Elementary Particles
Quarks
Quarks (in lavender, above) refers to a group of subatomic particles that appear to be one of the fundamental constituents of matter in the universe. 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. The SNF is carried by a steady stream of gluons that passes between fused quarks, maintaining their indivisibility. Unlike larger particles (hadrons like baryons and mesons, described below), quarks are seemingly fundamental and indivisible. Quarks are zero-dimensional point-structures without a measurable size; but they possess other properties (such as mass, charge, or spin) which emerge from the particles' interactions with various fields, are intrinsic to the particles, and serve to differentiate them.
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.
| quark flavor | baryon number | charge | spin | strangeness | charm | bottom | top | mass (MeV) |
|---|---|---|---|---|---|---|---|---|
| down (d) | 1/3 | -(1/3)e | 1/2 | 0 | 0 | 0 | 0 | 5-15 |
| up (u) | 1/3 | +(2/3)e | 1/2 | 0 | 0 | 0 | 0 | 2-8 |
| strange (s) | 1/3 | -(1/3)e | 1/2 | -1 | 0 | 0 | 0 | 100-300 |
| charm (c) | 1/3 | +(2/3)e | 1/2 | 0 | 1 | 0 | 0 | 1000-1600 |
| bottom (b)
[beauty (b)] |
1/3 | -(1/3)e | 1/2 | 0 | 0 | -1 | 0 | 4100-4500 |
| top (t)
[truth (t)] |
1/3 | +(2/3)e | 1/2 | 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.
Quarks are spin '-1/2' particles, which means they are fermions according to the spin–statistics theorem. They are subject to the Pauli exclusion principle, which states that no two identical fermions can simultaneously occupy the same quantum state. This is in contrast to bosons (particles with integer spin), of which any number can be in the same state. Unlike leptons, quarks possess color charge, which causes them to engage in the strong interaction.
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.
Antiparticles of quarks are called antiquarks, and are denoted by a bar over the symbol for the corresponding quark: anti-up quark: uˉ; anti-down quark: dˉ; anti-charm quark: cˉ; anti-strange quark: sˉ; anti-top quark: tˉ (sometimes referred to as anti-truth); anti-bottom quark: bˉ (sometimes referred to as anti-beauty) As with antimatter in general, antiquarks have the same mass, mean lifetime, and spin as their respective quarks, but the charges have the opposite sign.
The mass hierarchy problem, which questions why there are such vast differences in the masses of fundamental particles, remains one of the key challenges in the field. The top quark is the heaviest of all observed elementary particles and is about 40,000 times more massive than the up quark; these differences have profound implications for the stability of matter and the formation of the observable universe and are as yet unexplained.
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").