Quark

There are six different types of quarks, known as flavors: up, down, charm, strange, top, and bottom.^[3] The charm, strange, top, and bottom flavors are unstable and decay rapidly, and can be created and studied only under special conditions, such as in particle accelerators and in cosmic rays. However, the up and down flavors are very common in the universe and are generally stable. For every quark flavor there is an antiparticle, called an antiquark, that differs from quarks only in that some of their properties are of opposite sign.

Due to a phenomenon known as color confinement, quarks do not exist as free particles in nature.^[4] They are always bound together in composite particles named hadrons.^[4] There are two types of hadrons: mesons (particles made of one quark and one antiquark) and baryons (particles made of three quarks).^[5] Various combinations of the six flavors account for all of the known hadrons. Hadrons are differentiated by the specific quarks they contain and the manner in which they are combined.^[3] The most famous and best known hadrons are protons and neutrons which make up the atomic nucleus. Since quarks are not found in isolation, their properties can only be deduced from experiments on hadrons.^[4] An exception to this rule is the top quark, which decays so rapidly that it does not produce hadrons at all, and instead observed through the identification of the particles it has decayed into.^[6]

Physicists Murray Gell-Mann and George Zweig independently proposed the quark model in 1964.^[7] There was little evidence for the theory until 1968, when electron-proton scatterings indicated the existence of small substructures within the proton.^[8]^[9] By 1995, when the top quark was observed at Fermilab, all the six flavors had been observed and proven.

History

Murray Gell-Mann in 2007. Gell-Mann and George Zweig first proposed the quark model in 1964.

The Gell-Mann?Zweig model predicted three quarks, which they named up, down and strange (, , ). At the time, the pair of physicists ascribed various properties and values to the three new proposed particles, such as electrical charge and spin.^[11] The initial reaction of the physics community to the proposal was mixed, many having reservations regarding the actual physicality of the quark concept. They believed the quark was merely an abstract concept that could be used temporarily to help explain certain concepts that were not well understood, rather than an actual entity that existed in the way that Gell-Mann and Zweig had envisioned.^[10]

In less than a year, extensions to the Gell-Mann?Zweig model were proposed when another duo of physicists, Sheldon Lee Glashow and James Bjorken, predicted the existence of a fourth flavor of quark, which they referred to as charm (). The addition was proposed because it expanded the power and self consistency of the theory: it allowed a better description of the weak interaction (the mechanism that allows quarks to decay); equalized the number of quarks with the number of known leptons; and implied a mass formula that correctly reproduced the masses of the known mesons.^[12]

In 1968, deep inelastic scattering experiments at the Stanford Linear Accelerator Center showed that the proton had substructure.^[13]^[8]^[9] However, while the concept of hadron substructure had been proven, there was still apprehension towards the quark model; the substructures became known at the time as partons, "and it was unfashionable to identify them explicitly with quarks".^[14] These partons were later identified as up and down quarks.^[15] Their discovery also validated the existence of a third strange quark, because it was necessary to the model Gell-Mann and Zweig had proposed.^[16]

In a 1970 paper,^[17] Glashow, John Iliopoulos, and Luciano Maiani gave more compelling theoretical arguments for the as-yet undiscovered charm quark.^[18] The number of proposed quark flavors grew to the current six in 1973, when Makoto Kobayashi and Toshihide Maskawa noted that the experimental observation of CP violation could be explained if there were another pair of quarks. They named the two additional quarks top () and bottom ().^[11]

It was the observation of the charm quark that finally convinced the physics community of the quark model's correctness.^[14] Following a decade without empirical evidence supporting the flavor's existence, it was created and observed almost simultaneously by two teams in November 1974: one at the Stanford Linear Accelerator Center under Samuel Ting and one at Brookhaven National Laboratory under Burton Richter. The two parties had assigned the discovered particle two different names, J and ?. The particle hence became formally known as the J/? meson and it was considered a quark?antiquark pair of the charm flavor that Glashow and Bjorken had predicted, or the charmonium.^[10]

