Quarks

Quarks and Leptons are the building blocks which build up matter, i.e., they are seen as the "elementary particles". In the present standard model, there are six "flavors" of quarks. They can successfully account for all known mesons andbaryons (over 200). The most familiar baryons are the proton and neutron, which are each constructed from up and down quarks. Quarks are observed to occur only in combinations of two quarks (mesons), three quarks (baryons), and the recently discovered particles with five quarks (pentaquark).

Quark	Symbol	Spin	Charge	Baryon Number	S	C	B	T	Mass*
Up	U	1/2	+2/3	1/3	0	0	0	0	360 MeV
Down	D	1/2	-1/3	1/3	0	0	0	0	360 MeV
Charm	C	1/2	+2/3	1/3	0	+1	0	0	1500 MeV
Strange	S	1/2	-1/3	1/3	-1	0	0	0	540 MeV
Top	T	1/2	+2/3	1/3	0	0	0	+1	174 GeV
Bottom	B	1/2	-1/3	1/3	0	0	-1	0	5 GeV

*The masses should not be taken too seriously, because the confinement of quarks implies that we cannot isolate them to measure their masses in a direct way. The masses must be implied indirectly from scattering experiments. The masses quoted for the U and D are about 1/3 the mass of a proton, since we know the proton has three quarks. But in other combinations they contribute different masses. 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 masses of C and S are from Serway, and the T and B masses 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 undergotransformations by the exchange of W bosons, and those transformations determine the rate and nature of the decay of hadrons by the weak interaction.