## Thursday, December 31, 2009

### F(rench)-theory

Ok, everybody made speculations about the meaning of the F in F-theory. Possibly the most accepted one was that it was due to Cumrumm Va(F)a. But an article appearing now in arxiv has shown it's real origin.

The authors of the article are Adil Belha and Leila Medari. It is titled "Superstrings, Phenomenology and F-theory". the abstract reads:

We give brief ideas on building gauge models in superstring theory, especially the four-dimensional models obtained from the compactification of F-theory. According to Vafa, we discuss the construction of F-theory to approach non-perturbative aspects of type IIB superstring. Then, we present local models of F-theory, which can generate new four-dimensional gauge models with applications to phenomenology.

It is based on invited talks given by A. Belhaj in Oviedo, Rabat, Zaragoza.

Untill here nothing seems to support my claim of the explanation of the origin of the name. But if you go and see the paper, available in: http://arxiv.org/pdf/0912.5295 one finds that it is written in French. That explains all it ;-).

Fortunately I have a relatively good knowledge of French and I could make a quick reading of the article. It is a good introduction to the topic, from the very beguining explaining the basics of string theory, D-branes and all that. Later it explains the basics of F-theory, of local models and of local F-theory GUT models. All of it in a short article of 15 pages.

Despite the name it doesn't dive too much into phenomenology. But still it gives a good introduction to many aspects of the subject for non initiated people. In that sense it is far better than the blog entry of Jackes Distler about the first big paper of Vafa. And, definitively, it looks like a good chance for Spanish people people interested in the subject but not speaking English and maybe speaking French.

By the way, for those that didn't read the Spanish entry about the CDMS announcement just say that F-theory GUTS predicts that the LSP (lightest supersymmetric partner) is the gravitino, which is not a viable candidate for a WIMP. The CDMS two events finding (irrespective of how statistically significant it could be) is kind of a hint that the LSP is a WIMP (maybe a neutralino) so if confirmed the actual Vafa models of F-theory GUT would become invalidated. Possibly the experts on the subject could recook some aspects of the more phenomenological aspects of the theory (mainly the supersymmetry breaking mechanism) to fit the new data. But certainly the best aspect of the whole construction, reproducing the standard model and make concrete predictions, would go away.

But, as Vafa said in the strings 2009 conferences. That's the bad point of making predictions, that they could be invalidated.

If someone is interested in knowing it I must say that since the CDMS announcement I have decided to study in more detail what heterotic phenomenology can offer. It doesn't mean that F-theory is not interesting any more, but irrespectively of the CDMS I needed to pay more attention to heterotic theories. The CDMS is just a good excuse.

Also I am reading (and in some cases rereading) a lot of articles in black holes (stringy and not stringy ones). You can read about it in my other blog (if you speak Spanish). Still I guess that I will also talk about the subject in this blog in a near future, when I have finished reading carefully a few bunch of articles. For example, today there is an article about the subject of B-h creation in particle collisions: http://arxiv.org/abs/0912.5481.

Other interesting articles today in arxiv are: Unification of Residues and Grassmannian Dualities by Nima Arkani-Hamed, Jacob Bourjaily, Freddy Cachazo and Jaroslav Trnka. The article continuate the MHV program to give a twistorial technique to find scattering amplitudes. I must admit that although I recognize it's interest I am not following too much that developments. Still I think some readers can find it more attractive than me.

Also I would note two papers in dark energy:

Inverse problem - reconstruction of dark energy models

Abstract:

We review how we can construct the gravity models which reproduces the arbitrary development of the universe. We consider the reconstruction in the Einstein gravity coupled with generalized perfect fluid, scalar-Einstein gravity, scalar-Einstein-Gauss-Bonnet gravity, Einstein-$F(G)$-gravity, and $F(R)$-gravity. Very explicit formulas are given to reconstruct the models, which could be used when we find the detailed data of the development of the universe by future observations. Especially we find the formulas using e-foldings, which has a direct relation with observed redshift. As long as we observe the time development of the Hubble rate $H$, there exists a variety of models describing the arbitrary development of universe.

The F(R) theories of the subject refers to approaches where one consider gravity theories with terms in the lagrangian that contain higher order terms in the curvature that appear as counterterms in the renormaliztion program of conventinal quantum gravity (the theory actually is not enormalizable because of the need of infinite diferent terms). There was recently a good review article about the subject and if I have time to read it I will post about that kind of theories.

Also about dark energy is a paper by A. M. Polyakov: Decay of Vacuum Energy .

