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.