33.5 Quarks: Is That All There Is?

The Standard Model is a powerful theory that describes the fundamental particles and forces of our universe. It categorizes particles into fermions (quarks and leptons) and bosons (force carriers), explaining how they interact through strong, weak, and electromagnetic forces.

Quarks are the building blocks of hadrons like protons and neutrons. They come in six flavors and combine in specific ways to form particles with unique properties. Understanding quark composition helps predict particle behavior and interactions in the subatomic world.

Fundamental Particles and the Standard Model

Fundamental particles in Standard Model

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• Most basic building blocks of matter cannot be broken down into smaller components
• Standard Model theory describes properties and interactions of fundamental particles
  • Includes strong, weak, and electromagnetic forces (three of the four fundamental forces)
  • Does not include gravity
• Categorizes fundamental particles into two main groups:
  • Fermions include quarks and leptons
  • Bosons mediate forces between particles (force carriers)

Quarks vs antiquarks

• Quarks are fundamental particles that make up hadrons (protons, neutrons)
  • Have fractional electric charges: +2/3 or -1/3
  • Participate in strong, weak, and electromagnetic interactions
• Antiquarks are antiparticles of quarks
  • Same mass but opposite charge and other quantum numbers compared to corresponding quarks
  • When quark and antiquark collide, they annihilate each other, releasing energy

Six quark flavors

• Up (u), down (d), charm (c), strange (s), top (t), and bottom (b)
  • Each flavor has corresponding antiquark: anti-up ($\bar{u}$), anti-down ($\bar{d}$), anti-charm ($\bar{c}$), anti-strange ($\bar{s}$), anti-top ($\bar{t}$), anti-bottom ($\bar{b}$)
• Quarks combine to form hadrons categorized into two main groups:
  • Baryons composed of three quarks (qqq) or three antiquarks ($\bar{q}\bar{q}\bar{q}$) (protons, neutrons)
  • Mesons composed of quark and antiquark ($q\bar{q}$) (pions, kaons)
• Specific combination of quark flavors determines properties of resulting hadron

Quark composition of hadrons

• Protons composed of two up quarks and one down quark (uud)
• Neutrons composed of one up quark and two down quarks (udd)
• Pions ($\pi^+$, $\pi^-$, $\pi^0$) are examples of mesons:
  • $\pi^+$ composed of up quark and anti-down quark ($u\bar{d}$)
  • $\pi^-$ composed of down quark and anti-up quark ($d\bar{u}$)
  • $\pi^0$ is superposition of $u\bar{u}$ and $d\bar{d}$ states

Quantum numbers from quark composition

• Quantum numbers (electric charge, baryon number, strangeness) calculated based on quark composition of particle
  • Electric charge: sum of individual quark charges
  • Baryon number: +1/3 for each quark, -1/3 for each antiquark; baryons have baryon number +1, mesons have baryon number 0
  • Strangeness: +1 for each strange quark, -1 for each anti-strange quark
• Conservation laws based on quantum numbers help predict particle behavior and allowed interactions
  • Total electric charge, baryon number, and strangeness must be conserved in particle interactions

Quark interactions and properties

• Quarks possess a property called color charge, which is the source of the strong nuclear force
• Gluons are the force carriers of the strong interaction between quarks
• Quantum chromodynamics (QCD) is the theory describing strong interactions between quarks and gluons
• Quark confinement explains why quarks are never observed in isolation
• Asymptotic freedom describes how the strong force between quarks becomes weaker at very short distances