View source for Proton

{{Short description|Subatomic particle with positive charge}}
{{hatgrp|{{Other uses}}
{{redirect|P+|the record label|Jean Dawson}}}}
{{pp-move}}
{{Infobox particle
| bgcolour       =
| classification = [[Baryon]]
| name           = Proton
| image          = Quark structure proton.svg
| caption        = The [[valence quark]] content of a proton. The [[Color charge|color assignment]] of individual quarks is arbitrary, but all three colors must be present. Forces between quarks are mediated by [[gluons]].
| num_types      =
| composition    = 2 [[up quark]]s (u), 1 [[down quark]] (d)
| statistics     = [[Fermionic]]
| group          = [[Hadron]]
| generation     =
| interaction    = [[Gravity]], [[Electromagnetic interaction|electromagnetic]], [[Weak interaction|weak]], [[Strong interaction|strong]]
| antiparticle   = [[Antiproton]]
| theorized      = [[William Prout]] (1815)
| discovered     = Observed as H<sup>+</sup> by [[Eugen Goldstein]] (1886). Identified in other nuclei (and named) by [[Ernest Rutherford]] (1917–1920).
| symbol         = {{SubatomicParticle|Proton}}, {{SubatomicParticle|Proton+}}, {{SubatomicParticle|Nucleon+}}, {{chem|1|1|H|+}}
| mass           = {{physconst|mp}}<br />{{physconst|mp_Da}}<br />{{physconst|mpc2_MeV|unit={{val|ul=MeV/c2}}}}
| mean_lifetime   = > {{val|3.6|e=29|u=years}}<ref>{{Cite journal|last1=The SNO+ Collaboration|last2=Anderson|first2=M.|last3=Andringa|first3=S.|last4=Arushanova|first4=E.|last5=Asahi|first5=S.|last6=Askins|first6=M.|last7=Auty|first7=D. J.|last8=Back|first8=A. R.|last9=Barnard|first9=Z.|last10=Barros|first10=N.|last11=Bartlett|first11=D.|date=2019-02-20|title=Search for invisible le modes of nucleon decay in water with the SNO+ detector|url=https://link.aps.org/doi/10.1103/PhysRevD.99.032008|journal=Physical Review D|volume=99|issue=3|pages=032008|doi=10.1103/PhysRevD.99.032008|arxiv=1812.05552 |bibcode=2019PhRvD..99c2008A |s2cid=96457175 }}</ref> (stable)
| electric_charge = {{val|p=+|1|ul=e}}
| charge_radius = {{val|0.8414|(19)|u=[[Femtometre|fm]]}}<ref name="CODATA2018" />
| electric_dipole_moment = < {{val|2.1|e=-25|u=''e''⋅cm}}<ref>{{Cite journal|last=Sahoo|first=B. K.|date=2017-01-17|title=Improved limits on the hadronic and semihadronic $CP$ violating parameters and role of a dark force carrier in the electric dipole moment of $^{199}\mathrm{Hg}$|url=https://link.aps.org/doi/10.1103/PhysRevD.95.013002|journal=Physical Review D|volume=95|issue=1|pages=013002|doi=10.1103/PhysRevD.95.013002|arxiv=1612.09371 |s2cid=119344894 }}</ref>
| electric_polarizability = {{val|0.00112|(4)|u=fm<sup>3</sup>}}
| magnetic_moment = {{physconst|mup}}<br />{{physconst|mup/muB|after=&nbsp;[[Bohr magneton|''μ''<sub>B</sub>]]}}<br />{{physconst|mup/muN|after=&nbsp;[[Nuclear magneton|''μ''<sub>N</sub>]]}}
| magnetic_polarizability = {{val|1.9|(5)|e=-4|u=fm<sup>3</sup>}}
| spin = {{sfrac|1|2}}&nbsp;[[reduced Planck constant|''ħ'']]
| isospin = {{sfrac|1|2}}
| parity = +1
| condensed_symmetries = ''[[Isospin|I]]''(''[[Total angular momentum|J]]''<sup>''[[Parity (physics)|P]]''</sup>) = {{sfrac|1|2}}({{sfrac|1|2}}<sup>+</sup>)
}}
A '''proton''' is a stable [[subatomic particle]], symbol {{SubatomicParticle|Proton}}, [[Hydron (chemistry)|H<sup>+</sup>]], or <sup>1</sup>H<sup>+</sup> with a positive [[electric charge]] of +1&nbsp;''e'' ([[elementary charge]]). Its mass is slightly less than the mass of a [[neutron]] and approximately {{val|1,836}} times the mass of an [[electron]] (the [[proton-to-electron mass ratio]]). Protons and neutrons, each with a mass of approximately one [[atomic mass unit]], are jointly referred to as ''[[nucleon]]s'' (particles present in atomic nuclei).

One or more protons are present in the [[Atomic nucleus|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]] (represented by the symbol ''Z''). Since each [[chemical element|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 chemical characteristics of the element.

The word ''proton'' is [[Greek language|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 (known to be the lightest nucleus) could be extracted from the nuclei of [[nitrogen]] by atomic collisions.<ref name="Britannica" /> 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 quark]]s, and together with [[neutron]]s are now classified as [[hadron]]s. Protons are composed of two [[up quark]]s of charge +{{sfrac|2|3}}''e'' each, and one [[down quark]] of charge −{{sfrac|1|3}}''e''. The [[rest mass]]es of quarks contribute only about 1% of a proton's mass.<ref name="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 [[root mean square]] [[charge radius]] of a proton is about 0.84–0.87&nbsp;[[femtometre|fm]] ({{val|1|u=fm}} = {{val|e=-15|u=m}}).<ref name="PSI" /><ref name="Antognini2013" /> In 2019, two different studies, using different techniques, found this radius to be 0.833&nbsp;fm, with an uncertainty of ±0.010 fm.<ref name="ReferenceA">{{Cite journal|last1=Bezginov|first1=N.|last2=Valdez|first2=T.|last3=Horbatsch|first3=M.|last4=Marsman|first4=A.|last5=Vutha|first5=A. C.|last6=Hessels|first6=E. A.|s2cid=201845158|date=2019-09-06|title=A measurement of the atomic hydrogen Lamb shift and the proton charge radius|journal=Science|volume=365|issue=6457|pages=1007–1012|doi=10.1126/science.aau7807|pmid=31488684|issn=0036-8075|bibcode=2019Sci...365.1007B|doi-access=free}}</ref><ref name="ReferenceB">{{Cite journal|last1=Xiong|first1=W.|last2=Gasparian|first2=A.|last3=Gao|first3=H.|last4=Dutta|first4=D.|last5=Khandaker|first5=M.|last6=Liyanage|first6=N.|last7=Pasyuk|first7=E.|last8=Peng|first8=C.|last9=Bai|first9=X.|last10=Ye|first10=L.|last11=Gnanvo|first11=K.|s2cid=207831686|date=November 2019|title=A small proton charge radius from an electron–proton scattering experiment|journal=Nature|volume=575|issue=7781|pages=147–150|doi=10.1038/s41586-019-1721-2|pmid=31695211|bibcode=2019Natur.575..147X|osti=1575200|issn=1476-4687}}</ref>

