high-energy physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and b ...

, a pseudoscalar meson is a

meson In particle physics, a meson ( or ) is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles ...

with total spin 0 and odd

parity Parity may refer to: * Parity (computing) ** Parity bit in computing, sets the parity of data for the purpose of error detection ** Parity flag in computing, indicates if the number of set bits is odd or even in the binary representation of the r ...

(usually notated as Pseudoscalar mesons are commonly seen in

proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...

-proton scattering and proton-antiproton annihilation, and include the

pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more gene ...

(),

kaon KAON (Karlsruhe ontology) is an ontology infrastructure developed by the University of Karlsruhe and the Research Center for Information Technologies in Karlsruhe. Its first incarnation was developed in 2002 and supported an enhanced version of ...

(),

eta Eta (uppercase , lowercase ; grc, ἦτα ''ē̂ta'' or ell, ήτα ''ita'' ) is the seventh letter of the Greek alphabet, representing the close front unrounded vowel . Originally denoting the voiceless glottal fricative in most dialects, ...

(), and eta prime () particles, whose masses are known with great precision. Among all of the mesons known to exist, in some sense, the pseudoscalars are the most well studied and understood.

History

The

() was first proposed to exist by Yukawa in the 1930s as the primary force carrying

boson In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0,1,2 ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have odd half-integer s ...

of the

Yukawa potential In particle, atomic and condensed matter physics, a Yukawa potential (also called a screened Coulomb potential) is a potential named after the Japanese physicist Hideki Yukawa. The potential is of the form: :V_\text(r)= -g^2\frac, where is a m ...

in nuclear interactions, and was later observed at nearly the same mass that he originally predicted for it. In the 1950s and 1960s, the pseudoscalar mesons began to proliferate, and were eventually organized into a

multiplet In physics and particularly in particle physics, a multiplet is the state space for 'internal' degrees of freedom of a particle, that is, degrees of freedom associated to a particle itself, as opposed to 'external' degrees of freedom such as the ...

according to

Murray Gell-Mann Murray Gell-Mann (; September 15, 1929 – May 24, 2019) was an American physicist who received the 1969 Nobel Prize in Physics for his work on the theory of elementary particles. He was the Robert Andrews Millikan Professor of Theoretical ...

's so-called " Eightfold Way". Gell-Mann further predicted the existence of a ninth resonance in the pseudoscalar multiplet, which he originally called . Indeed, this particle was later found and is now known as the eta prime meson (). The structure of the pseudoscalar meson multiplet, and also the ground state baryon multiplets, led Gell-Mann (and Zweig, independently) to create the well known

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 E ...

The puzzle

Despite the pseudoscalar mesons' masses being known to high precision, and being the most well studied and understood mesons, the decay properties of the pseudoscalar mesons, particularly of eta () and eta-prime (), are somewhat contradictory to their mass hierarchy: While the meson is much more massive than the meson, the meson is thought to contain a larger component of the relatively heavy strange and anti-strange

quarks 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. All commonly o ...

, than the η′ meson does, which appears contradictory. This failure of the quark model to explain this mass difference is called the " puzzle". The presence of an (1405) state also brings

glueball In particle physics, a glueball (also gluonium, gluon-ball) is a hypothetical composite particle. It consists solely of gluon particles, without valence quarks. Such a state is possible because gluons carry color charge and experience the strong ...

mixing into the discussion. It is possible that the and mesons mix with the pseudoscalar

which should occur somewhere above the scalar glueball in mass, as an unmixed state. This is one of a few ways in which the unexpectedly large mass of 957.78 MeV/c² can be explained, relative to its model-predicted mass around 250–300 MeV/c².

List of pseudoscalar mesons

Footnotes

References

Mesons Bosons Subatomic particles with spin 0 {{particle-stub

History

The puzzle

List of pseudoscalar mesons

See also

Footnotes

References