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The
J/ψ
(J/psi)meson/ˈdʒeɪˈsaɪˈmiːzɒn/is asubatomic particle,aflavor-neutralmesonconsisting of acharm quarkand a charmantiquark.Mesons formed by abound stateof a charm quark and a charm anti-quark are generally known as "charmonium"or psions.[1]The
J/ψ
is the most common form of charmonium, due to itsspinof 1 and its lowrest mass.The
J/ψ
has a rest mass of3.0969GeV/c2,just above that of the
η
c(2.9836GeV/c2), and amean lifetimeof7.2×10−21s.This lifetime was about a thousand times longer than expected.[2]
 | |
| Composition | c c |
|---|---|
| Statistics | bosonic |
| Family | meson |
| Interactions | strong,weak,electromagnetic,gravity |
| Symbol | J/ψ |
| Antiparticle | self |
| Discovered | SLAC:Burton Richteret al. (1974) BNL:Samuel Tinget al. (1974) |
| Types | 1 |
| Mass | 5.5208×10−27kg 3.096916GeV/c2 |
| Decay width | 92.9keV |
| Decays into | 3 g or γ +2 g or γ |
| Electric charge | 0e |
| Spin | 1ħ |
| Isospin | 0 |
| Hypercharge | 0 |
| Parity | −1 |
| C parity | −1 |
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 TingofMIT.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 physicsat the time have become collectively known as the "November Revolution".Richter and Ting were awarded the 1976Nobel Prize in Physics.
Background to discovery
editThe background to the discovery of the
J/ψ
was both theoretical and experimental. In the 1960s, the firstquarkmodels ofelementary particle physicswere proposed, which said thatprotons,neutrons,and all otherbaryons,and also allmesons,are made fromfractionallycharged particles, the "quarks", originally with three types or "flavors", calledup,down,andstrange.(Later the model was expanded to six quarks, adding thecharm,topandbottomquarks.) 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 scatteringexperiments atSLACrevealed 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 theorieswithbroken symmetrybecame the first fully viable contenders for explaining theweak interactionafterGerardus 't Hooftdiscovered in 1971 how to calculate with them beyondtree level.The first experimental evidence for theseelectroweak unificationtheories was the discovery of theweak neutral currentin 1973. Gauge theories with quarks became a viable contender for thestrong interactionin 1973, when the concept ofasymptotic freedomwas 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-changingdecays 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 and named the particle the ψmeson.Ting named it the "J meson" in his simultaneous discovery.[4]
Decay modes
editHadronic decay modes of
J/ψ
are strongly suppressed because of theOZI rule.This effect strongly increases the lifetime of the particle and thereby gives it its very narrowdecay widthof just93.2±2.1 keV.Because of this strong suppression, electromagnetic decays begin to compete with hadronic decays. This is why the
J/ψ
has a significantbranching fractionto leptons.
The primary decay modes[5]are:
c c → 3 g |
64.1%±1.0% | |
c c → γ + 2 g |
8.8%±1.1% | |
c c → γ |
~25.5% | |
γ → hadrons |
13.5%±0.3% | |
γ → e+ + e− |
5.971%±0.032% | |
γ → μ+ + μ− |
5.961%±0.033% |
J/ψ
melting
edit
In a hotQCDmedium,when the temperature is raised well beyond theHagedorn temperature,the
J/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 Synchrotronand atBNL'sRelativistic Heavy Ion Colliderhave studied this phenomenon without a conclusive outcome as of 2009. This is due to the requirement that the disappearance of
J/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 some
J/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 of
J/ψ
is expected[7]inheavy ion experiments at LHCwhere the quark-combinant production mechanism should be dominant given the large abundance of charm quarks in the QGP. Aside of
J/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]
Name
editBecause of the nearly simultaneous discovery, the
J/ψ
is the only particle to have a two-letter name. Richter named it "SP", after theSPEARaccelerator used atSLAC;however, none of his coworkers liked that name. After consulting with Greek-bornLeo Resvanisto see whichGreek letterswere still available, and rejecting "iota"because its name implies insignificance, Richter chose" psi "– a name which, asGerson Goldhaberpointed out, contains the original name "SP", but in reverse order.[10]Coincidentally, laterspark chamberpictures often resembled the psi shape. Ting assigned the name "J" to it, saying that the more stable particles, such as theW and Z bosonshad Roman names, as opposed to classical particles, which had Greek names. He also cited the symbol for electromagnetic currentwhich 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 the
J/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.Othervectorcharm–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 namecharmoniumis used for the
J/ψ
and other charm–anticharm bound states.[b]This is by analogy withpositronium,which also consists of a particle and its antiparticle (anelectronandpositronin the case of positronium).
See also
editFootnotes
edit- ^ Glenn Everhart, Terry Rhoades, Min Chen, and Ulrich Becker, atBrookhavenfirst to discerned the 3.1 GeV peak in pair-production rates.