Free protons occur occasionally on Earth: [[thunderstorm]]s can produce protons with energies of up to several tens of [[Electronvolt|MeV]].<ref name="Kohn2015" /><ref name="Kohn2017" /> At sufficiently low temperatures and kinetic energies, free protons will bind to [[electron]]s. However, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, until it is captured by the [[electron cloud]] of an atom. The result is a diatomic or [[polyatomic ion]] containing hydrogen. In a vacuum, when free electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral [[hydrogen atom]], which is chemically a [[free radical]]. Such "free hydrogen atoms" tend to react chemically with many other types of atoms at sufficiently low energies. When free hydrogen atoms react with each other, they form neutral hydrogen molecules (H<sub>2</sub>), which are the most common molecular component of [[molecular clouds]] in [[interstellar medium|interstellar space]].<ref>{{Cite journal |last=Schlemmer |first=Stephan |date=2011-02-08 |title=H<sub>2</sub> Generation in the Early Universe Governs the Formation of the First Stars |url=http://dx.doi.org/10.1002/anie.201005920 |journal=Angewandte Chemie International Edition |volume=50 |issue=10 |pages=2214–2215 |doi=10.1002/anie.201005920 |pmid=21305679 |issn=1433-7851}}</ref>

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 ==
{{Nuclear physics}}
{{unsolved|physics|How do the quarks and gluons carry the spin of protons?}}
Protons are [[spin-½|spin-{{sfrac|1|2}}]] [[fermion]]s and are composed of three valence quarks,<ref name="Adair1989" /> making them [[baryon]]s (a sub-type of [[hadron]]s). The two [[up quark]]s and one [[down quark]] of a proton are held together by the [[strong interaction|strong force]], mediated by [[gluon]]s.<ref name="Cottingham1986" />{{rp|21–22}} A modern perspective has a proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of [[sea quark]]s. Protons have a positive charge distribution, which decays approximately exponentially, with a root mean square [[charge radius]] of about 0.8&nbsp;fm.<ref name="Basdevant2005" />

Protons and [[neutron]]s are both [[nucleon]]s, which may be bound together by the [[nuclear force]] to form [[atomic nuclei]]. The nucleus of the most common [[isotope]] of the [[hydrogen atom]] (with the [[chemical symbol]] "H") 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]] proposed that all atoms are composed of hydrogen atoms (which he called "protyles"), based on a simplistic interpretation of early values of [[atomic weight]]s (see [[Prout's hypothesis]]), which was disproved when more accurate values were measured.<ref name="Lecturer2006" />{{rp|39–42}}
[[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]] (also known as anode rays) and showed that they were positively charged particles (ions) produced from gases. However, since particles from different gases had different values of [[charge-to-mass ratio]] (''q''/''m''), they could not be identified with a single particle, unlike the negative [[electron]]s discovered by [[J. J. Thomson]]. [[Wilhelm Wien]] in 1898 identified the hydrogen ion as the particle with the highest charge-to-mass ratio in ionized gases.<ref name="Wien1904" />

Following the discovery of the atomic nucleus by [[Ernest Rutherford]] in 1911, [[Antonius van den Broek]] proposed that the place of each element in the [[periodic table]] (its atomic number) is equal to its nuclear charge. This was confirmed experimentally by [[Henry Moseley]] in 1913 using [[X-ray spectroscopy|X-ray spectra]] (More details in [[Atomic number]] under Moseley's 1913 experiment).

In 1917, Rutherford performed experiments (reported in 1919 and 1925) which proved that the hydrogen nucleus is present in other nuclei, a result usually described as the discovery of protons.<ref name="Petrucci2002" /> These experiments began after Rutherford observed that when [[alpha particles]] would strike air, Rutherford could detect scintillation on a [[zinc sulfide]] screen produced at a distance well beyond the distance of alpha-particle range of travel but instead corresponding to the range of travel of hydrogen atoms (protons).<ref>{{cite web |title=How Rutherford detects proton and life of Rutherford |url=https://www.sciencedirect.com/topics/mathematics/rutherford |website=ScienceDirect |access-date=6 December 2023 |archive-date=5 December 2023 |archive-url=https://web.archive.org/web/20231205185436/https://www.sciencedirect.com/topics/mathematics/rutherford |url-status=live }}</ref> After experimentation, Rutherford traced the reaction to the nitrogen in air and found that when alpha particles were introduced into pure nitrogen gas, the effect was larger. In 1919, Rutherford assumed that the alpha particle merely knocked a proton out of nitrogen, turning it into carbon. After observing Blackett's cloud chamber images in 1925, Rutherford realized that the alpha particle was absorbed. If the alpha particle were not absorbed, then it would knock a proton off of nitrogen creating 3 charged particles (a negatively charged carbon, a proton, and an alpha particle). It can be shown<ref>{{cite journal |title=Blackett's cloud chamber |journal=Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character |date=2 February 1925 |volume=107 |issue=742 |pages=349–360 |doi=10.1098/rspa.1925.0029 |doi-access=free }}</ref> that the 3 charged particles would create three tracks in the cloud chamber, but instead only 2 tracks in the cloud chamber were observed. The alpha particle is absorbed by the nitrogen atom. After capture of the alpha particle, a hydrogen nucleus is ejected, creating a net result of 2 charged particles (a proton and a positively charged oxygen) which make 2 tracks in the cloud chamber. Heavy oxygen (<sup>17</sup>O), not carbon or fluorine, is the product. This was the first reported [[nuclear reaction]], {{chem2|^{14}N + α → ^{17}O + p}}. Rutherford at first thought of our modern "p" in this equation as a hydrogen ion, {{chem2|H+}}.

Depending on one's perspective, either 1919 (when it was seen experimentally as derived from another source than hydrogen) or 1920 (when it was recognized and proposed as an elementary particle) may be regarded as the moment when the proton was 'discovered'.

Rutherford knew hydrogen to be the simplest and lightest element and was influenced by [[Prout's hypothesis]] that hydrogen was the building block of all elements. Discovery that the hydrogen nucleus is present in other nuclei as an elementary particle led Rutherford to give the hydrogen nucleus {{chem2|H+}} a special name as a particle, since he suspected that hydrogen, the lightest element, contained only one of these particles. He named this new fundamental building block of the nucleus the ''proton'', after the neuter singular of the Greek word for "first", {{lang|grc|πρῶτον}}. However, Rutherford also had in mind the word ''protyle'' as used by Prout. Rutherford spoke at the [[British Association for the Advancement of Science]] at its [[Cardiff]] meeting beginning 24 August 1920.<ref name="NatureScience" /> At the meeting, he was asked by [[Oliver Lodge]] for a new name for the positive hydrogen nucleus to avoid confusion with the neutral hydrogen atom. He initially suggested both ''proton'' and ''prouton'' (after Prout).<ref name="Romer1997" /> Rutherford later reported that the meeting had accepted his suggestion that the hydrogen nucleus be named the "proton", following Prout's word "protyle".<ref name="Footnote" /> The first use of the word "proton" in the scientific literature appeared in 1920.<ref>OED{{OED|term=proton|access-date=24 March 2021}}</ref><ref name="Pais1986" />

