# proton

Proton (p + )

classification
Fermion
Baryon
Nucleon
properties
electric charge e
(+1.602 10 −19  C )
Dimensions 1.007 276 466 583 (15) (29)  u
1.672 621 923 69 (51) · 10 −27  kg
1836.152 673 43 (11)  m e
Resting energy 938,272,088 16 (29)  MeV
Compton wavelength 1.321 409 855 39 (40) 10 −15  m
magnetic moment 1.410 606 797 36 (60) 10 −26  J  /  T
2.792 847 344 63 (82)  μ N
g factor 5.585 694 6893 (16)
gyromagnetic
ratio
2.675 221 8744 (11) 10 8  1 / ( s T )
Spin parity 1/2 +
Isospin 1/2 (z component +1/2)
average lifespan stable
Interactions strong
weak
electromagnetic
gravitation
Quark
composition
1 down, 2 up

The proton [ ˈproːtɔn ] ( plural protons [ proˈtoːnən ]; from ancient Greek τὸ πρῶτον to prōton 'the first') is a stable, electrically positively charged hadron . Its symbol is . The proton, along with the neutron and the electron, is one of the building blocks of the atoms from which all everyday matter is composed. ${\ displaystyle \ mathbf {p}}$

The atomic nucleus of ordinary hydrogen is a single proton, so the proton is also known as the hydrogen nucleus or hydrogen ion . However, these names are ambiguous because there are isotopes of hydrogen that also contain one or two neutrons in the nucleus.

## construction

The proton consists of two up quarks and one down quark (formula uud). These three valence quarks are surrounded by a “lake” of gluons and quark-antiquark pairs. Only about 1 percent of the mass of the proton comes from the masses of the valence quarks. The rest comes from the kinetic and binding energy between quarks and gluons; the gluons, as force exchange particles, convey the strong force between the quarks. The diameter of a free proton is about 1.7 · 10 −15  m. Like the neutron, the proton is a baryon .

## properties

The proton is the only stable hadron and the lightest baryon . Since a decay can only lead to lighter particles, the proton has to be stable because of the baryon number conservation according to the standard model . Experiments on the Kamiokande suggest a half-life of at least 10 32 years. The search for proton decay is of particular importance to physics as it would offer the opportunity to test theories beyond the Standard Model.

The magnetic moment can be determined by the simplified model at the level of curd constituent quark to calculate. Here is the nuclear magneton ; are the moments relating to the masses of the respective constituent quark with the g-factor 2. The result roughly agrees with the measured values. ${\ displaystyle {\ vec {\ mu _ {\ rm {p}}}} = {\ tfrac {4} {3}} {\ vec {\ mu _ {\ rm {u}}}} - {\ tfrac {1} {3}} {\ vec {\ mu _ {\ rm {d}}}} = + 2 {,} 79 \, {\ vec {\ mu _ {\ mathrm {N}}}}}$${\ displaystyle \ mu _ {\ mathrm {N}}}$${\ displaystyle \ mu _ {\ rm {u}}, \ mu _ {\ rm {d}}}$

Protons can arise from the beta decay of neutrons:

${\ displaystyle \ mathrm {n} \ rightarrow \ mathrm {p} + \ mathrm {e} ^ {-} + {\ bar {\ nu}} _ {e} +0 {,} 78 \, \ mathrm {MeV }}$

The reverse process occurs e.g. B. on the formation of a neutron star and is theoretically possible even under normal conditions, but statistically extremely rare, since three particles with precisely coordinated energies would have to collide at the same time. However, a proton bound in a very proton-rich atomic nucleus can turn into a neutron through beta-plus decay or electron capture .

The antimatter particle ( antiparticle ) to the proton is the antiproton , which was first artificially created in 1955 by Emilio Segrè and Owen Chamberlain , which earned the discoverers the 1959 Nobel Prize in Physics . It has the same mass as the proton, but has a negative electrical charge.

## Protons as components of atomic nuclei

The atomic nucleus of almost all nuclides consists of protons and neutrons , the nucleons ; the only exception is the most common hydrogen atom 1 H, whose atomic nucleus only consists of a single proton (see also Proton (chemistry) ). The number of protons in the atomic nucleus is called the atomic number , it determines the number of electrons in the atomic shell and thus the chemical properties of the element . Atoms with the same number of protons but different numbers of neutrons are called isotopes and have almost identical chemical properties.

