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Does there exist any good proposal on how the gauge coupling unification can be fixed in non-supersymmetric GUTs? If not, can we assert that non-supersymmetric GUTs have been experimentally ruled out? Of course, I'm referring to the category of GUTs not already ruled out by proton decay rate.

P.S. Somehow people take this issue as evidence for supersymmetry, but not as a death blow to non-supersymmetric GUTs. Why is it the case? Is there a simple fix?

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Bump... Is my question too obscure or too naive? I thought this question should be straightforward for experts. –  felix Apr 26 '11 at 5:47
    
The GUT scale is close enough to the Planck scale that you can fix the running with non-renormalizable corrections to the Lagrangian which are ignored in standard analyses, the mismatch is not too far off to fix. It's not so natural to fix the matching in SU(5) gut, the natural term doesn't push the couplings in the right direction, there's an answer of mine looking at that, but it might be good in SO(10) or a flipped SU(5). –  Ron Maimon Aug 19 '13 at 20:03
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up vote 3 down vote accepted

There is the possibility to have particles at an intermediate scale which correct this dilemma, but this is not very appealing since the new scale is essentially a free parameter which takes away the cool GUT prediction for the strong coupling. Another alternative to supersymmetry is having other additional light particles at the TeV scale in the correct amount (for example four weak scalar doublets or something like that has been discussed before) which can very much improve the precision of unification in nonsupersymmetric models, and since there is more or less only a discrete parameter (the number of light new fields) it kind of is predictive and falsifiable, since those fields should in principle be observable.

I suppose the main reason why many people are scared away from such ideas is that one explicitely introduces new physics at the gut scale without solving the higgs hierarchy problem. That means that at the GUT scale, you really do have one parameter in your theory that has to be tuned to many digits precision in order to get a small electroweak scale.

Hope this helps

Cheers

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