Particle Physics

Standard model particles
The fundamental particles of the standard model of particle physics (Click to enlarge)

The fundamental particles of nature are divided into two main groups, known as fermions (after Enrico Fermi) and bosons (after Satyendra Nath Bose).

All fermionic particles have quantum spin numbers (see quantum mechanics) that are fractions of an integer e.g. 1/2, 1/3, 3/2, 2/3, etc, whereas Bosons have integer spin numbers e.g. 0, 1, 2, 3, 4, etc. The Pauli exclusion principal says that no two particles with a fractional spin number can share the same quantum state and position. Since Fermions obey this principal, fundamental fermions can be considered to be the particles that make up the matter of the universe. Bosons do not obey the Pauli exclusion principal, and the fundamental bosonic particles can be considered to be the particles which mediate the fundamental forces in quantum field theory.

Fundamental Fermions

Fermions include the following fundamental particles, which are the building blocks of all matter in the Standard Model of particle physics.

Quarks

There are six known types, or “flavours” of quark (pronounced quork). All quarks are spin 1/2 particles, i.e. they can exist in either a spin up +1/2 or spin down -1/2 state.
The six flavours of quark, in order of increasing mass, are:

  • Up quarks (u)
    electric charge: +2/3 e (where e is the elementary electric charge of a proton)
    mass: 2.3 MeV/c2 (N.B mass is in units of mega electronvolts over the speed of light squared. One MeV/c2 is equal to 1.78266184 × 10-36 kilograms).
  • Down quarks (d)
    electric charge: -1/3 e
    mass: 4.8 MeV/c2
  • Strange quarks (s)
    electric charge: -1/3 e,
    mass: 95 MeV/c2
    Strange quarks are assigned a ‘Strangeness’ quantum number of S = 1, which is preserved in all interactions.
  • Charm quarks (c)
    electric charge +2/3 e
    mass: 1,200 MeV/c2
    Like Strangness, Charm number (C) is also preserved in interactions
  • Bottom or beauty quarks (b)
    electric charge: -1/3 e
    mass: 4,180 MeV/c2.
  • Top or truth quarks (t)
    electric charge: +2/3 e
    mass: 173,070 MeV/c2
    Bottom and Top quantum numbers are also preserved in quantum interactions.

Quarks also possess a property known as colour or colour charge. This is not a colour in the usual sense of the word but is a quantum property that can take one of six different values known as red, anti-red, green, anti-green, blue and anti-blue. The analogy with colour comes from the principal that quarks only combine to form composite particles which have an overall colour of “white”. This means that three quarks, with the different colours of red, green and blue (or anti-red, anti-green and anti-blue) can combine to form particles known as baryons (for example, a proton, which consists of two up quarks and a down quark, or a neutron, which consists of two down quarks and an up quarks) or two quarks of opposite colour, e.g. red and anti-red can combine to form particles known as mesons. Collectively, these particles consisting of quarks are known as hadrons and the theory which deals with colour charge is known as quantum chromodynamics.

The spelling of the word quark comes from a line in James Joyce’s Finnegans Wake “three quarks for Muster Mark”. Quark could either mean the cry of a seagull or a quart of beer, fitting the pronunciation ‘quork’ which had already been suggested as a name for the particle. Since quarks occur in threes within baryonic particles it seemed fitting to use James Joyce’s spelling.

Leptons

Leptons are the second set of fundamental fermions. The most familiar to us is the electron, which is the carrier of electric current in our power supplies and which account for the different properties of the chemical elements.

The electron neutrino is an associated lepton of vanishingly small but non-zero mass and no electric charge, which hardly interacts with normal matter at all. Neutrinos were first discovered due to their production in radioactive beta decay, where they were not detected directly but account for seemingly “missing energy” and preserve the overall quantum spin during the reaction.

Two more “generations” of leptons also exist, with similar properties to the electron and neutrino but higher masses.
These three lepton generations are:

  • The electron (e or β) and electron neutrino (νe)

Electrons are spin 1/2 particles with an electric charge of around −1.602×10−19 Coulombs, or -1 e. Electrons have a mass of around 0.511 MeV/c2 or 9.109×10−31 kg

  • The muon (μ) and muon neutrino (νμ)

Muons are heavier versions of the Electron (mass ~105 MeV/c2), also with spin 1/2 and an electric charge of -1 e. Muons also have an associated neutrino.

  • The tauon or tau lepton (τ) and tau neutrino (ντ)

Tauons are heavier still than than Muons with a mass of around 1,777 MeV/c2. There is also an associated Tau neutino.

Fundamental Bosons

Bosons include the following fundamental force-carrying particles, also known as gauge bosons in gauge field theory:

Photons

The photon is the quantum particle of light and all other wavelengths of electromagnetic radiation. A photon can also be though of as a quantum vibration of the electromagnetic gauge field and, hence, the particle which mediates the electromagnetic force. For more information on photons see the page on light and electromagnetic radiation. Photons have zero spin, no electrical charge and zero rest mass.

Gluons

Gluons are the particles that mediate the strong nuclear force (see the fundamental forces page). Gluons bind together quarks to form hadrons and also hold protons and neutrons together in the nuclei of atoms. Gluons are spin 1 particles and theories predict that they should be massless. Gluons have no electrical charge although, like quarks, they posses colour charge. There are eight different types of gluon according to the theory of quantum chromodynamics.

W+, W and Z0 particles

The W and Z particles are spin 1 particles responsible for mediating the weak nuclear force, which is involved in the radioactive decay of hadrons. These particles have relatively large masses, almost 100 times the mass of a proton, and are heavier than an atom of iron, which is the reason that they act only over very short distances (see fundamental forces). W particles have an electric charge of +1 e or – 1 e, while the Z particle is electrically neutral.

Higg’s Boson

The Higg’s boson is a spin 0 boson, which was proposed to explain why some of the fundamental particles have mass. The Higg’s Boson is named after Peter Higgs’s, one of the theorists who first proposed its existence.

According to theory, a Higg’s field permeates all of space. When some particles move through this field they interact with it causing them to acquire mass. The Higg’s particle is a quantum excitation of this field.

Experiments at the Large Hadron Collider (a particle accelerator near Geneva) provided the first evidence of the existence of the Higg’s Boson in 2011.

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