Editing Neutron (section)

== Properties ==

=== Mass ===
The mass of a neutron cannot be directly determined by [[mass spectrometry]] since it has no electric charge. But since the masses of a proton and of a [[deuteron]] can be measured with a mass spectrometer, the mass of a neutron can be deduced by subtracting proton mass from deuteron mass, with the difference being the mass of the neutron plus the [[binding energy]] of deuterium (expressed as a positive emitted energy). The latter can be directly measured by measuring the energy (<math>B_d</math>) of the single {{val|2.224|u=MeV}} gamma photon emitted when a deuteron is formed by a proton capturing a neutron (this is exothermic and happens with zero-energy neutrons). The small recoil kinetic energy (<math>E_{rd}</math>) of the deuteron (about 0.06% of the total energy) must also be accounted for.
: <math>m_n= m_d - m_p + B_d - E_{rd}</math>

The energy of the gamma ray can be measured to high precision by X-ray diffraction techniques, as was first done by Bell and Elliot in 1948. The best modern (1986) values for neutron mass by this technique are provided by Greene, et al.<ref>{{cite journal |last1= Greene |first1= GL |display-authors= etal |year= 1986 |title= New determination of the deuteron binding energy and the neutron mass |journal= [[Physical Review Letters]]|volume= 56 |issue= 8|pages= 819–822 |doi=10.1103/PhysRevLett.56.819 |bibcode=1986PhRvL..56..819G |pmid=10033294}}</ref> These give a neutron mass of:
: ''m''<sub>neutron</sub> = {{physconst|mn_Da}}
The value for the neutron mass in MeV is less accurately known, due to less accuracy in the known conversion of [[Dalton (unit)|Da]] to MeV/''c''<sup>2</sup>:<ref name="Byrne_NNM"/>{{rp|18–19}}
: ''m''<sub>neutron</sub> = {{physconst|mnc2_MeV|unit=MeV/c2}} 

Another method to determine the mass of a neutron starts from the beta decay of the neutron, when the momenta of the resulting proton and electron are measured.

=== Spin ===
The neutron is a spin&nbsp;{{small|{{sfrac|1|2}}}} particle, that is, it is a [[fermion]] with intrinsic angular momentum equal to {{small|{{sfrac|1|2}}}}&nbsp;{{mvar|ħ}}, where {{mvar|ħ}} is the [[reduced Planck constant]]. For many years after the discovery of the neutron, its exact spin was ambiguous. Although it was assumed to be a spin&nbsp;{{small|{{sfrac|1|2}}}} [[Dirac particle]], the possibility that the neutron was a spin&nbsp;{{small|{{sfrac|3|2}}}} particle lingered. The interactions of the neutron's magnetic moment with an external magnetic field were exploited to finally determine the spin of the neutron.<ref name="Byrne">{{cite book |title=Neutrons, Nuclei and Matter: An exploration of the physics of slow neutrons |author=J. Byrne |isbn= 978-0486482385 |year=2011 |location=Mineola, NY |publisher=Dover Publications |pages=28–31}}</ref> In 1949, Hughes and Burgy measured neutrons reflected from a ferromagnetic mirror and found that the angular distribution of the reflections was consistent with spin&nbsp;{{small|{{sfrac|1|2}}}}.<ref>{{cite journal |last1=Hughes |first1=D.J. |last2=Burgy |first2=M.T. |year=1949 |title=Reflection and polarization of neutrons by magnetized mirrors |journal=[[Physical Review]] |volume=76 |issue=9 |pages=1413–1414 |doi=10.1103/PhysRev.76.1413 |bibcode=1949PhRv...76.1413H |url=http://physics.princeton.edu/~mcdonald/examples/EP/hughes_pr_76_1413_49.pdf |access-date=2016-06-26 |url-status=dead |archive-url=https://web.archive.org/web/20160813204055/http://physics.princeton.edu/~mcdonald/examples/EP/hughes_pr_76_1413_49.pdf |archive-date=2016-08-13}}</ref> In 1954, Sherwood, Stephenson, and Bernstein employed neutrons in a [[Stern–Gerlach experiment]] that used a magnetic field to separate the neutron spin states. They recorded two such spin states, consistent with a spin&nbsp;{{small|{{sfrac|1|2}}}} particle.<ref name="Byrne"/><ref name="Sherwood">{{cite journal |last1=Sherwood |first1=J.E. |last2=Stephenson |first2=T.E. |first3=S. |last3=Bernstein |year=1954 |title=Stern-Gerlach experiment on polarized neutrons |journal=[[Physical Review]] |volume=96 |issue=6 |pages=1546–1548 |doi=10.1103/PhysRev.96.1546 |bibcode=1954PhRv...96.1546S }}</ref>

