J / ψ
The J / ψ is a meson, a subatomic particle. Its discovery in 1974 was of great importance, because it was proved the existence of a fourth quark, the charm quark.
The J / ψ is a bound state of a charm quark and an anti- charm quark, ie it is a charmonium. All its flavor quantum numbers are therefore zero. The J / ψ is the longest-lived and first discovered charmonium. It has a mass of 3097 MeV/c2 and a decay width of 92.9 keV/c2, which corresponds to a lifetime of 10-20. Its quantum numbers.
The J / ψ decays to 87.7 % over the strong or electromagnetic interaction in hadrons. The entire electromagnetic share from 25.4% to 13.5% is distributed hadronic final states and 6% each leptonic final states with two muons or two electrons.
For such a heavy meson, it is very unusual that the decay width is so small and that the electromagnetic decay can compete with the strong. This is because the usual way of the decay of heavy mesons by the addition of a light quark - antiquark pair for energetic reasons is not possible and that the annihilation of c and c of the strong interaction for the sake of parity conservation requires at least three gluons and therefore after the OZI rule is suppressed.
History of Research
The J / ψ in 1974 was almost simultaneously discovered by two groups that it called J or ψ - hence its peculiar double name. One group under Burton Richter discovered at the Stanford Linear Accelerator Center, the other group under Samuel Chao Chung Ting at Brookhaven National Laboratory. Richter and Ting presented their findings together in front of a press conference on 11 November 1974, the public. The two scientists were awarded for the discovery of this particle in 1976 with the Nobel Prize for Physics.
The discovery of the J / ψ was a sensation because its width ( energy spread ) is only about one 1000th of the other mesons in this energy and its life (after the energy-time uncertainty relation ) is therefore about 1000 times as long.
At this point, the quark model had only three quarks (u, d, s ); the only plausible explanation for durable Meson was a new, fourth quark. This " charm" quark had already been predicted theoretically, and could with the J / ψ its existence regarded as certain.
At the Cornell Electron-Positron Storage Ring ( CESR) at Cornell University there is a specially tuned to the mass production of charmonium particle accelerator in the ongoing experiment CLEO -c show the properties of these particles are studied ( and their decay products ).
One aim of the charmonium research is the exploration of the still not exactly known potential curve of the strong interaction. From the standpoint of the Coulomb force produces the charmonium is similar except for different charges and masses of the theoretically very well understood positronium. The potential of interaction is calculated from emission and Absortionsspektren of transitions between excited states of charmonium. After deduction of the Coulomb potential as the potential of the strong interaction remains and can be parameterized. In the simplest case for the quark - antiquark potential is obtained as a coulombartiges potential for small distances and a linear potential for greater distances.
Samuel Ting, who propagated the name " J " for the particles, is of Chinese descent. His family name ( "thing" in Pinyin ) is written with the character丁that a "J" looks very similar. Ting has so named his discovery after himself.