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The '''neutron electric dipole moment''' ('''nEDM'''), denoted ''d''<sub>n</sub>, is a measure for the distribution of positive and negative charge inside the [[neutron]]. A nonzero [[electric dipole moment]] can only exist if the centers of the negative and positive charge distribution inside the particle do not coincide. So far, no neutron EDM has been found. The current best measured limit for ''d''<sub>n</sub> is {{val|0.0|1.1|e=-26|u=[[Elementary charge|''e'']]⋅cm}}.<ref name=":0">{{cite journal
 | last = Abel | first = C. |display-authors=etal
 | year = 2020
 | title =  Measurement of the Permanent Electric Dipole Moment of the Neutron
 | journal = [[Physical Review Letters]]
 | volume = 124 | pages = 081803
 | doi = 10.1103/PhysRevLett.124.081803
 | issue = 8| pmid = 32167372 | arxiv = 2001.11966 | bibcode = 2020PhRvL.124h1803A | doi-access = free
 }}</ref>

== Theory ==
[[Image:NEDM P&T violation.png|thumb|right|Parity (P) and time-reversal (T) violation due to an electric dipole moment. μ denotes the magnetic dipole moment of the neutron, while d is the electric dipole moment. ]]
A permanent electric dipole moment of a fundamental particle violates both [[Parity (physics)|parity]] (P) and [[T-symmetry|time reversal symmetry]] (T). These violations can be understood by examining the neutron's [[Nucleon magnetic moment|magnetic dipole moment]] and hypothetical electric dipole moment. Under time reversal, the [[Magnetic moment|magnetic dipole moment]] changes its direction, whereas the electric dipole moment stays unchanged. Under parity, the electric dipole moment changes its direction but not the magnetic dipole moment. As the resulting system under P and T is not symmetric with respect to the initial system, these symmetries are violated in the case of the existence of an EDM. Having also [[CPT symmetry]], the combined symmetry [[CP-violation|CP]] is violated as well.

=== Standard Model prediction ===
As it is depicted above, in order to generate a nonzero nEDM one needs processes that [[CP violation|violate]] [[CP symmetry]]. CP violation has been observed in [[weak interaction]]s and is included in the [[Standard Model|Standard Model of particle physics]] via the CP-violating phase in the [[Cabibbo–Kobayashi–Maskawa matrix|CKM]] matrix. However, the amount of CP violation is very small and therefore also the contribution to the nEDM: {{nowrap|{{abs|''d''<sub>n</sub>}} ~ {{val|e=-31|u=''e''⋅cm}}}}.<ref>
{{cite arXiv
 | last = Dar | first = S.
 | year = 2000
 | title =  The Neutron EDM in the SM : A Review
 | eprint = hep-ph/0008248
}}</ref>

=== Matter&ndash;antimatter asymmetry ===
{{main|Baryogenesis}}

From the asymmetry between matter and antimatter in the universe, one suspects that there must be a sizeable amount of [[CP-violation]]. Measuring a neutron electric dipole moment at a much higher level than predicted by the Standard Model would therefore directly confirm this suspicion and improve our understanding of CP-violating processes.

=== Strong CP problem ===
{{main|CP-violation}}

As the neutron is built up of [[quark]]s, it is also susceptible to CP violation stemming from [[strong interaction]]s. [[Quantum chromodynamics]] – the theoretical description of the strong force – naturally includes a term that breaks CP-symmetry. The strength of this term is characterized by the angle ''θ''. The current limit on the nEDM constrains this angle to be less than 10<sup>−10</sup>&nbsp;[[radian]]s. This [[Fine-tuning (physics)|fine-tuning]] of the angle&nbsp;''θ'', which is naturally expected to be of order 1, is the [[strong CP problem]].