As a fermion, the neutron is subject to the [[Pauli exclusion principle]]; two neutrons cannot have the same quantum numbers. This is the source of the [[Neutron degeneracy|degeneracy pressure]] which counteracts gravity in [[neutron star]]s and prevents them from forming black holes.<ref name=Bombaci>{{cite journal |first=I. |last=Bombaci |date=1996 |title=The Maximum Mass of a Neutron Star |journal=[[Astronomy and Astrophysics]] |volume=305 | pages=871–877 |bibcode=1996A&A...305..871B}}</ref>

{{See also|Delta baryon}}

=== Magnetic moment ===
{{Main|Nucleon magnetic moment}}
Even though the neutron is a neutral particle, the magnetic moment of a neutron is not zero. The neutron is not affected by electric fields, but it is affected by magnetic fields. The value for the neutron's magnetic moment was first directly measured by [[Luis Walter Alvarez|Luis Alvarez]] and [[Felix Bloch]] at [[Berkeley, California]], in 1940.<ref name="Alvarez">{{cite journal |last1=Alvarez |first1=L.W |last2=Bloch |first2=F.
|year=1940 |title=A quantitative determination of the neutron magnetic moment in absolute nuclear magnetons
|journal=[[Physical Review]]|volume=57 |issue=2 |pages=111–122 |doi=10.1103/physrev.57.111|bibcode=1940PhRv...57..111A}}</ref> Alvarez and Bloch determined the magnetic moment of the neutron to be {{nowrap|1=''μ''<sub>n</sub>= {{val|-1.93|(2)|u=''μ''<sub>N</sub>}}}}, where ''μ''<sub>N</sub> is the [[nuclear magneton]]. The neutron's magnetic moment has a negative value, because its orientation is opposite to the neutron's spin.<ref name=Llewellyn>{{cite book |title=Modern Physics |author1=Tipler, Paul Allen |author2=Llewellyn, Ralph A. |url=https://books.google.com/books?id=tpU18JqcSNkC&pg=PA310 |page=310 |isbn=978-0-7167-4345-3 |year=2002 |edition=4 |publisher=[[Macmillan Publishers (United States)|Macmillan]] |access-date=2020-08-27 |archive-date=2022-04-07 |archive-url=https://web.archive.org/web/20220407104048/https://books.google.com/books?id=tpU18JqcSNkC&pg=PA310 |url-status=live }}</ref>

The magnetic moment of the neutron is an indication of its quark substructure and internal charge distribution.<ref name="ReferenceA">{{cite journal
|last1=Gell |first1=Y.
|last2=Lichtenberg |first2=D.B.
|year=1969
|title=Quark model and the magnetic moments of proton and neutron
|journal=[[Il Nuovo Cimento A]]|series=Series 10 |volume=61 |issue=1
|pages=27–40
|doi=10.1007/BF02760010
|bibcode= 1969NCimA..61...27G|s2cid=123822660
}}</ref> In the [[quark model]] for [[hadrons]], the neutron is composed of one up quark (charge +2/3&nbsp;''e'') and two down quarks (charge −1/3&nbsp;''e'').<ref name="ReferenceA"/> The magnetic moment of the neutron can be modeled as a sum of the magnetic moments of the constituent quarks.<ref name="Perk">{{cite book |author1-last= Perkins |author1-first= Donald H. |title= Introduction to High Energy Physics |pages= [https://archive.org/details/introductiontohi0000perk/page/201 201–202] |publisher= Addison Wesley, Reading, Massachusetts |date= 1982 |isbn= 978-0-201-05757-7 |url= https://archive.org/details/introductiontohi0000perk/page/201 }}</ref> The calculation assumes that the quarks behave like point-like Dirac particles, each having their own magnetic moment. Simplistically, the magnetic moment of the neutron can be viewed as resulting from the vector sum of the three quark magnetic moments, plus the orbital magnetic moments caused by the movement of the three charged quarks within the neutron.

