Neutron electric dipole moment

Last updated

The neutron electric dipole moment (nEDM), denoted dn, 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 dn is (0.0±1.1)×10−26  e⋅cm. [1]

Contents

Theory

Parity (P) and time-reversal (T) violation due to an electric dipole moment. m denotes the magnetic dipole moment of the neutron, while d is the electric dipole moment. NEDM P&T violation.png
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 (P) and time reversal symmetry (T). These violations can be understood by examining the neutron's magnetic dipole moment and hypothetical electric dipole moment. Under time reversal, the 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 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. CP violation has been observed in weak interactions and is included in the Standard Model of particle physics 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: |dn| ~ 10−31 e⋅cm. [2]

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. 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 quarks, it is also susceptible to CP violation stemming from strong interactions. 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 10−10  radians. This fine-tuning of the angle θ, which is naturally expected to be of order 1, is the strong CP problem.

SUSY CP problem

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 10−25e⋅cm and 10−28e⋅cm. [3] [4] As in the case of the strong interaction, the limit on the neutron EDM is already constraining the CP violating phases. The 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 in the presence of parallel and antiparallel magnetic and electric fields. The precession frequency for each of the two cases is given by

,

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:

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

The history of neutron EDM limits including the latest best result by the nEDM collaboration at PSI. The prediction stemming from the Standard Model is also indicated. NEDM-History-wiki.png
The history of neutron EDM limits including the latest best result by the nEDM collaboration at PSI. 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 thermal (and later cold) neutrons to conduct the measurement. It started with the experiment by James Smith, Purcell, and Ramsey in 1951 (and published in 1957) at 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 |dn| < 5×10−20 e⋅cm . [5] [6] 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 |dn| < 3×10−24 e⋅cm. [7]

After that, experiments with ultracold neutrons (UCN) took over. It started in 1980 with an experiment at the Leningrad Nuclear Physics Institute  [ ru ] (LNPI) obtaining a limit of |dn| < 1.6×10−24 e⋅cm. [8] This experiment and especially the experiment starting in 1984 at the Institut Laue-Langevin (ILL) pushed the limit down by another two orders of magnitude yielding the best upper limit in 2006, revised in 2015.

During these 70 years of experiments, six orders of magnitude have been covered, thereby putting stringent constraints on theoretical models. [9]

The latest best limit of |dn| < 1.8×10−26 e⋅cm has been published 2020 by the nEDM collaboration at Paul Scherrer Institute (PSI). [1]

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 10−28 e⋅cm over the next 10 years, thereby covering the range of prediction coming from supersymmetric extensions to the Standard Model.

The Cryogenic neutron EDM experiment or CryoEDM was under development at the Institut Laue-Langevin but its activities were stopped in 2013/2014. [21]

See also

Related Research Articles

<span class="mw-page-title-main">Neutron</span> Subatomic particle with no charge

The neutron is a subatomic particle, symbol
n
 or
n0
, which has a neutral charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, they are both referred to as nucleons. Nucleons have a mass of approximately one atomic mass unit, or dalton. Their properties and interactions are described by nuclear physics. Protons and neutrons are not elementary particles; each is composed of three quarks.

<span class="mw-page-title-main">T-symmetry</span> Time reversal symmetry in physics

T-symmetry or time reversal symmetry is the theoretical symmetry of physical laws under the transformation of time reversal,

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 exists a partner particle with different spin properties. There have been multiple experiments on supersymmetry that have failed to provide evidence that it exists in nature. If evidence is found, supersymmetry could help explain certain phenomena, such as the nature of dark matter and the hierarchy problem in particle physics.

An axion is a hypothetical elementary particle originally theorized in 1978 independently by Frank Wilczek and Steven Weinberg as the Goldstone boson of Peccei–Quinn theory, which had been proposed in 1977 to solve the strong CP problem in quantum chromodynamics (QCD). If axions exist and have low mass within a specific range, they are of interest as a possible component of cold dark matter.

In physics, mirror matter, also called shadow matter or Alice matter, is a hypothetical counterpart to ordinary matter.

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, the Peccei–Quinn theory is a well-known, long-standing proposal for the resolution of the strong CP problem formulated by Roberto Peccei and Helen Quinn in 1977. The theory introduces a new anomalous symmetry to the Standard Model along with a new scalar field which spontaneously breaks the symmetry at low energies, giving rise to an axion that suppresses the problematic CP violation. This model has long since been ruled out by experiments and has instead been replaced by similar invisible axion models which utilize the same mechanism to solve the strong CP problem.

