Charmed baryons
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'''Charmed baryons''' are a category of [[composite particle]]s comprising all [[baryon]]s with at least one unit of [[charm (quantum number)]]. In [[quark model]] terms, charmed baryons consist of three [[quarks]], one or more of which is a [[charm quark]]. This is in contrast to conventional [[baryon]]s that have three light [[quarks]];
the most obvious examples of such baryons are [[protons]] and [[neutrons]]. Charmed baryons were first observed in the 1970s; since then, a large number of distinct charmed baryon states have been identified. In 2002, the SELEX collaboration, based at [[Fermilab]] published evidence of a doubly charmed baryon (that is a three [[quark]]
state in which two of the quarks are [[charm quark]]s), but this claim is controversial and has yet to be confirmed by another experiment.
==Nomenclature==
The naming system for charmed baryons follows that of lower mass [[baryon]]s. Those comprising one charmed quark
with two up or down quarks are known as {{SubatomicParticle|Charmed Lambda+}} or
{{SubatomicParticle|Charmed Sigma}} particles,
those with one [[charm quark]], one strange quark and one up or down quark are denoted
{{SubatomicParticle|Charmed Xi}}s, and those with one [[charm quark]] and two strange quarks are called
{{SubatomicParticle|Charmed Omega}}s. The differentiation between {{SubatomicParticle|Charmed Lambda}}
and {{SubatomicParticle|Charmed Sigma}} is made on the basis of their [[isospin]]. The former are isospin zero
(I=0) and the latter isospin one (I=1). This statement is equivalent to saying that there is only one
charge state of the {{SubatomicParticle| Charmed Lambda}}, that is the singly positive charged state
{{SubatomicParticle| Charmed Lambda+}}, whereas there are three charge states of the
{{SubatomicParticle| Charmed Sigma}}, the
{{SubatomicParticle| Charmed Sigma0}}, the
{{SubatomicParticle| Charmed Sigma+}}, and the
{{SubatomicParticle| Charmed Sigma++}}. The {{SubatomicParticle|Charmed Xi}} is [[isospin]] one-half,
and thus exists as two different charged states (neutral and singly charged). The
{{SubatomicParticle| Charmed Omega0}} is I=0 and only exists as a neutral particle.
The important parameters of charmed baryons to be studied are firstly the mass, secondly the lifetime
for those that have a measurable lifetime, thirdly the intrinsic width (those particles that have too short
a lifetime to measure have a measurable spread in mass due to Heisenberg's [[uncertainty principle]], and this spread
is known as their width), and lastly their decay modes. Compilations of measurements of these may
be found in the publications of the [[Particle Data Group]].
==Methods of Production and Detection==
Charmed baryons are formed in high energy particle collisions, such as those produced by [[particle accelerators]]. The general method to find them is to detect their decay products, identify what particles they are,
and measure their
momenta. If all the decay products are found and measured correctly, the mass of the parent particle may
be measured. As an example, a favored decay of the {{SubatomicParticle|Charmed Lambda+}} is into
a [[proton]], a [[kaon]] and a [[pion]]. The momenta of these (rather stable) particles are measured by
the detector and using the usual rules of 4-[[momentum]] using the correct relativistic equations, this
gives a measure of the mass of the parent particle. In particle collisions, [[protons]], [[kaons]] and
[[pions]] are all rather commonly produced, and only a fraction of these combinations will have come
from a charmed baryon. Thus, it is important to measure many such combinations. A plot of the calculated
parent mass will then have a peak at the mass of the {{SubatomicParticle|Charmed Lambda+}}, but this is in
addition to a smooth "[[phase-space]]" background. The width of the peak will be governed by the resolution of
the detector, provided that the charmed baryon is reasonably stable (such as the
{{SubatomicParticle|Charmed Lambda+}} which has a lifetime of around 2x10<sup>-13</sup> s, which is a long
time for this purpose!). Other, higher states of charmed baryon which decay by the [[strong interaction]],
typically have large intrinsic widths and this makes the peak stand up less definitively against the
background combinations. First observations of particles by this method are notoriously difficult - over-zealous
interpretation of statistical fluctuations or effects that produce false "peaks" mean that several
published results were later found to be false. However, with more data collected by more experiments over
the years, the [[spectroscopy]] of the charmed baryons states has now reached a mature level.
=={{SubatomicParticle|Charmed Lambda+}} History==
The first charmed baryon to be discovered was the {{SubatomicParticle|Charmed Lambda+}}. It is not entirely clear when the particle was first observed; there were a number of experiments which published evidence for the state beginning in 1975, but the reported masses were frequently lower than the value now known. Since then, {{SubatomicParticle|Charmed Lambda+}} have been produced and studied at many experiments, notably fixed-target experiments (such as FOCUS and [[SELEX]]) and {{SubatomicParticle|Electron}}{{SubatomicParticle|Positron}} B-factories ([[ARGUS (experiment)|ARGUS]], [[CLEO (particle detector)|CLEO]], [[BaBar experiment|BABAR]], [[Belle experiment|BELLE]]).
===Mass===
The definitive mass measurement has been made by the
[[BaBar experiment|BABAR]], which report as mass of {{val|2286.46|u=MeV/c2}} with a small
uncertainty. To put this in context, it is more than twice as heavy as the [[proton]]. The excess
mass is easily explained by the large constituent mass of the [[charm quark]], which by itself is
more than that the [[proton]].
===Lifetime===
The lifetime of the {{SubatomicParticle|Charmed Lambda+}} is presently measured to be almost exactly
0.2 ps (i.e. 2x10<sup>-13</sup> s). This is a typical lifetime for particles that decay via the
[[weak interaction]], taking into account the large available [[phase space]]. The lifetime measurement has contributions
from a number of experiments, notably FOCUS, [[SELEX]] and [[CLEO (particle detector)|CLEO]].