=== SUSY CP problem ===
[[Supersymmetry|Supersymmetric]] extensions to the Standard Model, such as the [[Minimal Supersymmetric Standard Model]], generally lead to a large CP-violation. Typical predictions for the neutron EDM arising from the theory range between {{nowrap|10<sup>−25</sup> ''e''⋅cm}} and {{nowrap|10<sup>−28</sup> ''e''⋅cm}}.<ref>{{cite journal
 | last1 = Abel | first1 = S. | last2 = Khalil
 | last3 = Lebedev
 | year = 2001
 | title =  EDM constraints in supersymmetric theories
 | journal = [[Nuclear Physics B]]
 | volume = 606 | pages = 151–182
 | doi = 10.1016/S0550-3213(01)00233-4 | first2 = S. | first3 = O.
| issue = 1–2 |arxiv = hep-ph/0103320 |bibcode = 2001NuPhB.606..151A | s2cid = 14168743 }}</ref><ref>
{{cite journal
 | last1 = Pospelov | first1 = M. | last2 = Ritz
 | year = 2005
 | title =  Electric dipole moments as probes of new physics
 | journal = [[Annals of Physics]]
 | volume = 318 | pages = 119–169
 | doi = 10.1016/j.aop.2005.04.002 | first2 = A.
| issue = 1 |arxiv = hep-ph/0504231 |bibcode = 2005AnPhy.318..119P | s2cid = 13827759 }}
</ref> As in the case of the [[strong interaction]], the limit on the neutron EDM is already constraining the CP violating phases. The [[Fine-tuning (physics)|fine-tuning]] is, however, not as severe yet.

== Experimental technique ==
In order to extract the neutron EDM, one measures the [[Larmor precession]] of the neutron [[Spin (physics)|spin]] in the presence of parallel and antiparallel magnetic and electric fields. The precession frequency for each of the two cases is given by

:<math> h\nu = 2\mu_\text{n} B \pm 2d_\text{n} E </math>,

the addition or subtraction of the frequencies stemming from the precession of the [[magnetic moment]] around the [[magnetic field]] and the precession of the electric dipole moment around the [[electric field]]. From the difference of those two frequencies one readily obtains a measure of the neutron EDM:

:<math> d_\text{n} = \frac{h\,\Delta\nu}{4E} </math>

The biggest challenge of the experiment (and at the same time the source of the biggest systematic false effects) is to ensure that the [[magnetic field]] does not change during these two measurements.

== History ==
[[File:NEDM-History-wiki.png|thumb|300x300px|The history of neutron EDM limits including the latest best result by the nEDM collaboration at PSI.<ref name=":0" /> The prediction stemming from the Standard Model is also indicated.]]
The first experiments searching for the electric dipole moment of the neutron used beams of [[Neutron temperature|thermal]] (and later [[Neutron temperature|cold]]) neutrons to conduct the measurement. It started with the experiment by [[James H. Smith (physicist)|James Smith]], [[Edward Mills Purcell|Purcell]], and [[Norman Foster Ramsey, Jr.|Ramsey]] in 1951 (and published in 1957) at [[Oak Ridge National Laboratory|ORNL's]] Graphite Reactor (as the three researchers were from [[Harvard University]], this experiment is called ORNL/Harvard or something similar, see figure in this section), obtaining a limit of {{nowrap|{{abs|''d''<sub>n</sub>}} < {{val|5|e=-20|u=''e''⋅cm}} .}}<ref>{{cite journal |last1=Smith |first1=J.H. |last2=Purcell |first2=E.M. |last3=Ramsey |first3=N.F. |year=1957 |title=Experimental limit to the electric dipole moment of the neutron |journal=[[Physical Review]] |volume=108 |issue=1 |pages=120–122 |doi=10.1103/PhysRev.108.120 |bibcode = 1957PhRv..108..120S }}</ref><ref>{{cite web | url=https://nedm.ornl.gov/early-fundamental-neutron-experiments-at-ornl/ | title=Early Fundamental Neutron Experiments at ORNL – ORNL &#124; nEDM }}</ref> Beams of neutrons were used until 1977 for nEDM experiments. At this point, systematic effects related to the high velocities of the neutrons in the beam became insurmountable. The final limit obtained with a neutron beam amounts to {{nowrap|{{abs|''d''<sub>n</sub>}} < {{val|3|e=-24|u=''e''⋅cm}}}}.<ref>
{{cite journal |last1=Dress |first1=W.B. |display-authors=etal |year=1977 |title=Search for an electric dipole moment of the neutron |journal=[[Physical Review D]] |volume=15 |issue=1 |pages=9–21 |doi=10.1103/PhysRevD.15.9 |bibcode=1977PhRvD..15....9D}}</ref>