==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 exist in [[plasma (physics)|plasmas]] in which temperatures are too high to allow them to combine with [[electron]]s.<ref name=Ebeling2021>{{cite journal |title=Equation of state of hydrogen, helium, and solar plasmas |first=W. |last=Ebeling |first2=H. |last2=Reinholz |first3=G. |last3=Röpke |date=2021 |journal=Contributions to Plasma Physics |doi=10.1002/ctpp.202100085}}</ref> Free protons of high energy and velocity make up 90% of [[cosmic ray]]s, which propagate through the [[interstellar medium]].<ref name=goddard-2012>{{cite web|title=What are cosmic rays?|publisher=NASA|department=Goddard Space Flight Center|url=http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html|access-date=31 October 2012|url-status=dead|archive-url=https://web.archive.org/web/20121028154200/http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html|archive-date=28 October 2012}}{{cite web|title=mirror copy, also archived|url=http://certificate.ulo.ucl.ac.uk/modules/year_one/NASA_GSFC/goddard/imagine.gsfc.nasa.gov/docs/science/know_l2/cosmic_rays.html|archive-url=https://web.archive.org/web/20160304080140/http://certificate.ulo.ucl.ac.uk/modules/year_one/NASA_GSFC/goddard/imagine.gsfc.nasa.gov/docs/science/know_l2/cosmic_rays.html|archive-date=4 March 2016}}
</ref> Free protons are [[proton emission|emitted directly]] from [[atomic nucleus|atomic nuclei]] in some rare types of [[radioactive decay]].<ref>{{Cite book |url=http://link.springer.com/10.1007/978-94-010-0708-5 |title=Nuclei Far from Stability and Astrophysics |publisher=Springer Netherlands |year=2001 |isbn=978-0-7923-6937-0 |editor-last=Poenaru |editor-first=Dorin N. |location= Dordrecht |pages=79–81 |language=English |doi=10.1007/978-94-010-0708-5 |editor-last2=Rebel |editor-first2=Heinigerd |editor-last3=Wentz |editor-first3=Jürgen}}</ref> Protons also result (along with electrons and [[antineutrino]]s) from the [[free neutron decay|radioactive decay of free neutrons]], which are unstable.<ref name=Gonzalez-2021>{{Cite journal|last1=UCNτ Collaboration|last2=Gonzalez|first2=F. M.|last3=Fries|first3=E. M.|last4=Cude-Woods|first4=C.|last5=Bailey|first5=T.|last6=Blatnik|first6=M.|last7=Broussard|first7=L. J.|last8=Callahan|first8=N. B.|last9=Choi|first9=J. H.|last10=Clayton|first10=S. M.|last11=Currie|first11=S. A.|date=2021-10-13|title=Improved Neutron Lifetime Measurement with UCNτ|url=https://par.nsf.gov/servlets/purl/10304438|journal=Physical Review Letters|volume=127|issue=16|page=162501|arxiv=2106.10375|doi=10.1103/PhysRevLett.127.162501|pmid=34723594|bibcode=2021PhRvL.127p2501G|s2cid=235490073|access-date=2024-04-01|archive-date=2024-04-01|archive-url=https://web.archive.org/web/20240401134040/https://par.nsf.gov/servlets/purl/10304438|url-status=live}}</ref>

== Stability ==
{{Main|Proton decay}}
{{unsolved|physics|Are protons fundamentally stable? Or do they decay with a finite lifetime as predicted by some extensions to the standard model?}}
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 theory|grand unified theories]] (GUTs) of particle physics predict that [[proton decay]] should take place with lifetimes between 10<sup>31</sup> and 10<sup>36</sup> years. Experimental searches have established lower bounds on the [[mean lifetime]] of a proton for various assumed decay products.<ref name="Bucella1989" /><ref name="Lee1995" /><ref name="Kamioka" />

Experiments at the [[Super-Kamiokande]] detector in Japan gave lower limits for proton [[mean lifetime]] of {{val|6.6|e=33|u=years}} for decay to an [[antimuon]] and a neutral [[pion]], and {{val|8.2|e=33|u=years}} for decay to a [[positron]] and a neutral pion.<ref name="Nishino2009" />
Another experiment at the [[Sudbury Neutrino Observatory]] in Canada searched for [[gamma ray]]s resulting from residual nuclei resulting from the decay of a proton from oxygen-16. This experiment was designed to detect decay to any product, and established a lower limit to a proton lifetime of {{val|2.1|e=29|u=years}}.<ref name="Ahmed2004" />

However, protons are known to transform into [[neutron]]s 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:
: {{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron|link=yes}} → {{SubatomicParticle|Neutron|link=yes}} + {{SubatomicParticle|Electron neutrino|link=yes}}

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 [[Positron emission|beta plus decay]] (β+ decay).

According to [[quantum field theory]], the mean proper lifetime of protons <math>\tau_\mathrm{p}</math> becomes finite when they are accelerating with [[proper acceleration]] <math>a</math>, and <math>\tau_\mathrm{p}</math> decreases with increasing <math>a</math>. Acceleration gives rise to a [[S-matrix|non-vanishing probability]] for the transition {{nowrap|{{SubatomicParticle|Proton+}} → {{SubatomicParticle|Neutron}} + {{SubatomicParticle|Positron}} + {{SubatomicParticle|Electron neutrino}}}}. This was a matter of concern in the later 1990s because <math>\tau_\mathrm{p}</math> is a scalar that can be measured by the inertial and [[Rindler coordinates|coaccelerated observers]]. In the [[Inertial frame of reference|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 [[Unruh effect|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: 
# {{nowrap|{{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron}} → {{SubatomicParticle|Neutron}} + {{SubatomicParticle|Neutrino}}}}, 
# {{nowrap|{{SubatomicParticle|Proton+}} + {{SubatomicParticle|Antineutrino}} → {{SubatomicParticle|Neutron}} + {{SubatomicParticle|Positron}}}} and
#  {{nowrap|{{SubatomicParticle|Proton+}} + {{SubatomicParticle|Electron}} + {{SubatomicParticle|Antineutrino}} → {{SubatomicParticle|Neutron}}}}. 
Adding the contributions of each of these processes, one should obtain <math>\tau_\mathrm{p}</math>.<ref>{{Cite journal |last1=Vanzella |first1=Daniel A. T. |last2=Matsas |first2=George E. A. |date=2001-09-25 |title=Decay of Accelerated Protons and the Existence of the Fulling–Davies–Unruh Effect |url=https://link.aps.org/doi/10.1103/PhysRevLett.87.151301 |journal=Physical Review Letters |volume=87 |issue=15 |pages=151301 |doi=10.1103/PhysRevLett.87.151301|pmid=11580689 |arxiv=gr-qc/0104030 |bibcode=2001PhRvL..87o1301V |hdl=11449/66594 |s2cid=3202478 }}</ref><ref>{{Cite journal |last1=Matsas |first1=George E. A. |last2=Vanzella |first2=Daniel A. T. |date=1999-03-16 |title=Decay of protons and neutrons induced by acceleration |url=https://link.aps.org/doi/10.1103/PhysRevD.59.094004 |journal=Physical Review D |volume=59 |issue=9 |pages=094004 |doi=10.1103/PhysRevD.59.094004 |arxiv=gr-qc/9901008 |bibcode=1999PhRvD..59i4004M |hdl=11449/65768 |s2cid=2646123 |access-date=2022-07-24 |archive-date=2023-12-30 |archive-url=https://web.archive.org/web/20231230134640/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.59.094004 |url-status=live }}</ref><ref>{{Cite journal |last1=Vanzella |first1=Daniel A. T. |last2=Matsas |first2=George E. A. |date=2000-12-06 |title=Weak decay of uniformly accelerated protons and related processes |url=https://link.aps.org/doi/10.1103/PhysRevD.63.014010 |journal=Physical Review D |volume=63 |issue=1 |pages=014010 |doi=10.1103/PhysRevD.63.014010 |arxiv=hep-ph/0002010 |bibcode=2000PhRvD..63a4010V |hdl=11449/66417 |s2cid=12735961 |access-date=2022-07-24 |archive-date=2023-12-30 |archive-url=https://web.archive.org/web/20231230134615/https://journals.aps.org/prd/abstract/10.1103/PhysRevD.63.014010 |url-status=live }}</ref><ref>{{Cite journal |last1=Matsas |first1=George E. A. |last2=Vanzella |first2=Daniel a. T. |date=2002-12-01 |title=The fulling–davies–unruh effect is mandatory: the proton's testimony |url=https://www.worldscientific.com/doi/abs/10.1142/S0218271802002918 |journal=International Journal of Modern Physics D |volume=11 |issue=10 |pages=1573–1577 |doi=10.1142/S0218271802002918 |arxiv=gr-qc/0205078 |s2cid=16555072 |issn=0218-2718 |access-date=2022-07-24 |archive-date=2022-07-24 |archive-url=https://web.archive.org/web/20220724015304/https://www.worldscientific.com/doi/abs/10.1142/S0218271802002918 |url-status=live }}</ref>