In 1977, the bottom quark was observed by Leon Lederman and a team at Fermilab.^[7] This indicated that a top quark probably existed, because the bottom quark was without a partner. However, it was not until eighteen years later, in 1995, that the top quark was finally observed. The top quark's discovery was quite significant, because it proved to be far more massive than expected, almost as heavy as a gold atom. Reasons for the top quark's extremely large mass remain unclear.^[19]

Etymology

Gell-Mann originally named the quark after the sound ducks make.^[20] For some time, Gell-Mann was undecided on an actual spelling for the term he had coined, until he found the word quark in James Joyce's book Finnegans Wake:

Gell-Mann went into further detail regarding the name of the quark in his book, The Quark and the Jaguar: Adventures in the Simple and the Complex, saying that the pronunciation for quark had been derived from quart, which fitted perfectly with the three-quark theory in that one might have "three quarts of drinks at a bar."^[21]

George Zweig, the co-proposer of the theory, preferred the name ace for the particle he had theorized, but Gell-Mann's terminology came to prominence once the quark model had been commonly accepted.^[22]

Properties

Flavor

Quarks come in six types, or "flavors".^[23] This term has nothing to do with the typical human experience of flavor, but is an arbitrarily named property that comes from a simple everyday word that is easy to comprehend and work with.^[24]

The six flavors are named up, down, charm, strange, top and bottom; the top and bottom flavors are also known as truth and beauty, respectively.^[4] Typically, only the stable up and down flavors are in common natural occurrence; heavier quarks can only be created in high-energy conditions, such as in cosmic rays, and quickly decay into lighter quarks and other particles. Most studies conducted on heavier quarks have been performed in artificially-created conditions such as in particle accelerators.

Flavors are grouped into three generations: the first generation comprises up and down quarks, the second comprises charm and strange, and the third comprises top and bottom. Quarks of higher generations have greater masses and thus are generally less stable than quarks of lower generations.^[24] Leptons are similarly divided into three generations.

For every quark flavor, there is a corresponding antiquark (denoted by the letter for the quark with an overbar, for example for an up antiquark). Much like antimatter in general, antiquarks have the same mass and spin of their respective quarks, but the electric charge and other charges have the opposite sign.^[25] Various quark flavor combinations result in the formation of composite particles known as hadrons. There are two types of hadrons: baryons (made of three quarks) and mesons (made of a quark and an antiquark). The building blocks of the atomic nucleus?the proton and the neutron?are baryons.^[24] There are a great number of known hadrons, and most of them are differentiated by their quark content and the properties that these constituent quarks confer upon them.^[4]

Weak interaction

A depiction of the six quarks' most likely decay modes, with mass increasing from left to right. Decay refers to the process whereby one elementary particle transforms into another elementary particle.

A quark of one flavor can transform, or decay, into a quark of a different flavor by the weak interaction. A quark can decay into a lighter quark by emitting a W boson, or can absorb a W boson to turn into a heavier quark. This mechanism causes the radioactive process known as beta decay, in which a neutron "splits" into a proton, an electron and an antineutrino. This occurs when one of the down quarks in the neutron (composed by ) decays into an up quark by emitting a boson, transforming the neutron into a proton (). The boson then decays into an electron () and an electron antineutrino ().^[26]

Electric charge

A quark can only hold a charge of fractional or non-integer value, either ?1/3 or +2/3 (measured in elementary charges), but the charge of an antiquark can be either +1/3 or ?2/3. The up, charm and top quarks all have charge of +2/3, while the down, strange and bottom quarks have ?1/3. The electrical charge of a hadron is determined by the sum of the charges of the constituent quarks;^[27] the total is always an integer.

The structure of the proton. With two up quarks, each with a charge of +2/3, and one down quark, with a charge of ?1/3, the proton has a +1 charge.