<span class="mw-page-title-main">Baryon asymmetry</span> Imbalance of matter and antimatter in the observable universe

In physical cosmology, the baryon asymmetry problem, also known as the matter asymmetry problem or the matter–antimatter asymmetry problem, is the observed imbalance in baryonic matter and antibaryonic matter in the observable universe. Neither the standard model of particle physics nor the theory of general relativity provides a known explanation for why this should be so, and it is a natural assumption that the universe is neutral with all conserved charges. The Big Bang should have produced equal amounts of matter and antimatter. Since this does not seem to have been the case, it is likely some physical laws must have acted differently or did not exist for matter and/or antimatter. Several competing hypotheses exist to explain the imbalance of matter and antimatter that resulted in baryogenesis. However, there is as of yet no consensus theory to explain the phenomenon, which has been described as "one of the great mysteries in physics".

Pran Nath is a theoretical physicist working at Northeastern University, with research focus in elementary particle physics. He holds a Matthews Distinguished University Professor chair.

The electron electric dipole momentde is an intrinsic property of an electron such that the potential energy is linearly related to the strength of the electric field:

<span class="mw-page-title-main">Ultracold neutrons</span> Free neutrons stored in very small traps

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.

In physics, vector meson dominance (VMD) was a model developed by J. J. Sakurai in the 1960s before the introduction of quantum chromodynamics to describe interactions between energetic photons and hadronic matter.

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 EDM experiment. The previous name nEDM is also sometimes used, but should be avoided where there may be ambiguity. The project follows the Sussex/RAL/ILL nEDM experiment, which set the current best upper limit of 2.9×10−26ecm. To reach the improved sensitivity, cryoEDM uses a new source of ultracold neutrons (UCN), which works by scattering cold neutrons in superfluid helium.

<span class="mw-page-title-main">Hughes–Drever experiment</span>

Hughes–Drever experiments are spectroscopic tests of the isotropy of mass and space. Although originally conceived of as a test of Mach's principle, they are now understood to be an important test of Lorentz invariance. As in Michelson–Morley experiments, the existence of a preferred frame of reference or other deviations from Lorentz invariance can be tested, which also affects the validity of the equivalence principle. Thus these experiments concern fundamental aspects of both special and general relativity. Unlike Michelson–Morley type experiments, Hughes–Drever experiments test the isotropy of the interactions of matter itself, that is, of protons, neutrons, and electrons. The accuracy achieved makes this kind of experiment one of the most accurate confirmations of relativity.

In electromagnetism, a toroidal moment is an independent term in the multipole expansion of electromagnetic fields besides magnetic and electric multipoles. In the electrostatic multipole expansion, all charge and current distributions can be expanded into a complete set of electric and magnetic multipole coefficients. However, additional terms arise in an electrodynamic multipole expansion. The coefficients of these terms are given by the toroidal multipole moments as well as time derivatives of the electric and magnetic multipole moments. While electric dipoles can be understood as separated charges and magnetic dipoles as circular currents, axial toroidal dipoles describes toroidal (donut-shaped) charge arrangements whereas polar toroidal dipole correspond to the field of a solenoid bent into a torus.

David P. DeMille is an American physicist and Professor of Physics at the University of Chicago. He is best known for his use of polar diatomic molecules to search for symmetry-violating effects within the molecules and as a means for manipulating the external properties of the molecules.

Neutrons are spin 1/2 particles that interact with magnetic induction fields via the Zeeman interaction. This interaction is both rather large and simple to describe. Several neutron scattering techniques have been developed to use thermal neutrons to characterize magnetic micro and nanostructures.

Thorium monoxide, is the binary oxide of thorium having chemical formula ThO. The covalent bond in this diatomic molecule is highly polar. The effective electric field between the two atoms has been calculated to be about 80 gigavolts per centimeter, one of the largest known internal effective electric fields.

<span class="mw-page-title-main">Yannis K. Semertzidis</span>

Yannis K. Semertzidis is a physicist exploring axions as a dark matter candidate, precision physics in storage rings including muon g-2 and proton electric dipole moment (pEDM). The axion and the pEDM are intimately connected through the strong CP problem. Furthermore, if the pEDM is found to be non-zero, it can help resolve the matter anti-matter asymmetry mystery of our universe. During his research career, he held a number of positions in the Department of Physics in Brookhaven National Laboratory, including initiator and co-spokesperson of the Storage Ring Electric Dipole Moment Collaboration. He is the founding director of the Institute for Basic Science (IBS) Center for Axion and Precision Physics Research, is a professor in the Physics Department of KAIST, and a Fellow of the American Physical Society.

Geoffrey L. Greene is an American neutron physicist. Greene received his bachelor's degree from Swarthmore College in 1971 and his doctorate from Harvard University in 1974 with Norman Ramsey, a Nobel laureate in physics. There he worked on low-energy (cold) neutrons, which were then first available in intense beams. As a post-doctoral fellow he was at the Rutherford Appleton Laboratory and the Institut Laue-Langevin. He was an assistant professor at Yale University and then at the National Institute of Standards and Technology (NIST). He has held various management positions at Los Alamos National Laboratory. Since 2002, he is a professor at the University of Tennessee and Oak Ridge National Laboratory.

