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| Composition | cc |
|---|---|
| Statistics | bosonic |
| Family | meson |
| Interactions | strong,weak,electromagnetic,gravity |
| Symbol | J/ψ |
| Antiparticle | self |
| Discovered | SLAC:Burton Richter et al. (1974) BNL:Samuel Ting et al. (1974) |
| Types | 1 |
| Mass | 5.5208×10−27 kg 3.096916 GeV/c2 |
| Decay width | 92.9 keV |
| Decays into | 3g orγ+2g orγ |
| Electric charge | 0 e |
| Spin | 1 ħ |
| Isospin | 0 |
| Hypercharge | 0 |
| Parity | −1 |
| C parity | −1 |
TheJ/ψ (J/psi)meson/ˈdʒeɪˈsaɪˈmiːzɒn/ is asubatomic particle, aflavor-neutralmeson consisting of acharm quark and a charmantiquark. Mesons formed by abound state of a charm quark and a charm anti-quark are generally known as "charmonium" or psions.[1] TheJ/ψ is the most common form of charmonium, due to itsspin of 1 and its lowrest mass. TheJ/ψ has a rest mass of3.0969 GeV/c2, just above that of theη
c (2.9836 GeV/c2), and amean lifetime of7.2×10−21 s. This lifetime was about a thousand times longer than expected.[2]
Its discovery was made independently by two research groups, one at theStanford Linear Accelerator Center, headed byBurton Richter, and one at theBrookhaven National Laboratory, headed bySamuel Ting ofMIT. They discovered that they had found the same particle, and both announced their discoveries on 11 November 1974. The importance of this discovery[citation needed] is highlighted by the fact that the subsequent, rapid changes inhigh-energy physics at the time have become collectively known as the "November Revolution". Richter and Ting were awarded the 1976Nobel Prize in Physics.
The background to the discovery of theJ/ψ was both theoretical and experimental. In the 1960s, the firstquark models ofelementary particle physics were proposed, which said thatprotons,neutrons, and all otherbaryons, and also allmesons, are made fromfractionally charged particles, the "quarks", originally with three types or "flavors", calledup,down, andstrange. (Later the model was expanded to six quarks, adding thecharm,top andbottom quarks.) Despite the ability of quark models to bring order to the "elementary particle zoo", they were considered something like mathematical fiction at the time, a simple artifact of deeper physical reasons.[3]
Starting in 1969,deep inelastic scattering experiments atSLAC revealed surprising experimental evidence for particles inside of protons. Whether these were quarks or something else was not known at first. Many experiments were needed to fully identify the properties of the sub-protonic components. To a first approximation, they indeed were a match for the previously described quarks.
On the theoretical front,gauge theories withbroken symmetry became the first fully viable contenders for explaining theweak interaction afterGerardus 't Hooft discovered in 1971 how to calculate with them beyondtree level. The first experimental evidence for theseelectroweak unification theories was the discovery of theweak neutral current in 1973. Gauge theories with quarks became a viable contender for thestrong interaction in 1973, when the concept ofasymptotic freedom was identified.
However, a naive mixture of electroweak theory and the quark model led to calculations about known decay modes that contradicted observation: In particular, it predictedZ boson-mediatedflavor-changing decays of a strange quark into a down quark, which were not observed. A 1970 idea ofSheldon Glashow,John Iliopoulos, andLuciano Maiani, known as theGIM mechanism, showed that the flavor-changing decays would be strongly suppressed if there were a fourth quark (now called thecharm quark) that was a complementary counterpart to thestrange quark. By summer 1974 this work had led to theoretical predictions of what a charm + anticharm meson would be like.
The group atBrookhaven,[a] were the first to discern a peak at 3.1 GeV in plots of production rates. Ting named it the "J meson".[4]
Hadronic decay modes ofJ/ψ are strongly suppressed because of theOZI rule. This effect strongly increases the lifetime of the particle and thereby gives it its very narrowdecay width of just93.2±2.1 keV. Because of this strong suppression, electromagnetic decays begin to compete with hadronic decays. This is why theJ/ψ has a significantbranching fraction to leptons.
The primary decay modes[5] are:
| cc → 3g | 64.1%±1.0% | |
| cc →γ + 2g | 8.8%±1.1% | |
| cc →γ | ~25.5% | |
| γ → hadrons | 13.5%±0.3% | |
| γ →e+ +e− | 5.971%±0.032% | |
| γ →μ+ +μ− | 5.961%±0.033% |
In a hotQCDmedium, when the temperature is raised well beyond theHagedorn temperature, theJ/ψ and its excitations are expected to melt.[6] This is one of the predicted signals of the formation of thequark–gluon plasma. Heavy-ion experiments atCERN'sSuper Proton Synchrotron and atBNL'sRelativistic Heavy Ion Collider have studied this phenomenon without a conclusive outcome as of 2009. This is due to the requirement that the disappearance ofJ/ψ mesons is evaluated with respect to the baseline provided by the total production of all charm quark-containing subatomic particles, and because it is widely expected that someJ/ψ are produced and/or destroyed at time ofQGPhadronization. Thus, there is uncertainty in the prevailing conditions at the initial collisions.
In fact, instead of suppression, enhanced production ofJ/ψ is expected[7] inheavy ion experiments at LHC where the quark-combinant production mechanism should be dominant given the large abundance of charm quarks in the QGP. Aside ofJ/ψ,charmed B mesons (B
c), offer a signature that indicates that quarks move freely and bind at-will whencombining to form hadrons.[8][9]
Because of the nearly simultaneous discovery, theJ/ψ is the only particle to have a two-letter name. Richter named it "SP", after theSPEAR accelerator used atSLAC; however, none of his coworkers liked that name. After consulting with Greek-bornLeo Resvanis to see whichGreek letters were still available, and rejecting "iota" because its name implies insignificance, Richter chose "psi" – a name which, asGerson Goldhaber pointed out, contains the original name "SP", but in reverse order.[10] Coincidentally, laterspark chamber pictures often resembled the psi shape. Ting assigned the name "J" to it, saying that the more stable particles, such as theW and Z bosons had Roman names, as opposed to classical particles, which had Greek names. He also cited the symbol for electromagnetic current which much of their previous work was concentrated on to be one of the reasons.[4]
Much of the scientific community considered it unjust to give one of the two discoverers priority, so most subsequent publications have referred to the particle as the "J/ψ".
The first excited state of theJ/ψ was called the ψ′; it is now called the ψ(2S), indicating its quantum state. The next excited state was called the ψ″; it is now called ψ(3770), indicating mass inMeV/c2. Othervector charm–anticharm states are denoted similarly with ψ and the quantum state (if known) or the mass.[11] The "J" is not used, since Richter's group alone first found excited states.
The namecharmonium is used for theJ/ψ and other charm–anticharm bound states.[b] This is by analogy withpositronium, which also consists of a particle and its antiparticle (anelectron andpositron in the case of positronium).
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