== Quarks and the mass of a proton ==
In [[quantum chromodynamics]], the modern theory of the nuclear force, most of the mass of protons and [[neutron]]s 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 [[quark]]s, while the [[gluon]]s have zero rest mass. The extra energy of the [[quark]]s and [[gluon]]s 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 [[Mass–energy equivalence#Composite systems|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]] [[quantum field theory|particle field]] surrounding the quark.<ref name="Waston2004" />{{rp|285–286}}<ref name="Smith2003" />{{rp|150–151}} These masses typically have very different values. The kinetic energy of the quarks that is a consequence of confinement is a contribution (see ''[[Mass in special relativity]]''). Using [[lattice QCD]] calculations, the contributions to the mass of the proton are the quark condensate (~9%, comprising the up and down quarks and a sea of virtual strange quarks), the quark kinetic energy (~32%), the gluon kinetic energy (~37%), and the anomalous gluonic contribution (~23%, comprising contributions from condensates of all quark flavors).<ref>{{cite magazine|author=André Walker-Loud|title=Dissecting the Mass of the Proton|magazine=Physics|date=19 November 2018|volume=11|page=118|doi=10.1103/Physics.11.118|bibcode=2018PhyOJ..11..118W|url=https://physics.aps.org/articles/v11/118|access-date=2021-06-04|doi-access=free|archive-date=2021-06-05|archive-url=https://web.archive.org/web/20210605002635/https://physics.aps.org/articles/v11/118|url-status=live}}</ref>

The constituent quark model wavefunction for the proton is
<math display="block">\mathrm{|p_\uparrow\rangle = \tfrac{1}{\sqrt {18}} \left(2| u_\uparrow d_\downarrow u_\uparrow \rangle + 2| u_\uparrow  u_\uparrow d_\downarrow \rangle + 2| d_\downarrow u_\uparrow   u_\uparrow \rangle - | u_\uparrow u_\downarrow d_\uparrow\rangle  -| u_\uparrow d_\uparrow u_\downarrow\rangle  - | u_\downarrow d_\uparrow u_\uparrow\rangle
- | d_\uparrow u_\downarrow u_\uparrow\rangle  - |d_\uparrow  u_\uparrow u_\downarrow\rangle-| u_\downarrow u_\uparrow d_\uparrow\rangle\right)}.</math>

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<ref name="SMAnswers" /><ref name="Fodor2008" /> claim that the mass is determined to better than 4% accuracy, even to 1% accuracy (see Figure S5 in Dürr ''et al.''<ref name="Fodor2008" />). 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.<ref name="Perdrisat2007" /> It is hard to tell whether these errors are controlled properly, because the quantities that are compared to experiment are the masses of the [[hadron]]s, 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&nbsp;..."<ref name="Boffi2007" />
More conceptual approaches to the structure of protons are: the [[skyrmion|topological soliton]] approach originally due to [[Tony Skyrme]] and the more accurate [[AdS/QCD|AdS/QCD approach]] that extends it to include a [[string theory]] of gluons,<ref name="Erlich2008" /> 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.<ref name="Calangelo2000" /> These methods do not have the same accuracy as the more brute-force lattice QCD methods, at least not yet.

== Charge radius ==
{{Main|Proton radius puzzle}}

The [[CODATA]] recommended value of a proton's [[charge radius]] is {{physconst|rp|after=.}} The radius of the proton is defined by a formula that can be calculated by [[quantum electrodynamics]] and be derived from either atomic spectroscopy or by electron–proton scattering. The formula involves a form-factor related to the two-dimensional [[Parton (particle physics)|parton]] diameter of the proton.<ref>{{cite journal |last=Miller |first=Gerald A. |date=2019-03-07 |title=Defining the proton radius: A unified treatment |url=https://link.aps.org/doi/10.1103/PhysRevC.99.035202 |journal=Physical Review C |language=en |volume=99 |issue=3 |page=035202 |doi=10.1103/PhysRevC.99.035202 |issn=2469-9985|arxiv=1812.02714 |bibcode=2019PhRvC..99c5202M }}</ref>

A value from before 2010 is based on scattering electrons from protons followed by complex calculation involving scattering cross section based on [[Marshall Rosenbluth|Rosenbluth]] equation for [[momentum-transfer cross section]]), and based on studies of the atomic [[energy level]]s of hydrogen and deuterium.
In 2010 an international research team published a proton charge radius measurement via the [[Lamb shift]] in muonic hydrogen (an [[exotic atom]] made of a proton and a negatively charged [[muon]]). As a muon is 200 times heavier than an electron, resulting in a smaller [[atomic orbital]], it is much more sensitive to the proton's charge radius and thus allows a more precise measurement.<ref name="Pohl2010" /> Subsequent improved scattering and electron-spectroscopy measurements agree with the new small radius. Work continues to refine and check this new value.<ref name=KarrReview>{{cite journal |last1=Karr |first1=Jean-Philippe |last2=Marchand |first2=Dominique |last3=Voutier |first3=Eric |date=November 2020 |title=The proton size |url=https://www.nature.com/articles/s42254-020-0229-x |journal=Nature Reviews Physics |language=en |volume=2 |issue=11 |pages=601–614 |doi=10.1038/s42254-020-0229-x |bibcode=2020NatRP...2..601K |issn=2522-5820 |access-date=2024-03-28 |archive-date=2024-03-28 |archive-url=https://web.archive.org/web/20240328000755/https://www.nature.com/articles/s42254-020-0229-x |url-status=live }}</ref>

=== Pressure inside the proton ===
Since the proton is composed of quarks confined by gluons, an equivalent [[pressure]] that acts on the quarks can be defined. The size of that pressure and other details about it are controversial.

In 2018 this pressure was reported to be on the order 10<sup>35</sup>&nbsp;Pa, which is greater than the pressure inside a [[neutron star]]. It was said to be maximum at the centre, positive (repulsive) to a radial distance of about 0.6&nbsp;fm, negative (attractive) at greater distances, and very weak beyond about 2&nbsp;fm. These numbers were derived by a combination of a theoretical model and experimental 
[[Compton scattering]] of high-energy electrons.<ref name="Burkert2018" /><ref>{{Cite web |url=https://www.quantamagazine.org/swirling-forces-crushing-pressures-measured-in-the-proton-20240314/ |title=Wood C. Swirling Forces, Crushing Pressures Measured in the Proton. ''Quanta Magazine'' March 14, 2024 |access-date=April 1, 2024 |archive-date=April 1, 2024 |archive-url=https://web.archive.org/web/20240401060328/https://www.quantamagazine.org/swirling-forces-crushing-pressures-measured-in-the-proton-20240314/ |url-status=live }}</ref><ref>{{cite journal | last1=Burkert | first1=V. D. | last2=Elouadrhiri | first2=L. | last3=Girod | first3=F. X. | last4=Lorcé | first4=C. | last5=Schweitzer | first5=P. | last6=Shanahan | first6=P. E. |display-authors=1| title=Colloquium : Gravitational form factors of the proton | journal=Reviews of Modern Physics | volume=95 | issue=4 | date=2023-12-22 | issn=0034-6861 | doi=10.1103/RevModPhys.95.041002 | page=041002|arxiv=2303.08347| bibcode=2023RvMP...95d1002B }}</ref> However, these results have been challenged as also being consistent with zero pressure<ref>{{Cite journal |last=Kumerički |first=Krešimir |date=June 2019 |title=Measurability of pressure inside the proton |url=https://www.nature.com/articles/s41586-019-1211-6 |journal=Nature |language=en |volume=570 |issue=7759 |pages=E1–E2 |doi=10.1038/s41586-019-1211-6 |pmid=31168113 |bibcode=2019Natur.570E...1K |issn=0028-0836 |access-date=2024-03-27 |archive-date=2024-03-27 |archive-url=https://web.archive.org/web/20240327172609/https://www.nature.com/articles/s41586-019-1211-6 |url-status=live }}</ref> and as effectively providing the pressure profile shape by selection of the model.<ref>{{Cite journal |last1=Dutrieux |first1=H. |last2=Lorcé |first2=C. |last3=Moutarde |first3=H. |last4=Sznajder |first4=P. |last5=Trawiński |first5=A. |last6=Wagner |first6=J. |date=April 2021 |title=Phenomenological assessment of proton mechanical properties from deeply virtual Compton scattering |url=https://link.springer.com/10.1140/epjc/s10052-021-09069-w |journal=The European Physical Journal C |language=en |volume=81 |issue=4 |page=300 |doi=10.1140/epjc/s10052-021-09069-w |issn=1434-6044|arxiv=2101.03855 |bibcode=2021EPJC...81..300D }}</ref>

