The eta () and eta prime meson () are isosinglet
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 ...
s made of a mixture of
up,
down and
strange
Strange may refer to:
Fiction
* Strange (comic book), a comic book limited series by Marvel Comics
* Strange (Marvel Comics), one of a pair of Marvel Comics characters known as The Strangers
* Adam Strange, a DC Comics superhero
* The title c ...
quark
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 nucleus, atomic nuclei ...
s and their
antiquark
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 ...
s. The
charmed eta meson () and
bottom eta meson () are similar forms of
quarkonium
In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavor (physics), flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. The name "quarko ...
; they have the same
spin and
parity as the (light) defined, but are made of
charm quark
The charm quark, charmed quark, or c quark is an elementary particle found in composite subatomic particles called hadrons such as the J/psi meson and the charmed baryons created in particle accelerator collisions. Several bosons, including th ...
s and
bottom quark
The bottom quark, beauty quark, or b quark, is an elementary particle of the third generation. It is a heavy quark with a charge of − ''e''.
All quarks are described in a similar way by electroweak interaction and quantum chromodynamic ...
s respectively. The
top quark
The top quark, sometimes also referred to as the truth quark, (symbol: t) is the most massive of all observed elementary particles. It derives its mass from its coupling to the Higgs field. This coupling is very close to unity; in the Standard ...
is too heavy to form a similar meson, due to its very fast decay.
General
The eta was discovered in
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 ...
–
nucleon
In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number.
Until the 1960s, nucleons were thought to be ele ...
collisions at the
Bevatron
The Bevatron was a particle accelerator — specifically, a Weak focusing, weak-focusing proton synchrotron — located at Lawrence Berkeley National Laboratory, U.S., which began operations in 1954. The antiproton was discovered there in ...
in 1961 by
Aihud Pevsner et al. at a time when the proposal of the
Eightfold Way was leading to predictions and discoveries of new particles from symmetry considerations.
[
The difference between the mass of the and that of the is larger than the ]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 ...
can naturally explain. This " puzzle" can be resolved[ by the 't Hooft ]instanton
An instanton (or pseudoparticle) is a notion appearing in theoretical and mathematical physics. An instanton is a classical solution to equations of motion with a finite, non-zero action, either in quantum mechanics or in quantum field theory. M ...
mechanism,[ whose realization is also known as the Witten–Veneziano mechanism.][ Specifically, in QCD, the higher mass of the is very significant, since it is associated with the axial U(1) classical symmetry, which is ''explicitly broken'' through the ]chiral anomaly
In theoretical physics, a chiral anomaly is the anomalous nonconservation of a chiral current. In everyday terms, it is analogous to a sealed box that contained equal numbers of left and right-handed bolts, but when opened was found to have mor ...
upon quantization; thus, although the "protected" mass is small, the is not.
Quark composition
The particles belong to the "pseudo-scalar" nonet of mesons which have spin and negative parity,[ and and have zero total isospin, , and zero ]strangeness
In particle physics, strangeness (symbol ''S'') is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic interactions that occur in a short period of time. The strangeness of a ...
, and hypercharge
In particle physics, the hypercharge (a portmanteau of hyperonic and charge (physics), charge) ''Y'' of a subatomic particle, particle is a quantum number conserved under the strong interaction. The concept of hypercharge provides a single charg ...
. Each quark which appears in an particle is accompanied by its antiquark, hence all the main quantum numbers are zero, and the particle overall is "flavourless".
The basic SU(3)
In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant 1.
The matrices of the more general unitary group may have complex determinants with absolute value 1, rather than real 1 i ...
symmetry theory of quarks for the three lightest quarks, which only takes into account the strong force
In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interactions. It confines quarks into protons, neutrons, and other hadron particles, an ...
, predicts corresponding particles
:
and
:
The subscripts are labels that refer to the fact that belongs to a singlet (which is fully antisymmetrical) and is part of an octet. However, the electroweak interaction
In particle physics, the electroweak interaction or electroweak force is the unified description of two of the fundamental interactions of nature: electromagnetism (electromagnetic interaction) and the weak interaction. Although these two force ...
– which can transform one flavour of quark into another – causes a small but significant amount of " mixing" of the eigenstates
In quantum physics, a quantum state is a mathematical entity that embodies the knowledge of a quantum system. Quantum mechanics specifies the construction, evolution, and measurement of a quantum state. The result is a prediction for the system re ...
(with mixing angle [ so that the actual quark composition is a linear combination of these formulae. That is:
:
The unsubscripted name refers to the real particle which is actually observed and which is close to the . The is the observed particle close to .][
The and particles are closely related to the better-known neutral ]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 ...
where
:
In fact, , and are three mutually orthogonal, linear combinations of the quark pairs , , and ; they are at the centre of the pseudo-scalar nonet of mesons[ with all the main quantum numbers equal to zero.
]
η′ meson
The η′ meson () is a flavor SU(3) singlet, unlike the . It is a different superposition of the same quarks as the eta meson (), as described above, and it has a higher mass, a different decay state, and a shorter lifetime.
Fundamentally, it results from the direct sum decomposition of the approximate SU(3) flavor symmetry among the 3 lightest quarks, , where 1 corresponds to η1 before s light quark mixing yields .
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 ...
* Quarkonium
In particle physics, quarkonium (from quark and -onium, pl. quarkonia) is a flavor (physics), flavorless meson whose constituents are a heavy quark and its own antiquark, making it both a neutral particle and its own antiparticle. The name "quarko ...
* Special unitary group
In mathematics, the special unitary group of degree , denoted , is the Lie group of unitary matrices with determinant 1.
The matrices of the more general unitary group may have complex determinants with absolute value 1, rather than real 1 ...
References
External links
Eta
an
Eta' meson summaries
at the Particle Data Group
{{Authority control
Mesons