In one of the early successes of the Standard Model, in 1964 Mirza A.B. Beg, [[Benjamin W. Lee]], and [[Abraham Pais]] calculated the ratio of proton to neutron magnetic moments to be −3/2 (or a ratio of −1.5), which agrees with the experimental value to within 3%.<ref name="Greenberg">
{{citation
|last=Greenberg |first=O.W.
|chapter=Color Charge Degree of Freedom in Particle Physics
|year=2009
|title=Compendium of Quantum Physics
|publisher=Springer Berlin Heidelberg
|pages=109–111
|doi=10.1007/978-3-540-70626-7_32
|arxiv=0805.0289
|isbn=978-3-540-70622-9
|s2cid=17512393
}}</ref><ref name="Beg">{{cite journal |last1=Beg |first1=M.A.B. |last2=Lee |first2=B.W.|last3=Pais |first3=A.
|year=1964 |title=SU(6) and electromagnetic interactions
|journal=[[Physical Review Letters]]|volume=13 |issue=16 |pages=514–517, erratum 650 |doi=10.1103/physrevlett.13.514|bibcode= 1964PhRvL..13..514B}}</ref><ref name="Sakita">{{cite journal |last1=Sakita |first1=B.
|year=1964 |title=Electromagnetic properties of baryons in the supermultiplet scheme of elementary particles
|journal=[[Physical Review Letters]]|volume=13 |issue=21
|pages=643–646 |doi=10.1103/physrevlett.13.643|bibcode= 1964PhRvL..13..643S}}</ref> The measured value for this ratio is {{val|-1.45989805|(34)}}.<ref name="2014 CODATA" />

The above treatment compares neutrons with protons, allowing the complex behavior of quarks to be subtracted out between models, and merely exploring what the effects would be of differing quark charges (or quark type). Such calculations are enough to show that the interior of neutrons is very much like that of protons, save for the difference in quark composition with a down quark in the neutron replacing an up quark in the proton.

The neutron magnetic moment can be roughly computed by assuming a simple [[special relativity|nonrelativistic]], quantum mechanical [[wavefunction]] for [[baryon]]s composed of three quarks. A straightforward calculation gives fairly accurate estimates for the magnetic moments of neutrons, protons, and other baryons.<ref name="Perk"/> For a neutron, the result of this calculation is that the magnetic moment of the neutron is given by {{nowrap|1=''μ''<sub>n</sub>= 4/3 ''μ''<sub>d</sub> − 1/3 ''μ''<sub>u</sub>}}, where ''μ''<sub>d</sub> and ''μ''<sub>u</sub> are the magnetic moments for the down and up quarks, respectively. This result combines the intrinsic magnetic moments of the quarks with their orbital magnetic moments, and assumes the three quarks are in a particular, dominant quantum state.

{| class="wikitable" style="text-align:center;"
|-
! Baryon
! Magnetic moment<br/>of quark model
! Computed<br/>(<math>\mu_\mathrm{N}</math>)
! Observed<br/>(<math>\mu_\mathrm{N}</math>)
|-
| p
| 4/3 ''μ''<sub>u</sub> − 1/3 ''μ''<sub>d</sub>
| 2.79
| 2.793
|-
| n
| 4/3 ''μ''<sub>d</sub> − 1/3 ''μ''<sub>u</sub>
| −1.86
| −1.913
|}

The results of this calculation are encouraging, but the masses of the up or down quarks were assumed to be 1/3 the mass of a nucleon.<ref name="Perk"/> The masses of the quarks are actually only about 1% that of a nucleon.<ref name="Mass">{{cite web |url=https://www.science.org/content/article/mass-common-quark-finally-nailed-down |title=Mass of the Common Quark Finally Nailed Down |last1=Cho |first1=Adrian |date=2 April 2010 |website=Science |publisher=American Association for the Advancement of Science |access-date=27 September 2014 |archive-date=27 August 2015 |archive-url=https://web.archive.org/web/20150827120227/http://news.sciencemag.org/physics/2010/04/mass-common-quark-finally-nailed-down |url-status=live }}</ref> The discrepancy stems from the complexity of the Standard Model for nucleons, where most of their mass originates in the [[gluon]] fields, virtual particles, and their associated energy that are essential aspects of the [[strong force]].<ref name="Mass"/><ref name="Wilczek">{{cite journal |last1=Wilczek |first1=F. |year=2003 |title=The Origin of Mass |journal=[[MIT Physics Annual]] |pages=24–35 |url=http://web.mit.edu/physics/news/physicsatmit/physicsatmit_03_wilczek_originofmass.pdf |archive-date=June 20, 2015 |archive-url=https://web.archive.org/web/20150620011542/http://web.mit.edu/physics/news/physicsatmit/physicsatmit_03_wilczek_originofmass.pdf |url-status=live }}</ref> Furthermore, the complex system of quarks and gluons that constitute a neutron requires a relativistic treatment.<ref>
{{cite journal
 |last1=Ji |first1=Xiangdong
 |year=1995
 |title=A QCD Analysis of the Mass Structure of the Nucleon
 |journal=[[Physical Review Letters]]|volume=74 |issue=7
 |pages=1071–1074
 |doi=10.1103/PhysRevLett.74.1071
 |pmid=10058927
 |arxiv= hep-ph/9410274 |bibcode= 1995PhRvL..74.1071J|s2cid=15148740
}}</ref> But the nucleon magnetic moment has been successfully computed numerically from [[first principle]]s, including all of the effects mentioned and using more realistic values for the quark masses. The calculation gave results that were in fair agreement with measurement, but it required significant computing resources.<ref>
{{cite journal
 |last1=Martinelli |first1=G.
 |last2=Parisi |first2=G.
 |last3=Petronzio |first3=R.
 |last4=Rapuano |first4=F.
 |year=1982
 |title=The proton and neutron magnetic moments in lattice QCD
 |journal=[[Physics Letters B]]
 |volume=116
 |issue=6
 |pages=434–436
 |doi=10.1016/0370-2693(82)90162-9
 |bibcode=1982PhLB..116..434M
 |url=https://cds.cern.ch/record/138281/files/198207343.pdf
 |access-date=2019-08-25
 |archive-date=2020-04-20
 |archive-url=https://web.archive.org/web/20200420144400/https://cds.cern.ch/record/138281/files/198207343.pdf
 |url-status=live
}}</ref><ref name="MagMom">{{cite web |url=http://phys.org/news/2015-02-magnetic-moments-nuclear.html |title=Pinpointing the magnetic moments of nuclear matter |last1=Kincade |first1=Kathy |date=2 February 2015 |website=[[Phys.org]] |access-date=May 8, 2015 |archive-date=2 May 2015 |archive-url=https://web.archive.org/web/20150502123656/http://phys.org/news/2015-02-magnetic-moments-nuclear.html |url-status=live }}</ref>

