Topness

Last updated

Topness (T, also called truth), a flavour quantum number, represents the difference between the number of top quarks (t) and number of top antiquarks (t) that are present in a particle:

Top quark type of quark

The top quark, also known as the t quark or truth quark, is the most massive of all observed elementary particles. Like all quarks, the top quark is a fermion with spin 1/2, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. It has an electric charge of +2/3 e. It has a mass of 172.44 ± 0.13 (stat) ± 0.47 (syst)GeV/c2, which is about the same mass as an atom of tungsten. The antiparticle of the top quark is the top antiquark, which differs from it only in that some of its properties have equal magnitude but opposite sign.

Contents

By convention, top quarks have a topness of +1 and top antiquarks have a topness of 1.The term "topness" is rarely used; most physicists simply refer to "the number of top quarks" and "the number of top antiquarks".

Conservation

Like all flavour quantum numbers, topness is preserved under strong and electromagnetic interactions, but not under weak interaction. However the top quark is extremely unstable, with a half-life under 1023 s, which is the required time for the strong interaction to take place. For that reason the top quark does not hadronize, that is it never forms any meson or baryon, so the topness of a meson or a baryon is always zero. By the time it can interact strongly it has already decayed to another flavour of quark (usually to a bottom quark).

Strong interaction force between hadrons that remains constant at any distance as the hadrons travel within a nucleus

In particle physics, the strong interaction is the mechanism responsible for the strong nuclear force (also called the strong force, nuclear strong force, or colour force), and is one of the four known fundamental interactions, with the others being electromagnetism, the weak interaction, and gravitation. At the range of 10−15 m (1 femtometer), the strong force is approximately 137 times as strong as electromagnetism, a million times as strong as the weak interaction, and 1038 times as strong as gravitation. The strong nuclear force holds most ordinary matter together because it confines quarks into hadron particles such as the proton and neutron. In addition, the strong force binds neutrons and protons to create atomic nuclei. Most of the mass of a common proton or neutron is the result of the strong force field energy; the individual quarks provide only about 1% of the mass of a proton.

Weak interaction the fundamental interaction responsible for beta decay and nuclear fission

In particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is the mechanism of interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak interaction serves an essential role in nuclear fission, and the theory regarding it in terms of both its behavior and effects is sometimes called quantum flavordynamics (QFD). However, the term QFD is rarely used because the weak force is better understood in terms of electroweak theory (EWT). In addition to this, QFD is related to quantum chromodynamics (QCD), which deals with the strong interaction, and quantum electrodynamics (QED), which deals with the electromagnetic force.

Half-life is the time required for a quantity to reduce to half its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo, or how long stable atoms survive, radioactive decay. The term is also used more generally to characterize any type of exponential or non-exponential decay. For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the human body. The converse of half-life is doubling time.

Related Research Articles

Baryon proton or any other hadron that is composed of three quarks

In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks. Baryons belong to the hadron family of particles, which are the quark-based particles. They are also classified as fermions, i.e., they have half-integer spin.

Elementary particle quantum particle having no known substructure; quark, electron, photon, etc.

In particle physics, an elementary particle or fundamental particle is a subatomic particle with no sub structure, thus not composed of other particles. Particles currently thought to be elementary include the fundamental fermions, which generally are "matter particles" and "antimatter particles", as well as the fundamental bosons, which generally are "force particles" that mediate interactions among fermions. A particle containing two or more elementary particles is a composite particle.

In particle physics, a hadron is a composite particle made of two or more quarks held together by the strong force in a similar way as molecules are held together by the electromagnetic force. Most of the mass of ordinary matter comes from two hadrons, the proton and the neutron.

Meson subatomic particle; hadron made of equal number of quark(s) and antiquark(s): normally one of each

In particle physics, mesons are hadronic subatomic particles composed of one quark and one antiquark, bound together by strong interactions. Because mesons are composed of quark subparticles, they have physical size, notably a diameter of roughly one femtometer, which is about 1.2 times the size of a proton or neutron. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. Charged mesons decay to form electrons and neutrinos. Uncharged mesons may decay to photons. Both of these decays imply that color is no longer a property of the byproducts.

