Quark

Quark is one of the elementary particles that are building blocks of matter. According to current standard model, there are six "flavours" of quarks. They account for all known mesons and Baryons (over 200). The most familiar baryons are the proton and neutron, both of which are constructed from up and down quarks. Quarks are observed to occur only in combinations of two quarks(mesons), three quarks(baryons). There was a recent claim of observation of particles with five quarks(pentaquark), but there was no further experimentation borne out of it.
Quark 	Symbol	Spin 	Charge 	Baryon Number	S 	C 	B 	T 	Mass
Up 	U 	\frac{1}{2}	+\frac{2}{3}	\frac{1}{3} 	0 	0 	0 	0 	1.7 - 3.3 MeV
Down 	D 	\frac{1}{2}	-\frac{1}{3}	\frac{1}{3} 	0 	0 	0 	0 	4.1 - 5.8 MeV
Charm 	C 	\frac{1}{2}	+\frac{2}{3}	\frac{1}{3} 	0 	+1	0 	0 	1270 MeV
Strange	S 	\frac{1}{2}	-\frac{1}{3}	\frac{1}{3} 	-1	0 	0 	0 	101 MeV
Top 	T 	\frac{1}{2}	+\frac{2}{3}	\frac{1}{3} 	0 	0 	0 	+1	172 GeV
Bottom 	B 	\frac{1}{2}	-\frac{1}{3}	\frac{1}{3} 	0 	0 	-1	0 	4.19 GeV (MS) 4.67 GeV (1S)
Each of six "flavours" of quarks can have three different "colours". The quark forces are attractive only in "colourless" combinations of three quarks (baryons), quark-antiquark pairs (mesons) and possibly larger combinations such as the pentaquark that could also meet the colourless condition. Quarks undergo transformations by the exchange of W bosons, and those transformations determine the rate and nature of the decay of hadrons by the weak interaction.
==Types of Quarks==
They are listed as follows:-
===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. The free neutron decay and nuclei decay by beta decay in the processes are thought to be the result of a more fundamental quark process:-
 d \rightarrow u + e^- + \overline{v}_e
===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 a much longer time than expected: 10-10 seconds instead of the expected 10 seconds! This particle was named the lambda particle () and the property which caused 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 10-23 seconds was expected because the lambda as a baryon participates in the strong interaction, and that usually leads to such very short lifetimes. The long observed lifetime helped develop a new conservation law for such decays called the "conservation of strangeness". The presence of a strange quark in a particle is denoted by a quantum number S=-1. Particle decay by the strong or electromagnetic interactions preserve 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 processes show that strangeness is not conserved:
===Charm Quark===
In 1974 a meson called the J/Psi particle was discovered by experimenters at Stanford (Richter) and Brookhaven National Laboratory (Ting). 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. Richter and Ting shared the 1976 Nobel Prize for their discovery.
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 \Lambda^{+}_{c}. It has a composition udc and a mass of 2281 MeV/c. 
===Top Quark===
Convincing evidence for the observation 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.
==Confinement of Quarks==
There are good reasons for the lack of direct observation. Apparently the color force does not drop off with distance like the other observed forces. It is postulated that it may actually increase with distance at the rate of about 1 GeV per fermi. A free quark is not observed because by the time the separation is on an observable scale, the energy is far above the pair production energy for quark-antiquark pairs. For the U and D quarks the masses 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 observed.
Basically, you can't see an isolated quark because the color force does not let them go, and the energy required to separate them produces quark-antiquark pairs long before they are far enough apart to observe separately.
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 expressed 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 increases 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 case of the barrier facing the quark, the energy barrier does not drop off with distance, but in fact increases.
==See also==
* proton
* Neutron
* Leptons
Category:Quantum Physics