what is proton ???

The proton is a subatomic hadron particle with the symbol p or p+ and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The proton is also stable by itself. Free protons are emitted directly in some rare types of radioactive decay, and result from the decay of free neutrons from other radioactivity. They soon pick up an electron and become neutral hydrogen, which may then react chemically. Free protons may exist in plasmas or in cosmic rays in vacuum. The proton particle is composed of three fundamental particles: two up quarks and one down quark. It is about 1.6–1.7 fm in diameter. Description Protons are spin-½ fermions and are composed of three quarks,making them baryons (a sub-type of hadrons). The two up quarks and one down quark of the proton are held together by the strong force, mediated by gluons.The proton has an approximately exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm. Protons and neutrons are both nucleons, which may be bound by the nuclear force into atomic nuclei. The nucleus of the most common isotope of the hydrogen atom is a lone proton. The nuclei of the heavy hydrogen isotopes deuterium and tritium contain one proton bound to one and two neutrons, respectively. All other types of atoms are composed of two or more protons and various numbers of neutrons. The number of protons in the nucleus determines the chemical properties of the atom and, thus, which chemical element is represented; it is the number of both neutrons and protons in a nuclide that determine the particular isotope of an element. Stability The spontaneous decay of free protons has never been observed, and the proton is therefore considered a stable particle. However, some grand unified theories of particle physics predict that proton decay should take place with lifetimes of the order of 1036 yr, and experimental searches have established lower bounds on the mean lifetime of the proton for various assumed decay products. Experiments at the Super-Kamiokande detector in Japan gave lower limits for proton mean lifetime of 6.6×1033 yr for decay to an antimuon and a neutral pion, and 8.2×1033 yr for decay to a positron and a neutral pion. Another experiment at the Sudbury Neutrino Observatory in Canada searched for gamma rays resulting from residual nuclei resulting from the decay of a proton from oxygen-16. This experiment was designed to detect decay to any product whatever, and established a lower limit to the proton lifetime of 2.1×1029 yr. However, protons are known to transform into neutrons through the process of electron capture (also called inverse beta decay). For free protons, this process does not occur spontaneously but only when energy is supplied. The equation is: p+ + e− → n + ν e The process is reversible; neutrons can convert back to protons through beta decay, a common form of radioactive decay. In fact, a free neutron decays this way, with a mean lifetime of about 15 minutes.