Proton

A proton is a stable subatomic particle, symbol, H⁺, or ¹H⁺ with a positive electric charge of +1 e. Its mass is slightly less than the mass of a neutron and approximately times the mass of an electron. Protons and neutrons, each with a mass of approximately one dalton, are jointly referred to as nucleons.
One or more protons are present in the nucleus of every atom. They provide the attractive electrostatic central force which binds the atomic electrons. The number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number. Since each element is identified by the number of protons in its nucleus, each element has its own atomic number, which determines the number of atomic electrons and consequently the identity and chemical characteristics of the element.
The word proton is Greek for "first", and the name was given to the hydrogen nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the hydrogen nucleus could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a fundamental or elementary particle, and hence a building block of nitrogen and all other heavier atomic nuclei.
Although protons were originally considered to be elementary particles, in the modern Standard Model of particle physics, protons are known to be composite particles, containing three valence quarks, and together with neutrons are now classified as hadrons. Protons are composed of two up quarks of charge +e each, and one down quark of charge −e. The rest masses of quarks contribute only about 1% of a proton's mass. The remainder of a proton's mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. The proton charge radius is around but two different kinds of measurements give slightly different values.
At sufficiently low temperatures and kinetic energies, free protons will bind electrons in any matter they traverse.
Free protons are routinely used for accelerators for proton therapy or various particle physics experiments, with the most powerful example being the Large Hadron Collider.

Description

Protons are spin- fermions and are composed of three valence quarks, making them baryons. The two up quarks and one down quark of a proton are held together by the strong force, mediated by gluons. A modern perspective has a proton composed of the valence quarks, the gluons, and transitory pairs of sea quarks. Protons have a positive charge distribution, which decays approximately exponentially, with a root mean square charge radius of about 0.8 fm.
Protons and neutrons are both nucleons, which may be bound together by the nuclear force to form 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 atomic nuclei are composed of two or more protons and various numbers of neutrons.

History

The concept of a hydrogen-like particle as a constituent of other atoms was developed over a long period. As early as 1815, William Prout used early values of atomic weight to devise what later researchers called Prout's hypothesis: all atoms are composed of integer combinations of hydrogen atoms. When more accurate values of the atomic weights were measured, the integer relationship failed. Nevertheless the concept continued to intrigue scientists and would eventually emerge again a century later.
File:Rutherford 1911 Solvay.jpg|thumb|150px|Ernest Rutherford at the first Solvay Conference, 1911
File:Proton detected in an isopropanol cloud chamber.jpg|thumb|Proton detected in an isopropanol cloud chamber
In 1886, Eugen Goldstein discovered canal rays exiting from perforations in the discharge tube. Wilhelm Wien in 1898 showed that these rays had a charge opposite to the negative electrons discovered by J. J. Thomson, but with a much higher mass to charge ratio. Later that year Thomson was able to determine a value for the magnitude of the electric charge, e, and show that the canal rays included material with charge-to-mass ratio consistent with the hydrogen ion.
Following the discovery of the atomic nucleus by Ernest Rutherford in 1913, Antonius van den Broek proposed that the place of each element in the periodic table is equal to its nuclear charge. Van den Broek speculated that the nucleus contained alpha particles with four positive charges and two electrons, the first version of the nuclear-electron hypothesis.. Also in 1913 Niels Bohr presented an theory of atomic structure which predicted electronic transitions related to nuclear charge. This was confirmed experimentally by Henry Moseley in 1913 when he showed that the energy of X-ray spectra lines of many elements followed a pattern based on atomic number.
In 1919, after a long series of sporadic experiments interrupted by WWI, Rutherford discovered what he called artificial disintegration of nitrogen atoms. Using alpha particles from radium to strike air, Rutherford detected scintillation on a zinc sulfide screen at a distance, up to 28 cm, well beyond the distance of alpha-particle range of travel but instead corresponding to the range of travel of hydrogen atoms. By 1920 he concluded that these hydrogen nuclei were a constituent part of the nitrogen nucleus. This result has been described as the discovery of protons.
When Rutherford described his results at the British Association for the Advancement of Science August 1920 he was asked by Oliver Lodge for a new name for the positive hydrogen nucleus to avoid confusion with the neutral hydrogen atom. Rutherford initially suggested both proton and prouton. Rutherford later reported that the meeting had accepted his suggestion that the hydrogen nucleus be named the "proton", following Prout's word "protyle". The first use of the word "proton" in the scientific literature appeared in 1920.
Rutherford initially assumed that the alpha particle merely knocked a proton out of nitrogen, turning it into carbon. Patrick Blackett's cloud chamber images in 1925 demonstrated that the alpha particle was absorbed. If the alpha particle were not absorbed, then 3 charged particles, a negatively charged carbon, a proton, and an alpha particle, would be expected. The 3 charged particles would create three tracks in the cloud chamber, but only 2 tracks in the cloud chamber were observed. Blackett proposed that the alpha particle is absorbed by the nitrogen atom. Heavy oxygen, not carbon, was the product. This was the first reported nuclear reaction,.

