2007 Schools Wikipedia Selection. Related subjects: General Physics
In physics, the proton ( Greek πρῶτον proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1.602 × 10−19 coulomb), a diameter of about 1.5×10−15 m, and a mass of 938.3 MeV/c2 ( 1.6726 × 10−27 kg), or about 1836 times the mass of an electron. The proton is observed to be stable, although some theories predict that the proton may decay. The proton has a density of about 2.31 × 1017 kg m−3.
Protons are spin-1/2 fermions and are composed of three quarks, making them baryons. The two up quarks and one down quark of the proton are also held together by the strong nuclear force, mediated by gluons. Protons may be transmuted into neutrons by inverse beta decay (that is, by capturing an electron); since neutrons are heavier than protons, this process does not occur spontaneously but only when energy is supplied. The proton's antimatter equivalent is the antiproton, which has the same magnitude charge as the proton but the opposite sign.
Protons and neutrons are both nucleons, which may be bound by the nuclear force into atomic nuclei. The most common isotope of the hydrogen atom is a single proton. The nuclei of other atoms are composed of various numbers of protons and neutrons. The number of protons in the nucleus determines the chemical properties of the atom and which chemical element it is.
The antiproton is the antiparticle of the proton. It was discovered in 1955 by Emilio Segre and Owen Chamberlain, for which they were awarded the 1959 Nobel Prize in Physics.
CPT-symmetry puts strong constraints on the relative properties of particles and antiparticles and, therefore, is open to stringent tests. For example, the charges of the proton and antiproton must sum to exactly zero. This equality has been tested to one part in 108. The equality of their masses is also tested to better than one part in 108. By holding antiprotons in a Penning trap, the equality of the charge to mass ratio of the proton and the antiproton has been tested to 1 part in 9×1011. The magnetic moment of the antiproton has been measured with error of 8×10−3 nuclear Bohr magnetons, and is found to be equal and opposite to that of the proton.
Due to their stability and large mass (compared to electrons), protons are well suited to use in particle colliders such as the Large Hadron Collider at CERN and the Tevatron at Fermilab. Protons also make up a large majority of the cosmic rays which impinge on the Earth's atmosphere. Such high-energy proton collisions are more complicated to study than electron collisions, due to the composite nature of the proton. Understanding the details of proton structure requires quantum chromodynamics.