Quarks are the building blocks of matter, forming protons and neutrons. These tiny particles come in six flavors and have unique properties like fractional electric charges and color charge. Understanding quarks is crucial for grasping the fundamental structure of the universe.

Particle physics explores the interactions between quarks and other elementary particles. The Standard Model, which includes quarks, leptons, and force-carrying bosons, provides a framework for understanding these interactions. The discovery of the Higgs boson in 2012 further validated this model.

Quarks and Particle Physics

Properties and role of quarks

• Elementary particles that make up hadrons
  • Have fractional electric charges: +2/3 or -1/3 of the elementary charge ($e$)
  • Possess color charge, the source of the strong nuclear force
  • Have intrinsic spin of 1/2, making them fermions
• Come in six flavors: up, down, charm, strange, top, and bottom
  • Each flavor has a corresponding antimatter counterpart with opposite properties
• Combine to form hadrons through the strong nuclear force
  • Baryons composed of three quarks (protons and neutrons)
  • Mesons composed of a quark and an antiquark (pions and kaons)
• Combination of quarks in hadrons must result in a colorless state
  • Achieved through color confinement, where quarks are bound together by gluons
• Exhibit asymptotic freedom, where the strength of the strong force between quarks decreases at very short distances

Hadrons vs leptons

• Hadrons are composite particles made of quarks, leptons are elementary particles
  • Hadrons participate in the strong nuclear interaction, leptons do not
  • Examples of hadrons: protons, neutrons, and mesons
  • Examples of leptons: electrons, muons, and neutrinos
• Hadrons have internal structure and can be divided into smaller constituents (quarks)
  • Leptons have no known internal structure and are considered point-like particles
• Both hadrons and leptons participate in the weak nuclear and electromagnetic interactions (if charged)
  • Only hadrons are affected by the strong nuclear force due to their quark composition

Matter and antimatter composition

• Matter particles are the ordinary particles that make up the observable universe
  • Examples: protons (two up quarks and one down quark) and neutrons (one up quark and two down quarks)
• Antimatter particles are the counterparts of matter particles, with opposite properties like electric charge
  • Examples: antiprotons (two anti-up quarks and one anti-down quark) and antineutrons (one anti-up quark and two anti-down quarks)
• When a matter particle and its corresponding antimatter particle collide, they annihilate each other
  • Their mass is converted into pure energy according to Einstein's equation $E=mc^2$

Quarks in standard model

• The standard model describes the fundamental particles and their interactions
  • Includes three generations of quarks and leptons, and gauge bosons that mediate the fundamental forces
• Quarks play a crucial role as the building blocks of hadrons
  • The six flavors of quarks (up, down, charm, strange, top, bottom) are arranged in three generations, with increasing mass
• The standard model also includes the Higgs boson, responsible for the mass generation of quarks and other particles
• Interactions between quarks are described by the theory of quantum chromodynamics (QCD)
  • QCD explains the strong nuclear force and the behavior of quarks within hadrons
• Quarks experience quantum confinement, which prevents them from being observed individually in nature

Higgs boson and quark mass

• The Higgs boson is a scalar particle predicted by the standard model and discovered in 2012
  • Associated with the Higgs field, which permeates all of space
• Quarks and other particles acquire mass through their interaction with the Higgs field
  • The strength of a particle's interaction with the Higgs field determines its mass
• Discovery of the Higgs boson confirmed the mechanism of mass generation in the standard model
  • Provided an explanation for the origin of quark masses and differences between quark flavors
• The Higgs boson's existence and properties have been a crucial test of the standard model's validity
  • Its discovery has opened up new avenues for research into the nature of mass and potential for new physics beyond the standard model

Quark theory and high-energy physics

• Murray Gell-Mann proposed the quark model in 1964, revolutionizing our understanding of particle physics
• Partons, originally proposed to explain the internal structure of hadrons, were later identified as quarks and gluons
• Under extreme conditions of high temperature and density, quarks and gluons can form a state of matter called quark-gluon plasma