Summary
- Fermions
- Fermions…
- are the particles of matter
- obey Fermi-Dirac statistics
- have half integral spin quantum numbers (±½ for elementary particles; ±1½, ±2½, … for composite particles)
- come in one of twelve flavors
- belong to one of three generations
- ordinary matter (the parts needed to make an atom)
- exotic matter (produced in high energy collisions)
- very exotic matter (produced in very high energy collisions)
- The elementary fermions are either quarks or leptons.
- Quarks (q)…
- have spin ½
- come in one of six flavors
- three in the up group, each with an electric charge of +⅔ e
- up (u)
- strange (s)
- top (t)
- three in the down group, each with an electric charge of −⅓ e
- down (d)
- charm (c)
- bottom (b)
- have a property called color or color charge
- Color charge is something like electric charge.
- All quarks can be found in any one of three colors.
- Color in this context has nothing to do with human vision or visible light.
- Only quarks and gluons are colored.
- are always bound to other quarks (by the strong force)
- Leptons (l)…
- have spin ½
- come in one of six flavors
- the three heavy leptons, each with an electric charge of −1 e
- electron (e)
- muon (μ, mu)
- tau (τ, tau)
- can be found free or bound to other charged fermions (by the electromagnetic force)
- and three corresponding neutrinos, with no electric charge
- electron neutrino (νe, nu sub e)
- muon neutrino (νμ, nu sub mu)
- tau neutrino (ντ, nu sub tau)
- only interact with themselves and other particles via the weak force
- are exceptionally lightweight
- The composite fermions arranged in order of increasing complexity are
- Hadrons
- Mesons
- are quark-antiquark pairs
- have spin 0 or 1 making them bosons
- have charge 0 e or ±1 e
- are color neutral
- include exotic particles like the pion (π), eta (η), rho (ρ), etc.
- At least 148 different mesons have been identified.
- Baryons
- are quark triplets
- have spin ½, 1½, … making them fermions
- have charge +2 e, +1 e, 0 e, −1 e
- are color neutral
- subgroups
- nucleons: protons and neutrons
- protons are made of two up quarks and one down quark (uud)
- neutrons are made of one up quark and two down quarks (udd)
- hyperons: exotic particles like the lambda (Λ), sigma (Σ), omega (Ω), etc.
- At least 125 different baryons have been identified.
- Nuclei
- are groups of protons and neutrons
- have a positive integral amount of elementary charge
- are color neutral
- Atoms
- are nuclei with bound electrons (one electron for every proton)
- are electrically neutral
- are color neutral
- Molecules
- are atoms sharing electrons
- Bosons
- Bosons…
- are the particles that mediate interactions
- obey Bose-Einstein statistics
- have integral spin quantum numbers (±0, ±1, ±2, …)
- Vector bosons…
- have spin 1
- are also known as gauge bosons
- belong to one of three types each associated with a fundamental force
- The photon (γ, the Greek letter gamma)
- carries the electromagnetic force between particles with charge
- has an infinite range
- is massless
- exerts a force that is moderately strong relative to the other fundamental forces
- has no charge or color
- is described by the theory of quantum electrodynamics (QED)
- is discussed in more detail in a another section of this book
- Gluons (g)
- carry the strong force between particles with color (only quarks and gluons)
- have a short range (~ 10−15 m, about the diameter of a nucleon)
- are massless
- exert a force which is strong relative to the other fundamental forces
- come in one of eight color combinations, but carry no charge
- are described by the theory of quantum chromodynamics (QCD)
- are discussed in more detail in a another section of this book
- The intermediate vector bosons (W+, W−, Z0)
- carry the weak force between certain particles with flavor
- are also known as the weak bosons
- have an extremely short range (~ 10−18 m, smaller than any known object)
- have mass
- exert a force that is moderately weak relative to the other fundamental forces
- come in charged and uncharged varieties, but are not colored
- W+ has a charge of +1e
- W− has a charge of −1e
- Z0 has no charge
- are described by electroweak theory (EWT), which unites electromagnetism with weak interactions
- are discussed in more detail in a another section of this book
- Scalar bosons…
- have spin 0
- The Higgs boson (H0)
- mediates the Higgs mechanism
- gives mass to all fundamental particles with mass (including itself)
- has mass
- Tensor bosons…
- are hypothetical particles beyond the standard model
- would have spin 2
- The graviton (G), if it exists,
- would carry the gravitational force between particles with mass-energy
- would have infinite range
- would be massless
- would be extremely weak
- may one day be described by a theory of quantum gravitation or a theory of everything