Episode 2: the Standard Model of Particle Physics

Continuing our look into the subatomic realm that we began last week, we kick off Episode 2 by discussing how physicists tell particles apart. Essentially the idea is that we pick invariants – particle properties that remain unchanged under symmetry transformations, i.e., changes in perspective. The three types of invariant used to classify subatomic particles are: the mass m, the spin s, and the force-charges q. Mass can be anything from zero to large, but spin is quantized in units of ½h-bar, where h-bar is a constant of Nature known as Planck’s constant. Numerically h-bar~1.05 x 10^-34Joule-seconds.

We draw an important distinction between bosons (which have spin 0, 1, 2,…) and fermions (which have spin 1/2, 3/2, 5/2, …). At high temperatures where particles race around with a lot of average kinetic energy, bosons and fermions behave pretty much the same, but this is not true at low temperatures. The Pauli Exclusion Principle (PEP) is a crucially important property of fermions: it says that no two fermions can be in the same quantum state at the same time. In plain language, this means that fermions have elbows. This has profound consequences which we discuss.

Matter (stuff) in particle physics is composed of fermions, while force-transmitters are bosons. The electromagnetic and two nuclear forces have spin one messengers, while gravity has a spin two messenger particle. The mysterious hypothesised Higgs boson (responsible for mass of quarks, leptons and the weak bosons) has spin zero. Another important dichotomy is that between hadrons, i.e.  baryons and mesons, made of quarks and gluons, which feel the strong force, and leptons which are not affected by the colour force. Leptons, composed of electrons, muons, taus and their associated neutrinos, do feel the weak nuclear force. Every particle possessing energy feels gravity, including photons.

We discuss properties of the four different forces – gravity, electromagnetic, strong nuclear and weak nuclear – including how they differ in range and strength. In particular, we discuss which forces act on which particles. We also develop in some detail the ice skater analogy, which motivates why particle physicists can successfully model force transmission in terms of exchange of messenger particles.

We close by giving a lightning tour of Fermilab, the Fermi National Accelerator Laboratory in the USA, and the Large Hadron Collider (LHC) based at CERN near Geneva, Switzerland.

Here is a PDF file of my slides from Episode 2.