A Brief History of Modern Physics

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1 A Brief History of Modern Physics Modern 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, ) ~ phenomena at very short distance scales structure of the atom behavior of light, subatomic particles

2 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?

3 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

4 Constituents of Matter (II) How can we probe the structure of the atom? Perform scattering experiments with high energy particles h with de Broglie wavelength λ = p Rutherford Scattering Expts (1910) Projectiles: α particles (He nucleus) produced in radioactive decays e.g. 232 Th 228 Ra + α Kinetic energy of α particle K = 4 MeV λ m Observations: ~1 in 10 4 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

5 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 m moving close to the speed of light: v = c Electrons do not see 2 mile-long linac but a contracted length of only 1 in.!

6 Constituents of Matter (IV) Scattering experiments at SLAC established the existence of quarks as fundamental constituents of protons and neutrons What do we currently know about the structure of matter? Atom Nucleus = bound system of positive nucleus + orbiting electrons ~ m = bound system of protons + neutrons (nucleons) ~ m Nucleons = bound system of up and down quarks Quarks Name =? no known structure down to < m Spin Charge up (u) ½ +⅔ e down (d) ½ strange (s) ½ ⅓ e charmed (c) ½ +⅔ e bottom (b) ½ ⅓ e top (t) ½ +⅔ e ⅓ e mass

7 Forces How 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 Gravitational ~10-3 fm Forces mediated by particles:

8 4 Forces

9 Probing short distance scales (high energy) uncovers deep regularities, symmetries and can lead to unified descriptions of different phenomena

10 Particle accelerators allow us to peer into the earliest moments of the Universe Forces believed to be unified at extreme energies (or tiny distance scales)

11 News from the Cosmos Quarks and leptons make up only 5% of the Universe! Deep mystery: what is dark energy and dark matter? Antimatter: 0%

12 Matter-antimatter Asymmetry (I) Baryogenesis Puzzle Early Universe: matter and antimatter created in equal amounts Universe Today: no antimatter! Big Bang time Mystery: Where did the antimatter go? Why is there any matter left today?

13 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 reactions 3. Reactions occur out of thermal equilibrium (Big Bang) Violation of CP-invariance, C-asymmetry and baryon asymmetry of the Universe Sakharov s paper summary in verse: From S. Okubo s effect At high temperature A coat is tailored for the Universe To fit its skewed shape

14 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 (mirror symmetry) 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 K 0 L π µ + ν µ is slightly higher than that for K 0 L π + µ ν µ (rate asymmetry = 0.3%)

15 Matter-antimatter Asymmetry (IV) Does the Standard Model provide Sakharov s three ingredients? YES! How much asymmetry do we need? 1 in 10 9 baryons must survive annihilation to generate the baryon asymmetry observed today: (n B n B ) / n γ = 6 x (WMAP) Can the Standard Model do that? NO! Amount of CP violation is too small by ~10 orders of magnitude

16 e + e - ϒ(4S) B B with E(e+) = 3.1 GeV and E(e - ) = 9.0 GeV

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