=== Charge radius in solvated proton, hydronium ===
The radius of the hydrated proton appears in the [[Born equation]] for calculating the hydration enthalpy of [[hydronium]].

== Interaction of free protons with ordinary matter ==
Although protons have affinity for oppositely charged electrons, this is a relatively low-energy interaction and so free protons must lose sufficient velocity (and [[kinetic energy]]) in order to become closely associated and bound to electrons. High energy protons, in traversing ordinary matter, lose energy by collisions with [[atomic nuclei]], and by [[ionization]] of atoms (removing electrons) until they are slowed sufficiently to be captured by the [[electron cloud]] in a normal atom.

However, in such an association with an electron, the character of the bound proton is not changed, and it remains a proton. The attraction of low-energy free protons to any electrons present in normal matter (such as the electrons in normal atoms) causes free protons to stop and to form a new chemical bond with an atom. Such a bond happens at any sufficiently "cold" temperature (that is, comparable to temperatures at the surface of the Sun) and with any type of atom. Thus, in interaction with any type of normal (non-plasma) matter, low-velocity free protons do not remain free but are attracted to electrons in any atom or molecule with which they come into contact, causing the proton and molecule to combine. Such molecules are then said to be "[[protonated]]", and chemically they are simply compounds of hydrogen, often positively charged. Often, as a result, they become so-called [[Brønsted acid]]s. For example, a proton captured by a water molecule in water becomes [[hydronium]], the [[aqueous]] [[cation]] {{H3O+}}.

== Proton in chemistry ==

=== Atomic number ===
In [[chemistry]], the number of protons in the [[atomic nucleus|nucleus]] of an atom is known as the [[atomic number]], which determines the [[chemical element]] to which the atom belongs. For example, the atomic number of [[chlorine]] is 17; this means that each chlorine atom has 17 protons and that all atoms with 17 protons are chlorine atoms. The chemical properties of each atom are determined by the number of (negatively charged) [[electron]]s, which for neutral atoms is equal to the number of (positive) protons so that the total charge is zero. For example, a neutral chlorine atom has 17 protons and 17 electrons, whereas a Cl<sup>−</sup> [[anion]] has 17 protons and 18 electrons for a total charge of −1.

All atoms of a given element are not necessarily identical, however. The [[number of neutrons]] may vary to form different [[isotope]]s, and energy levels may differ, resulting in different [[nuclear isomer]]s. For example, there are two stable [[isotopes of chlorine]]: {{nuclide|Chlorine|35}} with 35 − 17 = 18 neutrons and {{nuclide|Chlorine|37}} with 37 − 17 = 20 neutrons.

=== Hydrogen ion ===
{{See also|Hydron (chemistry)}}
[[File:Hydrogen.svg|thumb|220px|Protium, the most common isotope of hydrogen, consists of one proton and one electron (it has no neutrons). The term ''hydrogen ion'' ({{chem|H|+}}) implies that that H-atom has lost its one electron, causing only a proton to remain. Thus, in chemistry, the terms ''proton'' and ''hydrogen ion'' (for the protium isotope) are used synonymously.]]

{{quote box|width=20%|align=left|quote=The proton is a unique chemical species, being a bare nucleus. As a consequence it has no independent existence in the condensed state and is invariably found bound by a pair of electrons to another atom.|salign=right|source=Ross Stewart, ''The Proton: Application to Organic Chemistry'' (1985, p. 1)}}
In chemistry, the term ''proton'' refers to the hydrogen ion, {{chem|H|+}}. Since the atomic number of hydrogen is 1, a hydrogen ion has no electrons and corresponds to a bare nucleus, consisting of a proton (and 0 neutrons for the most abundant isotope ''protium'' {{nuclide|Hydrogen|1|link=yes}}). The proton is a "bare charge" with only about 1/64,000 of the radius of a hydrogen atom, and so is extremely reactive chemically. The free proton, thus, has an extremely short lifetime in chemical systems such as liquids and it reacts immediately with the [[electron cloud]] of any available molecule. In aqueous solution, it forms the [[hydronium ion]], H<sub>3</sub>O<sup>+</sup>, which in turn is further [[solvation|solvated]] by water molecules in [[Hydrogen ion#Cation (positively charged)|clusters]] such as [H<sub>5</sub>O<sub>2</sub>]<sup>+</sup> and [H<sub>9</sub>O<sub>4</sub>]<sup>+</sup>.<ref name="Headrick2005" />

The transfer of {{chem|H|+}} in an [[Brønsted–Lowry acid–base theory|acid–base reaction]] is usually referred to as "proton transfer". The [[acid]] is referred to as a proton donor and the [[base (chemistry)|base]] as a proton acceptor. Likewise, [[biochemistry|biochemical]] terms such as ''[[proton pump]]'' and ''[[proton channel]]'' refer to the movement of hydrated {{chem|H|+}} ions.

The ion produced by removing the electron from a [[deuterium]] atom is known as a ''deuteron'', not a proton. Likewise, removing an electron from a [[tritium]] atom produces a ''triton''.

=== Proton nuclear magnetic resonance (NMR) ===
Also in chemistry, the term ''[[proton NMR]]'' refers to the observation of hydrogen-1 nuclei in (mostly [[organic chemistry|organic]]) molecules by [[nuclear magnetic resonance]]. This method uses the [[Quantization (physics)|quantized]] [[spin magnetic moment]] of the proton, which is due to its angular momentum (or [[Spin (physics)|spin]]), which in turn has a magnitude of one-half the reduced [[Planck constant]]. (<math>\hbar/2</math>). The name refers to examination of protons as they occur in [[Hydrogen-1|protium]] (hydrogen-1 atoms) in compounds, and does not imply that free protons exist in the compound being studied.

