Way cool, huh?
What the hell is it?
I even follow the logic that it’s existence is possibly an explanation for the existence of the universe, what with the imbalance of matter/anti-matter. (follow the logic, not understand) But what I don’t understand, follow, or even begin to have a handle on is just what the heck it is.
Is this one of those folded up dimension things?
Lost again in the zoo of particle physics.
Send a search party.
Tris
Nothing so exotic as folded-up dimensions. At the most intuitive level, a dipole moment is essentially a way of describing how the charge is distributed inside an object. For example, if I have two equal and opposite charges sitting some distance apart, I have zero net charge. However, I do have a dipole moment because there are constituent charges that make up this zero net charge. Dipole moments have magnitude of charge times distance; in the case of two equal and opposite charges, it happens to be the magnitude of the charges times the distance separating them.
Now, very few credible scientists really believe that the electron is itself made up of smaller sub-charges like in the situation above; to the best we can tell, electrons are point particles. (Credible scientists might speculate about it, but there’s no experimental evidence for it.) However, it’s possible that the electron could have an inherent dipole moment even though it’s a point particle, just like it has angular momentum and charge. That’s what these guys are looking for.
There are also some theoretical reasons to believe that the electron dipole moment is actually exactly zero, and it’s certainly small, or it would have been detected. The most likely result of this experiment is that they don’t see anything. But experiments like this are still important: If nothing else, they’ll tighten up the upper bound on the dipole moment, and of course there’s always a chance they’ll get a positive result.
In the standard model of particle physics, the electron’s electric dipole moment (EDM) should be somewhere around 10[sup]-38[/sup] e-cm. Current experimental limits sit around 10[sup]-27[/sup] e-cm. This new material, the authors say, should help the solid-state experimental technique push this limit down, initially to 10[sup]-28[/sup] e-cm, but groups are already aiming at 10[sup]-32[/sup] e-cm and beyond.
Why do we care?
A non-zero electron EDM is a fundamentally CP-violating phenomenon. Since the only known CP violation in the standard model comes from the weak interactions of quarks, and since electrons don’t interact with quarks directly via the weak interaction, a non-zero electron EDM comes about only through so-called loop corrections – smaller and smaller terms in the calculation’s perturbation series. [If it helps, note that a Taylor series is a type of perturbation series.] Each term in the series is representable by a Feynman diagram with some number of virtual particle loops, and the more loops you have, the smaller the term (more or less).
It turns out that if you add up all the 0-loop, 1-loop, and 2-loop contributions to the EDM, you get zero. You have to go to three loops before the standard model CP violation stops canceling out in the calculation. This is why the predicted value is so small. (That, plus the CP violation present in the quark sector isn’t all that great to begin with.)
The complete series involves all permutations of loops and particles and interactions. So, if you have a Feynman diagram that has an up-quark loop, you also have that same one with a top-quark loop. Since the up and top quarks have different masses, the terms that their diagrams represent have different values. (General rule: heavier loop particle, larger term).
Now, the calculation that leads to 10[sup]-38[/sup] e-cm includes all known particles and interactions in its list of Feynman diagram “permutations”. But, if there are unknown particles out there, perhaps ones too heavy to have yet been observed directly, they would add additional terms to the series. Some run-of-the-mill supersymmetry models, for example, include particles that, when included in the loop tallies, bring the EDM expectation up near the experimental limit (10[sup]-27[/sup] e-cm or so). [To be sure, there are models that would predict an EDM even larger, but they have already been ruled out.]
So, if one observes a larger-than-standard-model value for the electron EDM, it points immediately to new physics (one example of which I’ve hinted at above, although scores of possibilities are discussed.)
Shouldn’t that be the other way around? A heavier loop particle would typically be further off mass shell, and so those diagrams should be more difficult.
Why wouldn’t having a dipole moment constitute evidence that the electron is not a point particle?
Is there actually any reason to believe that the electron is a structureless point beyond Ockham’s Razor as applied to the fact that that there is no evidence that it is not a point?
This is true at tree level (i.e., in diagrams with no loops), where momentum conservation constrains the virtual particle to be (potentially) far off-shell. The resulting propagator then looks like either 1/m[sup]2[/sup] or (for large momentum q) 1/q[sup]2[/sup], which would be small.
Inside a loop, though, the momentum is not constrained. One must integrate over all possible momenta on each leg of the loop, subject to conservation constraints at the external vertices. The result is that even a heavy loop particle will get to contribute near mass shell at some point in the integral’s range, leaving the usually mass-dependent numerators around to fight it out.
In the case of the electron EDM, a 10% change in the top mass results in a ~20% change in the expected EDM value, indicating the strong dependence on the mass of the heaviest quark.
I think MikeS was simply making a more general statement that there isn’t any evidence for substructure at this point. But to your question specifically: the electron is assumed to be a point-like particle in the standard model, yet the standard model predicts a non-zero EDM. That is to say: a non-zero EDM does not immediately imply substructure, although substructure is one way to produce electric dipole moments.
Ah. I see.
Thanks.
Tris
The lack of structure of the electron isn’t just an article of faith; it’s been probed extensively, and if there is any structure there, it’s very, very small. In fact, the upper bound on the size of the electron is one of two contenders for the smallest distance ever measured.
What’s the other one?
A quark, maybe?
Actually, the other one is the measured movement of the test masses at the LIGO detectors, which is a completely different sort of measurement in every way. So far as I can tell, it’s pure coincidence that the two numbers are so close.
One could get a grant based on a coincidence like that.
Tris
Isn’t the detectable movement even smaller for resonant mass gravity wave detectors, like MiniGrail? The strain sensitivity is supposed to be something like an order of magnitude larger than LIGO, but the length of the antenna is about three orders of magnitude less. I don’t know the latest actual measurements.