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

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Discussion Overview

The discussion centers around the differences in measurability between octupole and dipole deformations in atomic nuclei, exploring the underlying physics and implications of these phenomena. Participants seek to understand why octupole deformation can be observed while dipole deformation has not been detected, despite both being expected to vanish in stationary states under certain conditions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the intuition behind the observation of octupole deformation in nuclei, suggesting that while both octupole and dipole deformations should theoretically be present due to parity violation, the expectation is that dipole deformation would be more significant based on multipole expansion logic.
  • Another participant points out the distinction between the shape of the nucleus and the shape of the charge distribution, using U-238 as an example of a cigar-shaped nucleus that does not have a directional dipole moment.
  • Several participants express confusion regarding the relationship between nuclear shape and charge distribution, particularly in the context of measuring electric dipole moments and the implications of such measurements for new physics.
  • A participant raises a question about the necessity of actual negative charge for the existence of an electric dipole moment, suggesting a scenario where protons and neutrons are distributed unevenly within a nucleus.
  • Another participant clarifies that an electric dipole moment can exist without the presence of both positive and negative charges, emphasizing that it is the charge distribution that matters.
  • There is a repeated inquiry into why octupole deformation has been measured while dipole deformation has not, despite both being associated with parity violation.

Areas of Agreement / Disagreement

Participants express varying levels of confusion and disagreement regarding the concepts of nuclear shape, charge distribution, and the implications of electric dipole moments. No consensus is reached on the reasons for the measurability of octupole deformation versus the non-detection of dipole deformation.

Contextual Notes

The discussion highlights limitations in understanding the relationship between nuclear structure and observable physical phenomena, particularly regarding the definitions and implications of electric dipole and octupole moments. There are unresolved questions about the conditions under which these deformations can be measured.

Malamala
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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!
 
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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.
 
Vanadium 50 said:
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?
 
You are confusing the shape of the nucleus with the shape of the charge distribution.
 
Vanadium 50 said:
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?
 
I am confused by your confusion. I have no idea what you are talking about.
 
Vanadium 50 said:
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?
 
Malamala said:
Why can a nucleus have an electric octupole moment, but not an electric dipole moment?

Why do you think it does?
 
Vanadium 50 said:
Why do you think it does?
Well if I knew I wouldn't ask here...
 
  • #10
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?
 
  • #11
snorkack said:
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?
 

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