4S) (4S) →→ B BB Bwith with E(eE(e+) = 3.1 GeV+) = 3.1 GeV andand E(eE(e--) = 9.0 GeV) = 9.0 GeV