Definition of Quarks in Physics

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A quark is one of the fundamental particles in physics. They join to form hadrons, such as protons and neutrons, which are components of the nuclei of atoms. The study of quarks and the interactions between them through the strong force is called particle physics.

The antiparticle of a quark is the antiquark. Quarks and antiquarks are the only two fundamental particles that interact through all four fundamental forces of physics: gravitation, electromagnetism, and the strong and weak interactions.

Quarks and Confinement

A quark exhibits confinement, which means that the quarks are not observed independently but always in combination with other quarks. This makes determining the properties (mass, spin, and parity) impossible to measure directly; these traits must be inferred from the particles composed of them.

These measurements indicate a non-integer spin (either +1/2 or -1/2), so quarks are fermions and follow the Pauli Exclusion Principle.

In the strong interaction between quarks, they exchange gluons, which are massless vector gauge bosons that carry a pair of color and anticolor charges. When exchanging gluons, the color of the quarks change. This color force is weakest when the quarks are close together and becomes stronger as they move apart.

Quarks are so strongly bound by the color force that if there is enough energy to separate them, a quark-antiquark pair is produced and binds with any free quark to produce a hadron. As a result, free quarks are never seen alone.

Flavors of Quarks

There are six flavors of quarks: up, down, strange, charm, bottom, and top. The flavor of the quark determines its properties.

Quarks with a charge of +(2/3)e are called up-type quarks, and those with a charge of -(1/3)e are called down-type.

There are three generations of quarks, based on pairs of weak positive/negative, weak isospin. The first generation quarks are up and down quarks, the second-generation quarks are strange, and charm quarks, the third generation quarks are top and bottom quarks.

All quarks have a baryon number (B = 1/3) and a lepton number (L = 0). The flavor determines certain other unique properties, described in individual descriptions.

The up and down quarks make up protons and neutrons, seen in the nucleus of ordinary matter. They are the lightest and most stable. The heavier quarks are produced in high-energy collisions and rapidly decay into up and down quarks. A proton is composed of two up quarks and a down quark. A neutron is composed of one up quark and two down quarks.

First-Generation Quarks

Up quark (symbol u)

  • Weak Isospin: +1/2
  • Isospin (Iz): +1/2
  • Charge (proportion of e): +2/3
  • Mass (in MeV/c2): 1.5 to 4.0 

Down quark (symbol d)

  • Weak Isospin: -1/2
  • Isospin (Iz): -1/2
  • Charge (proportion of e): -1/3
  • Mass (in MeV/c2): 4 to 8 

Second Generation Quarks

Charm quark (symbol c)

  • Weak Isospin: +1/2
  • Charm (C): 1
  • Charge (proportion of e): +2/3
  • Mass (in MeV/c2): 1150 to 1350 

Strange quark (symbol s)

  • Weak Isospin: -1/2
  • Strangeness (S): -1
  • Charge (proportion of e): -1/3
  • Mass (in MeV/c2): 80 to 130 

Third Generation Quarks

Top quark (symbol t)

  • Weak Isospin: +1/2
  • Topness (T): 1
  • Charge (proportion of e): +2/3
  • Mass (in MeV/c2): 170200 to 174800 

Bottom quark (symbol b)

  • Weak Isospin: -1/2
  • Bottomness (B'): 1
  • Charge (proportion of e): -1/3
  • Mass (in MeV/c2): 4100 to 4400 
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Jones, Andrew Zimmerman. "Definition of Quarks in Physics." ThoughtCo, Aug. 28, 2020, Jones, Andrew Zimmerman. (2020, August 28). Definition of Quarks in Physics. Retrieved from Jones, Andrew Zimmerman. "Definition of Quarks in Physics." ThoughtCo. (accessed March 25, 2023).