After that, experiments with [[ultracold neutrons]] (UCN) took over. It started in 1980 with an experiment at the {{ill|Leningrad Nuclear Physics Institute|ru|Петербургский институт ядерной физики имени Б. П. Константинова}} (LNPI) obtaining a limit of {{nowrap|{{abs|''d''<sub>n</sub>}} < {{val|1.6|e=-24|u=''e''⋅cm}} }}.<ref>{{cite journal |last1=Altarev |first1=I.S. |display-authors=etal |year=1980 |title=A search for the electric dipole moment of the neutron using ultracold neutrons |journal=[[Nuclear Physics A]] |volume=341 |pages=269–283 |doi=10.1016/0375-9474(80)90313-9 |issue=2 |bibcode=1980NuPhA.341..269A }}</ref> This experiment and especially the experiment starting in 1984 at the [[Institut Laue-Langevin]] (ILL) pushed the limit down by another two [[Order of magnitude|orders of magnitude]] yielding the best upper limit in 2006, revised in 2015.

During these 70&nbsp;years of experiments, six [[Order of magnitude|orders of magnitude]] have been covered, thereby putting stringent constraints on theoretical models.<ref>{{cite journal |last=Ramsey |first=N.F. |year=1982 |title=Electric-dipole moments of particles |journal=[[Annu. Rev. Nucl. Part. Sci.]] |volume=32 |issue=1 |pages=211–233 |doi=10.1146/annurev.ns.32.120182.001235 |bibcode=1982ARNPS..32..211R|doi-access=free }}</ref>

The latest best limit of {{nowrap|{{abs|''d''<sub>n</sub>}} < {{val|1.8|e=-26|u=''e''⋅cm}} }} has been published 2020 by the nEDM collaboration at [[Paul Scherrer Institute]] (PSI).<ref name=":0" />

== Current experiments ==
Currently, there are at least six experiments aiming at improving the current limit (or measuring for the first time) on the neutron EDM with a sensitivity down to {{nowrap|{{val|e=-28|u=''e''⋅cm}}}} over the next 10 years, thereby covering the range of prediction coming from [[Supersymmetry|supersymmetric]] extensions to the Standard Model.

