Elementary Particles

Quarks

Quarks are zero-dimension point-like structures without a measurable size. This does not mean they lack other properties, such as mass, charge, or spin; these properties emerge from the particles' interactions with various fields, are intrinsic to the particles, and serve to differentiate them.

Quarks consist of several types of subatomic particles that serve as one of the fundamental constituents of matter. They are associated with one another by way of the strong nuclear force and combine to make up larger subatomic particles, just as those particles combine at larger scales to create atomic nuclei. Unlike these larger hadrons (baryons and mesons, described below), quarks do not appear to have their own constituent building blocks - they are seemingly fundamental, and indivisible. They always occur in combinations with other versions or types of the same elemental particle. Quark types are referred to as flavors, and they are divided into three pairs: up versus down; charm versus strange; and top versus bottom.

Flavors of Quark

quark flavor	baryon number	charge	strangeness	charm	bottom	top	mass (MeV)
down (d)	1/3	-(1/3)e	0	0	0	0	5-15
up (u)	1/3	+(2/3)e	0	0	0	0	2-8
strange (s)	1/3	-(1/3)e	-1	0	0	0	100-300
charm (c)	1/3	+(2/3)e	0	1	0	0	1000-1600
bottom (b)	1/3	-(1/3)e	0	0	-1	0	4100-4500
top (t)	1/3	+(2/3)e	0	0	0	1	180000

Hadrons (composite subatomic particles) have two categories based on quark composition: all mesons (electrons, neutrinos, photons, et cetera) consist of a quark and an antiquark in combination, while all baryons (protons, neutrons, et cetera) are composed of three combined quarks.

• Proton: up + up + down; charge: 2/3 + 2/3 - 1/3 = 3/3 = 1
• Neutron: up + down + down; charge: 2/3 - 1/3 - 1/3 = 0/3 = 0

Quarks possess a fractional electrical charge. Unlike protons and electrons which have whole integer charges (+1 and -1, respectively), quarks have fractional charges that are either +2/3 or -1/3 of the elementary charge (e). This fractional charge is not observed in isolation due to a phenomenon known as confinement; quarks are always found in combinations that result in particles with whole number charges. These charges also play a significant role in determining the structure of hadrons and their interactions via the strong nuclear force, mediated by gluons, responsible for holding quarks together within hadrons.

The concept of color charge in QCD is analogous to electric charge in electromagnetism but is more complex due to its non-abelian nature. Quarks carry one of three color charges (red, green, or blue), and antiquarks carry anticolors. In a baryon, the three quarks must combine in a way that their color charges 'cancel out,' resulting in a color-neutral (or 'white') particle. This is achieved by combining one red, one green, and one blue quark. In a meson, the quark and antiquark must have corresponding color and anticolor charges, which also results in a color-neutral/white combination.

Quarks are permanently confined within hadrons due to color charge and the SNF. The SNF becomes stronger as quarks move apart, a phenomenon known as color confinement; as such, quarks cannot be isolated and observed individually under normal conditions. They exist as part of composite particles (hadrons). The principle of color confinement posits that particles observable at low energies, like those we encounter in everyday physics, must be color-neutral. This is why quarks cannot be observed in isolation under normal conditions; they are always found in combinations that produce color-neutral hadrons.

The etymology of the term is obscure - it comes from an odd line in James Joyce's novel, Finnegan's Wake ("Three quarks for Muster Mark").

Bosons

Bosons are particles that mediate fundamental forces in the universe. Unlike fermions, multiple bosons can occupy the same quantum state. There are several types of bosons, each associated with a fundamental force:

• Photons (γ) are responsible for carrying the electromagnetic force, which governs interactions between electrically charged particles. They are massless and travel at the speed of light. Photons transmit (or broadcast the passage of) electromagnetic radiation. The photon is its own antiparticle, because it is electrically neutral.
• Gluons (g) mediate the strong nuclear force, which binds quarks together within hadrons (such as in mesons like protons and neutrons). The SNF is the fundamental force that fuses parts of atomic nuclei together. Gluons carry the color charge of the SNF and come in combinations that are their own antiparticles, as well as combinations that have distinct antiparticles. There are eight types of gluons in quantum chromodynamics (QCD), and their antiparticles are defined by color and anticolor charges.
• Bosons (W+, W-, Z0) are responsible for mediating the weak nuclear force, which governs certain types of particle interactions like radioactive decay. The WNF is involved in processes that change one type of quark into another and is crucial for the energy production of stars. The W+ is the antiparticle of the W− and vice versa; they are charged bosons, and their charge determines the distinction between particle and antiparticle. The Z boson is neutral and is its own antiparticle.
  • Higgs Boson (H0) is associated with the Higgs field, which gives particles mass. Its discovery confirmed the existence of the Higgs mechanism, a fundamental aspect of the Standard Model. The Higgs boson is electrically neutral and has no quantum numbers that would differentiate it from an opposite, so it is considered its own antiparticle.