- ^
There are two different regimes of flavorless, neutralmesons:Low mass and high mass.
π0
,the lightest of all mesons), the
η
and
η′
,
ρ0
,
ω0
,and so-on. Whether high or low mass, since all of the flavorless mesons’ quantum numbers are zero they can only be distinguished by their masses. Generally their quark content is invisible, especially the low-mass flavorless mesons, not only because their very similar small masses can be easily confused, but also because the low-mass particles themselves do actually exist as mixtures. For example the lowest mass of all mesons is the neutralpion;it is approximately an equal mix ofddanduumatching quark–antiquark pairs.- cc="charmonium"=
J/ψ
meson - bb= "bottomonium"=
ϒ0
- cc="charmonium"=
References
edit- ^Kapusta, J.; Müller, B.; Rafelski, J. (9 December 2003).Quark-Gluon Plasma: Theoretical Foundations: An Annotated Reprint Collection.p. 462.ISBN9780444511102.Retrieved25 September2014– via Google Books.
- ^ "Shared Physics prize for elementary particle"(Press release).The Royal Swedish Academy of Sciences.18 October 1976.Retrieved23 April2012.
- ^ Pickering, A. (1984).Constructing Quarks.University of Chicago Press.pp. 114–125.ISBN978-0-226-66799-7.
- ^abWe discussed the name of the new particle for some time. Someone pointed out to me that the really exciting stable particles are designated by Roman characters – like the postulated W0,the intermediate vector boson, the Z0,etc. – whereas the “classical” particles have Greek designations like ρ, ω etc. This, combined with the fact that our work in the last decade had been concentrated on the electromagnetic currentgave us the idea to call this particle the J particle.Samuel Ting,The Discovery of the J ParticleNobel prize lecture, 11. December 1976[1]
- ^Nakamura, K.; et al. (Particle Data Group) (2022)."J/ψ(1S)"(PDF).Particle Data Group.Journal of Physics G.37(7A). Lawrence Berkeley Laboratory: 075021.Bibcode:2010JPhG...37g5021N.doi:10.1088/0954-3899/37/7A/075021.
- ^Matsui, T.; Satz, H. (1986). "J/ψ suppression by quark–gluon plasma formation".Physics Letters B.178(4): 416–422.Bibcode:1986PhLB..178..416M.doi:10.1016/0370-2693(86)91404-8.OSTI1118865.
- ^Thews, R. L.; Schroedter, M.;Rafelski, J.(2001). "Enhanced J/ψ production in deconfined quark matter".Physical Review C.63(5): 054905.arXiv:hep-ph/0007323.Bibcode:2001PhRvC..63e4905T.doi:10.1103/PhysRevC.63.054905.S2CID11932902.
- ^Schroedter, M.; Thews, R.L.; Rafelski, J. (2000). "Bc-meson production in ultrarelativistic nuclear collisions ".Physical Review C.62(2): 024905.arXiv:hep-ph/0004041.Bibcode:2000PhRvC..62b4905S.doi:10.1103/PhysRevC.62.024905.S2CID119008673.
- ^ Fulcher, L.P.; Rafelski, J.; Thews, R.L. (1999). "Bcmesons as a signal of deconfinement ".arXiv:hep-ph/9905201.
- ^ Zielinski, L (8 August 2006)."Physics Folklore".QuarkNet.Retrieved13 April2009.
- ^
Roos, M; Wohl, CG; (Particle Data Group) (2004)."Naming schemes for hadrons"(PDF).Retrieved13 April2009.
{{cite web}}:CS1 maint: multiple names: authors list (link)
Sources
edit- Glashow, S. L.; Iliopoulos, J.; Maiani, L. (1970). "Weak Interactions with Lepton–Hadron Symmetry".Physical Review D.2(7): 1285–1292.Bibcode:1970PhRvD...2.1285G.doi:10.1103/PhysRevD.2.1285.
- Aubert, J.; et al. (1974)."Experimental Observation of a Heavy Particle J".Physical Review Letters.33(23): 1404–1406.Bibcode:1974PhRvL..33.1404A.doi:10.1103/PhysRevLett.33.1404.
- Augustin, J.; et al. (1974)."Discovery of a Narrow Resonance in e+e−Annihilation ".Physical Review Letters.33(23): 1406–1408.Bibcode:1974PhRvL..33.1406A.doi:10.1103/PhysRevLett.33.1406.
- Bobra, M. (2005)."Logbook: J/ψ particle".Symmetry Magazine.2(7): 34.
- Yao, W.-M.; et al. (Particle Data Group) (2006)."Review of Particle Physics: Naming Scheme for Hadrons"(PDF).Journal of Physics G.33(1): 108.arXiv:astro-ph/0601168.Bibcode:2006JPhG...33....1Y.doi:10.1088/0954-3899/33/1/001.