J / ψ meson

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J / ψ

classification
Boson
Hadron
Meson
properties
electric charge neutral
Resting energy 3096,900 (6)  MeV
Spin parity 1 -
Isospin 0 (z component 0)
average lifespan 7.09 (21) · 10 −21  s
Decay width 0.0929 (28)  MeV
Interactions strong
weak
electromagnetic
gravitation
Quark
composition 		1 charm and 1 anti-charm

The J / ψ (also called psion ) is a meson (unstable subatomic particles ). Its discovery in 1974 was of great importance because it proved the existence of a fourth quark , the charm quark.

properties

The J / ψ is a bound state of a charm quark and an anti- charm quark , i.e. H. it is a charmonium . All his Flavor - quantum numbers are therefore zero. The J / ψ is the longest-lived and first-discovered Charmonium. It has a mass of 3097 MeV / c 2 and a decay width of 92.9 keV / c 2 , which corresponds to a lifetime of 10 −20  s. Its quantum numbers are .

The J / ψ decays to 87.7% via the strong or the electromagnetic interaction into hadrons. The total electromagnetic component of 25.4% is divided into 13.5% hadronic end states and 6% each leptonic end states with 2 muons or 2 electrons .

It is very unusual for such a heavy meson that the decay width is so narrow and that the electromagnetic decay can compete with the strong one. This is because the usual way of decaying heavy mesons through the addition of a light quark-antiquark pair is not possible for energetic reasons and that the annihilation of c and c via the strong interaction requires at least three gluons to preserve parity and therefore is suppressed according to the OZI rule .

Research history

The J / ψ was discovered almost simultaneously in 1974 by two groups who called it J and ψ - hence the peculiar double name. One group under Burton Richter discovered it at the Stanford Linear Accelerator Center , the other group under Samuel Chao Chung Ting at Brookhaven National Laboratory . Richter and Ting presented their results to the public at a press conference on November 11, 1974. The two scientists were awarded the Nobel Prize in Physics in 1976 for their discovery of this particle .

The discovery of the J / ψ was a sensation because its width (energy uncertainty) is only a thousandth of that of other mesons in this energy range and its lifetime (according to the energy-time uncertainty relation ) is a good 1000 times as long.

At that time the quark model only knew three quarks (u, d, s); the only plausible explanation for such a long-lived meson was a new, fourth quark. This “charm” quark had already been theoretically predicted, and with the J / ψ its existence could be considered certain. The discovery of the J / ψ thus triggered the so-called “November Revolution” of particle physics, as a result of which further mesons and baryons with charm quantum numbers were discovered.

One goal of Charmonium research is to investigate the still not exactly known potential of the strong interaction. From the point of view of the Coulomb force , charmonium is similar to positronium, which is theoretically very well understood, apart from different charges and masses . The potential of the interaction is calculated from the emission and absorption spectra of the transitions between excited states of the charmonium. After subtracting the Coulomb potential , the potential of the strong interaction remains and can be parameterized . In the simplest case, a coulomb-like potential for the quark-antiquark potential is obtained for small ranges and a linear potential for larger distances.

Trivia

Samuel Ting, who advocated the name "J" for the particle, is of Chinese descent. His family name ("Dīng" in Pinyin transcription) is written with the character丁, which looks very similar to a "J". So Ting may have named his discovery after himself.

See also

Individual evidence

  1. ↑ a b The information on the particle properties (info box) are, unless otherwise stated, taken from: C. Patrignani et al. ( Particle Data Group ): 2017 Review of Particle Physics. In: Chin. Phys. C, 40, 100001 (2016) and 2017 update. Particle Data Group, accessed May 22, 2018 .
  2. ↑ Bogdan Povh et al .: Particles and Cores. 6th edition. Springer-Verlag GmbH, 2004, ISBN 3-540-21065-2
  3. ↑ SLAC-SP-017 Collaboration (JE Augustin et al.): Discovery of a Narrow Resonance in e + e - Annihilation. In: Physical Review Letters. Volume 33, 1974, pp. 1406-1408 ( online )
  4. ↑ E598 Collaboration (JJ Aubert et al.): Experimental Observation Of A Heavy Particle J. In: Physical Review Letters. Volume 33, 1974, pp. 1404-1406 ( online )
  5. ^ The Nobel Prize in Physics 1976 Press release. NobelPrize.org, October 18, 1976, accessed September 18, 2018 .

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