Tuesday, 26 June 2012

String Phenomenology: Day 1

Here is a brief overview of the more interesting points of day 1. I will link to the abstracts of each talk; hopefully, in time, the same pages will also include the slides and video from the talks.

Ben Allanach kicked things off by describing some of the lates experimental results, and what they might mean for supersymmetry (SUSY) in particular (and hence for string model building, basically all of which is supersymmetric). His most important points (I think) were the following:

  • Discovery of a standard-model-like Higgs, with a mass of around 125 GeV, could be just around the corner. In many popular realisations of SUSY breaking, a Higgs mass of 125 GeV is right at, or just beyond, the maximum possible value, assuming that superpartner masses are kept below several TeV.
  • There is an unexplained anomaly in the Tevatron data, in the 'forward-backward asymmetry' in the production of top-anti-top pairs. The Tevatron collided protons and anti-protons, and this variable measures the number of tops which are produced travelling in the same direction as the initial proton, compared to the number travelling in the direction of the anti-proton. The measured value disagrees with the standard model prediction by something like $3\sigma$.
  • LHCb is beginning to measure certain rare meson decays for the first time. Ben talked in particular about $B_s \to \mu^+\,\mu^-$, which can be particularly sensitive to new physics contributions (since a bottom quark must annihilate with an anti-strange-quark, this is an interaction involving a 'flavour-changing neutral current'; these are highly suppressed in the standard model by the GIM mechanism).
  • The Daya Bay neutrino oscillation experiment recently made the first conclusive measurement showing that the neutrino mixing parameter $\theta_{13}$ is non-zero. This rules out, in particular, the popular tri-bi-maximal mixing scenario for neutrinos. To be honest, very few, if any, string models can make such precise predictions anyway, so although this is important, I doubt the audience here feels one way or the other about it.
Ben also has a couple of recent papers exploring R-parity-violating supersymmetry. This can apparently explain the forward-backward asymmetry, as well as effectively 'hiding' the superpartners for longer at the LHC, due to the lack of the classic missing energy signatures of supersymmetry with R-parity.

Bobby Acharya then talked about his work on 'generic' predictions from string theory, which I mentioned here. Their reasoning seems to start with the following argument:

  • In spontaneously-broken supergravity theories with moduli (scalars which are massless in the supersymmetric limit, and have Planck-suppressed couplings), the moduli cannot all be made significantly heavier than the gravitino.
  • All moduli must be heavier than about 30 TeV, otherwise they decay during or after big bang nucleosynthesis, and hence ruin its successful predictions. By the above point, the gravitino mass must therefore be at least 30 TeV. The gravitino mass is roughly $m_{3/2} \sim \frac{M_{\text{SUSY}}^2}{M_{\text{Pl}}}$, where $M_{\text{SUSY}}$ is the fundamental SUSY-breaking scale. So $M_{\text{SUSY}}$ is bounded by this argument.
The picture which apparently follows from the above is that sleptons and squarks have masses of tens of TeV, leaving only gluinos, charginos, and neutralinos at LHC-accessible energies.

The only clear way out of the above argument seems to be to have a late period of low-scale inflation, which dilutes the moduli and prevents them from causing problems with cosmology. However, Fernando Quevedo also seemed to think that the first point above may not be completely watertight. The overall reaction to the claims of Acharya et al seems to be scepticism, but I haven't heard any concrete arguments against them yet. Gordy Kane will be talking more about this on day two.


  1. Hi Rhys - looks like one line of your LaTeX code hasn't compiled properly. Or at least it doesn't display for me (in the bullet point beginning "All moduli must be ...").

    1. Thanks Sesh, it's fixed now. I was missing a closing bracket...


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