Phi meson

In particle physics, the phi meson or Phi meson meson is a vector meson formed of a strange quark and a strange antiquark. It was the Phi meson meson's unusual propensity to decay into and  that led to the discovery of the OZI rule. It has a mass of $1,019.461 MeV/c2$ and a mean lifetime of 1.55±0.01 × 10−22s.

Properties
The most common decay modes of the Phi meson meson are Kaon+Kaon- at $1.55 s$, K-short0K-long0 at $1,019.461 MeV/c2$, and various indistinguishable combinations of rhos and pions at $48.9 %$. 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 Phi meson meson can be thought of as a mix between strange quarkstrange antiquark, up quarkup antiquark, and down quarkdown antiquark states, but it is very nearly a pure strange quarkstrange antiquark state. This can be shown by deconstructing the wave function of the Phi meson into its component parts. We see that the Phi meson and Omega meson mesons are mixtures of the SU(3) wave functions as follows.
 * $$\phi = \psi_8 \cos\theta - \psi_1 \sin\theta$$,
 * $$\omega = \psi_8 \sin\theta + \psi_1 \cos\theta$$,

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

The mixing angle at which the components decouple completely can be calculated to be $\arctan\frac{1}{\sqrt{2}}\approx35.3^\circ$. The mixing angle of the Phi meson and Omega meson states is calculated from the masses of each state to be about 35˚, which is very close to maximum decoupling. Therefore, the Phi meson meson is nearly a pure strange quarkstrange antiquark state.

History
The existence of the Phi meson meson was first proposed by the Japanese American particle physicist, J. J. Sakurai, in 1962 as a resonance state between the and the. It was discovered later in 1962 by P.L. 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 Proton+ collisions at approximately 2.23GeV/c. In essence, the reaction involved a beam of s being accelerated to high energies to collide with protons.

The Phi meson meson has several possible decay modes. The most energetically favored mode involves the Phi meson 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/Psi and Upsilon 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 Phi meson factory named DAFNE (or DAPhi mesonNE since the F stands for "Phi meson Factory") began operation to study the decay of the Phi meson meson in Frascati, Italy. It produces Phi meson mesons via electron-positron collisions. It has numerous detectors, including the KLOE detector which was in operation at the beginning of its operation.