Product Code Database
| \left(\mathrm{u\bar{u} + d\bar{d} + s\bar{s}}\right)}}
}}
| statistics = Bosonic
| group =
| interaction = Strong, Weak interaction, Gravity, electromagnetic
| antiparticle = Self
| status =
| theorized =
| discovered = Aihud Pevsner et al. (1961)
| symbol = ,
| mass = :| electric_charge = | spin = 0 ħ | strangeness = | charm = | bottomness = | topness = | isospin = 0 | hypercharge = 0 | parity = −1 | c_parity = +1
or
:
The eta () and eta prime meson () are isosinglet made of a mixture of up quark, down quark and strange quark and their . The charmed eta meson () and bottom eta meson () are similar forms of quarkonium; they have the same spin and parity as the (light) defined, but are made of and respectively. The top quark is too heavy to form a similar meson, due to its very fast decay.
The difference between the mass of the and that of the is larger than the quark model can naturally explain. This "– puzzle" can be resolved by the 't Hooft instanton 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 upon quantization; thus, although the "protected" mass is small, the is not.
The basic SU(3) symmetry theory of quarks for the three lightest quarks, which takes into account only the strong force, predicts corresponding particles
The subscripts are labels that refer to the fact that η belongs to a singlet (which is fully antisymmetrical) and η is part of an octet. However, the electroweak interaction – which can transform one flavour of quark into another – causes a small but significant amount of "mixing" of the eigenstates (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 η. The is the observed particle close to η.
The and particles are closely related to the better-known neutral pion , where
In fact, , η, and η 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.
Fundamentally, it results from the direct sum decomposition of the approximate SU(3) flavor symmetry among the three lightest quarks, , where 1 corresponds to η1 before slight quark mixing yields .
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, and hypercharge. 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".
and
\cos\theta_\mathrm{P} & - \sin\theta_\mathrm{P} \\
\sin\theta_\mathrm{P} & ~~\cos\theta_\mathrm{P}
\end{array}\right) \left(\begin{array}{c} \mathrm{\eta}_8 \\
\mathrm{\eta}_1 \end{array}\right) =
\left(\begin{array}{c} \mathrm{\eta} \\ \mathrm{\eta'} \end{array}\right) ~.
η′ meson
The eta prime 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.
See also
External links
|
|
