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Neutron Dipole Moment

April 27, 2020

The concept of matter was simple in Aristotle's time, around 350 BC, there being just four elements; Earth (Γη), Air (ʾΑηρ), Fire (Πυρ), and Water (ʿΥδωρ). All things were composites of different proportions of these four fundamental elements. The problem with Aristotle's approach, however, is that none of this was determined by experiment, just pure thought.

The four classic elements, and the four qualities of matter.

There were four qualities, Dry ξηρον, Wet ʿυγρον, Hot θερμον, and Cold ψυχρον associated with the four classical elements, Earth ΓΗ, Air ʾΑΗΡ, Fire ΠΥΡ, and Water ʿΥΔΩΡ.

Fire is both hot and dry, Earth is both cold and dry, Air is both hot and wet, and water is both cold and wet.

Composite materials built from these pure elements had qualities in proportion to their Earth-Air-Fire-Water content.

(Created using Inkscape. Click for larger image.)

A contrary view, that matter is composed of indivisible particles called atoms, was popular even before Aristotle's time; but, that theory (atomism) was also just a product of pure thought. Much later, a multitude of actual chemical elements was discovered using scientific methods, starting with the discovery of phosphorous by German Alchemist, Hennig Brand in 1649. In the 19th century, English chemist, John Dalton, gave scientific evidence that atoms do exist, and that chemical reactions involve the separation or rearrangement of atoms.

Eventually, physicists began to wonder whether the atoms of chemical elements were composites of other things. In 1897, the first constituent of atoms, the electron was discovered by Joseph John Thomson (1856-1940, commonly known as "J.J. Thomson"), for which Thomson was awarded the 1906 Nobel Prize for Physics. Thomson's model of matter was the so-called plum-pudding model in which the electrons were embedded like raisins in a positively-charged pudding. The electron raisins were allowed to orbit within the positive pudding.

Interestingly, while most scientists in the later part of the 19th century conjectured that any constituent parts of atoms should also be of atomic size, Thomson's experiments showed that the negatively-charged cathode rays he had discovered were a thousand times smaller. This result indicated a need for a deeper probing of the atom.

Experiments by Ernest Rutherford in which he analyzed the scattering of alpha particles striking gold foils revealed the existence of a compact positively-charged nucleus. In 1911, Rutherford proposed a planetary model of the atom, refined by Niels Bohr (1885-1962), whose model explained the emission spectra of atoms. In experiments from 1917-1925, Rutherford proved that the atomic nucleus contains the nuclei of hydrogen atoms, which marks the discovery of the proton.

Figure caption

Ernest Rutherford (1871-1937).

While Rutherford is famous for the discovery of the proton, his 1908 Nobel Prize was in chemistry "For his investigations into the disintegration of the elements, and the chemistry of radioactive substances."

Rutherford was Director of the Cavendish Laboratory of Cambridge University when James Chadwick discovered the neutron in 1932.

(Wikimedia Commons image, modified for artistic effect.)

Experimental science advances as improved laboratory equipment becomes available. It took James Chadwick (1891-1974) just two weeks of experiments with neutrons to feel confident enough to send a letter to Nature in February, 1932, entitled, "Possible Existence of a Neutron". He followed the letter with a detailed article in the Proceedings of the Royal Society A entitled, "The Existence of a Neutron." Chadwick was quickly awarded the 1935 Nobel Prize in Physics for this discovery.

At the time of its discovery, there was the idea that neutrons might be composite particles consisting of an electron bound to a proton, the equal charges balancing to zero. Chadwick and his colleague, Maurice Goldhaber (1911-2011), found that the neutron mass of 1.0084-1.0090 daltons (now known to be 1.008645 daltons) was too large for that possibility; so, at that time, electrons, protons, and neutrons were considered to be elementary particles and not composite particles.

As science advanced into the 1960s and 1970s, there was a realization that protons and neutrons might be composite particles, In 1968, accelerator experiments indicated that protons might contain smaller objects. Eventually, physicists developed what's called the Standard Model that has protons and neutrons constructed from three tightly-bound particles called quarks (see figure). Everything needs a name, and the quark was named by physicist, Murray Gell-Mann (1929-2019), who took the word from a phrase in Finnegans Wake by James Joyce (Three quarks for Muster Mark!, which is commonly considered to mean, Three quarts for Mister Mark!, the quart being a measure of beer. Since three quarks make a proton, the appearance of "three" in that phrase probably helped.)

