Composition |
|
---|---|
Statistics | Bosonic |
Family | Mesons |
Interactions | Strong, Weak, Gravitation, Electromagnetic |
Symbol | η , η′ |
Antiparticle | Self |
Discovered | Aihud Pevsner et al. (1961) |
Types | 2 |
Mass | η : 547.862±0.018 MeV/c2 [1] η′ : 957.78±0.06 MeV/c2 [1] |
Mean lifetime | η : (5.0±0.3)×10−19 s, η′ : (3.2±0.2)×10−21 s |
Decays into | |
Electric charge | 0 e |
Spin | 0 |
Isospin | 0 |
Hypercharge | 0 |
Parity | -1 |
C parity | +1 |
The eta (
η
) and eta prime meson (
η′
) are isosinglet
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 similar forms of
quarkonium; they have the same
spin and
parity as the (light)
η
defined, 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.
The eta was discovered in pion– nucleon collisions at the Bevatron 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. [2]
The difference between the mass of the
η
and that of the
η′
is larger than the
quark model can naturally explain. This "
η
–
η′
puzzle" can be resolved
[3]
[4]
[5] by the 't Hooft
instanton mechanism,
[6] whose 1/ N realization is also known as the Witten–Veneziano mechanism.
[7]
[8] Specifically, in QCD, the higher mass of the
η′
is very significant, since it is associated with the axial UA(1) classical symmetry, which is explicitly broken through the
chiral anomaly upon quantization; thus, although the "protected"
η
mass is small, the
η′
is not.
The
η
particles belong to the "pseudo-scalar" 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, hence all the main quantum numbers are zero, and the particle overall is
"flavourless".
The basic SU(3) symmetry theory of quarks for the three lightest quarks, which only takes into account the strong force, predicts corresponding particles
and
The subscripts are labels that refer to the fact that η1 belongs to a singlet (which is fully antisymmetrical) and η8 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 θP = −11.5°), [11] so that the actual quark composition is a linear combination of these formulae. That is:
The unsubscripted name
η
refers to the real particle which is actually observed and which is close to the η8. The
η′
is the observed particle close to η1.
[10]
The
η
and
η′
particles are closely related to the better-known neutral
pion
π0
, where
In fact,
π0
, η1, and η8 are three
mutually orthogonal, linear combinations of the quark pairs
u
u
,
d
d
, and
s
s
; they are at the centre of the pseudo-scalar nonet of mesons
[9]
[10] with all the main quantum numbers equal to zero.
The η′ meson (
η′
) is a flavor SU(3) singlet, unlike the
η
. It is a different superposition of the same quarks as the eta meson (
η
), 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
η′
.