* n2EDM of the nEDM collaboration<ref name=":1" /> under construction at the UCN source at the [[Paul Scherrer Institute]].<ref name=":1">[https://www.psi.ch/nedm/ nEDM Collaboration at PSI Website: https://www.psi.ch/nedm/]</ref> In February 2022 the apparatus was being set up at PSI, and commissioning with neutrons expected in late 2022.<ref>{{cite web | url=https://www.psi.ch/en/ltp-ucn-physics/scientific-highlights/cnrs-movie-on-n2edm | title=CNRS movie on n2EDM &#124; UCN Physics &#124; Paul Scherrer Institut (PSI) | date=23 February 2022 }}</ref> The apparatus is expected to reach sensitivity of {{nowrap|{{val|e=-27|u=''e''⋅cm}}}} after 500 days of operation.<ref>{{Cite journal|arxiv = 2101.08730|doi = 10.1140/epjc/s10052-021-09298-z|title = The design of the n2EDM experiment|year = 2021|last1 = Ayres|first1 = N. J.|last2 = Ban|first2 = G.|last3 = Bienstman|first3 = L.|last4 = Bison|first4 = G.|last5 = Bodek|first5 = K.|last6 = Bondar|first6 = V.|last7 = Bouillaud|first7 = T.|last8 = Chanel|first8 = E.|last9 = Chen|first9 = J.|last10 = Chiu|first10 = P.-J.|last11 = Clément|first11 = B.|last12 = Crawford|first12 = C. B.|last13 = Daum|first13 = M.|last14 = Dechenaux|first14 = B.|last15 = Doorenbos|first15 = C. B.|last16 = Emmenegger|first16 = S.|last17 = Ferraris-Bouchez|first17 = L.|last18 = Fertl|first18 = M.|last19 = Fratangelo|first19 = A.|last20 = Flaux|first20 = P.|last21 = Goupillière|first21 = D.|last22 = Griffith|first22 = W. C.|last23 = Grujic|first23 = Z. D.|last24 = Harris|first24 = P. G.|last25 = Kirch|first25 = K.|last26 = Koss|first26 = P. A.|last27 = Krempel|first27 = J.|last28 = Lauss|first28 = B.|last29 = Lefort|first29 = T.|last30 = Lemière|first30 = Y.|journal = The European Physical Journal C|volume = 81|issue = 6|page = 512|pmid = 34720721|pmc = 8550164|bibcode = 2021EPJC...81..512A|display-authors = 1}}</ref>
* TUCAN, a UCN nEDM experiment under construction at [[TRIUMF]]<ref>[http://meetings.aps.org/link/BAPS.2012.NWS.C3.3 TRIUMF Ultracold Neutron Source]</ref>
* nEDM@SNS experiment under construction (as of 2022) at the [[Spallation Neutron Source]]<ref>{{Cite web |url=http://Nedm.ornl.gov |title=nEDM Experiment at the Spallation Neutron Source}}</ref><ref>{{cite journal |last=Ahmed |first=M.W. |year=2019 |title=A New Cryogenic Apparatus to Search for the Neutron Electric Dipole Moment |journal=[[Journal of Instrumentation]] |volume=14 |issue=11 |pages=P11017 |doi=10.1088/1748-0221/14/11/P11017|arxiv = 1908.09937|bibcode=2019JInst..14P1017A |s2cid=201646389 }}</ref><ref>https://nedm.ornl.gov/</ref>
* PNPI nEDM experiment awaiting operation approval at the [[Institut Laue-Langevin]]<ref>[http://nrd.pnpi.spb.ru/LabSereb/neutronedm.htm nrd.pnpi.spb.ru Neutron EDM page]</ref>
* PanEDM experiment being built at the Institut Laue-Langevin<ref>{{cite journal |first=D.|last=Wurm |display-authors=etal|year=2019 |title=The PanEDM neutron electric dipole moment experiment at the ILL |journal=[[EPJ Web Conf.]] |volume=219 |pages=02006 |doi=10.1051/epjconf/201921902006|arxiv=1911.09161 |bibcode=2019EPJWC.21902006W |s2cid=208202103 }}</ref>
* LANL Electric Dipole Moment (LANL nEDM) at [[Los Alamos National Laboratory]]<ref>{{Cite web |url=https://lansce.lanl.gov/facilities/ultracold-neutrons/experiments.php |title=UCN Experiments}}</ref>
* Beam EDM at [[University of Bern]], Switzerland<ref>{{Cite web |url=https://www.lhep.unibe.ch/research/neutron_and_precision/beam_edm/index_eng.html |title=Beam EDM - Pulsed Neutron EDM Experiment}}</ref>

The [[Cryogenic neutron EDM experiment]] or CryoEDM was under development at the Institut Laue-Langevin but its activities were stopped in 2013/2014.<ref>{{Cite web |title=hepwww.rl.ac.uk Cryogenic EDM |url=http://hepwww.rl.ac.uk/EDM/index_files/CryoEDM.htm |url-status=dead |archive-url=https://web.archive.org/web/20120216171059/http://hepwww.rl.ac.uk/EDM/index_files/CryoEDM.htm |archive-date=2012-02-16 |access-date=2009-01-22}}</ref>

==See also==
* [[Anomalous electric dipole moment]]
* [[Anomalous magnetic dipole moment]]
* [[Axion]] – a hypothetical particle proposed to explain the strong force's unexpected preservation of CP
* [[Electric dipole spin resonance]]
* [[Electron electric dipole moment]] – another electric dipole which should exist, but also should be too small to have yet been measured
* [[Electron magnetic moment]]
* [[Nucleon magnetic moment]] – the corresponding magnetic property, which has been measured

==References==
<references/>

{{DEFAULTSORT:Neutron Electric Dipole Moment}}
[[Category:Electric dipole moment]]
[[Category:Electromagnetism]]
[[Category:Particle physics]]