The protons in the atomic nucleus contribute to the total atomic mass. The strong interaction between protons and neutrons is responsible for the maintenance and stability of the atomic nucleus. While the positively charged protons experience both attractive (strong interaction) and repulsive forces ( electromagnetic interaction ) with one another, there is no electrostatic force between neutrons and between neutrons and protons.

The diproton , the fictitious helium isotope 2 He, the nucleus of which would consist of only two protons, is not "particle stable" because, due to the Pauli principle - in contrast to the proton and neutron in the deuteron - two protons can only form a singlet - State with antiparallel spins. Due to the strong spin dependence of the nucleon-nucleon interaction, however, this is energetically increased and therefore not bound. Only with a further neutron in the nucleus is the stable 3 He obtained.

Via the nuclear photo effect , protons can be released from the nucleus by high-energy photons , as well as in other nuclear reactions by collisions with fast protons, neutrons or alpha particles . In the case of nuclei with a particularly high or particularly low number of neutrons, spontaneous nucleon emission, i.e. proton or neutron emission, can occur. One speaks here of proton or neutron radiation. The half-lives here are always very short. In the case of an extreme excess of protons (such as the iron isotope 45 Fe), two-proton decay can occur, in which even two protons are emitted at the same time (see the main article on radioactivity ).

## Scattering processes from or on protons

Scattering experiments with protons on other nucleons are carried out in order to investigate the properties of the nucleon-nucleon interactions. In neutron scattering, the strong interaction is the dominant force; the electromagnetic and especially the weak interaction are negligible here. If protons are scattered on protons, the Coulomb force must also be taken into account. The core forces also depend on the spin. A result of the comparison of the pp-scattering with the nn-scattering is that the nuclear forces are independent of the charge state of the nucleons (the proportion of the Coulomb force in the cross-section of the pp-scattering is subtracted in order to compare only the effect of the nuclear forces).

The form factor of the proton can be determined with elastic or quasi-elastic scattering of electrons on protons . By scattering a polarized 1.16 GeV electron beam on protons, their weak charge was precisely measured. The fact that parity is not maintained applies only to weak interaction .

## Further reactions of the proton in astrophysics

Proton-proton reactions are one of two fusion reactions in hydrogen burning .

In the case of proton attachment in the p-process , a fast proton overcomes the repulsion by the Coulomb force and becomes part of the atomic nucleus that is hit.

## Current research areas

One explores the properties of the proton u. a. in systems such as the Super Proton Synchrotron (SPS) and the Large Hadron Collider (LHC) at CERN , the Tevatron in Fermilab or HERA . Research with proton-antiproton collisions serves, among other things, to search for physics beyond the standard model.

Measurements of the Lamb shift on the muonic hydrogen , i.e. on the bound system of muon and proton, in 2010 resulted in a 4% lower value for the charge radius of the proton than previously assumed. a. was determined from scattering tests on electron accelerators. Since the muon is much heavier than the electron, it comes much closer to the proton. In the case of muonic atoms, this makes the influence of the proton expansion on the spectrum more precisely measurable. The difference in proton radius was in the range of four standard deviations. This attracted a great deal of attention at the time, as it raises questions about quantum electrodynamics , which is actually considered the best-researched physical theory, which for example predicts the energy levels in the hydrogen atom with an accuracy of 12 decimal places. Deviations from the standard model were also discussed, but one of the physicists involved ( Randolf Pohl ) considers a deviation of the Rydberg constant from previously accepted values ​​to be more likely. In 2016, the deviation was also confirmed in muonic deuterium atoms. In 2017, a deviation from the hydrogen standard data was also discovered in measurements on ordinary hydrogen (in the amount of 3.3 standard deviations for both the proton radius and the Rydberg constant ). For this, two transitions had to be measured (in addition to 2s-1s the transition 2s-4p). The experiment represents one of the most accurate measurements in laser spectroscopy to date.

In terrestrial gamma-ray bursts, protons with energies of up to 30 MeV could occur alongside other mass particles. However, the time scale on which terrestrial proton beams can be measured is significantly longer than for terrestrial gamma-ray bursts.

## Technical applications

Accelerated protons are used in medicine as part of proton therapy to treat tumor tissue. Compared to conventional X-ray radiation, this is a gentler therapy, since the protons essentially only release their energy in a narrowly limited depth range in the tissue ( Bragg peak ). The tissue that is on the way there is significantly less stressed (factor 3 to 4), the tissue behind it is relatively less stressed compared to X-ray radiotherapy .

Protons with kinetic energies in the range 10 to 50 MeV from cyclotrons are used, for. B. also for the production of proton-rich radionuclides for medical purposes or for the superficial activation of machine parts for the purpose of later wear measurements.

