In
high-energy physics
Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the scale of protons and neutrons, while the stu ...
, a pseudoscalar meson is a
meson
In particle physics, a meson () 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, the ...
with
total spin 0 and odd
parity (usually notated as
Pseudoscalar mesons are commonly seen in
proton
A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
–proton scattering and proton–antiproton annihilation, and include the
pion
In particle physics, a pion (, ) or pi meson, denoted with the Greek alphabet, Greek letter pi (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 ...
(),
kaon (),
eta
Eta ( ; uppercase , lowercase ; ''ē̂ta'' or ''ita'' ) is the seventh letter of the Greek alphabet, representing the close front unrounded vowel, . Originally denoting the voiceless glottal fricative, , in most dialects of Ancient Greek, it ...
(), 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
pion
In particle physics, a pion (, ) or pi meson, denoted with the Greek alphabet, Greek letter pi (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 ...
() 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 half odd-intege ...
of the
Yukawa potential 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 according to
Murray Gell-Mann
Murray Gell-Mann (; September 15, 1929 – May 24, 2019) was an American theoretical physicist who played a preeminent role in the development of the theory of elementary particles. Gell-Mann introduced the concept of quarks as the funda ...
'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 that give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Eig ...
.
η–η′ 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, 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 mixing into the discussion. It is possible that the and mesons mix with the pseudoscalar
glueball 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 can be explained, relative to its model-predicted mass around .
List of pseudoscalar mesons
:
See also
*
List of mesons
: ''This list is of all known and predicted scalar, pseudoscalar and vector mesons. See list of particles for a more detailed list of particles found in particle physics.''
This article contains a list of mesons, unstable subatomic particles ...
*
Vector meson
In high energy physics, a vector meson is a meson with total spin 1 and odd parity (usually noted as ). Vector mesons have been seen in experiments since the 1960s, and are well known for their spectroscopic pattern of masses.
The vector mes ...
*
Pseudovector meson
*
Pseudoscalar boson
Footnotes
References
Mesons
Bosons
Subatomic particles with spin 0
Murray Gell-Mann
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