== Human exposure ==
{{Main|Effect of spaceflight on the human body}}
{{See also|Proton therapy}}

The [[Apollo Lunar Surface Experiments Package]]s (ALSEP) determined that more than 95% of the particles in the [[solar wind]] are electrons and protons, in approximately equal numbers.<ref name="LPI2009" /><ref name="BNL2007" />

{{Blockquote|Because the Solar Wind [[Spectrometer]] made continuous measurements, it was possible to measure how the [[Earth's magnetic field]] affects arriving solar wind particles. For about two-thirds of each orbit, the [[Moon]] is outside of the Earth's magnetic field. At these times, a typical proton density was 10 to 20 per cubic centimeter, with most protons having velocities between 400 and 650 kilometers per second. For about five days of each month, the Moon is inside the Earth's geomagnetic tail, and typically no solar wind particles were detectable. For the remainder of each lunar orbit, the Moon is in a transitional region known as the [[magnetosheath]], where the Earth's magnetic field affects the solar wind, but does not completely exclude it. In this region, the particle flux is reduced, with typical proton velocities of 250 to 450 kilometers per second. During the lunar night, the spectrometer was shielded from the solar wind by the Moon and no solar wind particles were measured.<ref name="LPI2009" />}}

Protons also have extrasolar origin from galactic [[cosmic ray]]s, where they make up about 90% of the total particle flux. These protons often have higher energy than solar wind protons, and their intensity is far more uniform and less variable than protons coming from the Sun, the production of which is heavily affected by [[solar proton event]]s such as [[coronal mass ejection]]s.

Research has been performed on the dose-rate effects of protons, as typically found in [[Human spaceflight|space travel]], on human health.<ref name="BNL2007" /><ref name="Shukitt-Hale" /> To be more specific, there are hopes to identify what specific chromosomes are damaged, and to define the damage, during [[cancer]] development from proton exposure.<ref name="BNL2007" /> Another study looks into determining "the effects of exposure to proton irradiation on neurochemical and behavioral endpoints, including [[dopaminergic]] functioning, [[amphetamine]]-induced conditioned taste aversion learning, and spatial learning and memory as measured by the [[Morris water maze]].<ref name="Shukitt-Hale" /> Electrical charging of a spacecraft due to interplanetary proton bombardment has also been proposed for study.<ref name="Green2006" /> There are many more studies that pertain to space travel, including [[galactic cosmic rays]] and their [[Health threat from cosmic rays|possible health effects]], and [[solar proton event]] exposure.

The [[STS-65#Mission parameters|American Biostack and Soviet Biorack]] space travel experiments have demonstrated the severity of molecular damage induced by heavy ions<!-- is it relevant to protons, indeed? --> on [[microorganism]]s including [[Artemia]] cysts.<ref name="Planel2004" />

== Antiproton ==
{{Main|Antiproton}}
[[CPT-symmetry]] puts strong constraints on the relative properties of particles and [[antiparticle]]s and, therefore, is open to stringent tests. For example, the charges of a proton and antiproton must sum to exactly zero. This equality has been tested to one part in {{val|e=8}}. The equality of their masses has also been tested to better than one part in {{val|e=8}}. By holding antiprotons in a [[Penning trap]], the equality of the charge-to-mass ratio of protons and antiprotons has been tested to one part in {{val|6|e=9}}.<ref name="Gabrielse2006" /> The [[magnetic moment]] of antiprotons has been measured with an error of {{val|8|e=-3}} nuclear [[Bohr magneton]]s, and is found to be equal and opposite to that of a proton.<ref>{{Cite web |title=BASE precisely measures antiproton's magnetic moment |url=https://home.cern/news/news/physics/base-precisely-measures-antiprotons-magnetic-moment |access-date=2022-03-04 |website=CERN |language=en |archive-date=2022-03-04 |archive-url=https://web.archive.org/web/20220304125452/https://home.cern/news/news/physics/base-precisely-measures-antiprotons-magnetic-moment |url-status=live }}</ref>

== See also ==
{{Portal|Physics}}
{{Div col|colwidth=22em}}
* [[Fermionic field]]
* [[Hydrogen]]
* [[Hydron (chemistry)]]
* [[List of particles]]
* [[Proton–proton chain]]
* [[Quark model]]
* [[Proton spin crisis]]
* [[Proton therapy]]
{{div col end}}

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<ref name="Headrick2005">{{cite journal |last1=Headrick |first1=J. M. |last2=Diken |first2=E. G. |last3=Walters |first3=R. S. |last4=Hammer |first4=N. I. |last5=Christie |first5=R. A. |last6=Cui |first6=J. |last7=Myshakin |first7=E. M. |last8=Duncan |first8=M. A. |last9=Johnson |first9=M. A. |last10=Jordan |first10=K. D. |s2cid=40852810 |year=2005 |title=Spectral Signatures of Hydrated Proton Vibrations in Water Clusters |journal=[[Science (journal)|Science]] |volume=308 |pages=1765–1769 |doi=10.1126/science.1113094 |pmid=15961665 |issue=5729 |bibcode=2005Sci...308.1765H}}</ref>

<ref name="LPI2009">{{cite web |year=2009 |title=Apollo 11 Mission |url=http://www.lpi.usra.edu/lunar/missions/apollo/apollo_11/experiments/swc/ |publisher=[[Lunar and Planetary Institute]] |access-date=2009-06-12 |archive-date=2012-08-07 |archive-url=https://web.archive.org/web/20120807050129/http://www.lpi.usra.edu/lunar/missions/apollo/apollo_11/experiments/swc/ |url-status=live }}</ref>

<ref name="BNL2007">{{cite web |date=12 December 2007 |title=Space Travel and Cancer Linked? Stony Brook Researcher Secures NASA Grant to Study Effects of Space Radiation |url=http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=07-X17 |publisher=[[Brookhaven National Laboratory]] |access-date=2009-06-12 |archive-url=https://web.archive.org/web/20081126121719/http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=07-X17 |archive-date=26 November 2008 |url-status=dead }}</ref>

<ref name="Shukitt-Hale">{{cite journal |last1=Shukitt-Hale |first1=B. |last2=Szprengiel |first2=A. |last3=Pluhar |first3=J. |last4=Rabin |first4=B. M. |last5=Joseph |first5=J. A. |title=The effects of proton exposure on neurochemistry and behavior |journal=Advances in Space Research |year=2004 |volume=33 |issue=8 |pages=1334–9 |doi=10.1016/j.asr.2003.10.038 |url=http://biblioteca.universia.net/ficha.do?id=43176300 |pmid=15803624 |bibcode=2004AdSpR..33.1334S |access-date=2009-06-12 |archive-url=https://web.archive.org/web/20110725132049/http://biblioteca.universia.net/ficha.do?id=43176300 |archive-date=2011-07-25 |url-status=dead }}</ref>

<ref name="Planel2004">{{cite book |last=Planel |first=H. |year=2004 |title=Space and life: an introduction to space biology and medicine |url=https://books.google.com/books?id=rnUFZ24RUdYC&pg=PA135 |publisher=[[CRC Press]] |pages=135–138 |isbn=978-0-415-31759-7 |access-date=2020-07-13 |archive-date=2020-08-09 |archive-url=https://web.archive.org/web/20200809004918/https://books.google.com/books?id=rnUFZ24RUdYC&pg=PA135 |url-status=live }}</ref>

<ref name="Britannica">{{Cite encyclopedia |url=https://www.britannica.com/science/proton-subatomic-particle |title=proton {{!}} Definition, Mass, Charge, & Facts |encyclopedia=Encyclopedia Britannica |access-date=2018-10-20 |archive-date=2023-08-22 |archive-url=https://web.archive.org/web/20230822075524/https://www.britannica.com/science/proton-subatomic-particle |url-status=live }}</ref>

<ref name="Adair1989">{{cite book |last=Adair |first=R. K. |year=1989 |title=The Great Design: Particles, Fields, and Creation |page=214 |publisher=[[Oxford University Press]] |bibcode=1988gdpf.book.....A}}</ref>

<ref name="Wien1904">{{cite journal |last=Wien |first=Wilhelm |year=1904 |title=Über positive Elektronen und die Existenz hoher Atomgewichte |journal=[[Annalen der Physik]] |volume=318 |issue=4 |pages=669–677 |doi=10.1002/andp.18943180404 |bibcode=1904AnP...318..669W |url=https://zenodo.org/record/2190505 |access-date=2020-07-13 |archive-date=2020-07-13 |archive-url=https://web.archive.org/web/20200713133516/https://zenodo.org/record/2190505 |url-status=live }}</ref>

