Why are physicists trying to measure the neutron’s electric dipole moment?


I received this note from a chemist correspondent of mine, and I thought I would post my answer for readers in cyberspace too.

Copied this from a cosmology site I loosely follow.  But it sounds a bit off to me.  What’s your take?

“Although the neutron has no net charge, it is composed of charged quarks, and any permanent spatial offset between positively and negatively charged ones could, in theory, give rise to a dipole moment that is sensitive to electric fields. Experiments to look for such an EDM were first carried out in the 1950s and their sensitivity has since improved by more than six orders of magnitude. But physicists continue to push sensitivities ever higher, in the hope of discovering an effect that would violate so-called charge–parity symmetry, thereby explaining how matter came to dominate over antimatter in the early universe.”

I’m pleased that you (my chemist correspondent) are paying attention to the cosmology news.

First of all, the site’s author points out correctly (and contrary to your ideas) that neutrons (and protons) are composite particles formed from three quarks. Neutrons are not composites of a proton and an electron as you claim. So I hope that this wasn’t why you thought the matter was a bit off.
Consider the neutron:
One up quark, charge +(2/3)e and spin 1/2, and two down quarks, charge -(1/3)e and spin 1/2, so the neutron charge is 0 and the three spins add (in the quantum way) to 1/2. ( The e is the magnitude of the electron’s electric charge.) The wiggly lines represent the force of quantum chromodynamics, carried by gluons, and in our school days known as the strong nuclear force. The little picture is not intended to suggest that the quarks exist in fixed position held together as if the neutron were a crystalline solid. The picture is also not intended to suggest that the quarks are little balls nor that they are blue, red, or green.Indeed, if the neutron did have its quarks in some more or less fixed position, as shown, the neutron would have an electron dipole moment, as there is a charge of (2/3)e in the upper left, and 2 charges of -(1/3)e to the lower right. This would produce an electric dipole moment, a vector pointing from the upper left to the lower right with a magnitude proportional to the charges and to the distance between them. You could, for example, note that a water molecule has an electric dipole moment because the oxygen atom pulls the hydrogens’ electrons to it, leaving a bit of positive charge behind.

Water Molecule

Well, in this (incorrect) static picture, the quarks might be arranged this way: d-u-d, (as the oxygens and carbon are in carbon dioxide). Then the neutron would have a zero electric dipole moment, but it would have an electric quadrupole moment.

Carbon dioxide
Now, quarks, as spin 1/2 particles, are fermions. The spin states would have to be u, spin up, and one of the ds spin up and the other spin down, which is what produces the neutron’s spin 1/2. The two negatively charged quarks, the ds, would tend to be pushed away from each other, and the positively charged quark would like to be close to each of them. You’d guess that this would tend to be like a carbon dioxide molecule.There is another major question in cosmology and in particle physics. Why is the Universe formed nearly entirely from “matter”? At first blush, our observations are that matter and anti-matter particles appear in precisely equal numbers when energy transforms into particles. Thus, the Big Bang ought to have produced equal numbers of electrons and of positrons, and of protons and anti-protons, and so on. Then those particles should have annihilated each other, leaving the Universe as a sea of photons, electromagnetic energy.

Today our estimate is that there are about about billion photons for every material particle. Thus there must be something that doesn’t quite balance. You could have in mind that, for some reason, a world made from mostly anti-particles would be somehow different from our world, made from particles. (Of course, we just mean there are two types of matter, one of which, the common one in our world, we call particles or matter, and the other type, which is uncommon in our world, we call anti-particles and anti-matter.)

Indeed, physicists have found such an asymmetry, first in tiny differences in the behavior of certain particles, kaons, and their anti-particles. But when researchers put in the numbers, so and so many particles of all types appear in the Big Bang, they are busy annihilating each other, except the kaons don’t quite cancel each other, leaving a one part in a billion excess of matter particles, the measured imbalance in kaons is not enough by several factors of ten to produce the observed billion to one ratio of photons to particles.

In the years since this first observation of obscure differences in the world and the anti-world (known in the lingo as CP violation, a topic for another blog post) physicists have found other small mis-matches, but none are sufficient by themselves or in combination to leave enough particles around as the Big Bang cools.

At this point, I’ve told you the context for physicists looking for a neutron’s electric dipole moment. So far, the mis-match between the world and the anti-world have all arisen in the mysterious force or interaction known as the weak nuclear force. In so far as this force is known and has noticeable effects, it produces so-called beta decay. This type of radioactivity involves a neutron decaying to produce a proton, an electron (the beta ray), and an electron anti-neutrino.

Beta decay appears in radioactivity of certain nuclear isotopes in which an excess neutron turns into a proton within the nucleus, emitting the beta ray and the neutrino. This increases the atomic number (the number of protons in the nucleus) by one. Beta decay might also occur in certain nuclear isotopes in which an excess proton turns into a neutron, emitting a positron and an electron neutrino (not an anti-neutrino). This also changes the atomic number of the isotope, reducing it by one. The proton in the nucleus has to get some energy from someplace to do this, since the neutron has more mass than the original proton, and there has to be some energy around to create the positron and neutrino.

Particle physicists, or quantum field theorists, don’t see any reason why the strong nuclear force doesn’t sometimes violate the matter-anti-matter symmetry in some obscure ways. It is a fact, however, that no one has ever observed such a violation. And they’ve looked.

It turns out that this violation, matter-anti-matter asymmetry, would appear in the forces between quarks as a neutron electric dipole moment. (It’s complicated to explain just why.) Researchers have been seeking to measure the neutron’s electric dipole moment for more than 60 years, but they keep getting zero. Or at least, the neutron’s electric dipole moment must be smaller than the experiments can see, if it is not zero. The experiments have gotten more than a million times more precise over the years, and there are half a dozen experiments of different types looking for this effect.

The so-called Standard Model, our best theory of everything so far, predicts that the strong nuclear force’s violation of the matter-anti-matter symmetry should be not zero, but, alas, still too small to solve the matter-anti-matter problem for the Universe.

If the experiments find that the neutron’s electric dipole moment is as the Standard Model predicts, then our confidence in the Standard Model would increase, and our confidence that the Standard Model is missing something would also increase. Well, that’s strange. Either way, that is, we need some new ideas and some new observations.


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