The electric charge of quarks is important in the construction of atoms. The hadron constituents of the atom, the neutron and proton, have charges of 0 and +1 respectively; the neutron is composed of two down quarks and one up quark, and the proton of two up quarks and one down quark. The total electric charge of a nucleus, that is, the number of protons in it, is known as the atomic number, and it is the main difference between atoms of different chemical elements. Atoms usually have as many electrons as protons; since the electric charge of an electron is ?1, the net electric charge of an atom is typically 0. When this is not the case, the atom is ionized.^[28]

Spin

The term spin denotes a property of physical particles corresponding to the rate and speed of a particle's rotation around its own axis. This concept is different in fundamental particles such as quarks, in that spin is an intrinsic property of point-like particles, rather than one derived from smaller components. The spin property is measured in units of h/(2?), where h is the Planck constant. This unit is often denoted by ?, and called the "reduced Planck constant" or the Dirac constant. The component of the spin of a quark along any axis is always either ?/2 or its negative, ??/2; for this reason quarks are referred to as spin-1/2 particles, or fermions.^[29]

In quarks, spin notation uses up arrows ? and down arrows ?, and general quark flavor notation. The flavor of the quark is first denoted using the first character of the flavor name, followed by either ? or ? to signify the values of +1/2 or ?1/2, respectively. For example, an up quark with a positive spin of 1/2 along a given axis would be denoted u?.^[30] The quark's spin value contributes to the overall spin of the parent hadron, much as quark's electrical charge does to the overall charge of the hadron. Varying combinations of quark spins result in the total spin value that can be assigned to the hadron.^[31]

Color

In addition to the electric charge, quarks carry another type of charge called color charge. Despite its name, color charge is not related to color of visible light.^[32] There are three types of color charge a quark can carry, named blue, green and red; each of them is complemented by an anti-color: antiblue, antigreen and antired, respectively. While a quark can have red, green or blue charge, an antiquark can have antired, antigreen, or antiblue charge.

The system of attraction and repulsion between quarks charged with any of the three colors (called strong interaction, and described by quantum chromodynamics) is as follows: a quark charged with one color value will be attracted to an antiquark carrying with the corresponding anticolor, while three quarks all charged with differing colors will similarly be forced together. In any other case, a force of repulsion will come into effect.^[33] Quarks initiate these color interactions via the exchange of a particle known as a gluon, a concept which is discussed below.

It is when the process of hadronization occurs that the three color types become relevant. The products of both instances of attraction will be color neutrality; a quark with n charge plus an antiquark of ?n charge will result in a color charge of 0, or "white". The combination of all three color charge types will similarly result in a canceling out of all color, yielding the same white color type as the interaction between the quark and antiquark. These two methods of color neutral hadronization represent the same ways the two types of hadrons are formed (hadrons must be color neutral); a meson, comprised of two particles, is the result of the binding of a quark and antiquark color charged oppositely, while a baryon, containing three particles, arises from the hadronization of three quarks all charged with different colors.^[34]

Mass

There are two different terms used when describing a quark's mass; current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark.^[35] These two values are typically very different in their relative size, for several reasons.

In a hadron most of the mass comes from the gluons that bind the constituent quarks together, rather than from the individual quarks; the mass of the quarks is almost negligible compared to the mass derived from the gluons' energy. While gluons are inherently massless, they possess energy, and it is this energy that contributes so greatly to the overall mass of the hadron. This is demonstrated by a common hadron?the proton. Composed of one and two quarks, the proton has an overall mass of approximately 938 MeV/c², of which the three quarks contribute around 15 MeV/c², the remainder is from the energy of the gluons.^[36]^[37]

This makes the calculation of quark mass difficult. Often, mass values can be derived after calculating the difference in mass between two related hadrons that have opposing or complementary quark components; for example, the proton to the neutron, where the difference between the two is one down quark to one up quark, the relative masses and the mass differences of which can then be measured by the difference in the overall mass of the two hadrons.^[36]