Quark elementary particle of which the proton or any other other baryon or meson is composed

A quark is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons, which include baryons and mesons. For this reason, much of what is known about quarks has been drawn from observations of hadrons.

Pentaquark hypothetical type of subatomic particle

A pentaquark is a subatomic particle consisting of four quarks and one antiquark bound together.

Annihilation quantum field theoretic process of particle–antiparticle annihilation

In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy and momentum of the initial pair are conserved in the process and distributed among a set of other particles in the final state. Antiparticles have exactly opposite additive quantum numbers from particles, so the sums of all quantum numbers of such an original pair are zero. Hence, any set of particles may be produced whose total quantum numbers are also zero as long as conservation of energy and conservation of momentum are obeyed.

In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as

In particle physics, strangeness (S) is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactions which occur in a short period of time. The strangeness of a particle is defined as:

In particle physics, the hyperchargeY of a particle is related to the strong interaction, and is distinct from the similarly named weak hypercharge, which has an analogous role in the electroweak interaction. The concept of hypercharge combines and unifies isospin and flavour into a single charge operator.

In nuclear physics and particle physics, isospin (I) is a quantum number related to the strong interaction. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions of baryons and mesons.

Charm is a flavour quantum number representing the difference between the number of charm quarks and charm antiquarks that are present in a particle:

In physics, bottomness or beauty is a flavour quantum number reflecting the difference between the number of bottom antiquarks and the number of bottom quarks that are present in a particle:

In particle physics, flavour or flavor refers to the species of an elementary particle. The Standard Model counts six flavours of quarks and six flavours of leptons. They are conventionally parameterized with flavour quantum numbers that are assigned to all subatomic particles. They can also be described by some of the family symmetries proposed for the quark-lepton generations.

Exotic hadron

Exotic hadrons are subatomic particles composed of quarks and gluons, but which - unlike "well-known" hadrons such as protons, neutrons and mesons - consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.

Quark model

In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Eightfold Way, the successful classification scheme organizing the large number of lighter hadrons that were being discovered starting in the 1950s and continuing through the 1960s. It received experimental verification beginning in the late 1960s and is a valid effective classification of them to date. The quark model was independently proposed by physicists Murray Gell-Mann, and George Zweig in 1964. Today, the model has essentially been absorbed as a component of the established quantum field theory of strong and electroweak particle interactions, dubbed the Standard Model.

The Lambda baryons are a family of subatomic hadron particles containing one up quark, one down quark, and a third quark from a higher flavour generation, in a combination where the quantum wave function changes sign upon the flavour of any two quarks being swapped. They are thus baryons, with total isospin of 0, and have either neutral electric charge or the elementary charge +1.

The Sigma baryons are a family of subatomic hadron particles which have two quarks from the first flavour generation, and a third quark from higher flavour generations, in a combination where the wavefunction does not swap sign when any two quark flavours are swapped. They are thus baryons, with Total Isospin of 1, and can either be neutral or have an elementary charge of +2, +1 or −1. They are closely related to the Lambda baryons, which differ only in the wavefunction's behaviour upon flavour exchange.

References

    Further reading

    Francis Louis Halzen is a Belgian-American particle physicist, known for the development of the IceCube Neutrino Observatory at the Amundsen-Scott South Pole Station in Antarctica which has been operational since 2010.

    arXiv online digital archive for electronic preprints of scientific papers

    arXiv is a repository of electronic preprints approved for posting after moderation, but not full peer review. It consists of scientific papers in the fields of mathematics, physics, astronomy, electrical engineering, computer science, quantitative biology, statistics, mathematical finance and economics, which can be accessed online. In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv repository. Begun on August 14, 1991, arXiv.org passed the half-million-article milestone on October 3, 2008, and had hit a million by the end of 2014. By October 2016 the submission rate had grown to more than 10,000 per month.