Quarks Quarks and Leptons are the building blocks which build up matter, i.e., they are en as the "elementary particles". In the prent standard model, there are six "flavors" of quarks. They can successfully account for all known mesons and baryons (over 200). The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks. Quarks are obrved to occur only in combinations of two quarks (mesons), three quarks (baryons). There was a recent claim of obrvation of particles with five quarks (pentaquark), but further experimentation has not borne it out.
*The mass should not be taken too riously, becau the confinement of quarks implies that we cannot isolate them to measure their mass in a direct way. The mass must be implied indirectly from scattering experiments. The numbers in the table are very different from numbers previously quoted and are bad on the July 2010 summary in Journal of Physics G, Review of Particle Physics, Particle Data Group. A summary can be found on the LBL site. The mass reprent a strong departure from earlier approaches which treated the mass for the U and D as about 1/3 the mass of a proton, since in the quark model the proton has three quarks. The mass quoted are model dependent, and the mass of the bottom quark is quoted for two different models. But in other combinations they contribute different mass. In the pion, an up and an anti-down quark yield a particle of only 139.6 MeV of mass energy, while in the rho vector meson the same combination of quarks has a mass of 770 MeV! The mass of C and S are from Serway, and the T and B mass are from descriptions of the experiments in which they were discovered. Each of the six "flavors" of quarks can have three different "colors". The quark forces are attractive only in "colorless" combinations of three quarks (baryons), quark-antiquark pairs (mesons) and possibly larger combinations such as the pentaquark that could also meet the colorless condition. Quarks undergo transformations by the exchange of W bosons, and tho transformations determine the rate and nature of the decay of hadrons by the weak interaction.
| Index Particle concepts References Serway Ch. 47 Rohlf Ch. 17 Griffiths Ch. 1 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Why "Quark"? The name "quark" was taken by Murray Gell-Mann from the book "Finnegan's Wake" by James Joyce. The line "Three quarks for " appears in the fanciful book. Gell-Mann received the 1969 Nobel Prize for his work in classifying elementary particles. | Index Particle concepts | ||||
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Up and Down Quarks The up and down quarks are the most common and least massive quarks, being the constituents of protons and neutrons and thus of most ordinary matter.
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The Strange Quark In 1947 during a study of cosmic ray interactions, a product of a proton collision with a nucleus was found to live for much longer time than expected: 10-10 conds instead of the expected 10-23 conds! This particle was named the lambda particle (Λ0) and the property which caud it to live so long was dubbed "strangeness" and that name stuck to be the name of one of the quarks from which the lambda particle is constructed. The lambda is a baryon which is made up of three quarks: an up, a down and a strange quark. The shorter lifetime of 10-23vrp conds was expected becau the lambda as a baryon participates in the strong interaction, and that usually leads to such very short lifetimes. The long obrved lifetime helped develop a new conrvation law for such decays called the "conrvation of strangeness". The prence of a strange quark in a particle is denoted by a quantum number S=-1. Particle decay by the strong or electromagnetic interactions prerve the strangeness quantum number. The decay process for the lambda particle must violate that rule, since there is no lighter particle which contains a strange quark - so the strange quark must be transformed to another quark in the process. That can only occur by the weak interaction, and that leads to a much longer lifetime. The decay process show that strangeness is not conrved: The quark transformations necessary to accomplish the decay process can be visualized with the help of Feynmann diagrams.
Conrvation of strangeness is not in fact an independent conrvation law, but can be viewed as a combination of the conrvation of charge, isospin, and baryon number. It is often expresd in terms of hypercharge Y, defined by: Isospin and either hypercharge or strangeness are the quantum numbers often ud to draw particle diagrams for the hadrons.
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The Charm Quark In 1974 a meson called the J/Psi particle was discovered. With a mass of 3100 MeV, over three times that of the proton, this particle was the first example of another quark, called the charm quark. The J/Psi is made up of a charm-anticharm quark pair. The lightest meson which contains a charm quark is the D meson. It provides interesting examples of decay since the charm quark must be transformed into a strange quark by the weak interaction in order for it to decay. One baryon with a charm quark is a called a lambda with symbol Λ+c . It has a composition udc and a mass of 2281 MeV/c2.
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The Top Quark Convincing evidence for the obrvation of the top quark was reported by Fermilab 's Tevatron facility in April 1995. The evidence was found in the collision products of 0.9 TeV protons with equally energetic antiprotons in the proton-antiproton collider. The evidence involved analysis of trillions of 1.8 TeV proton-antiproton collisions. The Collider Detector Facility group had found 56 top candidates over a predicted background of 23 and the D0 group found 17 events over a predicted background of 3.8. The value for the top quark mass from the combined data of the two groups after the completion of the run was 174.3 +/- 5.1 GeV. This is over 180 times the mass of a proton and about twice the mass of the next heaviest fundamental particle, the Z0 vector boson at about 93 GeV. The interaction is envisioned as follows:
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Confinement of Quarks How can one be so confident of the quark model when no one has ever en an isolated quark? There are good reasons for the lack of direct obrvation. Apparently the color force does not drop off with distance like the other obrved forces. It is postutated that it may actually increa with distance at the rate of about 1 GeV per fermi. A free quark is not obrved becau by the time the paration is on an obrvable scale, the energy is far above the pair productionenergy for quark-antiquark pairs. For the U and D quarks the mass are 10s of MeV so pair production would occur for distances much less than a fermi. You would expect a lot of mesons (quark-antiquark pairs) in very high energy collision experiments and that is what is obrved. Basically, you can't e an isolated quark becau the color force does not let them go, and the energy required to parate them produces quark-antiquark pairs long before they are far enough apart to obrve parately. One kind of visualization of quark confinement is called the "bag model". One visualizes the quarks as contained in an elastic bag which allows the quarks to move freely around, as long as you don't try to pull them further apart. But if you try to pull a quark out, the bag stretches and resists. Another way of looking at quark confinement is expresd by Rohlf. "When we try to pull a quark out of a proton, for example by striking the quark with another energetic particle, the quark experiences a potential energy barrier from the strong interaction that increas with distance." As the example of alpha decay demonstrates, having a barrier higher than the particle energy does not prevent the escape of the particle - quantum mechanical tunneling gives a finite probability for a 6 MeV alpha particle to get through a 30 MeV high energy barrier. But the energy barrier for the alpha particle is thin enough for tunneling to be effective. In the ca of the barrier facing the quark, the energy barrier does not drop off with distance, but in fact increas.
| Index Particle concepts Reference Rohlf Sec 6-6 | ||||
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