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

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?

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

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 =

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.!

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

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:

4 Forces4 Forces

Probing short distance scales (high energy) uncovers deep regularities,

symmetries and can lead to unified descriptions of different phenomena

Particle acceleratorsallow us to peer intothe earliest momentsof the Universe

Forces believed tobe unified atextreme energies(or tiny distancescales)

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%

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

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

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)

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…

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