33.5 Quarks: Is That All There Is?

Fundamental Particles and the Standard Model

Fundamental particles in Standard Model

The Standard Model is the theory that describes the most basic building blocks of matter and how they interact. It covers three of the four fundamental forces: strong, weak, and electromagnetic. Gravity is not included in the Standard Model.

Fundamental particles are particles that cannot be broken down into anything smaller. The Standard Model sorts them into two main groups:

• Fermions include quarks and leptons. These are the matter particles.
• Bosons are force carriers that mediate interactions between fermions.

Quarks vs antiquarks

Quarks are fundamental particles that combine to form hadrons (composite particles like protons and neutrons). They have two distinctive features:

• They carry fractional electric charges: either $+\frac{2}{3}$ or $-\frac{1}{3}$ (in units of the elementary charge $e$)
• They participate in all three Standard Model interactions: strong, weak, and electromagnetic

Every quark has a corresponding antiquark. An antiquark has the same mass as its quark partner but opposite electric charge and opposite values for other quantum numbers. When a quark meets its matching antiquark, they can annihilate each other and release energy.

Six quark flavors

Quarks come in six flavors, arranged in three generations:

Each flavor has a corresponding antiquark, written with a bar: $\bar{u}$, $\bar{d}$, $\bar{c}$, $\bar{s}$, $\bar{t}$, $\bar{b}$.

Quarks combine to form hadrons, which fall into two categories:

• Baryons: made of three quarks ($qqq$) or three antiquarks ($\bar{q}\bar{q}\bar{q}$). Protons and neutrons are baryons.
• Mesons: made of one quark and one antiquark ($q\bar{q}$). Pions and kaons are mesons.

The specific combination of quark flavors determines the particle's charge, mass, and other properties.

Quark composition of hadrons

The most familiar hadrons are protons and neutrons:

• Proton = two up quarks + one down quark (uud). Charge: $+\frac{2}{3} + \frac{2}{3} - \frac{1}{3} = +1$
• Neutron = one up quark + two down quarks (udd). Charge: $+\frac{2}{3} - \frac{1}{3} - \frac{1}{3} = 0$

Pions ($\pi^+$, $\pi^-$, $\pi^0$) are common examples of mesons:

• $\pi^+$: up quark + anti-down quark ($u\bar{d}$). Charge: $+\frac{2}{3} + \frac{1}{3} = +1$
• $\pi^-$: down quark + anti-up quark ($d\bar{u}$). Charge: $-\frac{1}{3} - \frac{2}{3} = -1$
• $\pi^0$: a quantum superposition of $u\bar{u}$ and $d\bar{d}$ states. Charge: $0$

Notice how the charges always add up to whole numbers, even though individual quarks carry fractional charges. This is a key feature of quark confinement: you never observe a fractional charge in nature.

Quantum numbers from quark composition

Three important quantum numbers can be calculated directly from a particle's quark content:

• Electric charge: sum the charges of all quarks in the particle
• Baryon number: each quark contributes $+\frac{1}{3}$, each antiquark contributes $-\frac{1}{3}$. Baryons have baryon number $+1$, antibaryons have $-1$, and mesons have $0$.
• Strangeness: each strange quark ($s$) contributes $-1$, each anti-strange quark ($\bar{s}$) contributes $+1$. All other flavors contribute $0$.

These quantum numbers matter because of conservation laws. In any particle interaction, total electric charge, total baryon number, and strangeness must be conserved. (Strangeness is conserved in strong and electromagnetic interactions but can change in weak interactions.) If a proposed reaction violates one of these conservation rules, that reaction cannot happen through that force.

Quark interactions and properties

Quarks interact through the strong force, which is described by the theory of quantum chromodynamics (QCD). A few core ideas:

• Quarks carry a property called color charge (red, green, or blue). This is unrelated to visible color; it's just a label for the type of strong charge.
• Gluons are the force carriers of the strong interaction, similar to how photons carry the electromagnetic force. Gluons themselves carry color charge, which makes QCD much more complex than electromagnetism.
• Quark confinement: quarks are never found alone. The strong force between quarks actually gets stronger as you try to pull them apart. If you add enough energy to separate quarks, that energy creates new quark-antiquark pairs instead of freeing individual quarks.
• Asymptotic freedom: at very short distances (or equivalently, very high energies), the strong force between quarks becomes weaker. This means quarks inside a proton behave almost like free particles when probed at high enough energies, which is what experiments at particle accelerators observe.