The J/ψ (J/psi) meson or psion is a subatomic
particle, a flavor-neutral meson consisting
of a charm quark and a charm antiquark. Mesons
formed by a bound state of a charm quark and
a charm anti-quark are generally known as
"charmonium". The J/ψ is the most common
form of charmonium, due to its low rest mass.
The J/ψ has a rest mass of 3.0969 GeV/c2,
just above that of the ηc (2.9836 GeV/c2),
and a mean lifetime of 7.2×10−21 s. This
lifetime was about a thousand times longer
than expected.Its discovery was made independently
by two research groups, one at the Stanford
Linear Accelerator Center, headed by Burton
Richter, and one at the Brookhaven National
Laboratory, headed by Samuel Ting of MIT.
They discovered they had actually found the
same particle, and both announced their discoveries
on 11 November 1974. The importance of this
discovery is highlighted by the fact that
the subsequent, rapid changes in high-energy
physics at the time have become collectively
known as the "November Revolution". Richter
and Ting were rewarded for their shared discovery
with the 1976 Nobel Prize in Physics.
== Background to discovery ==
The background to the discovery of the J/ψ
was both theoretical and experimental. In
the 1960s, the first quark models of elementary
particle physics were proposed, which said
that protons, neutrons and all other baryons,
and also all mesons, are made from three kinds
of fractionally-charged particles, the "quarks",
that come in three different types or "flavors",
called up, down, and strange. Despite the
capability of quark models to bring order
to the "elementary particle zoo", their status
was considered something like mathematical
fiction at the time, a simple artifact of
deeper physical reasons.
Starting in 1969, deep inelastic scattering
experiments at SLAC 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 subprotonic components. To a first approximation,
they were indeed the already-described quarks.
On the theoretical front, gauge theories with
broken symmetry became the first fully viable
contenders for explaining the weak interaction
after Gerardus 't Hooft discovered in 1971
how to calculate with them beyond tree level.
The first experimental evidence for these
electroweak unification theories was the discovery
of the weak neutral current in 1973. Gauge
theories with quarks became a viable contender
for the strong interaction in 1973 when the
concept of asymptotic 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 predicted Z boson-mediated
flavor-changing decays of a strange quark
into a down quark, which were not observed.
A 1970 idea of Sheldon Glashow, John Iliopoulos,
and Luciano Maiani, known as the GIM mechanism,
showed that the flavor-changing decays would
be strongly suppressed if there were a fourth
quark, charm, that paired with the strange
quark. This work led, by the summer of 1974,
to theoretical predictions of what a charm/anticharm
meson would be like. These predictions were
ignored. The work of Richter and Ting was
done for other reasons, mostly to explore
new energy regimes. In the Brookhaven group,
Glenn Everhart, Terry Rhoades, Min Chen, and
Ulrich Becker were the first to discern a
peak at 3.1 GeV in plots of production rates.
This was the first recognition of the "J".
== The name ==
Because of the nearly simultaneous discovery,
the J/ψ is the only particle to have a two-letter
name. Richter named it "SP", after the SPEAR
accelerator used at SLAC; however, none of
his coworkers liked that name. After consulting
with Greek-born Leo Resvanis to see which
Greek letters were still available, and rejecting
"iota" because its name implies insignificance,
Richter chose "psi" – a name which, as Gerson
Goldhaber pointed out, contains the original
name "SP", but in reverse order. Coincidentally,
later spark chamber pictures often resembled
the psi shape. Ting assigned the name "J"
to it, which is one letter away from "K",
the name of the already-known strange meson;
possibly by coincidence, "J" strongly resembles
the Chinese character for Ting's name (丁).
J is also the first letter of Ting's oldest
daughter's name, Jeanne.
Since the scientific community considered
it unjust to give one of the two discoverers
priority, 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 in MeV. Other vector charm-anticharm
states are denoted similarly with ψ and the
quantum state (if known) or the mass. The
"J" is not used, since Richter's group alone
first found excited states.
The name charmonium is used for the J/ψ and
other charm-anticharm bound states. This is
by analogy with positronium, which also consists
of a particle and its antiparticle (an electron
and positron in the case of positronium).
== J/ψ melting ==
In a hot QCD medium, when the temperature
is raised well beyond the Hagedorn temperature,
the J/ψ and its excitations are expected
to melt. This is one of the predicted signals
of the formation of the quark–gluon plasma.
Heavy-ion experiments at CERN's Super Proton
Synchrotron and at BNL's Relativistic Heavy
Ion Collider have 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
of QGP hadronization. Thus, there is uncertainty
in the prevailing conditions at the initial
collisions.
In fact, instead of suppression, enhanced
production of J/ψ is expected in heavy ion
experiments at LHC where 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 (Bc),
offer a signature that indicates that quarks
move freely and bind at-will when combining
to form hadrons.
== Decay modes ==
Hadronic decay modes of J/ψ are strongly
suppressed because of the OZI Rule. This effect
strongly increases the lifetime of the particle
and thereby gives it its very narrow decay
width of just 93.2±2.1 keV. Because of this
strong suppression, electromagnetic decays
begin to compete with hadronic decays. This
is why the J/ψ has a significant branching
fraction to leptons.
== See also ==
OZI Rule
List of multiple discoveries
== 
Notes ==
== 
Sources ==
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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.
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