# Testability of GUTs at the LHC

1. Jan 24, 2009

### Orbb

Hello,

my understanding of particle physics is very limited. I know that several GUTs involving various symmetries have been proposed. My question is, wether experiments at the LHC can help to rule out or even verify some of the proposed GUTs? Maybe someone has some specific answers. Thank you!

2. Jan 24, 2009

### Haelfix

Doubtful, the usual hypothetical GUT scale is at very high energies, near the Planck scale, many orders of magnitude away from what the LHC probes. The best you can do is get better experimental precision on the standard coupling constants (Electroweak, Strong) that way we can refine the interpolation when we run them down to that scale to see if they unify. Right now, they nearly hit but not quite (with Supersymmetry its even closer).

Barring that, we need better precision on the bounds of potential proton decay (a typical prediction of GuTs is that indeed protons do decay after a very long time) but the LHC won't help us there directly either.

Still sometimes there are roundabout ways of getting there, depending on which physics we see or don't see but I wouldn't count on it.

3. Jan 24, 2009

### bomanfishwow

I respectfully disagree with the post above. Many GUTs include decompositions such as (and as a very simple example):

$$SO(10) \to SU(5) \otimes U(1)_{GUT} \to SU(3) \otimes SU(2) \otimes U(1)_{Y} \otimes U(1)_{GUT}$$

The new U(1) would exhibit itself as a new neutral current process (so called Z primes). There are further decompositions where one finds new SU(2) x U(1), implying W primes as well as Z primes.

There are people actively looking for these things in various channels of potential interest, and some models provide striking signatures. Now, we will either make some exceedingly exciting discoveries or rule out certain models and model parameters.

4. Jan 26, 2009

### Haelfix

I agree that the discovery of say tev neutral current processes *might* shed some light on GUT processes if we are really lucky, but its far more probable a hadron collider won't be able to illuminate the specifics. The inverse problem is in full effect there since a number of potential non GUT models contain them. Afaik, they are hard to disentangle.

5. Jan 26, 2009

### bomanfishwow

Depends on just how much you want to disentangle. It's fairly easy (given enough integrated luminosity) to measure the spin and couplings of any new resonance from the decay kinematics, and this constrains things hugely. I.e. Spin 1 implies a new U(1) like thing, Spin 2 implies (amongst other things) KK models, same-sign events can imply exotic models with doubly-charged resonances etc etc. You can also add in further evidence - displaced decay vertices can imply models with B-L symmetry, boosted decay products can give further handles etc etc.

The argument is, of course, that a linear collider will be needed to really probe any new structure, but we'll definitely be able to say more than 'It's a bump at 1.5 TeV' if we find something.