Tuesday, April 22, 2014

Two string pheno papers

I hope you have survived the Easter if you had to undergo one. There are at least two interesting hep-th papers on string phenomenology today. Alon Faraggi wrote a 35-page review
String Phenomenology: Past, Present and Future Perspectives
which focuses on the old-fashioned heterotic string model building, especially the free fermionic ones. Those were the first research direction that convinced me more than 20 years ago that it had everything it needed to have to become a TOE.

Faraggi doesn't discuss inflation at all and it's questionable whether good inflation scenarios have been studied within the compactifications he prefers. That defect of his paper is more than compensated by the other paper I want to mention.

Luis E. Ibáñez and Irene Valenzuela wrote a paper on a realistic stringy explanation of the primordial gravitational waves apparently spotted by BICEP2,
The Inflaton as a MSSM Higgs and Open String Modulus Monodromy Inflation
The Higgs boson and the inflaton – two key players of recent experimental discoveries – are the two fundamental scalar fields in Nature whose existence is supported by the experimental data. The idea that they could be the same is very intriguing. However, the minimum incarnations of this sort seem to be excluded, especially after BICEP2, or they have severe problems, to say the least.

Ibáñez and his graduate student look at a slight modification of the minimum scenario. The inflaton isn't quite the light Higgs. Instead, it is one of the other Higgses and/or their superpartners. This heavy scalar – whose mass seems to be \(10^{13}\GeV\) if we want to realize the BICEP2 observations by the simplest Linde's quadratic potential – changes by trans-Planckian values as a field. The reason why it can do so is a special example of the axion monodromy inflation that Eva Silverstein has described a month ago. They argue that the appropriate scalars could be a modulus in heterotic \(\ZZ_{2N}\) orbifolds or an open-string D-brane modulus in type IIB orientifolds/orientifolds.

As you can see, they are avoiding the assumption that the superpartners must be near the \(\TeV\) scale that is accessible by the LHC. String theorists have always had mixed feelings about this question because the main reason for supersymmetry's existence according to string theory is much deeper and more fundamental – closer to the Planck scale – than some particular technical problem at a particular low-energy scale that would just happen to agree with the current collider scale. Phenomenologists tend to think that keeping the Higgs light is the key raison d'être for the SUSY's existence; string (formal) theorists such as these two authors prefer to think that SUSY has more important tasks before that – like stabilizing the Higgs potential (which doesn't imply that the Higgs has to be light).

In this Spanish scenario, the inflaton is as heavy as the SUSY breaking scale which is still about 3 orders of magnitude lighter than the GUT scale, \(10^{16}\GeV\), where they also approximately place the string scale and the compactification scale – and the numbers seem to make sense.

I think it's a good idea not to be excessively constrained by the phenomenologists' prejudice that SUSY had to be valid up to very low energies. The most natural picture suggested by string theory – and also by the experiments, including BICEP2 and the light Higgs mass near \(126\GeV\) that they seem to be compatible with – could be very different and hint at a SUSY breaking at an intermediate-to-high scalar not terribly far below the GUT scale. As you can see in this application of the SUSY scale to inflation, this high mass of the superpartners does not mean that SUSY has no implications for the experiments and observations.


  1. Must be the first time a Spanish work is reviewed here, I'm glad for it :) I studied at that university [where the authors of the paper come from] in the late eighties (and early nineties), and I remember very well that department of Theoretical Physics (at Universidad Autónoma de Madrid) had already deployed a team working on String Theory, I think they were among the first in Europe and certainly they were the first in Spain. I guess, I would like to think, that they have produced some good results since that time.

    I have seen that Luis E. Ibáñez has a book (String Theory and Particle Physics: An Introduction to String Phenomenology) with good reviews (e.g. from Juan Maldacena: "A clear exposition of the main ideas and ingredients necessary to connect string theory to the real world. An essential toolkit for the string theory model builder"). Any thoughts? Might be a good text for people coming from fields such as applied physics?

  2. Mind-boggling job mates, I take pleasure in longing your articles.bubblegum
    casting reviews

  3. What does exophopic in
    "exophobic heterotic-string vacua" ?

    I like it how nicely the things we have learned about inflation and the higgs and other things can be incorporated into string theory :-)

  4. Hi Dilaton! They seem to work very well.

    "Exophobic" means "containing no [approximately] massless exotic states". Exotic states are particles and fields outside the Standard Model - more often, people mean a more special subclass of particles and fields that don't fit into GUT (SU(5)) multiplets, usually because they seem to have fractional electric charges. Roughly speaking, "exotics" and "having fractional charges" are nearly synonyma. "Phobia" means a pathological fear of something so "exo-phobia" is a vacua that despises exotics. ;-)

  5. "The most natural picture suggested by string theory ..." What does such a picture predict for the precise number of new particles that need to be added to the SM?

  6. Dear David, in string theory - a consistent theory of quantum gravity - the number of particle species is strictly infinite.

    The number of particle species with masses below a bound M is always finite.

    Most of the very heavy particle species are black hole microstates. If the string coupling is sufficiently low (much smaller than one), there is an intermediate regime of particles that are not yet black hole microstates but they are excited string modes. Their number is also infinite in the free-string approximation. However, for too excited strings at nonzero couplings, self-gravity makes it invalid to use free string theory and the right interpretation of all these localized states is in terms of black hole microstates. The weaker coupling, the higher the free-string intuition may be used.

    All the finite numbers describing the particle spectrum and mentioned above depend on the compactification of string theory which is not yet fully known. But in virtually all types of studied compactifications, one may derive (even in practice) the number of light enough particle species and many more of their detailed properties.