EDM experiments are sensitive to electric field strength and observation time.

For electrons, you can get much higher fields inside a molecule.

For neutrons, you either need a very cleverly designed crystal or a bottle with ultracold neutrons. The crystal lets you get huge fields and huge particle counts, but relatively short observation times. Ultracold neutrons let you have long observation times (100-300 seconds) but lower fields and much lower particle counts.

Protons are probably the hardest of the lot. They need external fields, but they're charged so you can't just bottle them. You're stuck with a holding ring, and needing to make all those v x E corrections.

I'm working on the search for the EDM of the muon. Essentially it's much harder to search for the proton EDM than the neutron EDM. All EDM searches rely on a strong electric field applied to the particles. Because neutrons are neutral they are easily stored in some volume for a long time. You cannot so easily store protons because the moment you apply some E-field you start accelerating them. That's why you need to build a large storage ring with magnetic/electric focusing and so on. This brings numerous challenges that you don't have for the neutron. This, combined with the fact that we don't expect much different novel physics for the proton that won't be seen in the neutron has led to the focus on the neutron EDM, while the proton was left behind.

The usual quote is that for the proton we can reach sensitivities up to 10^-29 (around three orders of magnitude lower than the current nEDM limit), but thats only the statistical sensitivity. The systematic effects that would spoil that come much earlier and this limit is close to science fiction at this point. For example, if you have a magnetic field in the order of attotesla in the region of the storage ring it will dominate the measurement.

Would be happy to answer more edm questions :)

Whatever happened to Skrillex?

Thank you, this is why I love HN!

Ah, that makes sense! I thought neutrons were hard to store because they were neutral and go right through things, but I guess cold neutrons can be stored (at least until they decay into protons).

Is it expected that pEDM ~= nEDM, since they are uud and udd?

I cannot find the citation right now, but the p and n EDMs are expected to be close to each other ~1e-32 e.cm. One part is that they are uud and udd and the other thing to consider is that the quarks make up only ~2% of the proton/neutron mass. Most is binding energy and a soup of virtual quarks and gluons and in that regard they are even more similar I think. I am not very familiar how theoreticians calculate the EDM of such complex particles though.

On a side note, 'ultra cold neutrons' are a super interesting type of matter. Their energy is so low that they can be stored in bottles and are transported through tubes using turbines and mechanical valves.