Editing Proton (section)

== 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 |date=14 March 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]].