Experimental support of string theory?

[PAllen]

I'd like to hear what people have to say about the following paper, which is way beyond my level knowledge, but could be exciting(?):

http://arxiv.org/abs/1104.2302

 

[suprised]

I'd like to hear what people have to say about the following paper, which is way beyond my level knowledge, but could be exciting(?):

http://arxiv.org/abs/1104.2302

Thats actually quite simple to explain. There are some string models where the string scale is very low, like near the weak scale, so that the level spacing of the string resonances is small. Naturally one can see these resonances if their mass is small enough, that's the essence.

However, there is absolutley no reason why the string scale should be so low, it is only a remote possibility. So only if we were _extremely_ lucky, one could directly test strings in this manner.

 

[fzero]

The idea here is that there are classes of string models in which the Standard Model fields are entirely realized on a collection of a few D-branes. While the standard model gauge group is $$SU(3)\times SU(2)\times U(1)$$, D-branes have $$U(N)$$ gauge groups on them (if we include orientifolds, we can also realize $$SO(N)$$ and $$Sp(2N)$$ gauge groups). Therefore the $$SU(3)$$ color will actually come from a $$U(3)$$ group in a model which uses a minimal number of branes. It is easy to imagine an extra Higgs mechanism that breaks $$U(3)\rightarrow U(1)_3 \times SU(3)$$.

In order to figure out the spectrum of abelian gauge bosons, we should list all of the $$U(1)$$ groups that we have. These are

$$U(1)_3 \times U(1)_2 \times U(1)_1 \subset U(3)\times SU(2)\times U(1)_1$$

where $$U(1)_2 \subset SU(2)$$ is the diagonal subgroup. Let's denote the gauge boson of the $$U(1)_i$$ group by $$B^{(i)}_\mu$$. In the normal electroweak symmetry breaking, the photon is a linear combination

$$A_\mu = \sin\theta_W B^{(2)}_\mu + \cos\theta_W B^{(1)}_\mu,$$

while the Z is the orthogonal combination

$$Z_\mu = \cos\theta_W B^{(2)}_\mu - \sin\theta_W B^{(1)}_\mu.$$

Because of the new $$U(1)_3$$, in the brane models we now have 4 gauge bosons consisting of linear combinations

$$\tilde{Z}_\mu = \sin\vartheta_Z B^{(3)}_\mu + \cos\vartheta_Z Z_\mu$$

$$\tilde{Z'}_\mu = \cos\vartheta_Z B^{(3)}_\mu - \sin\vartheta_Z Z_\mu$$

$$\tilde{A}_\mu = \sin\vartheta_A B^{(3)}_\mu + \cos\vartheta_A A_\mu,$$

$$\tilde{Z''}_\mu = \cos\vartheta_A B^{(3)}_\mu - \sin\vartheta_A A_\mu$$

Up to labeling conventions, we get a massive gauge boson which we can identify with the physical Z, another massive gauge boson which is a Z', a massless gauge boson which we identify as the physical photon, and a 3rd massive gauge boson which we call Z''.

By exploring the parameter space, one of the bosons that I've labeled Z' or Z'' is being compared with the possible new observed particle.

I think it's interesting, and very much in the spirit of theoretical particle physics to explore all explanations of new physics, however remote. I also agree with surprised that low scale string models are only one small part of possible string vacua and therefore unlikely to be physical. But it would be extremely interesting to find evidence for them, since that would mean that other string physics might be within reach of the LHC.