References

  1. 1 2 3 Abel, C.; et al. (2020). "Measurement of the Permanent Electric Dipole Moment of the Neutron". Physical Review Letters . 124 (8): 081803. arXiv: 2001.11966 . Bibcode:2020PhRvL.124h1803A. doi: 10.1103/PhysRevLett.124.081803 . PMID   32167372.
  2. ↑ Dar, S. (2000). "The Neutron EDM in the SM : A Review". arXiv: hep-ph/0008248 .
  3. ↑ Abel, S.; Khalil, S.; Lebedev, O. (2001). "EDM constraints in supersymmetric theories". Nuclear Physics B . 606 (1–2): 151–182. arXiv: hep-ph/0103320 . Bibcode:2001NuPhB.606..151A. doi:10.1016/S0550-3213(01)00233-4. S2CID   14168743.
  4. ↑ Pospelov, M.; Ritz, A. (2005). "Electric dipole moments as probes of new physics". Annals of Physics . 318 (1): 119–169. arXiv: hep-ph/0504231 . Bibcode:2005AnPhy.318..119P. doi:10.1016/j.aop.2005.04.002. S2CID   13827759.
  5. ↑ Smith, J.H.; Purcell, E.M.; Ramsey, N.F. (1957). "Experimental limit to the electric dipole moment of the neutron". Physical Review . 108 (1): 120–122. Bibcode:1957PhRv..108..120S. doi:10.1103/PhysRev.108.120.
  6. ↑ "Early Fundamental Neutron Experiments at ORNL – ORNL | nEDM".
  7. ↑ Dress, W.B.; et al. (1977). "Search for an electric dipole moment of the neutron". Physical Review D . 15 (1): 9–21. Bibcode:1977PhRvD..15....9D. doi:10.1103/PhysRevD.15.9.
  8. ↑ Altarev, I.S.; et al. (1980). "A search for the electric dipole moment of the neutron using ultracold neutrons". Nuclear Physics A . 341 (2): 269–283. Bibcode:1980NuPhA.341..269A. doi:10.1016/0375-9474(80)90313-9.
  9. ↑ Ramsey, N.F. (1982). "Electric-dipole moments of particles". Annu. Rev. Nucl. Part. Sci. 32 (1): 211–233. Bibcode:1982ARNPS..32..211R. doi: 10.1146/annurev.ns.32.120182.001235 .
  10. 1 2 nEDM Collaboration at PSI Website: https://www.psi.ch/nedm/
  11. ↑ "CNRS movie on n2EDM | UCN Physics | Paul Scherrer Institut (PSI)". 23 February 2022.
  12. ↑ Ayres, N. J.; et al. (2021). "The design of the n2EDM experiment". The European Physical Journal C. 81 (6): 512. arXiv: 2101.08730 . Bibcode:2021EPJC...81..512A. doi:10.1140/epjc/s10052-021-09298-z. PMC   8550164 . PMID   34720721.
  13. ↑ TRIUMF Ultracold Neutron Source
  14. ↑ "nEDM Experiment at the Spallation Neutron Source".
  15. ↑ Ahmed, M.W. (2019). "A New Cryogenic Apparatus to Search for the Neutron Electric Dipole Moment". Journal of Instrumentation . 14 (11): P11017. arXiv: 1908.09937 . Bibcode:2019JInst..14P1017A. doi:10.1088/1748-0221/14/11/P11017. S2CID   201646389.
  16. ↑ "nEDM@SNS - The Neutron Electric Dipole Moment Experiment at the Spallation Neutron Source". nedm.ornl.gov. Oak Ridge National Laboratory. Retrieved 11 Sep 2024.
  17. ↑ nrd.pnpi.spb.ru Neutron EDM page
  18. ↑ Wurm, D.; et al. (2019). "The PanEDM neutron electric dipole moment experiment at the ILL". EPJ Web Conf. 219: 02006. arXiv: 1911.09161 . Bibcode:2019EPJWC.21902006W. doi:10.1051/epjconf/201921902006. S2CID   208202103.
  19. ↑ "Los Alamos Neutron Science Center (LANSCE) UCN Experiments". lansce.lanl.gov. Los Alamos National Laboratory. Retrieved 11 Sep 2024.
  20. ↑ "Beam EDM - Pulsed Neutron EDM Experiment".
  21. ↑ "hepwww.rl.ac.uk Cryogenic EDM". Archived from the original on 2012-02-16. Retrieved 2009-01-22.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.