Abstract:

This paper studies interacting massive particles on the de Sitter background. It is found that in some cases (depending on even/odd dimensionality of space, spins, masses and couplings of the involved particles etc) the vacuum acts as an inversely populated medium which is able to generate the stimulated radiation. This "cosmic laser" mechanism depletes the curvature and perhaps may help to solve the cosmological constant problem. The effect is more robust in the odd dimensional space-time, while in the even case additional assumptions are needed.

Polyakov is a very original thinker, and despite that sometimes it's ideas seems a bit non conventional it always worth reading him.

Possibly there are more interesting papers in axiv today, but I'll stop here.

Good new year to all readers.

## Friday, December 25, 2009

### Relation betwen the Sokolov–Ternov effect and the Unruh effect

I have been disucisong in my other (and in the miguis forum) the proposal of Crane to use a black hole as an starship impulsor, bases on his arxiv article: ARE BLACK HOLE STARSHIPS POSSIBLE?.

You can read (if you understand spanish) the three post about the suject: 1 , 2 and 3.

While discusing that papers I have ben reading in wikipedia about it's litle brother, the Unruh effect.

As explained there in detaill that effect consist of the observance of thermal radiation by an acelerated observed of what is vaccum for an stationary observer. The temperature of the radiation is proportional to the aceleration: $$T=ha/4\pi^2ck$$ (k is the bolstman constant, the other quantities have their obvious meaning).

To my surprise in the entry is mentioned that there is a claim that the radiation has been observed. In particular it has been claimed to be observed in the Sokolov–Ternov effect: the effect of self-polarization of relativistic electrons or positrons moving at high energy in a magnetic field. The self-polarization occurs through the emission of spin-flip synchrotron radiation. and, in particular:

it was shown that if one takes an accelerated observer to be an electron circularly orbiting in a constant external magnetic field, then the experimentally verified Sokolov-Ternov effect coincides with the Unruh effect.

This results date back to 2005, so they are not new at all. And I am almost sure that they are controversial or someone would have a nobel prize for it ;-). The whole thing is that despite I try to be informed, I have no idea about it. Maybe other readers of the blog also were unaware of it and they could be curious to know.

## Thursday, December 17, 2009

### Dark matter live webcast

Ok, a litle bit late, but still something is going on:

Fermilab webcast in dark matter CDMSresults

Or, if you prefer you can watch the other simultaneous conference:

http://www-group.slac.stanford.edu/kipac/cdms_live.html

As I am posting late just tell that the main announcement has been already made, two events. That means not a definitive discovering, because of statistical considerations, but certainlly something. Now they are preciselly discusing exactly how significant this is.

Update: If you want to see a summary of the results by the CDMS team, get it here (it is a two pages pdf, without formulae, readable for most people).

Quick summary, as said in CF: if these events are interpreted as signal, the lower bound on the WIMP mass for these recoil energies is roughly 0.5 GeV.

I would add, a good guess (it gives the best possible cross-section) is a 70 GeV WIMP. DAMA claims of dark matter discovering, via inelastic dark matter (that is the WIMP has excited energy state) is compatible with CDMS results in a reasonable parameter range.

I invite you to read the entries on the topic in many of the blogs in my link list (and possibly many others). Although not a discovering there will be a lot of discussion about these results in the near future. And new results are announced for the future, when the new superCDMS would be working.

Update: You can see the recorded video of one of the conferences from this website: http://online.kitp.ucsb.edu/online/dmatter_m09/cooley/

The arxiv paper, still not submitted when I am posting this, is availabe here

There are some discussion in the blogs about the actual relevance of the signal. The most accepted one is a 1.5 sigma result. The discrepancies differ in how to actually consider the background. The data of 1.3 goes with the blinded background (optimized background obtained without knowledge of the existence of the signals). If one use other background one could get as much as (almost) 3 sigmas, or as few as 0. By the way, the very use of "sigma" is more appropriate for gaussian distributions, but it is used commonly for non gaussian ones with the appropriated corrections.

For the future I have read that before de superCDMS it is expected to have data from another experiment, the XENON100. They talk about "early in the 2010". It remains to see what "early" exactly means, and -more important- what the results are.

If one wants to read an easy introduction to the detailss of how CDMS works one can read this entry in the old tomasso dorigo blog. Be aware that Dorigo dosn't like too much supersymmetry and it argues that the (previous) CDMS result convince him a little bit more about that. Curiously he hasn't any entry about this new CDMS dataset.

I had not time to answer Matti to a question in the previous post. I leave here a link to his own view of these results as a compensation: http://matpitka.blogspot.com/2009/12/dark-matter-particle-was-not-detected.html#comments

## Tuesday, December 08, 2009

### Se rumorea que se ha descubierto la materia oscura

Pues si, pues sí. La famosa materia oscura que forma el noventa y tantos por ciento de la masa del universo cuya presencia se infiere por el comportamiento de la materia visible pero de la que no había evidencia directa parece que al final ha sido descubierta en uno de los numerosos experimentos de laboratorio que actualmente se dedican a su búsqueda.