Quark model for proton and neutron

Quark model for the proton and neutron. The two types of quarks present in each are the "up" quark u and the "down" quark d. (Left, Wikimedia Commons image by Arpad Horvath. Right, Wikimedia Commons image by Jacek Rybak.)

Even with its quark structure, the neutron is firmly without charge; but, just like the early electron-proton model of the neutron in which internal charges balance to zero, could that also be the case for the neutron? When looking for something like that, physicists are looking for an electric dipole moment of the neutron, which reflects the distribution of positive and negative charge inside the neutron. A zero dipole moment indicates that any positive and negative charges internal to the neutron coincide.

A neutron electric dipole moment above a very small limit would reveal problems with our present understanding of particle physics. The experimental method for measuring the moment is facilitated by the neutron's having a magnetic moment that precesses at an easily detected frequency called the Larmor frequency. I did an undergraduate project in electron paramagnetic resonance that utilized this same principle. In the case of electrons, the frequency varies with magnetic field as 28,025 MHz/T, and microwave frequencies are employed with conventional electromagnets. For the neutron, the frequencies are the much smaller 42.576 MHz/T, so the larger magnetic fields from superconducting magnets are typically employed.

The dipole momemnt experiment involves measuring this frequency for neutrons in the presence of a magnetic field while flipping the direction of an applied electric field. If there's no dipole moment, there is no change in frequency. An accurate measurement is only obtained when the magnetic field is stable and the motion of neutrons in the measured neutron beam is small. The first such measurement was done in 1957 by a team of physicists that included Edward M. Purcell (1912-1997), who had shared the 1952 Nobel Prize for Physics for his 1946 discovery of nuclear magnetic resonance, and Norman F. Ramsey (1915-2011), who was subsequently awarded the 1989 Nobel Prize in Physics for work on atomic clocks.[1] This first measurement indicated that the neutron electric dipole moment was less than 5x10-20 e-cm, and it was the first of many such measurements of improved accuracy (see graph).[2]

Experimental upper limit of neutron electric dipole moment vs year

Experimental upper limit of neutron electric dipole moment vs year.

The first data point, at the top left of this graph, is the Smith-Purcell-Ramsey experiment of 1957. Sensitivity has increased about six orders of magnitude since that time.

(Created using Inkscape from data on page 10 of ref. 2.[2])

The most recent measurement of the neutron electric dipole moment was conducted by a huge international team of 84 scientists from 18 research institutions in Belgium, France, Germany, Poland, Serbia, Switzerland, the United Kingdom, and the United States.[3-4] Their result, that the moment is less than 1.8x10-26 e-cm, is published in an free, open access paper in Physical Review Letters.[3-4] This is about half the best previous value, and it was obtained by increasing the time that neutrons could be observed.[4] Control of the magnetic field needed to achieve this accuracy was facilitated by atomic magnetometers based on the 199Hg isotope and cesium vapor.[3] The research team for this experiment was led by Philipp Schmidt-Wellenburg of the Paul Scherrer Institute and Guillaume Pignol of the Laboratoire de Physique Subatomique et de Cosmologie in France.[4]

So, why is this important? The Standard Model is not complete, since it doesn't combine quantum mechanics and general relativity, and there's the further problem of why the universe has more matter than antimatter. It's thought that a more complete theory would show that charge-parity (CP) symmetry is not conserved in quark binding by the strong force.[4] Such CP violation in the energetic, early universe would give more matter than antimatter, and this possibility would be exposed in a neutron electric dipole moment.[4] The observed limit of the moment indicates that CP violation of the strong force may have occurred earlier than the time when electromagnetic and weak nuclear forces became distinct.[4]


  1. J. H. Smith, E. M. Purcell, and N. F. Ramsey, "Experimental Limit to the Electric Dipole Moment of the Neutron," Phys. Rev., vol. 108, no. 1 (October, 1957), pp. 120ff., DOI:https://doi.org/10.1103/PhysRev.108.120.
  2. J. Pretz, "Measurement of Permanent Electric Dipole Moments of Proton, Deuteron and Light Nuclei in Storage Rings," Forschungszentrum Jülich Website, June 18, 2012 (PDF File).
  3. C. Abel et al., "Measurement of the Permanent Electric Dipole Moment of the Neutron," Phys. Rev. Lett., vol. 124, no. 8 (February 28, 2020), Article no. 081803, https://doi.org/10.1103/PhysRevLett.124.081803. This is an open access paper with a PDF file here.
  4. Philip Ball, "Focus: New Limit on the Neutron’s Internal Charge Asymmetry," Physics, vol. 13, no. 25 (February 28, 2020).

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