The neutron electric dipole moment (nEDM), denoted ''d''_n, is a measure for the distribution of positive and negative charge inside the

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

. A nonzero

electric dipole moment The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system: that is, a measure of the system's overall Chemical polarity, polarity. The International System of Units, SI unit for electric ...

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''_n is .

Theory

A permanent electric dipole moment of a fundamental particle violates both parity (P) and

time reversal symmetry T-symmetry or time reversal symmetry is the theoretical symmetry of physical laws under the transformation of time reversal, : T: t \mapsto -t. Since the second law of thermodynamics states that entropy increases as time flows toward the future ...

(T). These violations can be understood by examining the neutron's

magnetic dipole moment In electromagnetism, the magnetic moment or magnetic dipole moment is the combination of strength and orientation of a magnet or other object or system that exerts a magnetic field. The magnetic dipole moment of an object determines the magnitude ...

and hypothetical electric dipole moment. Under time reversal, the

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 Charge, parity, and time reversal symmetry is a fundamental symmetry of physical laws under the simultaneous transformations of charge conjugation (C), parity transformation (P), and time reversal (T). CPT is the only combination of C, P, and ...

, the combined symmetry CP is violated as well.

Standard Model prediction

As it is depicted above, in order to generate a nonzero nEDM one needs processes that violate

CP symmetry In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge conjugation symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics s ...

. CP violation has been observed in

weak interaction In nuclear physics and particle physics, the weak interaction, weak force or the weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is th ...

s and is included in the

Standard Model of particle physics The Standard Model of particle physics is the theory describing three of the four known fundamental forces (electromagnetic, weak and strong interactions – excluding gravity) in the universe and classifying all known elementary particles. It ...

via the CP-violating phase in the CKM matrix. However, the amount of CP violation is very small and therefore also the contribution to the nEDM: .

Matter–antimatter asymmetry

From the asymmetry between matter and antimatter in the universe, one suspects that there must be a sizeable amount of

CP-violation In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge conjugation symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics s ...

. 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

As the neutron is built up of

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...

s, it is also susceptible to CP violation stemming from

strong interaction In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interaction, fundamental interactions. It confines Quark, quarks into proton, protons, n ...

Quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...

– 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 10⁻¹⁰

radian The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. It is defined such that one radian is the angle subtended at ...

s. This

fine-tuning Fine-tuning may refer to: * Fine-tuning (deep learning) * Fine-tuning (physics) * Fine-tuned universe The fine-tuned universe is the hypothesis that, because "life as we know it" could not exist if the fundamental physical constants, const ...

of the angle ''θ'', which is naturally expected to be of order 1, is the

strong CP problem The strong CP problem is a question in particle physics, which brings up the following quandary: why does quantum chromodynamics (QCD) seem to preserve CP-symmetry? In particle physics, CP stands for the combination of C-symmetry (charge conjugati ...

SUSY CP problem

Supersymmetric Supersymmetry is a theoretical framework in physics that suggests the existence of a symmetry between particles with integer spin (''bosons'') and particles with half-integer spin (''fermions''). It proposes that for every known particle, there ...

extensions to the Standard Model, such as the

Minimal Supersymmetric Standard Model The Minimal Supersymmetric Standard Model (MSSM) is an extension to the Standard Model that realizes supersymmetry. MSSM is the minimal supersymmetrical model as it considers only "the inimumnumber of new particle states and new interactions ...

, generally lead to a large CP-violation. Typical predictions for the neutron EDM arising from the theory range between and . As in the case of the

, the limit on the neutron EDM is already constraining the CP violating phases. The

is, however, not as severe yet.

Experimental technique

In order to extract the neutron EDM, one measures the

Larmor precession Sir Joseph Larmor (; 11 July 1857 – 19 May 1942) was an Irish mathematician and physicist who made breakthroughs in the understanding of electricity, dynamics, thermodynamics, and the electron theory of matter. His most influential work was ...

of the neutron

spin Spin or spinning most often refers to: * Spin (physics) or particle spin, a fundamental property of elementary particles * Spin quantum number, a number which defines the value of a particle's spin * Spinning (textiles), the creation of yarn or thr ...

in the presence of parallel and antiparallel magnetic and electric fields. The precession frequency for each of the two cases is given by :

h\nu = 2\mu_\text B \pm 2d_\text E

, the addition or subtraction of the frequencies stemming from the precession of the

magnetic moment In electromagnetism, the magnetic moment or magnetic dipole moment is the combination of strength and orientation of a magnet or other object or system that exerts a magnetic field. The magnetic dipole moment of an object determines the magnitude ...

around the

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

and the precession of the electric dipole moment around the

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

. From the difference of those two frequencies one readily obtains a measure of the neutron EDM: :

d_\text = \frac

The biggest challenge of the experiment (and at the same time the source of the biggest systematic false effects) is to ensure that the

does not change during these two measurements.