===Decays===
The {{SubatomicParticle|Charmed Lambda+}} decays into a multitude of different final states, according
the rules of [[weak decays]]. The decay into a [[proton]], [[kaon]] and [[pion]] (each of them charged)
is a favorite with experimenters as it is particularly easy to detect. It accounts for around 5% of all
decays; around 30 distinct decay modes have been measured. Studies of these branching ratios
enable theoreticians to disentangle the various fundamental diagrams contributing the decays
and is a window on weak interaction physics.
=={{SubatomicParticle|Charmed Sigma}} Quark Content==
As noted above, {{SubatomicParticle|Charmed Sigma}} particles, like {{SubatomicParticle|Charmed Lambda+}}
particles, comprise a [[charm quark]] and two [[light quarks]]. However, {{SubatomicParticle|Charmed Sigma}}
particles have [[isospin]] 1. This is equivalent to saying that they can exist in three charged states,
the doubly charged, the singly charged, and the neutral. The situation is directly analagous to the
strange [[baryon]] nomenclature. The ground state (that is, with no [[orbital angular momentum]])
baryons can also be pictured thus. Each quark is a spin 1/2 particle. The spins can be pointed up, or
down. In {{SubatomicParticle|Charmed Lambda+}} ground state, the two light quarks point up-down to
give a zero spin di-quark. This then combines with the [[charm quark]] to give a spin 1/2 particle.
In the {{SubatomicParticle|Charmed Sigma}}, the two light quarks combine to give a spin 1 di-quark,
which then combines with the [[charm quark]] to give either a spin 1/2 particle, or a spin 3/2 particle
(normally known as a {{SubatomicParticle|Charmed Sigma*}}). It is the rules of [[quantum mechanics]]
that make it possible for a {{SubatomicParticle|Charmed Lambda}} to exist only with three different
quarks (that is cud quarks), whereas the {{SubatomicParticle|Charmed Sigma}} can exist as cuu, cud or
cdd (thus the three different charges).
All {{SubatomicParticle|Charmed Sigma}} particles decay by the [[strong force]]. Typically this mean
the emission of a [[pion]] as it decays down to the comparatively stable {{SubatomicParticle|Charmed Lambda+}}. Thus their masses are not usually measured directly, but in terms of their mass differences,
M({{SubatomicParticle|Charmed Sigma}})-M({{SubatomicParticle|Charmed Lambda+}}). This is
experimentally easier to measure precisely, and theoretically easier to predict, than the absolute
value of the mass.
=={{SubatomicParticle|Charmed Xi}} History and Mass==
In the standard [[quark]] model, {{SubatomicParticle|Charmed Xi+}} comprises a csu quark combination
and the {{SubatomicParticle|Charmed Xi0}} comprises a csd quark combination. Both particles decay
via the [[weak]] interaction. The first observation of the {{SubatomicParticle|Charmed Xi+}} was in 1983
by the WA62 collaboration working at [[CERN]]. They found a significant peak in the decay mode
ΛK<sup>-</sup>π<sup>+</sup>π<sup>+</sup> at a mass of {{val|2460|u=MeV/c2}} with an uncertainty of
{{val|25|u=MeV/c2}}. The present value
for the mass is taken from an average of 6 experiments, and is {{val|2467.9|u=MeV/c2}} with an uncertainty
of {{val|0.4|u=MeV/c2}}.
The {{SubatomicParticle|Charmed Xi0}} was discovered in 1989 by the [[CLEO (particle detector)|CLEO]], who
measured a peak in the decay mode Ξ<sup>-</sup>π<sup>+</sup> with a mass of {{val|2471|u=MeV/c2}} with a
{{val|5|u=MeV/c2}} uncertainty.
The presently accepted value ([[Particle Data Group]] fit) is {{val|2471.0|u=MeV/c2}} and the uncertainty
in the mass measurement has been reduced to {{val|0.4|u=MeV/c2}}.
=={{SubatomicParticle|Charmed Omega}} History and Mass==
Not surprisingly, of the 4 weakly-decaying, singly-charmed [[baryons]], the {{SubatomicParticle|Charmed Omega}} (the css quark
combination),
was the last to be discovered and the least well measured. Its history is murky. Some authors claim that
in 1985 a cluster of 3 events observed at CERN was a signal, but this can now be excluded on the
grounds of its incorrect mass. The ARGUS experiment published a small peak
as a possible signal in 1993, but this can now be excluded on cross-section
grounds, as many experiments have operated in the same environment
as ARGUS with many more collisions. The E-687 experiment at [[Fermilab]]
published 2 papers, one in 1993 and the other in 1994. The former
one showed a small peak of marginal significance in the decay mode
Ώπ, and a larger, apparently robust signal in the decay mode
Σ<sup>+</sup>K<sup>-</sup>K<sup>-</sup>π<sup>+</sup>.
This latter observation is considered valid by the [[Particle Data Group]], but increasingly seems
odd in that this decay mode has not been observed by other experiments. The [[CLEO (particle detector)|CLEO]] experiment
then showed a peak of 40 events in the sum of a variety of decay modes and a
mass of {{val|2494.6|u=MeV/c2}}. Since then, 2 experiments, [[BaBar experiment|BABAR]] and
[[BaBar experiment|BELLE]] have taken a great deal of data, and have shown very strong signals
at a mass very similar to the [[CLEO (particle detector)|CLEO]] value. However, neither have done the necessary
studies to be able to quote a mass with an uncertainty. Therefore, though there is
no doubt the that particle has been discovered, there is no definitive measurement of its mass.
[[Category:Quantum mechanics]]
[[Category:Quantum field theory]]
[[Category:Baryons]]
[[Category:Nuclear physics]]