# Physics:Phi meson

Composition Feynman diagram of the most common ϕ meson decay ϕ0: ss Bosonic Strong, Weak, Gravity, Electromagnetism ϕ, ϕ0 Self J. J. Sakurai (1962) Connolly et al. (1962) 1 1019.461±0.020 MeV/c2 (1.55±0.01)×10−22 s 0 1 0 0 -1 -1

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 Kaon0 and Antikaon0 that led to the discovery of the OZI rule. It has a mass of 1019.461±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 Kaon+Kaon- at 48.9%±0.5%, K-short0K0L at 34.2%±0.4%, and various indistinguishable combinations of rhos and pions at 15.3%±0.3%.[1] 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.

Particle name Particle
symbol
Antiparticle
symbol
Quark
content
Rest mass (MeV/c2) IG JPC S C B' Mean lifetime (s) Commonly decays to

(>5% of decays)

Phi meson[2] ϕ(1020) Self Strange quarkStrange antiquark 1,019.461 ± 0.020 0 1−− 0 0 0 1.55 ± 0.01 × 10−22[f] Kaon+ + Kaon- or
K-short0 + K0L or
(rho + pion) / (pion+ + pion0 + pion-)

The quark composition of the ϕ meson can be thought of as a mix between ss, up quarkup antiquark, and down quarkdown antiquark states, but it is very nearly a pure ss state.[3] 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.

$\displaystyle{ \phi = \psi_8 \cos\theta - \psi_1 \sin\theta }$,
$\displaystyle{ \omega = \psi_8 \sin\theta + \psi_1 \cos\theta }$,

where

$\displaystyle{ \theta }$ is the nonet mixing angle,
$\displaystyle{ \psi_8 = \frac{u\overline{u} + d\overline{d} - 2s\overline{s}}{\sqrt{6}} }$ and
$\displaystyle{ \psi_1 = \frac{u\overline{u} + d\overline{d} + s\overline{s}}{\sqrt{3}} }$.

The mixing angle at which the components decouple completely can be calculated to be $\displaystyle{ \arctan\frac{1}{\sqrt{2}}\approx35.3^\circ }$. 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.[3]

## 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 Kaon0 and the Antikaon0.[4] 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 Kaon-Proton+ collisions at approximately 2.23 GeV/c.[5][6] In essence, the reaction involved a beam of Ks 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.[7] 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.[8][9][10] This rule is now known as the OZI rule and is also the currently accepted explanation for the unusually long lifetimes of the J/Psi and Upsilon mesons.[7] Namely, on average they last ~ 7 × 10−21 s and ~ 1.5 × 10−20 s respectively.[7] 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.[7]

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.[6] 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.