In particle physics, the phi meson or ϕ meson
is a vector meson formed of a strange quark
and a strange antiquark. It was the ϕ meson's
unusual propensity to decay into K0 and K0
that led to the discovery of the OZI rule.
It has a mass of 1019.445±0.020 MeV/c2 and
a mean lifetime of 1.55±0.01 × 10−22s.
== Properties ==
The most common decay modes of the ϕ meson
are K+K− at 48.9%±0.5%, K0SK0L at 34.2%±0.4%,
and various indistinguishable combinations
of ρs and pions at 15.3%±0.3%. In all cases,
it decays via the strong force. The pion channel
would naïvely be the dominant decay channel
because the collective mass of the pions is
smaller than that of the kaons, making it
energetically favorable; however, it is suppressed
by the OZI rule.
The quark composition of the ϕ meson can
be thought of as a mix between ss, uu, and
dd states, but it is very nearly a pure ss
state. This can be shown by deconstructing
the wave function of the ϕ into its component
parts. We see that the ϕ and ω mesons are
mixtures of the SU(3) wave functions as follows.
ϕ
=
ψ
8
cos
⁡
θ
−
ψ
1
sin
⁡
θ
{\displaystyle \phi =\psi _{8}\cos \theta
-\psi _{1}\sin \theta }
,
ω
=
ψ
8
sin
⁡
θ
+
ψ
1
cos
⁡
θ
{\displaystyle \omega =\psi _{8}\sin \theta
+\psi _{1}\cos \theta }
,
where
θ
{\displaystyle \theta }
is the nonet mixing angle,
ψ
8
=
u
u
¯
−
d
d
¯
−
2
s
s
¯
6
{\displaystyle \psi _{8}={\frac {u{\overline
{u}}-d{\overline {d}}-2s{\overline {s}}}{\sqrt
{6}}}}
and
ψ
1
=
u
u
¯
+
d
d
¯
+
s
s
¯
3
{\displaystyle \psi _{1}={\frac {u{\overline
{u}}+d{\overline {d}}+s{\overline {s}}}{\sqrt
{3}}}}
.
The mixing angle at which the components decouple
completely can be calculated to be about 35.3˚.
The mixing angle of the ϕ and ω states is
calculated from the masses of each state to
be about 35˚, which is very close to maximum
decoupling. Therefore, the ϕ meson is nearly
a pure ss state.
== History ==
The existence of the ϕ meson was first proposed
by the Japanese American particle physicist,
J. J. Sakurai, in 1962 as a resonance state
between the K0 and the K0. It was discovered
later in 1962 by Connolly, et al. in a 20-inch
hydrogen bubble chamber at the Alternating
Gradient Synchrotron (AGS) in Brookhaven National
Laboratory in Uptown, NY while they were studying
K−p+ collisions at approximately 2.23 GeV/c.
In essence, the reaction involved a beam of
K−s being accelerated to high energies to
collide with protons.
The ϕ meson has several possible decay modes.
The most energetically favored mode involves
the ϕ meson decaying into 3 pions, which
is what would naïvely be expected. However,
we instead observe that it decays most frequently
into 2 kaons. Between 1963 and 1966, 3 people,
Susumu Okubo, George Zweig and Jugoro Iizuka,
each independently proposed a rule to account
for the observed suppression of the 3 pion
decay. This rule is now known as the OZI rule
and is also the currently accepted explanation
for the unusually long lifetimes of the J/ψ
and ϒ mesons. Namely, on average they last
~ 7 × 10−21 s and ~ 1.5 × 10−20 s respectively.
This is compared to the normal mean lifetime
of a meson decaying via the strong force,
which is on the order of 10−23 s.In 1999,
a ϕ factory named DAFNE (or DAϕNE since
the F stands for "ϕ Factory") began operation
to study the decay of the ϕ meson in Frascati,
Italy. It produces ϕ mesons via electron-positron
collisions. It has numerous detectors, including
the KLOE detector which was in operation at
the beginning of its operation
