Eta meson
Composition 
η : ≈ \mathrm{\tfrac{u\bar{u} + d\bar{d}  2s\bar{s}}{\sqrt{6}}} η′ : ≈ \mathrm{\tfrac{u\bar{u} + d\bar{d} + s\bar{s}}{\sqrt{3}}} 

Statistics  Bosonic 
Interactions  Strong, Weak, Gravitation 
Symbol  η, η′ 
Antiparticle  Self 
Mass 
η : ±0.018 MeV/c^{2} 547.862^{[1]} η′ : ±0.06 MeV/c^{2} 957.78^{[1]} 
Mean lifetime  η: ±0.3)×10^{−19} s (5.0, η′: ±0.2)×10^{−21} s (3.2 
Decays into 
η : 
Electric charge  e 0 
Spin  Integer 
The eta (η) and eta prime meson (η′) are mesons made of a mixture of up, down and strange quarks and their antiquarks. The charmed eta meson (η
c) and bottom eta meson (η
b) are forms of quarkonium; they have the same spin and parity as the light eta but are made of charm quarks and bottom quarks respectively. The top quark is too heavy to form a similar meson, due to its very fast decay.
Contents
 General 1
 Quark composition 2
 Eta Prime Meson 3
 See also 4
 External links 5
 References 6
General
The eta was discovered in pionnucleon collisions at the Bevatron in 1961 by A. 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.^{[2]}
The difference between the mass of the η and that of the η' is larger than the quark model can naturally explain. This "ηη' puzzle" can be resolved^{[3]}^{[4]}^{[5]} by the 't Hooft instanton mechanism,^{[6]} whose 1/N realization is also known as WittenVeneziano mechanism.^{[7]}^{[8]}
Quark composition
The η particles belong to the "pseudoscalar" nonet of mesons which have spin J = 0 and negative parity,^{[9]}^{[10]} and η and η′ have zero total isospin, I, and zero strangeness and hypercharge. Each quark which appears in an η particle is accompanied by its antiquark (the particle overall is "flavourless") and all the main quantum numbers are zero.
The basic SU(3) symmetry theory of quarks for the three lightest quarks, which only takes into account the strong force, predicts corresponding particles
 \eta_1 = \mathrm{\tfrac{u\bar{u} + d\bar{d} + s\bar{s}}{\sqrt{3}}}, and
 \eta_8 = \mathrm{\tfrac{u\bar{u} + d\bar{d}  2s\bar{s}}{\sqrt{6}}}.
The subscripts refer to the fact that η_{1} belongs to a singlet (which is fully antisymmetrical) and η_{8} is part of an octet. However in this case the weak and electromagnetic forces, which can transform one flavour of quark into another, cause a significant, though small, amount of "mixing" of the eigenstates (with mixing angle θ_{P} = −11.5 degrees),^{[11]} so that the actual quark composition is a linear combination of these formulae. That is:
 \left( \begin{array}{cc} \cos\theta_\mathrm{P} &  \sin\theta_\mathrm{P} \\ \sin\theta_\mathrm{P} & \cos\theta_\mathrm{P} \end{array}\right) \left( \begin{array}{c} \eta_8 \\ \eta_1 \end{array}\right) = \left( \begin{array}{c} \eta \\ \eta' \end{array} \right).
The unsubscripted name η refers to the real particle which is actually observed and which is close to the η_{8}. The η′ is the observed particle close to η_{1}.^{[10]}
The η and η′ particles are closely related to the betterknown neutral pion π0, where
 \pi^0 = \mathrm{\tfrac{u\bar{u}  d\bar{d}}{\sqrt{2}}}.
In fact π^{0}, η_{1} and η_{8} are three mutually orthogonal linear combinations of the quark pairs uu, dd and ss; they are at the centre of the pseudoscalar nonet of mesons^{[9]}^{[10]} with all the main quantum numbers equal to zero.
Eta Prime Meson
The Eta Prime Meson is essentially a superposition of the Eta Meson, the only significant differences being a higher mass, a different decay state, and a shorter lifetime.
See also
External links
 Eta Meson at the Particle Data Group
References
 ^ ^{a} ^{b} Light Unflavored Mesons as appearing in
 ^
 ^
 ^
 ^
 ^
 ^
 ^
 ^ ^{a} ^{b} The WorldHeritage meson article describes the SU(3) pseudoscalar nonet of mesons including η and η′.
 ^ ^{a} ^{b} ^{c} Page 150 describes the SU(3) pseudoscalar nonet of mesons including η and η′. Page 154 defines η_{1} and η_{8} and explains the mixing (leading to η and η′).
 ^ Quark Model Review as appearing in