=== Electric charge ===
The total electric charge of the neutron is {{val|0|u=[[elementary charge|''e'']]}}. This zero value has been tested experimentally, and the present experimental limit for the charge of the neutron is {{val|-2|(8)|e=-22|u=[[elementary charge|''e'']]}},<ref name="PDGLIVE">
{{cite journal
 |last1=Olive |first1=K.A.
 |author2=(Particle Data Group)
 |year=2014
 |title=Review of Particle Physics
 |journal=[[Chinese Physics C]]
 |volume=38
 |issue=9
 |pages=1–708
 |doi=10.1088/1674-1137/38/9/090001
 |pmid=10020536
 |display-authors=etal
 |bibcode=2014ChPhC..38i0001O
 |url=http://scipp.ucsc.edu/%7Ehaber/pubs/Review_of_Particle_Physics_2014.pdf
 |arxiv=1412.1408
 |s2cid=118395784
 |access-date=2017-10-26
 |archive-date=2020-06-01
 |archive-url=https://web.archive.org/web/20200601115825/http://scipp.ucsc.edu/%7Ehaber/pubs/Review_of_Particle_Physics_2014.pdf
 |url-status=live
}}</ref> or {{val|-3|(13)|e=-41|ul=C}}. This value is consistent with zero, given the experimental [[uncertainty#In measurements|uncertainties]] (indicated in parentheses). By comparison, the charge of the proton is {{val|+1|u=[[elementary charge|''e'']]}}.

