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Quark

A four-by-four table of particles. Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (ν sub e) and electron (e), muon neutrino (ν sub μ) and muon (μ), and tau neutrino (ν sub τ) and tau (τ), and Z sup 0 and W sup ± weak force. Mass, charge, and spin are listed for each particle.
Six of the particles in the Standard Model are quarks (shown in purple). Each of the first three columns forms a generation of matter.

A quark[1] is an elementary particle which makes up hadrons, the most stable of which are protons and neutrons. Atoms are made of protons, neutrons and electrons. It was once thought that all three of those were fundamental particles, which cannot be broken up into anything smaller, but after the invention of the particle accelerator, it was discovered that electrons are fundamental particles, but neutrons and protons are not. Neutrons and protons are made up of quarks, which are held together by gluons.[2]

There are six types of quarks. The types are called flavours. The flavours are up (u), down (d), strange (s), charm (c), top (t), and bottom (b). Up, charm and top quarks have a charge of +2⁄3, while down, strange and bottom quarks have a charge of -1⁄3. Each quark has a matching antiquark. Antiquarks have a charge opposite to that of their quarks; meaning that up, charm and top antiquarks have a charge of -2⁄3 and down, strange and bottom antiquarks have a charge of +1⁄3.

Only up and down quarks are found inside of atoms of normal matter. Two up quarks and one down make a proton (2⁄3 + 2⁄3 - 1⁄3 = +1 charge) while two down quarks and one up make a neutron (2⁄3 - 1⁄3 - 1⁄3 = 0 charge). The other four flavours are not seen naturally on Earth, but they can be made in particle accelerators. Some of them may also exist inside of stars.

When two or more quarks are held together by the strong nuclear force, the particle formed is called a hadron. Quarks that make the quantum number of hadrons are named 'valence quarks'. The two families of hadrons are baryons (made of three valence quarks) and mesons (which are made from a quark and an antiquark). Some examples of baryons are protons and neutrons, and examples of mesons are pions and kaons.

When quarks are stretched farther and farther, the force that holds them together becomes bigger. When it comes to the point when quarks are separated, they form two sets of quarks, because the energy that is put into trying to separate them is enough to form two new quarks. So scientists think it is not possible to have one quark by itself.

Quarks also have color charge and react via the weak force. For baryons, each quark is green, red or blue. One can be one color at one time. For mesons, the quark is red, green or blue and the antiquark is antired (cyan), antiblue (yellow) or antigreen (pink). Quarks can change color by emitting a W boson. For example, if a green color-charged quark emits a red-antigreen W boson to a red quark, the green quark will become red and the red quark will become green. Therefore, W and Z bosons (which mediate the weak force for neutrinos) help particles react via the weak interaction.

This also causes the beta and beta-plus decay of atoms where a neutron decays into a proton, electron and electron antineutrino, and makes a proton turn into a neutron, positron and electron neutrino.

A picture of a neutron. The 'u' stands for an up quark, and the 'd' stands for a down quark. A neutron is made of three quarks, and is a baryon (a type of hadron). The colors represent the colour strong force in quantum theory.

The idea (or model) for quarks was proposed by physicists Murray Gell-Mann and George Zweig in 1964. Other scientists began searching for evidence of quarks, and succeeded in 1968 through the use of deep inelastic scattering; a process used to probe the inside of hadrons.

Quark Media

  • Murray Gell-Mann

  • George Zweig at Department of Physics, National Taiwan University

  • Feynman diagram of beta decay with time flowing upwards. The CKM matrix (discussed below) encodes the probability of this and other quark decays.

  • The strengths of the weak interactions between the six quarks. The "intensities" of the lines are determined by the elements of the CKM matrix.

  • All types of hadrons have zero total color charge.

  • The pattern of strong charges for the three colors of quark, three antiquarks, and eight gluons (with two of zero charge overlapping).

  • Current quark masses for all six flavors in comparison, as balls of proportional volumes. Proton (gray) and electron (red) are shown in bottom left corner for scale.

  • A qualitative rendering of the phase diagram of quark matter. The precise details of the diagram are the subject of ongoing research.

References

  1. ↑ Named after a word from Finnegans Wake by James Joyce.
  2. ↑ Tong, David. 2017. Quantum fields. Royal Institution presentation. [1]

Other websites

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