<ref name="Petrucci2002">{{cite book |last1=Petrucci |first1=R. H. |last2=Harwood |first2=W. S. |last3=Herring |first3=F. G. |year=2002 |title=General Chemistry |url=https://archive.org/details/generalchemistry00hill |url-access=registration |edition=8th |page=[https://archive.org/details/generalchemistry00hill/page/41 41] |publisher=Upper Saddle River, N.J. : Prentice Hall |isbn=978-0-13-033445-9 }}</ref>

<ref name="NatureScience">See [http://www.nature.com/nature/journal/v105/n2651/abs/105780a0.html meeting report] {{Webarchive|url=https://web.archive.org/web/20170318033543/http://www.nature.com/nature/journal/v105/n2651/abs/105780a0.html |date=2017-03-18 }} and [https://www.science.org/doi/abs/10.1126/science.51.1330.627.a announcement] {{Webarchive|url=https://web.archive.org/web/20221019190517/https://www.science.org/doi/abs/10.1126/science.51.1330.627.a |date=2022-10-19 }}</ref>

<ref name="Romer1997">{{cite journal |author=Romer A |journal=[[American Journal of Physics]]|volume=65 |page=707 |year=1997 |title=Proton or prouton? Rutherford and the depths of the atom |bibcode=1997AmJPh..65..707R |doi=10.1119/1.18640 |issue=8}}</ref>

<ref name="Footnote">Rutherford reported acceptance by the ''British Association'' in a footnote to {{Cite journal |doi=10.1080/14786442108636219 |title=XXIV. The constitution of atoms |journal=[[Philosophical Magazine]] |series=Series 6 |volume=41 |issue=242 |pages=281–285 |year=1921 |last1=Masson |first1=O. |url=https://zenodo.org/record/1430963 |access-date=2019-06-21 |archive-date=2019-06-21 |archive-url=https://web.archive.org/web/20190621193214/https://zenodo.org/record/1430963 |url-status=live }}</ref>

<ref name="Pais1986">{{cite book |last=Pais |first=A. |year=1986 |title=Inward Bound |url=https://archive.org/details/inwardboundofmat00pais_0 |url-access=registration |publisher=[[Oxford University Press]] |isbn=0-19-851997-4 |page=[https://archive.org/details/inwardboundofmat00pais_0/page/296 296]}} Pais believed the first science literature use of the word ''proton'' occurs in {{Cite journal |doi=10.1038/106357a0 |title=Physics at the British Association |journal=[[Nature (journal)|Nature]] |volume=106 |issue=2663 |pages=357–358 |year=1920 |bibcode=1920Natur.106..357.|doi-access=free }}</ref>

<!-- ♦♦♦ not used<ref name="Popkin2017">{{cite news |url=https://www.science.org/content/article/surprise-proton-lighter-we-thought |title=Surprise! The proton is lighter than we thought |first=Gabriel |last=Popkin |date=20 July 2017 |journal=[[Science (journal)|Science]]}}</ref>

<ref name="Heisse2017">{{cite journal |title=High-Precision Measurement of the Proton's Atomic Mass |first1=F. |last1=Heiße |first2=F. |last2=Köhler-Langes |first3=S. |last3=Rau |first4=J. |last4=Hou |first5=S. |last5=Junck |first6=A. |last6=Kracke |first7=A. |last7=Mooser |first8=W. |last8=Quint |first9=S. |last9=Ulmer |first10=G. |last10=Werth |first11=K. |last11=Blaum |first12=S. |last12=Sturm |s2cid=31683973 |journal=[[Physical Review Letters]] |volume=119 |issue=3 |page=033001 |date=18 July 2017 |doi=10.1103/PhysRevLett.119.033001 |pmid=28777624 |arxiv=1706.06780 |bibcode=2017PhRvL.119c3001H}}</ref>
♦♦♦ -->
<ref name="Bucella1989">{{Cite journal |doi=10.1016/0370-2693(89)90637-0 |title=An upper limit for the proton lifetime in SO(10) |journal=[[Physics Letters B]] |volume=233 |issue=1–2 |pages=178–182 |year=1989 |last1=Buccella |first1=F. |last2=Miele |first2=G. |last3=Rosa |first3=L. |last4=Santorelli |first4=P. |last5=Tuzi |first5=T. |bibcode=1989PhLB..233..178B}}</ref>

<ref name="Lee1995">{{Cite journal |doi=10.1103/PhysRevD.51.229 |pmid=10018289 |arxiv=hep-ph/9404238 |title=Predictions for the proton lifetime in minimal nonsupersymmetric SO(10) models: An update |journal=[[Physical Review D]] |volume=51 |issue=1 |pages=229–235 |year=1995 |last1=Lee |first1=D.G. |last2=Mohapatra |first2=R. |last3=Parida |first3=M. |last4=Rani |first4=M. |bibcode=1995PhRvD..51..229L|s2cid=119341478 }}</ref>

<ref name="Kamioka">{{cite web |title=Proton lifetime is longer than 10<sup>34</sup> years |publisher=[[Kamioka Observatory]] |date=November 2009 |url=http://www-sk.icrr.u-tokyo.ac.jp/whatsnew/new-20091125-e.html |access-date=2014-08-31 |archive-date=2011-07-16 |archive-url=https://web.archive.org/web/20110716144726/http://www-sk.icrr.u-tokyo.ac.jp/whatsnew/new-20091125-e.html |url-status=dead }}</ref>

<ref name="Nishino2009">{{Cite journal |doi=10.1103/PhysRevLett.102.141801 |pmid=19392425 |arxiv=0903.0676 |bibcode=2009PhRvL.102n1801N |title=Search for Proton Decay via p→e<sup>+</sup>π<sup>0</sup> and p→μ<sup>+</sup>π<sup>0</sup> in a Large Water Cherenkov Detector |journal=[[Physical Review Letters]] |volume=102 |issue=14 |pages=141801 |year=2009 |last1=Nishino |first1=H. |last2=Clark |first2=S. |last3=Abe |first3=K. |last4=Hayato |first4=Y. |last5=Iida |first5=T. |last6=Ikeda |first6=M. |last7=Kameda |first7=J. |last8=Kobayashi |first8=K. |last9=Koshio |first9=Y. |last10=Miura |first10=M. |last11=Moriyama |first11=S. |last12=Nakahata |first12=M. |last13=Nakayama |first13=S. |last14=Obayashi |first14=Y. |last15=Ogawa |first15=H. |last16=Sekiya |first16=H. |last17=Shiozawa |first17=M. |last18=Suzuki |first18=Y. |last19=Takeda |first19=A. |last20=Takenaga |first20=Y. |last21=Takeuchi |first21=Y. |last22=Ueno |first22=K. |last23=Ueshima |first23=K. |last24=Watanabe |first24=H. |last25=Yamada |first25=S. |last26=Hazama |first26=S. |last27=Higuchi |first27=I. |last28=Ishihara |first28=C. |last29=Kajita |first29=T. |last30=Kaneyuki |first30=K. |s2cid=32385768 |display-authors=1}}</ref>