En realidad hay un grupo experimental italiano que responde a las siglas DAMA que llevan un tiempo diciendo haberla encontrado. Pero por una parte su evidencia es un tanto circunstancial, habiendo hallado variaciones estacionales de cierto tipo de eventos posiblemente relacionados con algunos candidatos posibles a materia oscura. Por otro lado experimentos con una sensibilidad igual, o superior, al DAMA no han encontrado nada. E realidad hay diferencias sutiles entre los diversos tipos de detectores y es posible -pero muy improbable- un cierto tipo de materia oscura que sea detectable por DAMA y no el resto de detectores.

Pero no es DAMA lo que esta ahora en candelero ( a raíz de este post en el blog de Jesster, resonances) sino CDMS, siglas de cryogenic dark matter search. Este grupo ha puesto detectores en una mina de sodio enterrada profundamente en algún lugar de Minessota. En 2007 este grupo entrego un informe negativo dónde ponían unos límites experimentales a las características posibles que podía tener la materia oscura tipo WIMP (weakly interacting massive particles). Se esperaba que ya estuviese publicado el artículo con la nueva remesa de datos, más extensa y tomada con instrumentos de mejorada sensibilidad. Pero se han retrasado y han enviado l artículo a nature, y esta revista ha aceptado el artículo, lo que hace pensar que pueda ser importante. Nature es una de las pocas revistas que quedan actualmente que tiene un contrato de confidencialidad (o como quiera traducirse disclosure) y hasta el 18 de este mes no estará disponible el artículo. Posiblemente ese mismo día también haya otro artículo paralelo en arxiv (libre de descarga para todo el mundo por consiguiente).

Realmente esta sería una estupenda noticia para todo el mundo, excepto tal vez el físico de cuerdas Cunrum Vafa y colaboradores, que en los últimos dos años habían desrrollado un excelente y elaborado modelo basado en teoría de cuerdas que reproducía el modelo standard de partículas sin aditamentos exóticos comunes en otros modelos fenomenológicos, y, aparte, hacía algunas predicciones. Entre ellas que la materia oscura esta formada principalmente por el gravitino (compañero supersimétrico del gravitón), que no es una partícula tipo WIMP. Si se confirma el hallazgo habría que ver si pueden reacomodar su modelo para incorporar este hallazgo sin destruir el resto de características buenas de su teoría.

Por lo que yo tengo comprendido de la teoría F la mayoría de restricciones que utiliza para hacer predicciones se basa en su modelo de ruptura de la supersimetria. Allí usan un modelo de mediación gauge (una variante de algo conocido como modelo de guidicce-massiero usado en modelos de mediación gravitatoria), dónde el mensajero es un bosón asociado a una simetría gauge tipo Peccei-Quin,asociada al axión de la QCD. Es un modelo bastante minimalista dónde casi no hay "sector oscuro" supersimétrico y en ese sentido parece muy buena idea. Pero claro, si ahora deben acomodar un WIMP como partícula supersimétrica mas ligera deberían revisar las cosas -si ello es posible- y posiblemente ese mecanismo de ruptura de la supersimetría sea lo que mas se presta a ello. Otra posibilidad, que a mi me parece muy remota, es que ya que tiene un WIMP -el neutralino mas ligero (un neutralino es una combinación del zino, fotino y higgsino) como posible NLSP (aunque la mejor opcion es un stau)- tal vez haya un mecanismo extraño de decay que pueda llevar a que haya WIMPS sueltos por ahí, y que el gravitino siga siendo el LSP (y por tanto el componente mayoritario de la materia oscura). Por las características del CDMS no podría detectar el gravitino. En fin, estas son especulaciones rápidas, y posiblemente con mi aún paupérrimo entendimiento de esas partes de la teoría F quizás sean demasiado arriesgadas. Por si acaso le he preguntado a motl (que también ha posteado la noticia en su blog http://motls.blogspot.com/2009/12/cdms-dark-matter-directly-detected.html), y en su respuesta parece estar de acuerdo con de lo que yo digo.

Como quiera que sea, le pese a quien le pese, si realmente se ha descubierto la materia oscura estamos ante un acontecimiento histórico. Es más, podría tener consecuencias para el experimento del LHC pues posiblemente este debería ser capaz de producir esta partícula recién observada, y así tendríamos una doble confirmación (aparte de una guía muy exacta de como afinar los detectores del LHC, lo cuál hará mas fácil la detección).