History

The first experiments searching for the electric dipole moment of the neutron used beams of

thermal A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...

(and later

cold Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjectivity, subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute t ...

) neutrons to conduct the measurement. It started with the experiment by James Smith,

Purcell Henry Purcell (, rare: ; September 1659 – 21 November 1695) was an English composer of Baroque music, most remembered for his more than 100 songs; a tragic opera, ''Dido and Aeneas''; and his incidental music to a version of Shakespeare's ...

, and

Ramsey Ramsey may refer to: Companies *Ramsey (retailer), Turkish clothing retailer People * Ramsey (given name), including a list of people with the given name * Ramsey (surname), including a list of people with the surname * Baron de Ramsey, a title i ...

in 1951 (and published in 1957) at ORNL's Graphite Reactor (as the three researchers were from

Harvard University Harvard University is a Private university, private Ivy League research university in Cambridge, Massachusetts, United States. Founded in 1636 and named for its first benefactor, the History of the Puritans in North America, Puritan clergyma ...

, this experiment is called ORNL/Harvard or something similar, see figure in this section), obtaining a limit of 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 . After that, experiments with

ultracold neutrons Ultracold neutrons (UCN) are free neutrons which can be stored in traps made from certain materials. The storage is based on the reflection of UCN by such materials under any angle of incidence. Properties The reflection is caused by the coh ...

(UCN) took over. It started in 1980 with an experiment at the (LNPI) obtaining a limit of . This experiment and especially the experiment starting in 1984 at the

Institut Laue-Langevin An institute is an organizational body created for a certain purpose. They are often research organisations (research institutes) created to do research on specific topics, or can also be a professional body. In some countries, institutes ca ...

(ILL) pushed the limit down by another two

orders of magnitude In a ratio scale based on powers of ten, the order of magnitude is a measure of the nearness of two figures. Two numbers are "within an order of magnitude" of each other if their ratio is between 1/10 and 10. In other words, the two numbers are wi ...

yielding the best upper limit in 2006, revised in 2015. During these 70 years of experiments, six

have been covered, thereby putting stringent constraints on theoretical models. The latest best limit of has been published 2020 by the nEDM collaboration at

Paul Scherrer Institute The Paul Scherrer Institute (PSI) is a multi-disciplinary research institute for natural and engineering sciences in Switzerland. It is located in the Canton of Aargau in the municipalities Villigen and Würenlingen on either side of the Ri ...

(PSI).

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 over the next 10 years, thereby covering the range of prediction coming from

supersymmetric Supersymmetry is a theoretical framework in physics that suggests the existence of a symmetry between particles with integer spin (''bosons'') and particles with half-integer spin (''fermions''). It proposes that for every known particle, there ...

extensions to the Standard Model. * n2EDM of the nEDM collaboration under construction at the UCN source at the

.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. The apparatus is expected to reach sensitivity of after 500 days of operation. * TUCAN, a UCN nEDM experiment under construction at

TRIUMF Triumf may refer to: * TRIUMF, Canada's national particle accelerator centre * 14959 TRIUMF, a minor planet * S-400 Triumf, a Russian anti-aircraft weapon system developed in the 1990s * Triumf Riza (1979–2007), Kosovo police officer and member o ...

* nEDM@SNS experiment under construction (as of 2022) at the

Spallation Neutron Source The Spallation Neutron Source (SNS) is an accelerator-based neutron source facility in the U.S. that provides the most intense pulsed neutron beams in the world for scientific research and industrial development.In 2007, SNS was entered into th ...

* PNPI nEDM experiment at the

* PNPI nEDM experiment at the reactor PIK is expected to reach sensitivity of per year. * PanEDM experiment being built at the Institut Laue-Langevin * LANL Electric Dipole Moment (LANL nEDM) at

Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development Laboratory, laboratories of the United States Department of Energy National Laboratories, United States Department of Energy ...

* Beam EDM at

University of Bern The University of Bern (, , ) is a public university, public research university in the Switzerland, Swiss capital of Bern. It was founded in 1834. It is regulated and financed by the canton of Bern. It is a comprehensive university offering a br ...

, Switzerland The

Cryogenic neutron EDM experiment CryoEDM is a particle physics experiment aiming to measure the electric dipole moment (EDM) of the neutron to a precision of ~10−28ecm. The name is an abbreviation of ''cryogenic Neutron electric dipole moment, neutron EDM experiment''. The previo ...

or CryoEDM was under development at the Institut Laue-Langevin but its activities were stopped in 2013/2014.

References

{{DEFAULTSORT:Neutron Electric Dipole Moment Electric dipole moment Electromagnetism Particle physics