=== Electric dipole moment ===
{{Main|Neutron electric dipole moment}}

The Standard Model of particle physics predicts a tiny separation of positive and negative charge within the neutron leading to a permanent [[electric dipole moment]].<ref name="sussex">
{{cite press release
 |date= 20 February 2006
 |title= Pear-shaped particles probe big-bang mystery
 |url= http://www.sussex.ac.uk/press_office/media/media537.shtml
 |publisher= [[University of Sussex]]
 |access-date= 2009-12-14
 |archive-date= 2011-06-07
 |archive-url= https://web.archive.org/web/20110607140456/http://www.sussex.ac.uk/press_office/media/media537.shtml
 |url-status= live
}}</ref> But the predicted value is well below the current sensitivity of experiments. From several [[list of unsolved problems in physics#High-energy physics/particle physics|unsolved puzzles in particle physics]], it is clear that the Standard Model is not the final and full description of all particles and their interactions. New theories going [[beyond the Standard Model]] generally lead to much larger predictions for the electric dipole moment of the neutron. Currently, there are at least four experiments trying to measure for the first time a finite neutron electric dipole moment, including:
* [[Cryogenic neutron EDM experiment]] being set up at the [[Institut Laue–Langevin]]<ref>[http://hepwww.rl.ac.uk/EDM/index_files/CryoEDM.htm A cryogenic experiment to search for the EDM of the neutron] {{Webarchive|url=https://web.archive.org/web/20120216171059/http://hepwww.rl.ac.uk/EDM/index_files/CryoEDM.htm |date=2012-02-16 }}. Hepwww.rl.ac.uk. Retrieved on 2012-08-16.</ref>
* n2EDM experiment under construction at the UCN source at the [[Paul Scherrer Institute]]<ref>[http://nedm.web.psi.ch/ Search for the neutron electric dipole moment at PSI: The n2EDM Project of the nEDM collaboration] {{Webarchive|url=https://web.archive.org/web/20150925151115/http://nedm.web.psi.ch/ |date=2015-09-25 }}. Nedm.web.psi.ch (2001-09-12). Retrieved on 2012-08-16.</ref>
* nEDM experiment being envisaged at the [[Spallation Neutron Source]]<ref>[http://www.phy.ornl.gov/nedm/ US nEDM ORNL experiment public page] {{Webarchive|url=https://web.archive.org/web/20170430055915/http://www.phy.ornl.gov/nedm |date=2017-04-30 }}. Retrieved on 2017-02-08.</ref><ref>[http://p25ext.lanl.gov/edm/edm.html SNS Neutron EDM Experiment] {{Webarchive|url=https://web.archive.org/web/20110210021529/http://p25ext.lanl.gov/edm/edm.html |date=2011-02-10 }}. P25ext.lanl.gov. Retrieved on 2012-08-16.</ref>
* nEDM experiment being built at the [[Institut Laue–Langevin]]<ref>{{cite journal | last=A.P. Serebrov | display-authors=et al.| title=New Measurements of the Neutron Electric Dipole Moment with the Petersburg Nuclear Physics Institute Double-Chamber Electric Dipole Moment Spectrometer | journal=Physics of Particles and Nuclei Letters | volume=12 | year=2015 | doi=10.1134/S1547477115020193 | pages=286-296}}</ref>

=== Antineutron ===
{{Main|Antineutron}}

The antineutron is the [[antiparticle]] of the neutron. It was discovered by [[Bruce Cork]] in 1956, a year after the [[antiproton]] was discovered. Neutrons have [[baryon number]] equal to 1 while antineutrons have -1. While all measured particle interactions conserve baryon number, [[baryon asymmetry|matter dominates over antimatter in the cosmos]] suggesting that there must be some way to change the baryon number. One proposed mechanism is [[Neutral particle oscillation|neutron-antineutron oscillations]] which might be detectable.<ref>{{Cite journal |last=Phillips |first=D. G. |last2=Snow |first2=W. M. |last3=Babu |first3=K. |last4=Banerjee |first4=S. |last5=Baxter |first5=D. V. |last6=Berezhiani |first6=Z. |last7=Bergevin |first7=M. |last8=Bhattacharya |first8=S. |last9=Brooijmans |first9=G. |last10=Castellanos |first10=L. |last11=Chen |first11=M. -C. |last12=Coppola |first12=C. E. |last13=Cowsik |first13=R. |last14=Crabtree |first14=J. A. |last15=Das |first15=P. |date=2016-02-11 |title=Neutron-antineutron oscillations: Theoretical status and experimental prospects |url=https://linkinghub.elsevier.com/retrieve/pii/S0370157315004457 |journal=Physics Reports |volume=612 |pages=1–45 |doi=10.1016/j.physrep.2015.11.001 |issn=0370-1573}}</ref><ref>{{cite journal | last=A. Addazi | display-authors=et al.| title=New high-sensitivity searches for neutrons converting into antineutrons and/or sterile neutrons at the HIBEAM/NNBAR experiment at the European Spallation Source | journal=Journal of Physics G | volume=48 | year=2021 | doi=10.1088/1361-6471/abf429 | pages=070501}}</ref> The lower limit on the period of oscillations 0.86x10<sup>8</sup> s (90% CL) was obtained using cold neutrons.<ref>{{cite journal | last=M. Baldo-Ceolin | display-authors=et al.| title=A new experimental limit on neutron-antineutron oscillations | journal=Z. Phys. C | volume=63 | year=1994 | doi=10.1007/BF01580321 | pages=409-416}}</ref> [[Ultracold neutrons]] may increase the sensitivity by 10–40 times, depending on the model of neutron reflection from walls.<ref>{{cite journal | last=A.K. Fomin | display-authors=et al.| title=Experiment on search for neutron–antineutron oscillations using a projected UCN source at the WWR-M reactor | journal=Journal of Physics: Conference Series | volume=798 | year=2017 | doi=10.1088/1742-6596/798/1/012115 | pages=012115}}</ref>