Quark

For other uses of this term, see: Quark (disambiguation)

In particle physics, the quarks are one of the two families of subatomic particles thought to be elemental and indivisible (the other being the leptons). Objects made up of quarks are known as hadrons; well known examples are protons and neutrons.

Quarks are generally believed to never exist alone but only in groups of two or three (and, more recently, five); all searches for free quarks since 1977 have yielded negative results. Quarks are differentiated from leptons, the other family of elemental particles, by electric charge. Leptons (such as the electron or the muon) have integral charge (+1, 0 or −1) while quarks have +2/3 or −1/3 charge (antiquarks have −2/3 or +1/3 charge). All quarks have spin 1/2  .

Table of quarks

1. The estimates of u and d masses are not uncontroversial and still actively being investigated; in fact, there are even suggestions in literature that the u quark could be essentially massless.

Ordinary matter such as protons and neutrons are composed of quarks of the UP or DOWN variety only. A proton contains two UP quarks and one DOWN quark, giving a total charge of +1. A neutron is made of two DOWN quarks and one UP quark, giving a total charge of zero. The other varieties of quarks can only be produced in particle accelerators, and degenerate quickly to the UP and DOWN quarks. (Electrons do not contain quarks, but are of a different type of particle called leptons).

Families of quarks

All the quarks that appear in ordinary matter are either up or down quarks. However, in very high-energy situations, other quarks appear. The first "extra" quark discovered was called a strange quark; as higher-energy collisions became possible, the charm, bottom, and eventually top quarks were discovered. These extra quarks seem to provide higher-mass copies of ordinary quarks, just as the muon and the tau provide higher-mass copies of the electron.

A physicist hearing this description might wonder, with some trepidation, whether there are yet more copies of these quarks with even higher masses. Recent research at CERN has provided strong evidence that no such families exist. This experiment relied on accurate determination of the variation in masses of the Z boson; by a subtle series of calculations, the numbers obtained could be shown to contradict the possibility that more families of quarks exist. See [1] for more information.

The number of families of quarks also affects the only other really high-energy situation we know of - the early Universe. The initial distribution of elements can be predicted using the Standard Model; any model with more, heavier, quarks would lead to a fraction of initial Helium 4 that is different from what is observed. Thus the number of quarks is confirmed by astronomical observations as well. See [2] for more information.

Color

According to the theory of quantum chromodynamics (QCD), quarks possess another property that is metaphorically called "color charge", because the rules for adding such charges are the same as those of additive color mixing—quarks are much too small to exhibit color in the usual sense. Instead of just two different charge types (like + and − in electromagnetism), color charge comes in 6 types. Quarks are "red", "green", or "blue", while antiquarks are anti-red or "cyan", anti-green or "magenta", and anti-blue or "yellow". In the theory, only color-neutral or "white" particles can exist separately: particles possessing color must be part of a "white" composite. Particles composed of one red, one green and one blue quark are called baryons; the proton and the neutron are the most important examples. Particles composed of a quark and an anti-quark of the corresponding anti-color are called mesons.

Particles of different color charge are attracted and particles of like color charge are repelled by the strong nuclear force, which is transferred by gluons, particles that themselves carry color charge. Therefore, colors of quarks are not static, but are interchanged by gluons, always maintaining the result neutral. This interchange of color charge is thought to result in the strong nuclear force holding quarks together in mesons and baryons; a "secondary" effect of this strong nuclear force is to hold the protons and neutrons together in the atomic nucleus.

Due to the extremely strong nature of the strong force, quarks are never found free. They are always bound into baryons or mesons. The exception to this is the t quark (discovered April 23, 1994), which is so massive that it decays before getting a chance to form baryons or mesons. When we try to separate quarks in a meson or baryon, as happens in particle accelerators, the strong force actually becomes stronger as they get farther apart. At some point it is more energetically favorable to create two more quarks to cancel out the increasing force, and two new quarks (a quark and an anti-quark) pop out of the vacuum. This process is called hadronization or fragmentation, and is one of the least understood processes in particle physics. As a result of fragmentation, when quarks are produced in particle accelerators, instead of seeing the individual quarks in detectors, scientists see "jets" of many color-neutral particles (mesons and baryons), clustered together.

History

The theory behind quarks was first suggested by physicists Murray Gell-Mann and George Zweig, who found they could explain the properties of many particles by considering them to be composed of these elementary quarks. The name quark comes from "three quarks for Muster Mark", a phrase in James Joyce's Finnegans Wake. According to The American Heritage® Dictionary of the English Language: Fourth Edition, the word quark as used by Joyce is the standard English verb quark, meaning "to caw, croak." The reference to three quarks in Joyce's phrase caught Gell-Mann's attention as particularly suggestive of the particle's circumstances.

See also

• Rubik's Cube for an interesting parallel
• List of particles

External links

• Observation of the Top Quark at Fermilab
• A Positron Named Priscilla - A description of CERN's experiment to count the families of quarks
• Cosmological Models - A description of the Big Bang model, its predictions and problems.
• the original English word quark and its adaptation to particle physics