Why Is Octupole Deformation Measurable in Nuclei but Not Dipole Deformation?

[Malamala]

Hello! I don't know much about this, so maybe the answer to my questions follows directly from the math of it, but I was wondering if there is an answer providing more physics intuition to this, not just math: Why can a nucleus have an octupole deformation, as a ground state stationary state (https://www.nature.com/articles/nature12073), but no nucleus so far was found to have dipole deformation. I understand that both type of deformations would have to vanish in a stationary state if parity would not be violated. Given that parity is actually violated by the weak interaction (ignore beyond the SM physics for now), we expect to have (a small) octupole and dipole deformation in some nuclei (probably in all nuclei in principle, but for most of them it is too small to be measured). In all cases I encountered so far in physics, when one makes a multipole expansion, the higher the multipole the lower the given effect. So based on that logic I would expect that a dipole deformation to be bigger than an octupole one. Yet, the octupole one was measured, but no dipole one. Why is this the case? Why can the weak interaction lead to a measurable octupole deformation, but not to a dipole one? Thank you!

 

[Vanadium 50]

You are confusing the shape of the nucleus with the shape of the charge distribution. U-238, for example, is known to be cigar-shaped even though as a spin-0 nucleus the "cigar" has no direction in which to point.

 

[Malamala]

You are confusing the shape of the nucleus with the shape of the charge distribution. U-238, for example, is known to be cigar-shaped even though as a spin-0 nucleus the "cigar" has no direction in which to point.

But then my questions would be, why didn't we see a dipole shaped charge distribution, if we saw an octupole shaped one?

 

[Vanadium 50]

You are confusing the shape of the nucleus with the shape of the charge distribution.

 

[Malamala]

You are confusing the shape of the nucleus with the shape of the charge distribution.

That could be the case (I don't know much about this), so any help is greatly appreciated. So in that paper, with the octupoled deformed Radium, is that octupole the shape of the nucleus or the charge distribution? And, regardless of which one it is, why didn't we see the same one (charge or nuclear shape, whichever that is), having a dipole? I read in many papers the claim that a nucleus having an electric dipole moment, means signs of new physics (at least given our current detectors). So I am a bit confused. Could you explain that a bit to me?

 

[Vanadium 50]

I am confused by your confusion. I have no idea what you are talking about.

 

[Malamala]

I am confused by your confusion. I have no idea what you are talking about.

Why can a nucleus have an electric octupole moment, but not an electric dipole moment?

 

[Vanadium 50]

Why can a nucleus have an electric octupole moment, but not an electric dipole moment?

Why do you think it does?

 

[Malamala]

Why do you think it does?

Well if I knew I wouldn't ask here...

 

[snorkack]

Is actual negative charge needed to have a "electric dipole moment", or not needed? There is the negative charges inside nucleons, of course.
If a nucleus had protons concentrated in one end and neutrons in the other end, would the nucleus then have an electric dipole moment?

 

[Malamala]

Is actual negative charge needed to have a "electric dipole moment", or not needed? There is the negative charges inside nucleons, of course.
If a nucleus had protons concentrated in one end and neutrons in the other end, would the nucleus then have an electric dipole moment?

You don't need positive or negative charge to have an electric dipole moment. It is all about the charge distribution. For example there are lots of searches for the electron electric dipole moment. The electron doesn't have any positive charge but some theory beyond the standard model predict it actually has an electric dipole moment if its charge distribution has a certain shape. Same for the nucleus. For a given shape, the nucleus could have an electric dipole, but none has ever been measured. However, the octupole deformation was measured. So my questions is why were we able to measure an octupole but not a dipole deformation, given that both of them (the operators corresponding to the dipole and octupole) violate parity?