String theory is good for...phenomenology of particle physics

Yesterday the number of visits to this blog had a major increase. Most of the traffic came from this post in Miguis web/blog.

The post was a translation to Spanish of an article in new scientist about the good points of string theory. I had seen a discussion of that article in Lubos blog, concretely here.

Well, that article comes to say that string theory is nowadays a good theory because it´s math structure, through the AdS/CFT correspondence is useful in QCD and condensed matter physics. Well, I don't know too much about that applications but if the experts in that subjects say so is a good sign.

But, actually, I don't think that that image is quite right nowadays. Readers of this blog know that I have played attention to many alternative theories. Some of the proponents of that theories make claims against string theory. Others, who don't actually offer any theory, that is, Peter Woit, claims that string theory "makes no predictions". In his blog he usually bring attention mostly to the most speculative articles written by string theorists.

Well, I am following, as much as I can, the actual F-theory minirevolution. Doing so I have become very surprised, and impressed, by how close string theory has become of actual physics. Before going into it I must say that I somewhat understand the sceptics in string theory. If one reads the books on the subject one certainly gets the impression that actual predictions are far away. For example the 1999 (that is, not too old) book of Michio Kaku Introduction to superstrings and M-theory in its chapters about phenonenology show the results of heterotic compactifications. In that results the best one could get were the standard model plus some additional U(1) factors. Also it was stated that to achieve the right number of generations , given by n=1/2X(Cy), that is, one half of the Euler characteristic of the Calabi-Yau mainfold, was difficoult (if not almost impossible).

Other books, as Polchinsky´s two volumes book and the Clifford Jones "D-branes" don't say too much about realistic compactifications. There are good reasons for that. The books are mostly concerned about the D-brane revolutions and its consequences, the black hole entropy calculation and the AdS/CFT conjecture. The most recent book of Becker-Becker-Schwartz makes more in deep cover of compactifications. But , with a good criteria, somewhat cares more about technical issues such as the moduli space of the compactification, mirror symmetries among type II A and type II B, and flux compactifications, which are relevants for the very important issue of moduli stabilization and the KKLT like models (related to the landscape). And , of course, the all make introductions to dualities, M theory, and , to a least extent, F-theory.


In fact all that are important technical aspects, and it requires time to learn them (one must read some articles if he really wants to properly understand some aspects). But one gets the impression that everything is still to far from LHC phsyics and cosmology testable predictions. In fact there is a very recent book, by a Michel Dine which goes into phenomenology title "supersymmetry and superstrings". I must say that I find that book somewhat failed. It is to brief covering subjects that even with some previous knowledge are hard to appreciate properly.

Well, in definitive, a lot of text books and no a clear signal of actual testable physics. Certainly discouraging. Divulgative books are not too different. That, certainly, can explain why some people has the impression that string theory is far from it's objectives. Blogs from string theorists try to say, to whoever listen them, that string theory is "the only game in town". In fact there are not many blogs in string theory with a decent publication rate. However I had also the idea that string theory was far of phenomenology, and I had not purchased too mcuh that topic.

F-theory minirevolution has changed that. I have read at last the two big pioneer papers of Vafa (arXiv:0802.3391v1 and arXiv:0806.0102v1), and almost completed the reading of the F-theory Guts cosmology (arXiv:0812.3155v1). Also I have made partial readings of some subsequent papers, and a few previous papers needed to understand the formalism developed.

Certainly are hard to understand papers. But once one gets familiar with them one sees what kind of physics is discussed. The first thing to say is one need to know the details of GUT's and symmetry (and supersymmetry) breaking. F-theory local models, with the right decoupling from gravity, can give an SU(5) model, without any exotics. They offer it's own way to break SU(5) into the MSSM, through an U(1) flux of hyperchrge, That mechanism avoids some of the problems presents in purely field theoretic models. In particular they can avoid problems with the observed lifetime of the proton. Ulterior papers get values in the CKM matrix that are good to get the observed asymetry oof baryons in universe. They offer ways to advoid the singlet-tripplet spliting problem of GUT's (That is, requiring the existence of Higgs doublets (1, 2)±1/2 leads necessarily also to color triplets. However, there exist rather uncomfortable lower bounds on the mass of these triplets). They offer a natural way to get small neutrino masses. In cosmology, trough a late decay of the saxion (whose lifetimem is predicted, that is, properly bounded, by the theory), they can avoid some of the problems that symmetry breaking bring to cosmology (the gravitino problem) and gives a righ way to obtain reheating after an inflactionary phase and some extra things that I haven't finished to read.

As you can see these models are quite near the cutting edge phenomenology. They offer solutions to problems not available by other approaches. And F-theory is not alone. Seemingly M-theory is going also into the local models + gravity decoupling business, see for example the paper Hitchin’s Equations and M-Theory Phenomenology by Tony Pantev and Martijn Wijnholt.

As I said I hadn't followed previously phenomenology with too much attention. But, in fact, more traditional approaches also had made some advances. For example this 2008 short review article of heterotic compactifications, From strings to the MSSM also cares about some of the previously mentioned aspects.

Another very recent paper, Towards Realistic String Vacua From Branes At Singularities, by Joseph P. Conlon, Anshuman Maharana, Fernando Quevedo, use the D-brane approach to phenomenology, not related to the gravity decoupling approach. They offer the bonus of moduli stabilization (something more habitual in cosmological models). In the abstract the conclude saying: "We propose that such a gauge boson could be responsible for the ghost muon anomaly recently found at the Tevatron’s CDF detector". Well, there some serious doubts about the real existence of that anomalies (see the tomasso dorigo´s blog, linked in this web site, and search for discussion of that topic).

Well, certainly there are a few bunch of models inspired by string theory, and not all of them (if any) can be truth at once. Also not all models make firm predictions. But the point is that they are actually reproducing the MSSM, GUT´s supersymmetric models, and mechanism to enhance the purely particle physics models. Also , in cosmology there are many different points where string is enhancing purely field theoretic models.

But, such as I see it, string theory is actually dictating the construction of (at least some of) the models that are going to be checked in the near future. Also one must not forget about the RS models, inspired by string theory, where one could get black holes in the LHC (that models possibly are not compatible with F-theory GUTs).

With all of this I think that string theory is doing exactly what one would expect from a traditional fundamental theory of physics such it has been made traditionally. Certainly I am talking about very, very, recent developments, most of them from this year and the previous one. But, anyway, it looks like if string theory is definitively "landing" into experimental physics, that is what it was expected from it. And, still, it is doing progress into clarifying it`s theoretical aspects, and the description of black holes (a topic not too easy to study in laboratory, except if LCH produce black holes, that is).

I am not at all a radical and I understand if some people wants to keep doing alternative approaches. The point of this post is to say that, as far as I see, the "not even wrong" criticism of string theory doesn't make too much sense nowadays.

And please, remember that I am not in a faculty position getting money from doing research in string theory. I have no economic, doctrinal or political reason to favour one theory or another. It is just that, according to what I know just now, string theory seems a perfectly good theory for doing high energy physics, and I have tried to explain why.