There are three Standard Model constants that govern how strong the three Standard Model forces are and they vary in an exactly known way with energy scale. In the Standard Model as we know it, however, all three of these constants never take the same value at the same energy scale.

It is widely claimed that in Minimal Supersymmetric Standard Model (MSSM) which is one of the most studied version of Supersymmetry (SUSY) that there is an energy scale called the GUT scale at which all three coupling constants that govern the strength of the three forces of nature (other than gravity) do take the same numerical value (which is dimensionless).

The presentation cited below, at pages 38-40 (see also pages 57-59), however, argues that the widely touted claim that the MSSM gives rise to gauge unification isn't accurate. As Woit comments:

It is widely claimed that in Minimal Supersymmetric Standard Model (MSSM) which is one of the most studied version of Supersymmetry (SUSY) that there is an energy scale called the GUT scale at which all three coupling constants that govern the strength of the three forces of nature (other than gravity) do take the same numerical value (which is dimensionless).

The presentation cited below, at pages 38-40 (see also pages 57-59), however, argues that the widely touted claim that the MSSM gives rise to gauge unification isn't accurate. As Woit comments:

The question recently came up here (see this posting) of how good the SUSY GUT coupling constant unification prediction is. At a recent summer school lecture, Ben Allanach says the prediction is off by 5 sigma, i.e. that if you try and predict the strong coupling at the Z mass this way, you get 0.129 +/- 0.002, whereas the measured value is 0.119 +/- 0.002. Someone should tell Frank Wilczek…

In terms of "beauty" gauge unification is one of the most compelling arguments for SUSY, so this is a big deal.

Indeed, at this point it may take smaller tweaks to the running of the Standard Model constants for them to unify near the GUT scale than it does for the MSSM to produce this result.

Also, any theory that results in exact gauge unification in a pure GUT model without gravity almost certainly has to be wrong, because the addition of quantum gravity into a theory of gravity with its new force carrying boson, the graviton, necessarily tweaks the running all of the other experimentally determined constants in the Standard Model or any GUT that you want to devise.

## 6 comments:

It would make more sense that Allanach meant SM, not MSSM. Someone should ask him.

Mitchell can you verify what Allanach meant by contacting him?

@Mitchell Feel free to read to linked presentation (although I think that the statement quoted by Woit isn't in there verbatim and may have been from an oral part of the presentation). But, I don't think that it makes sense that Allanach meant SM, because everyone has always known that the SM coupling constants don't unify so it wouldn't make sense to say that a prediction didn't hold - there is not GUT scale to calculate back from in the SM. The MSSM coupling constants are widely expected to unify but it would be notable if they didn't.

andrew if Allanch is correct, does this falsify class of SUSY and GUTs that require the couplings to unite at a single numerical value?

@noe Yes. But, in many classes of SUSY theories and GUTs the gauge unification presents as a "coincidence" rather than a simple corollary of the theory's basic principles.

so without SUSY and GUT,

what happens at around 10^15 gev between the strong and electroweak force?

Post a Comment