## Thursday, 10 May 2012

### Preprint roundup

The sheer number of new preprints which are listed every day on the arXiv makes it very difficult to read everything which might be of interest, and harder still to write blog posts about every paper which one might wish to discuss! So today I offer a short list of recent papers which have grabbed my attention, with little in the way of commentary (and in no particular order):

• First a bit of self-promotion. I have a new paper out this morning, about Dirac gauginos in F-theory. It really only takes the first basic steps in the study of such models, but I'm hoping it rouses some interest in them. Dirac gauginos are one way in which reasonably natural low-energy supersymmetry might be 'saved' from the null results reported so far from the LHC, but as far as I can tell, they have received zero attention from the string model building community until now. Believe it or not, Luboš beat me to reporting this.

• Also released this morning was a study by a number of authors, of putative string vacua containing anti-branes in a warped throat. This has been a widely-accepted way to break supersymmetry at a reasonably low scale, and in a reliable way, since the so-called KKLT paper. The new results seem to throw doubt on the whole idea (although I'm no expert in this subject, and I only skimmed the paper). Usually, the anti-brane is treated in the probe approximation, where its backreaction is ignored, and in this formalism, it is found that the apparent singularity of the geometry is resolved by 'polarisation' of the anti-brane. The authors claim that this cannot occur if the backreaction is properly taken into account, making the existence of these type of vacua somewhat more doubtful.

• Last week brought a review of the string landscape, by Mike Douglas. It contains the telling line "By now it is almost a truism that string theory makes no definite predictions for LHC physics…", which is refreshingly honest, as I don't hear too many string theorists admit this in public. He attempts to give a careful discussion of what, in his opinion, string theory can tell us, as well as what the landscape says for the prospects of finding supersymmetry. I hope to write a more detailed post about the predictivity/utility of string theory in the future.

• One way out of the stringent bounds which the LHC has already placed on supersymmetry is to assume that the first two generations of squarks are very heavy (thus evading detection), but the third generation ones are light. The third generation squarks are harder to find, but they are the ones which are important for naturalness of the Higgs sector, since the third-generation Yukawa couplings are the largest. Still, the (relative) heaviness of the Higgs boson (assuming it really is at about 125 GeV) means that the stops must be quite heavy, ruining naturalness, unless there is a large stop mixing. A new paper claims that this large stop mixing arises automatically from the renormalisation group flow if we assume that the first two generations of squarks are heavy. This is nice, as it doesn't have to be built in separately.

• It didn't take long for the first model to appear which explains the gamma ray line apparently seen by Fermi.

1. Hi Giotis. There is a link at the bottom of the main page to a post feed, and a link at the bottom of each comment section to a comment feed. They are in Atom format, but you can apparently just add ?alt=rss to the end to get RSS. So this should be what you want:

I apologise that I'm not too savvy when it comes to this stuff. I don't use RSS/Atom feeds myself.

2. Thanks Rhys...

I was searching for RSS feeds only since I didn't know the Atom feeds. Nice to know...

Now regarding the geometry singularity in the KKLT anti-brane model although I'm not a physicist I can imagine that these are not good news for the multivesre idea in the context of String theory. The multiverse and the anthropic principle requires an abundance of de Sitter vacua to solve the CC problem and it seems that dS vacua are hard to find in String theory. I was watching a video of Strings 2011 where Gary Shiu, while presenting his work on the scarcity of dS vacua in IIA, was asked by Linde to tell if this was the case for IIB too (having in mind the KKLT model). Shiu left the question unanswered I guess mainly because the inclusion of non-perturbative effects was not covered by his latest work.

In any case I can imagine that if it is found that dS vacua are very rare in IIB too this would have severe consequences for the multiverse paradigm.

3. dS vacua are indeed difficult to find. But that doesn't necessarily mean that they're not out there — most attempts to construct them start with a supersymmetric compactification, and then perturb it to break supersymmetry and raise the cosmological constant to a positive value. It might be that there are plenty of intrinsically non-supersymmetric vacua, but constructing these (and verifying their consistency) is much much harder.

I have somehow missed that the debate over the consistency of these solutions has been ongoing for some time. Joe Conlon tells me that there was a workshop a couple of weeks ago to discuss exactly these issues, and that opinion is still divided.

It's probably clear, but just to be sure: I'm far from an expert on this stuff.

2. You can obtain dS vacua without using anti branes, it's not really a problem. One example is the G2 construction of Acharya et al, where dS vacua arise when SUSY gets spontaneously broken by F-terms of the hidden sector matter field. Of course, there is no explicit G2 manifold example where this scenario has been implemented but the construction seems to be general enough that it should not matter.

1. I've never really looked into the details of the $G_2$ stuff, so I can't comment on that. But either way, I think it's a pretty big deal if these uplift mechanisms don't work, if only because they have been the dominant paradigm for 'realistic' string model-building for many years now.

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