Subatomic particles are divided into three groups: Photons,Leptons and Hadrons
Elementary particles are the building blocks of matter. All photons and leptons are elementary particles. Hadrons are not elementary particles but are composed of elementary particles called quarks. Scientists now believe that all matter belongs to either the quarks group or the leptons group.
Hadrons:
Hadrons are particles that experience the strong nuclear force. In addition to protons, neutrons and mesons, are also hadrons. The particles equal in mass or greater than protons are called baryons. Particles lighter than protons are called mesons.
Leptons:
Leptons are particles that do not experience strong nuclear force.Electrons,muons and neutrinos are leptons.
Quarks:
According to quark theory initiated by Gell-Mann and Zweig, the qurks are proposed as the basic building blocks of the mesons and baryons. A pair of quark and anti-quark makes a meson and 3 quarks make a baryon.It is proposed taht there are six quarks with their names, symbols and charges given in table.
A proton is assumed to be made up of two up-quarks and one down-quark as shown in Fig
The neutron is assumed to be made of one up-quark and two down-quarks as shown in the Fig
Currently the hundreds of hadrons can be accounted for in term of six quarks and their anti-quarks.It is believed that quarks cannot exist on their own, their existence has, however been indirectly verified.
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