I What follows after SUSY

  • Thread starter kelly0303
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Hello! I am really not an expert in this so please correct me if I say something stupid. I read a few articles (e.g. https://journals.aps.org/rmp/pdf/10.1103/RevModPhys.90.025008) in which there are presented the implications of low energy physics on high energy physics. In the electric dipole moment (EDM) of fundamental particles section it is said that the limits on the electron EDM (eEDM) implies that possible SUSY particles should have a mass greater than 10 TeV. Also an improvement in the eEDM measurements sensitivity by 1 or 2 order of magnitudes (which is not an easy task experimentally by any means), with a null result, would rule out SUSY (almost) completely, in the sense that the masses of the particles needed to explain the value of the EDM would be too large for SUSY to be able to solve the problems it was originally created for (e.g. Higgs mass, baryon asymmetry). So, assuming I didn't missunderstand what I read, I have a few questions. I hear over and over again that CERN is still trying, as one of its main objectives to search for SUSY. Yet if the masses of the particles are bigger than 10 TeV they wouldn't be able to find it at the current energies, and the HL-LHC, will just give more statistics, but not more energy to produce these particles. So, what do they mean when they say they are searching for SUSY particles? Are there some loopholes that allow such a small EDM and yet small mass SUSY particles at the same time? And a second question, assuming the EDM will not be found after the sensitivity improvement, hence most of the SUSY models ruled out, what are the most viable models available at our energy level that the physics community is most likely to start to look for? Thank you!
 

phyzguy

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I'm far from an expert in this area, but it seems to me that the ν-MSM is a very promising extension to the standard model that potentially addresses all of the issue we know about. I'd be interested to hear the comments of the people on this forum who know a lot more about high-energy physics than I do.
 
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too large for SUSY to be able to solve the problems it was originally created for (e.g. Higgs mass, baryon asymmetry)
As far as I know, the first supersymmetry in physics was proposed by Miyazawa in 1966, to place mesons (bosons) and baryons (fermions) in the same multiplet. Then at the start of the 1970s, it was found mathematically within string theory and field theory, but not yet with any application in mind. The standard phenomenological applications (hierarchy problem, gauge unification, dark matter) only came as people studied what it could do. To this narrative I would add the construction of the mature forms of string theory - superstring theory, M theory - as the apex of supersymmetric theory development. String theory opened a new frontier, possibly a final frontier, for model-building, as significant as the 1970s revolution in renormalizable quantum field theory.

If you ask particle physicists, what comes next, if the paradigm of weak-scale supersymmetry hasn't worked out, most of the theorists will still say that the hierarchy problem remains the outstanding problem to solve. The standard way to solve it, supersymmetric or otherwise, is still to have extra particles that cancel most of the divergences, and with them the need for tuning; and so all such frameworks face the same problem, that the new particles are hiding away. There are longstanding deviations from the standard model (B decays, g-2), and I suppose natural model builders now focus on models that would produce those effects.

There are other proposals for how to solve the hierarchy problem. Some people have anthropic arguments. These can lead to extra predictions if the anthropic framework is sufficiently detailed. For example, you might think you know that the string theory landscape is dominated by certain versions of MSSM, and tuning the Higgs like so, implies a certain mass spectrum for all the superpartners. I suppose "split supersymmetry" is an example of this.

Then you have mechanisms in which the Higgs vev evolves over cosmological time, and gets pinned at its current value by some mechanism. The "relaxion" paper from a few years ago pioneered this. I have seen a few papers lately which try to deal with tuning of Higgs vev and cosmological constant at the same time, with both being the product of the same trip through a series of vacua or local minima during the early universe. The extra predictions of such models would mostly be cosmological.

The fact that the Higgs mass places the standard model on the boundary of stability and metastability often sounds like a clue to new physics, though it's bad for phenomenology in the sense that it implies any new physics comes only at very high energy scales. I think it's a clue that theorists have yet to interpret correctly - there's work on it, but none of it has achieved its final form.

My approach to these matters is definitely theory-centric, in the following sense: If we never had another scrap of new data, there are still plenty of things that theorists should be trying to explain. They need to account for the apparent tuning of the Higgs, the QCD theta parameter, and the cosmological constant; they should be hoping to explain where the fundamental constants get their values (and where the qualitative features of fundamental physics come from too, like the symmetry groups; but the unexplained numbers have the most bits of information); and then there's all the astronomical and cosmological data.

Hopefully someone else will give us an experimenter's perspective.
 

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