Phi meson

Feynman diagram of the most commonϕ meson decay
Composition	ϕ0 :ss
Statistics	Bosonic
Family	Mesons
Interactions	Strong,Weak,Gravity,Electromagnetism
Symbol	ϕ,ϕ0
Antiparticle	Self
Theorized	Sakurai (1962)
Discovered	Connolly et al. (1963)
Types	1
Mass	1019.461±0.020 MeV/c2
Mean lifetime	(1.55±0.01)×10−22 s
Decays into	K+ +K− K0 S +K0 L ρ +π π+ +π0 +π−
Electric charge	0
Spin	1
Isospin	0
Hypercharge	0
Parity	−1
C parity	−1

Inparticle physics, thephi meson orϕ meson is avectormeson formed of astrangequark and astrangeantiquark. It was theϕ meson's unexpected propensity to decay intoK⁰
andK⁰
that led to the discovery of theOZI rule. It has a mass of1019.461±0.020 MeV/c² and a mean lifetime of 1.55±0.01 × 10⁻²²s .

The most common decay modes of theϕ meson areK⁺
K⁻
at48.9%±0.5%,K⁰
_S+K⁰
_L at34.2%±0.4%, and various indistinguishable mixed combinations ofrho mesons andpions at15.3%±0.3%.^[1] In all cases, it decays via thestrong 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, that decay route is suppressed by the OZI rule.

Technically, the quark composition of theϕ meson can be thought of as a mix betweenss,uu, anddd states, but it is very nearly a puress state.^[2] This can be shown by deconstructing thewave function of theϕ into its component parts. We see that theϕ andω mesons are mixtures of theSU(3) wave functions as follows.

${\displaystyle \varphi ={\psi }_8\cos \theta -{\psi }_1\sin \theta }$,

${\displaystyle \omega ={\psi }_8\sin \theta +{\psi }_1\cos \theta }$,

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$\arctan \frac{1}{\sqrt{2}}\approx {35.3}^{\circ }.$ The mixing angle of theϕ andω states is calculated from the masses of each state to be about 35˚, which is very close to maximum decoupling. Therefore, theϕ meson is nearly a puress state.^[2]

The existence of theϕ meson was first proposed by the Japanese American particle physicist,J. J. Sakurai, in 1962 as a resonance state between theK⁰
and theK⁰
.^[3] It was discovered later byConnolly et al. (1963) in a 20 inch hydrogen bubble chamber at theAlternating Gradient Synchrotron (AGS) inBrookhaven National Laboratory inUpton, NY while they were studyingK⁻
p⁺
collisions at approximately 2.23 GeV/c.^[4][5] In essence, the reaction involved a beam ofK⁻
s being accelerated to high energies to collide with protons.

Theϕ meson has several possible decay modes. The most energetically favored mode involves theϕ meson decaying into threepions, which is what would naïvely be expected. However, we instead observe that it decays most frequently into twokaons.^[6] Between 1963 and 1966, three people,Susumu Okubo,George Zweig, and Jugoro Iizuka, each independently proposed a rule to account for the observed suppression of the three pion decay.^[7][8][9] This rule is now known as theOZI rule and is also the currently accepted explanation for the unusually long lifetimes of theJ/ψ andϒ mesons.^[6] Namely, on average they last ~ 7 × 10⁻²¹s and ~ 1.5 × 10⁻²⁰ s respectively.^[6] 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 .^[6]

In 1999, aϕ factory namedDAFNE (or DAϕNE since the F stands for "ϕ Factory") began operation to study the decay of theϕ meson inFrascati,Italy.^[5] It producesϕ mesons viaelectron-positron collisions. It has numerous detectors, including theKLOE detector which was in operation at the beginning of its operation.

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• Quark model

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• Subatomic particles with spin 1

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