Particle Physics in a Nutshell:

• There are numerous elementary particles in addition to electron, proton, neutron.
• Elementary particles have other types of "charges" in addition to electric charge.
• Every elementary particle has an "antiparticle" -- a particle with the same mass & spin but opposite charges.
• The positron is the antiparticle of the electron.
• The Standard Model (1973) organized the elementary particles into sets of categories.
• The Standard Model is to particle physics what the Periodic Table is to chemistry..
• There are 4 fundamental forces: Strong, Electromagnetic, Weak, and Gravity.
• The Strong, EM, and Weak forces have been unified (shown to be aspects of a more general force) in the Standard Model.
• Particle exerts a force on each other by exchanging a force-mediating particle.
• There are 2 types of particles: Fermions and Bosons.
• Fermions have spins (1/2, 3/2, ...) and obey the Pauli Exclusion Principle.
• Bosons have spins (0, 1, 2, ...) and do NOT obey the Pauli Exclusion Principle.
• The particles on which forces act are usually Fermions (e.g. electron, neutron, proton).
• Force-mediating particles for the fundamental forces are always bosons.
• The force-mediating particles for the 4 fundamental forces are shown below:

  Force	Force-mediating particle	Example of where observed
  Strong	gluon	Within particles holding quarks together
  Electromagnetic	photon	Within atoms holding electrons to nucleus
  Weak	Z and W	In radioactive beta decay such as that heating the core of the earth
  Gravity	graviton	In the solar system hold planets in orbit

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• According to the the Standard Model, the only indivisible fermions are quarks and leptons.
• Neutrons and protons are each made of 3 quarks.
• Electrons and positrons are leptons. A muon is an electron-like particle that is also a lepton.
• The hadrons are particles on which the strong force acts.
• A meson is a hadron whose mass is less than that of a proton.
• A baryon is a hadron whose mass is greater than or equal to that of a proton.
• Baryons are made of 3 quarks.
• Mesons are made of 2 quarks.
• Quarks do not exist in isolation; they are confined within particles.
• The nucleus is held together by the nuclear force, a NON-fundamental force mediated by the exchange of a pi meson (pion).
• According to the Standard Models the nuclear force is ultimately a manifestation of the strong force.
• All of the particles predicted by the Standard Model have been discovered except for one: the Higgs Boson.
• The role of the Higgs Boson in the Standard Model is to endow massive particles with their mass.
• The Large Hadron Collider near Geneva, Switzerland is a $6 billion machine designed to create and detect the Higgs Boson.

A Few Facts about Cosmology:

• In 1998 astronmers discovered that the expansion of the univese is accelerating.
• Only 4% of the universe is made of normal matter.
• 23% of the univese is "dark matter": invisible matter that only interacts gravitationally.
• 73% of the universe is "dark energy": an anti-gravitating substance that is accelerating the expansion of the universe.
• A black hole is a region of intense gravity from which nothing can escape.
• If a star is sufficiently massive, it can become a black hole after it exhausts its fuel (H, He).
• Black holes radiate thermal (black body) radiation, slowly shrinking until they disappear as a result.

Unanswered Questions in Physics:

• What is dark matter?
• What is dark energy?
• What happens to information that falls into a black hole after the black hole radiates away leaving nothing behind?
• Why do the elementary particles have the particular mass values that they have?
• How can gravity be unified with the other 3 fundamental forces?
• Why does spacetime have 4 dimensions?
• Why is gravity so much weaker than the other forces?