<ref name="Ahmed2004">{{Cite journal |doi=10.1103/PhysRevLett.92.102004 |pmid=15089201 |bibcode=2004PhRvL..92j2004A |arxiv=hep-ex/0310030 |title=Constraints on Nucleon Decay via Invisible Modes from the Sudbury Neutrino Observatory |journal=[[Physical Review Letters]] |volume=92 |issue=10 |pages=102004 |year=2004 |last1=Ahmed |first1=S. |last2=Anthony |first2=A. |last3=Beier |first3=E. |last4=Bellerive |first4=A. |last5=Biller |first5=S. |last6=Boger |first6=J. |last7=Boulay |first7=M. |last8=Bowler |first8=M. |last9=Bowles |first9=T. |last10=Brice |first10=S. |last11=Bullard |first11=T. |last12=Chan |first12=Y. |last13=Chen |first13=M. |last14=Chen |first14=X. |last15=Cleveland |first15=B. |last16=Cox |first16=G. |last17=Dai |first17=X. |last18=Dalnoki-Veress |first18=F. |last19=Doe |first19=P. |last20=Dosanjh |first20=R. |last21=Doucas |first21=G. |last22=Dragowsky |first22=M. |last23=Duba |first23=C. |last24=Duncan |first24=F. |last25=Dunford |first25=M. |last26=Dunmore |first26=J. |last27=Earle |first27=E. |last28=Elliott |first28=S. |last29=Evans |first29=H. |last30=Ewan |first30=G. |s2cid=119336775 |display-authors=1}}</ref>

<ref name="Waston2004">{{cite book |last=Watson |first=A. |title=The Quantum Quark |pages=285–286 |publisher=[[Cambridge University Press]] |year=2004 |isbn=978-0-521-82907-6}}</ref>

<!-- <ref name="Weise1984">{{cite book |last1=Weise |first1=W. |last2=Green |first2=A. M. |title=Quarks and Nuclei |pages=65–66 |publisher=[[World Scientific]] |year=1984 |isbn=978-9971-966-61-4}}</ref> -->

<!-- <ref name="Ball2014">{{Cite journal |last=Ball |first=Philip |title=Nuclear masses calculated from scratch |journal=[[Nature (journal)|Nature]] |date=Nov 20, 2008 |url=http://www.nature.com/news/2008/081120/full/news.2008.1246.html |access-date=Aug 27, 2014 |doi=10.1038/news.2008.1246}}</ref> -->

<!-- <ref name="Reynolds2009">{{cite journal |last=Reynolds |first=Mark |title=Calculating the Mass of a Proton |journal=[[CNRS International Magazine]] |issue=13 |date=Apr 2009 |url=http://www2.cnrs.fr/en/1410.htm |issn=2270-5317 |access-date=Aug 27, 2014}}</ref> -->

<ref name="SMAnswers">See [http://www.sciencenews.org/view/generic/id/38788/title/Standard_model_gets_right_answer_for_proton,_neutron_masses this news report] {{Webarchive|url=https://web.archive.org/web/20090416132731/http://www.sciencenews.org/view/generic/id/38788/title/Standard_model_gets_right_answer_for_proton,_neutron_masses |date=2009-04-16 }} and links</ref>

<ref name="Erlich2008">{{cite conference |first=Erlich |last=Joshua |title=Recent Results in AdS/QCD |book-title=Proceedings, 8th Conference on Quark Confinement and the Hadron Spectrum, September 1–6, 2008, Mainz, Germany |date=December 2008 |arxiv=0812.4976 |bibcode=2008arXiv0812.4976E}}</ref>

<ref name="Calangelo2000">{{Cite book |first1=Colangelo |last1=Pietro |first2=Khodjamirian |last2=Alex |s2cid=16053543 |chapter=QCD Sum Rules, a Modern Perspective |title=At the Frontier of Particle Physics: Handbook of QCD |publisher=[[World Scientific Publishing]] |pages=1495–1576 |editor-first=Shifman |editor-last=M. |date=October 2000 |arxiv=hep-ph/0010175 |bibcode=2001afpp.book.1495C |doi=10.1142/9789812810458_0033 |isbn=978-981-02-4445-3 |citeseerx=10.1.1.346.9301}}</ref>

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<ref name="Burkert2018">{{cite journal |title=The pressure distribution inside the proton |last1=Burkert |first1=V. D. |last2=Elouadrhiri |first2=L. |last3=Girod |first3=F. X. |s2cid=21724781 |pages=396–399 |date=16 May 2018 |journal=[[Nature (journal)|Nature]] |volume=557 |issue=7705 |doi=10.1038/s41586-018-0060-z |pmid=29769668 |bibcode=2018Natur.557..396B|osti=1438388 }}</ref>

<ref name="Green2006">{{cite journal |last1=Green |first1=N. W. |last2=Frederickson |first2=A. R. |title=A Study of Spacecraft Charging due to Exposure to Interplanetary Protons |journal=[[AIP Conference Proceedings]] |volume=813 |pages=694–700 |url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/39501/1/05-0657.pdf |access-date=2009-06-12 |url-status=dead |archive-url=https://web.archive.org/web/20100527113425/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/39501/1/05-0657.pdf |archive-date=2010-05-27 |bibcode=2006AIPC..813..694G |year=2006 |doi=10.1063/1.2169250 |citeseerx=10.1.1.541.4495}}</ref>

<ref name="Gabrielse2006">{{cite journal |last=Gabrielse |first=G. |year=2006 |title=Antiproton mass measurements |journal=[[International Journal of Mass Spectrometry]] |volume=251 |issue=2–3 |pages=273–280 |doi=10.1016/j.ijms.2006.02.013 |bibcode=2006IJMSp.251..273G}}</ref>

}}

== Further reading ==
* {{cite journal |last1=Ball |first1=Richard D. |last2=Candido |first2=Alessandro |last3=Cruz-Martinez |first3=Juan |last4=Forte |first4=Stefano |last5=Giani |first5=Tommaso |last6=Hekhorn |first6=Felix |last7=Kudashkin |first7=Kirill |last8=Magni |first8=Giacomo |last9=Rojo |first9=Juan |title=Evidence for intrinsic charm quarks in the proton |journal=Nature |date=August 2022 |volume=608 |issue=7923 |pages=483–487 |doi=10.1038/s41586-022-04998-2 |pmid=35978125 |pmc=9385499 |arxiv=2208.08372 |bibcode=2022Natur.608..483N |language=en |issn=1476-4687}}
* {{Cite journal |last1=Gao |first1=H. |last2=Vanderhaeghen |first2=M. |date=2022-01-21 |title=The proton charge radius |url=https://link.aps.org/doi/10.1103/RevModPhys.94.015002 |journal=Reviews of Modern Physics |language=en |volume=94 |issue=1 |page=015002 |doi=10.1103/RevModPhys.94.015002 |issn=0034-6861|arxiv=2105.00571 |bibcode=2022RvMP...94a5002G }}

== External links ==
* {{Commons category-inline}}
* [http://pdg.lbl.gov/ Particle Data Group] at [[Lawrence Berkeley National Laboratory|LBL]]
* [http://www.cern.ch/lhc/ Large Hadron Collider]
* {{cite web |author1-link= Laurence Eaves |last1= Eaves |first1= Laurence |title= The shrinking proton |url= http://www.sixtysymbols.com/videos/protonradius.htm |website= Sixty Symbols |publisher= [[Brady Haran]] for the [[University of Nottingham]] |last2= Copeland |first2= Ed |last3= Padilla |first3= Antonio (Tony) |year= 2010 }}
* MIT proton visualization project:
** [https://www.quantamagazine.org/inside-the-proton-the-most-complicated-thing-imaginable-20221019 Inside the Proton, the ‘Most Complicated Thing You Could Possibly Imagine’], [[Quanta Magazine]], Oct 19 2022
** [https://www.youtube.com/watch?v=G-9I0buDi4s Visualizing the Proton], Arts at MIT, 2022

{{particles}}

{{Authority control}}

[[Category:Proton| ]]
[[Category:Baryons]]
[[Category:Cations]]
[[Category:Nucleons]]
[[Category:Hydrogen physics]]
[[Category:1910s in science]]