In
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) a ...
, a pseudoscalar meson is a
meson 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 ...
(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 (),
kaon (),
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
pion () was first proposed to exist by
Yukawa in the 1930s as the primary force carrying
boson 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'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 eta () and eta prime meson () are isosinglet mesons made of a mixture of up, down and strange quarks and their antiquarks. The charmed eta meson () and bottom eta meson () are similar forms of quarkonium; they have the same spin and pari ...
(). 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 Ei ...
.
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 common ...
, 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 957.78 MeV/c
2 can be explained, relative to its model-predicted mass around 250–300 MeV/c
2.
List of pseudoscalar mesons
:
See also
*
List of mesons
*
Vector meson
*
Pseudovector meson
In high energy physics, a pseudovector meson or axial vector meson is a meson with total spin 1 and even parity (+) (usually noted as
:Compare to a vector meson, which has a total spin 1 and odd parity
Charge parity (C) in addition ...
*
Pseudoscalar boson
Footnotes
References
Mesons
Bosons
Subatomic particles with spin 0
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