Charmed baryon
Charmed baryons are a category of composite particles comprising all baryons made of at least one charm quark. Since their first observation in the 1970s, a large number of distinct charmed baryon states have been identified. Observed charmed baryons have masses ranging between and. In 2002, the SELEX collaboration, based at Fermilab published evidence of a doubly charmed baryon, containing two charm quarks) with a mass of ~, but has yet to be confirmed by other experiments. One triply charmed baryon has been predicted but not yet observed.
Nomenclature
The nomenclature of charmed baryons is based on both quark content and isospin. The naming follows the rules established by the Particle Data Group.- Charmed baryons composed of one charm quark and two up, one up and one down, or two down quarks are known as charmed Lambdas, or charmed Sigmas.
- Charmed baryons of isospin composed of one charm quark, and one up or down quark are known as charmed Xis and all have isospin.
- Charmed baryons composed of one charm quark and no up or down quarks are called charmed Omegas and all have isospin 0.
- Charmed baryons composed of two charm quarks and one up or down quark are called double charmed Xis and all have isospin ).
- Charmed baryons composed of two charm quarks and no up or down quarks are called double charmed Omega and all have isospin 0.
- Charmed baryons composed of three charm quarks are called triple charmed Omegas, and all have isospin 0.
Properties
The important parameters of charmed baryons, to be studied, consist of four properties. They are firstly the mass, secondly the lifetime for those with a measurable lifetime, thirdly the intrinsic width, and lastly their decay modes. Compilations of measurements of these may be found in the publications of the Particle Data Group.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, then the mass of the parent particle may be calculated. As an example, a favored decay of the is into a proton, a kaon and a pion. The momenta of these particles are measured by the detector and using the usual rules of four-momentum using the correct relativistic equations, this gives a measure of the mass of the parent particle.In particle collisions, the 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, 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. Other, higher states of charmed baryons, which decay by the strong interaction, typically have large intrinsic widths. This makes the peak stand up less definitively against the background combinations. First observations of particles by this method are notoriously difficult - overzealous 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.
Charmed lambda+ history
The first charmed baryon to be discovered was the. 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, have been produced and studied at many experiments, notably fixed-target experiments and B-factories.Mass
The definitive mass measurement has been made by the BaBar experiment, which reported a mass of with a small uncertainty. To put this in context, it is more than twice as heavy as a proton. The excess mass is easily explained by the large constituent mass of the charm quark, which by itself is more than that of the proton.Lifetime
The lifetime of the is measured to be almost exactly 0.2 picoseconds. 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.Decays
The decays into a multitude of different final states, according to the rules of weak decays. The decay into a proton, kaon and pion 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 decaysand is a window on weak interaction physics.
Orbital excitations
The quark model, together with quantum mechanics predicts that there should be orbital excitations of particles. The lowest lying of these states are ones where the two light quarks combine into a spin-0 state, one unit of orbital angular momentum is added, and this combines with the intrinsic spin of the charm quark to make a, pair of particles. The higher of these was discovered in 1993 by ARGUS. At first it was not clear what state had been discovered, but the subsequent discovery of the lower state by CLEO clarified the situation. The decay modes, the masses, the measured widths, and the decays via two charged pions rather than one charged and one neutral pion, all confirm the identification of the states.Charmed Sigma quark content
As noted above, charmed Sigma particles, like particles, comprise a charm quark and two light up, down, strange) quarks. However, 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 analogous to the strange baryon nomenclature. The ground state baryons can also be pictured thus. Each quark is a spin 1/2 particle. The spins can be pointed up, or down. In ground state, the two light quarks point up-down to give a zero spin diquark. This then combines with the charm quark to give a spin 1/2 particle. In the, the two light quarks combine to give a spin 1 diquark, which then combines with the charm quark to give either a spin 1/2 particle, or a spin 3/2 particle. It is the rules of quantum mechanics that make it possible for a to exist only with three different quarks, whereas the can exist as cuu, cud or cdd.All particles decay by the strong force. Typically this mean the emission of a pion as it decays down to the comparatively stable. Thus their masses are not usually measured directly, but in terms of their mass differences, m−m. This is experimentally easier to measure precisely, and theoretically easier to predict, than the absolute value of the mass.
(2455) history and mass
The lowest-mass was given the name "2455" by the Particle Data Group, using their convention that strongly decaying particles are known by a rough value of their mass. It was searched for since the early days of charmed baryon studies. Individual events in bubble chambers were several times touted by experiments as evidence of the particles, but it is unclear how one event of this sort can be used as evidence of a resonance. As early as 1979, there was reasonable evidence of the doubly charged state from the Columbia-Brookhaven collaboration. In 1987-89, a series of experiments with much larger statistics, found clear evidence for both the doubly charged and neutral states. It became clear that the mass difference m − m is around. The singly-charged state was harder to detect, not because it is harder to produce, but simply because its decay via a neutral pion has more background and inferior resolution when detected by most particle detectors. It was not found, until 1993 by CLEO.The intrinsic width of the is small by the standard of most strong decays, but has now been measured, at least for the neutral and doubly charged states, to be around by the CLEO and FOCUS detectors.
The next state up in mass is the spin state, usually known as the or the. These are clearly going to be "wider" because of the extra phase space of their decay, which like the is to one pion plus a ground-state. Again, large statistics are necessary to claim a signal above the large number of - pairs that are produced. Again, the neutral and doubly charged states are experimentally easier to detect, and these were discovered in 1997 by the CLEO Collaboration. The singly charged state had to wait till 2001 by the time they had collected more data.
history and mass
In the standard quark model, comprises a csu quark combination and the comprises a csd quark combination. Both particles decay via the weak interaction. The first observation of the was in 1983 by the WA62 collaboration working at CERN. They found a significant peak in the decay mode at a mass of. The present value for the mass is taken from an average of 6 experiments, and is.The was discovered in 1989 by the CLEO, who measured a peak in the decay mode of with a mass of. The accepted value is.