Constraints on BSM from EDM measurements

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Main Question or Discussion Point

Hello! I see that the experiments looking for electric dipole moment (EDM) of fundamental particles (especially for the electron) are able to place constraints on new BSM particles with masses of around 10 TeV or even more, in a model independent way i.e. we just need to measure a non-zero electron EDM to know that BSM physics exist, even if we won't be able to specify what this new physics is with a single experiment (ignore the standard model EDM for now). I was wondering to what extent do these searches make high energy experiments redundant? As far as I know at LHC (mainly CMS and ATLAS) they are usually looking for certain models of new physics (hence they have specific cuts) and their energy is well below 10 TeV. What kind of new physics can be found at LHC that couldn't be found using EDM experiments? Of course you need both kinds of experiments to approach the searches from different angles (at LHC you could actually produce that given particle, not just observe its effects indirectly) and confirm discoveries, but I was wondering if there are some models that are not sensitive at all to EDM but they are to high energy experiments.
 

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  • #2
ohwilleke
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Your statement and question are based on a false premise, even though the gist is not completely wrong.

Within the Standard Model of elementary particle physics, such a dipole is predicted to be non-zero but very small, at most 10−38 e⋅cm. . . . In the Standard Model, the electron EDM arises from the CP-violating components of the CKM matrix. The moment is very small because the CP violation involves quarks, not electrons directly, so it can only arise by quantum processes where virtual quarks are created, interact with the electron, and then are annihilated.
From here citing Pospelov, M.; Ritz, A. (2005). "Electric dipole moments as probes of new physics". Annals of Physics. 318 (1): 119–169. arXiv:hep-ph/0504231. Bibcode:2005AnPhy.318..119P. doi:10.1016/j.aop.2005.04.002.

From the same source:

To date, no experiment has found a non-zero electron EDM. The Particle Data Group publishes its value as |de| < 0.87×10−28 e⋅cm.
Citing this source. But, the exclusion is ten orders of magnitude greater than the theoretically expected value (i.e. a factor of ten billion greater).

Some supersymmetric and technicolor theories predict an electron EDM near the current threshold of detection (i.e. an electron EDM much higher than in the SM).

I was wondering to what extent do these searches make high energy experiments redundant?
Not really.

The conclusion that a very small electron EDM is inconsistent with very massive undiscovered fundamental particles greater than a particular mass is model dependent. But one of the purposes of high energy experiments is to determine if the existing model is an accurate description of the world. And, colliders are much closer to being able to see particles with masses in excess of 10 TeV (roughly a factor of ten) than EDM measurement experiments are to probing the SM prediction of the value of the electron EDM (roughly a factor of ten billion).

Also, high energy experiments do more than just look for new undiscovered fundamental particles. Determining the behavior of colliding particles at high energies based upon theoretical calculations alone is non-trivial, non-obvious, and something that scientists sometimes get wrong. They'd get it wrong more often if they didn't have experiments to nudge them in the right direction when their calculations produced results different from the experimental results.

For example, simply using calculations, you'd expect that glueballs (composite particles made entirely of gluons) would be much more common than they are to see in isolation (as opposed to mixed with other bosons). Experiments have shown, however, that it isn't that simple.
 
  • #3
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Your statement and question are based on a false premise, even though the gist is not completely wrong.



From here citing Pospelov, M.; Ritz, A. (2005). "Electric dipole moments as probes of new physics". Annals of Physics. 318 (1): 119–169. arXiv:hep-ph/0504231. Bibcode:2005AnPhy.318..119P. doi:10.1016/j.aop.2005.04.002.

From the same source:



Citing this source. But, the exclusion is ten orders of magnitude greater than the theoretically expected value (i.e. a factor of ten billion greater).

Some supersymmetric and technicolor theories predict an electron EDM near the current threshold of detection (i.e. an electron EDM much higher than in the SM).



Not really.

The conclusion that a very small electron EDM is inconsistent with very massive undiscovered fundamental particles greater than a particular mass is model dependent. But one of the purposes of high energy experiments is to determine if the existing model is an accurate description of the world. And, colliders are much closer to being able to see particles with masses in excess of 10 TeV (roughly a factor of ten) than EDM measurement experiments are to probing the SM prediction of the value of the electron EDM (roughly a factor of ten billion).

Also, high energy experiments do more than just look for new undiscovered fundamental particles. Determining the behavior of colliding particles at high energies based upon theoretical calculations alone is non-trivial, non-obvious, and something that scientists sometimes get wrong. They'd get it wrong more often if they didn't have experiments to nudge them in the right direction when their calculations produced results different from the experimental results.

For example, simply using calculations, you'd expect that glueballs (composite particles made entirely of gluons) would be much more common than they are to see in isolation (as opposed to mixed with other bosons). Experiments have shown, however, that it isn't that simple.
Thank you for your reply! I guess my question was not very clear, sorry for that. What I meant mainly is, for a given model, if that model predicts (say) a given electron EDM as a function of a new massive particle mass and the current limits for the electron EDM set the limit for that mass to be (say) greater than 30 TeV (this limit is actually based on this recent Nature review and the ACME collaboration results), is there any reason to test the same model using LHC? LHC currently can't go higher than 14 TeV, so if the limits sets by ACME is 30 TeV, LHC will never be able to see something, as they can't go higher than 30 TeV. Of course there are (I assume) models which don't predict an enhanced EDM, and these need to be tested by LHC (or other experiments), but these models for which the EDM experiments set a mass limit higher than 14 TeV, is there any reason to be further approached by LHC?
 
  • #4
ohwilleke
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The answer is still basically model dependence. The cap that EDM imposes on new massive particles is model dependent, and moreover is is basically dependent upon models like supersymmetry and technicolor that have lost all observationally driven motivation. EDM doesn't reliably tell you what particles are possible in other models.

Also, the 14 TeV energy scale of the LHC does not mean it can reliably rule out or detect particles in the 14 TeV range. In round numbers, the LHC can reliably exclude or detect new particles up to about 1.4 TeV, and a new collider in the 100 TeV range that is being proposed would exclude or detect new particles up to about 10 TeV.

All of this said, I'm about as bearish as they come about the scientific benefits of a 100 TeV collider. My Baysean prior expectation of the likelihood that it will do anything but further confirm the Standard Model is 5% or less, and there is no investment in the history of science that is pricier and would consume more prime HEP talent. Don't get me wrong. The benefit of a big new collider is not zero, and advances like pinning down the mass of the top quark and Higgs boson (and its decay fractions) more exactly, refining parton distribution function data, and confirming the running with energy scale of the Standard Model coupling constants to greater precision would be helpful in theory development.

But the Standard Model has lots of components that are globally influenced by everything else in the model, especially with virtual loops. If there was something big within a factor of ten of the energy scales we currently probe, we'd see something, maybe not a five sigma discovery of a new particle, but we'd have pervasive anomalies all over the place in bigger magnitudes and frequency than we are seeing them.

In my humble opinion the money and personnel necessary to do that would be better put on hold for a while as we develop other fields like computational tools, space telescopes, gravitational wave detectors, muon g-2, neutrino physics experiments, instrumentation, etc. to have a better idea of exactly what we should be looking for, and better tools to look for it, rather than running on autopilot on a SUSY driven agenda that has petered out and an all purpose scan for anything else anomalous.
 

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