Occurrence

One or more bound protons are present in the nucleus of every atom.
Free protons are found naturally in a number of situations in which energies or temperatures are high enough to separate them from electrons, for which they have some affinity. Free protons occur occasionally on Earth: thunderstorms can produce protons with energies of up to several tens of megaelectronvolt. Free protons exist in plasmas in which temperatures are too high to allow them to combine with electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which propagate through the interstellar medium. Free protons are emitted directly from atomic nuclei in some rare types of radioactive decay. Protons also result from the radioactive decay of free neutrons, which are unstable.

Stability

The spontaneous decay of free protons has never been observed, and protons are therefore considered stable particles according to the Standard Model. However, some grand unified theories of particle physics predict that proton decay should take place with lifetimes between 10³¹ and 10³⁶ years. The experimental lower bound for the mean lifetime is.
The mean lifetime measures decay to any product. Lifetimes for decay to specific products is also measured. For example, experiments at the Super-Kamiokande detector in Japan gave lower limits for proton mean lifetime of for decay to an antimuon and a neutral pion, and for decay to a positron and a neutral pion.
Protons are known to transform into neutrons through the process of electron capture. For free protons, this process does not occur spontaneously but only when energy is supplied. The equation is:
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. A proton can also transform into a neutron through beta plus decay.
According to quantum field theory, the mean proper lifetime of protons becomes finite when they are accelerating with proper acceleration, and decreases with increasing. Acceleration gives rise to a non-vanishing probability for the transition. This was a matter of concern in the later 1990s because is a scalar that can be measured by the inertial and coaccelerated observers. In the inertial frame, the accelerating proton should decay according to the formula above. However, according to the coaccelerated observer the proton is at rest and hence should not decay. This puzzle is solved by realizing that in the coaccelerated frame there is a thermal bath due to Fulling–Davies–Unruh effect, an intrinsic effect of quantum field theory. In this thermal bath, experienced by the proton, there are electrons and antineutrinos with which the proton may interact according to the processes:

Adding the contributions of each of these processes, one should obtain.

Quarks and the mass of a proton

In quantum chromodynamics, the modern theory of the nuclear force, most of the mass of protons and neutrons is explained by special relativity. The mass of a proton is about 80–100 times greater than the sum of the rest masses of its three valence quarks, while the gluons have zero rest mass. The extra energy of the quarks and gluons in a proton, as compared to the rest energy of the quarks alone in the QCD vacuum, accounts for almost 99% of the proton's mass. The rest mass of a proton is, thus, the invariant mass of the system of moving quarks and gluons that make up the particle, and, in such systems, even the energy of massless particles confined to a system is still measured as part of the rest mass of the system.
Two terms are used in referring to the mass of the quarks that make up protons: current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark. These masses typically have very different values. The kinetic energy of the quarks that is a consequence of confinement is a contribution. Using lattice QCD calculations, the contributions to the mass of the proton are the quark condensate, the quark kinetic energy, the gluon kinetic energy, and the anomalous gluonic contribution.
The constituent quark model wavefunction for the proton is
The internal dynamics of protons are complicated, because they are determined by the quarks' exchanging gluons, and interacting with various vacuum condensates. Lattice QCD provides a way of calculating the mass of a proton directly from the theory to any accuracy, in principle. The most recent calculations claim that the mass is determined to better than 4% accuracy, even to 1% accuracy. These claims are still controversial, because the calculations cannot yet be done with quarks as light as they are in the real world. This means that the predictions are found by a process of extrapolation, which can introduce systematic errors. It is hard to tell whether these errors are controlled properly, because the quantities that are compared to experiment are the masses of the hadrons, which are known in advance.
These recent calculations are performed by massive supercomputers, and, as noted by Boffi and Pasquini: "a detailed description of the nucleon structure is still missing because... long-distance behavior requires a nonperturbative and/or numerical treatment ..."
More conceptual approaches to the structure of protons are: the topological soliton approach originally due to Tony Skyrme and the more accurate AdS/QCD approach that extends it to include a string theory of gluons, various QCD-inspired models like the bag model and the constituent quark model, which were popular in the 1980s, and the SVZ sum rules, which allow for rough approximate mass calculations. These methods do not have the same accuracy as the more brute-force lattice QCD methods, at least not yet.