How many scalars are needed in GUT theories?

[Niklas Nielsen]

Hello,

I don't know much about GUT physics, but I've been wondering whether these models usually breaks the grand unified symmetry to the standard model all at once, or multiple times at different energies. And in the case of multiple breakings, how many Higgs-like scalars are needed?

Thanks,
Niklas

 

[ChrisVer]

What do you mean by breaking guts to the standard model?
In general a bigger group (a gut) would break to smaller ones... Now how this breaking is going to take place (to which group you might end up), depends on the Higgs' vevs...
Of course there can be some vevs which would be "preferred" to fall into lower states, and thus you get something like the bigger group going to an unstable universe and then that universe "falling" on the Standard Model one... That's really a lot of work, and I am not really sure whether I explained it correctly, maybe someone else could do a better job in that and/or correct me.
How many Higgses you need, depends on the theory... for example SuSy can have 5 Higgs scalars (or 2 Higgs chiral superfields).
The problem with some GUTs is that we don't understand reason why, the EW scale breaking is far less than the GUTs scale breaking...

 

[Niklas Nielsen]

Thanks a lot for your answer.

The problem with some GUTs is that we don't understand reason why, the EW scale breaking is far less than the GUTs scale breaking...

I guess his is exactly what I'm curious about.. which mechanisms can let the EWSB happen at different a different scale than GUT breaking?

 

[ChrisVer]

The problem is generally referred to as "gauge hierarchy problem". You can try to work with the theory in a way that will keep your SM Higgs at EW scale untouched. For example in SU(5) GUT you have the gauge bosons coupling the Higgs in the adjoint representation (24dim) to the Higgs in the spinor representation (5dim) in 1st order loops- that can destroy renormalizability which needs coupling between them at tree level.
If you try to save renormalizability (by let us saying modifying the vacuum- adding corrections which can be negliged at EW scale) you get the problem of the gauge hierarchy again (you need extreme fine tuning to keep the SM Higgs at the EW scale).
SUSY appears to resolve the problem of Higgs fine tuning because (roughly speaking) it allows bosonic and fermionic contributions to the Higgs mass corrections, keeping it "low"...

 

[Mordred]

The SU(5) model has its own problems, but what GUT model doesn't lol. A potentially good model is the SO(10) GUT model. From what I can tell its gaining some strong support.

here is a review of the SUSY model
http://arxiv.org/pdf/hep-ph/9402266v5.pdf

here is a couple showing how SO(10) I'm currently studying this model myself. So don't ask me too many questions on it

http://arxiv.org/pdf/1003.6102v1.pdf
http://cds.cern.ch/record/392392/files/9907211.pdf

Chronos posted this paper in which

"Lawrence Krauss suggests SU(10) is favored by BICEP2 data" very short paper though
http://arxiv.org/abs/1404.0634

in all fairness here is a review paper on the SU(10)
http://arxiv.org/abs/1212.3407

the paper points out there is the SUSY SO(10) as well as the SO(10) another paper I have has a seesawII mechanism but I would have to dig that up atm I'm focusing on the single seesaw lol