GUT viable w/o SUSY? Could GUT be physically wrong?

[ensabah6]

One argument in favor of SUSY is that without SUSY, the running gauge coupling unification do not meet for the three forces, but with SUSY they meet around 10^15 GEV. This implies that gauge coupling most meet for GUT to be viable. So without SUSY, the running gauge couplings do not meet.

If SUSY does not exist, then the SM gauging couplings do not meet. Does this imply that GUT are physically wrong?

[tom.stoer]

If SUSY does not exist, then the SM gauging couplings do not meet. Does this imply that GUT are physically wrong?
Afaik w/o SUSY the gauge couplings are calculated in some order in perturbation theory and are extrapolated over a huge energy range. So for me there is no reason why higher order corrections should not tune the running of couplings.

And as far as I understood GUT is not rules out b/c the couplings do not meet but because all GUTs cause the proton decay too fast.

[Haelfix]

A lot of this game is very sensitive to new physics and calculational sophistication between the electroweak scale and the GUT scale. Absent knowing that, its very hard to say much of anything exactly.

Keep in mind the couplings almost meet with just the standard model physics, so close that even if SuSY didn't exist, it wouldn't be a bad guess to assume that some sort of unification phenomenology takes over.

So of course, you have the famous selling point for the MSSM where the couplings unite almost perfectly. However I would say its oversold putting it that way, as its a little bit fishy b/c it really assumes very little new physics until the unification scale and could be purely coincidental that it seems perfect to us at this time given our limited knowledge of btSM physics. Instead I would say that SuSY has the attractive benefit that it tends to ameliorate coupling constant unification in a range of generic model(s) building. Thats still nontrivial, b/c a priori you might have expected the fit to simply get worse. That it doesn't is a good sign.