Monday, 2 April 2007

Surprise from Martin Schmaltz

We are done with April Fools hoaxes for this year. It is high time to resume boring theory blogging. I have a few delayed reports in store...

Martin Schmaltz flashed through CERN two weeks ago. Whenever he's around, there is action. The other day, when he talked about Little Higgs at CERN, somebody from the audience called him a little taliban. No wonder that, this time, nobody at CERN TH gave him a seminar slot. Luckily enough, he was adopted by unsuspecting experimentalists from the EP seminar.

Martin came with a talk MSSM scalar masses and hidden sector interactions. Although this title seems well-behaved, a look at the abstract promises a lot. Martin wrote ...prediction [concerning soft scalar masses] from commonly used spectrum codes (such as ISAJET, SPHENO, SOFTSUSY, SUSPECT) may be completely wrong... He bothered to enumerate all the popular codes in order to annoy a maximum number of people.

If the Minimal Supersymmetric Standard Model (MSSM) is discovered at the LHC and precisely measured at the ILC, then we will learn about the superparticle masses. We expect that those masses are determined by a high energy theory which is more fundametal than the MSSM. One possibility is that the high energy theory is a string theory. Thus, by measuring the MSSM parameters we could get a glimpse of the fundamental theory. If we are extremely lucky, the LHC and ILC measurements could point to some specific realization of string theory.

The relation between masses measured in low-energy experiments and the fundamental parameters is not straightforward. In colliders, we would measure the MSSM parameters at the 1 TeV scale. On the other hand, the fundamental theory directly predicts the mass parameters at the high scale, for example at the GUT scale 10^16 GeV. The two sets of parameters are related by renormalization group equations. Relating the low- and high-energy parameters is theoretically so important, that the task has been automatized in the publicly available codes mentioned before. The nice thing is that, within the standard MSSM paradigm, the renormalization group equations depend only on the quantities that can be measured at low energies. Therefore unambiguous predictions of high-energy parameters from low-energy inputs can be made. Or so it seemed.

In his recent paper, Martin pointed out that the relation between the high- and low-energy parameters can be obscured by the effects related to supersymmetry breaking. He claims that the renormalization group equations for the MSSM scalar masses contain a potentially large contribution that has been overlooked so far (the gaugino masses are not affected). This contribution comes from self-interactions in the hidden sector that is responsible for spontaneous supersymmetry. It has been assumed so far that this effect can change only the overall normalization of the scalar masses. Martin showed that this assumption is in general wrong and gave examples where the new effects dominate the standard MSSM contributions. The masses of the hidden sector particles are typically of order 10^11 GeV, so we will have no experimental access to the hidden sector parameters. Therefore collider measurements of scalar superparticle masses may reveal nothing about the fundamental theory.

According to Martin, a string theory fan after listening to his talk would look like this:
This is an artistic vision. However, there is still a large class of string models where the hidden sector renormalization is not dangerous. This is the case when supersymmetry breaking is mediated by light moduli, which includes the beloved KKLT. The moduli interact weakly, with gravitational strength, therefore they cannot mess up the MSSM renormalization group equations. On the other hand, in the case of dynamical supersymmetry breaking a-la Seiberg, the hidden sector interacts strongly and large effects are difficult to avoid.

For a comic book version of the story, have a look at the transparencies. For technical details, you should consult the paper.

1 comment:

Anonymous said...

In old fashioned string theory GUTs, the
different matter particles which form a
16 of SO(10) or 10 + 5 bar of SU(5) do not come from the same "points" in the extra dimensions anyway; unification occurs at the Kaluza-Klein scale and there is Wilson line breaking to 3-2-1 right there. So
Martin's very interesting work doesn't make those models look much different. It affects purely field theoretic models where over some range of scales one would have had e.g. SO(10) with the pieces of a 16 all coming from the "same" place. In Martin's story, the gauge couplings still unify, so one still sees a hint of unification, in any case, if it occurs.