## Research history

William Prout suspected in 1815 that all atoms were made up of hydrogen atoms.

Protons first appeared in research in 1898, when Wilhelm Wien discovered that the Geissler tube had to be filled with hydrogen in order to obtain canal beams with the greatest charge-to-mass ratio. This radiation consists of protons.

In 1913, Niels Bohr developed the model named after him for the hydrogen atom, in which an electron orbits a positively charged atomic nucleus. This nucleus is a proton.

In 1919 Ernest Rutherford discovered that there are atomic nuclei of hydrogen in the atomic nucleus of nitrogen . He then assumed that all atomic nuclei are made up of hydrogen nuclei and suggested the name proton for them. He referred to the word protyle , which denotes a hypothetical basic substance of all matter .

## swell

• Wolfgang Demtröder: Experimental Physics (Volume 4). 2nd Edition. Springer, Berlin 2005, ISBN 3-540-21451-8 .
• Donald H. Perkins: Introduction to high energy physics. 4th edition. Cambridge University Press, 2000, ISBN 0-521-62196-8 .

## Individual evidence

1. Proton lighter than expected. Retrieved July 27, 2017 . Proton mass in u . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
2. CODATA Recommended Values. National Institute of Standards and Technology, accessed July 21, 2019 . Proton mass in kg . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
3. CODATA Recommended Values. National Institute of Standards and Technology, accessed July 21, 2019 . Proton mass in multiples of the electron mass. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
4. CODATA Recommended Values. National Institute of Standards and Technology, accessed July 21, 2019 . Proton mass in MeV / c 2 . The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
5. CODATA Recommended Values. National Institute of Standards and Technology, accessed July 21, 2019 . Compton wavelength of the proton. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
6. CODATA Recommended Values. National Institute of Standards and Technology, accessed July 21, 2019 . Magnetic moment of the proton. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
7. CODATA Recommended Values. National Institute of Standards and Technology, accessed July 21, 2019 . g-factor of the proton. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
8. CODATA Recommended Values. National Institute of Standards and Technology, accessed July 21, 2019 . Gyromagnetic ratio of the proton. The numbers in brackets denote the uncertainty in the last digits of the value; this uncertainty is given as the estimated standard deviation of the specified numerical value from the actual value.
9. ^ Wilhelm Gemoll : Greek-German school and hand dictionary. Munich / Vienna 1965.
10. S. Dürr et al .: Ab initio determination of Light Hadron Masses. Science 322 (2008) pp. 1224-1227
11. ^ The Jefferson Lab Q-weak Collaboration: Precision measurement of the weak charge of the proton. Nature vol. 557 (2018) pages 207-211, doi: 10.1038 / s41586-018-0096-0
12. Search for physics outside the Standard Model in proton-antiproton collisions with leptons and jets in the final state; Thomas Nunnemann; Weblink to PDF lecture
13. Randolf Pohl et al .: The size of the proton . In: Nature . tape 466 , no. 7303 , 2010, p. 213-216 , doi : 10.1038 / nature09250 .
14. Natalie Wolchover, New Measurement Deepen's Proton Puzzle, Quanta Magazine, August 11, 2016
15. Shrunken Proton , Pro Physik, October 6, 2017
16. A. Beyer et al .: The Rydberg constant and proton size from atomic hydrogen, Science, Volume 358, 2017, p. 79
17. Köhn, C., Ebert, U .: Calculation of beams of positrons, neutrons and protons associated with terrestrial gamma-ray flashes. J. Geophys. Res. Atmos. (2015), vol. 23, doi : 10.1002 / 2014JD022229
18. Köhn, C., Diniz, G., Harakeh, MN: Production mechanisms of leptons, photons, and hadrons and their possible feedback close to lightning leaders. J. Geophys. Res. Atmos. (2017), vol. 122, doi : 10.1002 / 2016JD025445
19. Rutherford in a footnote to the article "The Constitution of Atoms." by Orme Masson in The Philosophical Magazine, Vol 41 (1921), pp. 281-285. : "... Finally the name" proton "met with general approval, particularly as it suggests the original term" protyle "given by Prout in his well-known hypothesis that all atoms are built up of hydrogen. The need of a special name for the nuclear unit of mass 1 was drawn attention to by Sir Oliver Lodge at the Sectional meeting, and the writer then suggested the name "proton."
20. ^ Wilhelm Wien: About positive electrons and the existence of high atomic weights. In: Annals of Physics. Volume 318 (4), 1904, pp. 669-677.