The masses of most quarks were within predicted ranges at the time of their discovery, with the notable exception of the top quark, which was found to have a mass approximately equal to that of a gold nucleus, around 200 times heavier than the hadron it was thought to form.^[38] Various theories have been offered to explain this very large mass; common predictions assert that the answer to the abnormality will be found when more is known about the top quark's interaction with the Higgs field, and how the Higgs boson produces mass and makes mass possible.^[19]

Table of properties

**Quark flavor properties**^[39]
Generation	Spin	Name	Electric charge	Mass (MeV/c²)	Antiparticle
1	1/2	Up	+2/3	1.5 to 3.3	Antiup
1	1/2	Down	?1/3	3.5 to 6.0	Antidown
2	1/2	Charm	+2/3	1160 to 1340	Anticharm
2	1/2	Strange	?1/3	70 to 130	Antistrange
3	1/2	Top	+2/3	169,100 to 173,300	Antitop
3	1/2	Bottom	?1/3	4130 to 4370	Antibottom

Color confinement and gluons

A phenomenon called color confinement comes into effect within hadrons. This refers to a quark's inability to be separated from its hadron, therefore rendering isolated observation impossible. This makes direct observation impossible for all quarks except the top; instead, what is known about quarks has been inferred from the effect they have on their parent hadron's properties.^[40]^[41] The top quark is an exception because its lifetime is so short that it does not have a chance to hadronize.^[6] One method used is comparing two hadrons that have all but one quark in common, the properties of the different quark are inferred from the difference in values between the two hadrons. Color confinement is primarily caused by interactions with particles known as gluons.

Quarks have an inherent relationship with the gluon, which is technically a massless vector gauge boson. Gluons are responsible for the color field, or the strong interaction, that ensures that quarks remain bound in hadrons and instigates color confinement, and are the subjects of the quantum chromodynamics research area.^[42] Gluons, roughly speaking, carry both a color charge and an anti-color charge, for example red?antiblue.^[43]^[44]

Gluons are constantly exchanged between quarks through an emission and reception process. These gluon exchange events between quarks are extremely frequent, occurring approximately 10²⁴ times every second.^[45] When a gluon is transferred between one quark and another, a color change comes into effect in the receiving and emitting quark.^[36]^[46] These constant switches in color within quarks are mediated by the gluons in such a way that a bound hadron will constantly retain a dynamic and ever-changing set of color types that will preserve the force of attraction, therefore forever disallowing quarks to exist in isolation.^[47]

The color field the gluon creates is structured with a mechanism that contributes to a hadron's indivisibility. This is demonstrated by the varying strength of the binding force between the constituent quarks of a hadron; as quarks come closer to each other, the binding force actually weakens (this is called asymptotic freedom), but while they drift further apart, the strength of the bind dramatically increases. This is because as the color field is stressed by the drifting away of a quark, much as an elastic band is stressed when pulled apart, a proportionate and necessary multitude of gluons of appropriate color property are created to strengthen the stretched field. In this way, an infinite amount of energy would be required to wrench a quark from its hadronized state.^[48]

These strong interactions are non-linear, because gluons can emit gluons and exchange gluons with other gluons. This property has led to postulations regarding the possible existence of a particle that is purely gluon?a glueball?despite previous observations indicating that gluons cannot exist without attached quarks.^[49]

Sea quarks

Those quarks that make up the core of the hadron are called valence quarks. These quarks are generally stable, and are the quarks that contribute to the quantum numbers of their hadrons. However, from the gluons' strong interaction field are born short-lived, virtual quark?antiquark () pairs, known as sea quarks. These sea quarks are much less stable, and they annihilate each other very quickly within the interior of the hadron. They are born from the splitting of a gluon, but when the sea quark is annihilated, new gluons are produced.^[50] There is a constant quantum flux of sea quarks that are born from the vacuum, and this allows for a constant cycle of gluon splits and rebirths. This flux is colloquially known as "the sea".^[51]