Monday, January 19, 2009

Hep-th papers on Monday

There are ten pretty interesting hep-th papers today.


The last one, by Frederik Denef, Mboyo Esole, and Megha Padi, elaborates on a very clever way to look at type IIB orientifold compactifications: translate them into black holes. To do so, they compactify the Universe on a three-torus, T-dualize all of it, and end up with a black hole in a Universe where a left shoe may become a right shoe after a round trip. ;-)

This allows them to count the vacua via the black hole entropy - they're able to say e.g. that a sector of compactifications contains 10^{777} vacua - and study some of their detailed properties, including a new orientifold variation of the OSV identity (which doesn't involve any squares of the partition sums). Very interesting.

All classical N=8 SUGRA amplitudes

The first paper, by J. Drummond, Mark Spradlin, Nastya Volovich, and Congkao Wen explains a recursive algorithm to calculate all tree level amplitudes in the N=8 supergravity: quite a powerful technical result (although the experts in this sub-field usually care about loop amplitudes more than they do about complicated tree-level ones). They build their new comprehensive algorithm on a similar recent algorithm for N=4 Yang-Mills theory: the only novelty is that some invariants have to be squared and new dressing factors have to be inserted.

Universal, inflating N=1 SUGRA

There are actually three papers today that deal with the N=8 SUGRA. The second one I mention, by James Gates and Sergei Ketov, argues that one chiral superfield coupled to the minimal flat space supergravity is equivalent to a higher-derivative supergravity built from the chiral curvature superfield. That allows them to view the early inflation and the present acceleration by the dark energy to have mathematically isomorphic roots - roots that they also try to trace to a dilaton-axion stringy origin.

3D toy model of N=8 SUGRA as a TOE

The third paper where the N=8 SUGRA is important was written by Jean Alexandre, John Ellis, and Nikolaos Mavromatos. They study the emergence of various composite fields in three-dimensional coset field theories - with holons and spinons being the elementary building blocks - and argue that these mechanisms are also important for qualitatively new physics of the N=8 SUGRA in four dimensions that may be relevant for its application as a "TOE", a statement that is surely both both exaggerated and somewhat obsolete.

Predicting a wrong, negative C.C.

George Ellis and Lee Smolin have arguably submitted the first paper co-authored by the second author that I could agree with, even though all the content can be summarized in the following sentence: if there are infinitely many semi-realistic vacua with the cosmological constant clustered around minus epsilon - and no counterparts with a positive epsilon, as suggested by some recent papers e.g. Shelton-Taylor-Wecht -, then it is fair to say that the weak anthropic principle (apparently incorrectly) predicts that the cosmological constant is negative.

Well, there are at least three facts that make this trivial (and probably obvious to many experts, because whether or not string theory predicted or predicts a negative C.C. has been discussed for years: of course not) conclusion weaker than tea. The stability and the physical character of the Shelton-Taylor-Wecht vacua has not really been established; it has not really been established that there are no corresponding positive-C.C. vacua; and the weak anthropic assumption is wrong which means that even an infinite multiplicative underrepresentation of a class of vacua doesn't kill it. ;-)

AdS/QCD: quark-gluon plasma

Johanna Erdmenger, Matthias Kaminski, and Felix Rust study an N=4 gauge theory coupled to N=2 matter, looking for mesons etc., and they claim that their results about their spectrum (and widths) are relevant for the quark-gluon plasma regime of ordinary QCD.

Dimensional reduction of monopoles

Brian Dolan and Richard Szabo consider the dimensional reduction of compactifications with spheres and focus on the effect of the reduction on the magnetic flux through the sphere, especially on the magnetic monopoles. They look at the Kaluza-Klein tower and its Yukawa interactions and make some tools more controllable by switching to a fuzzy sphere instead of the ordinary one.

No fixed points in Yukawa systems

Holger Gies and Michael Scherer study the UV properties of some toy models of the Higgs mechanism with various fermions and Yukawa couplings. They use the term "asymptotic safety" even though IMHO it should be reserved for the (unlikely) existence of UV fixed points in gravity. They show that some models admit no non-trivial UV fixed points.

QFT on quantum graphs

E. Ragoucy computes properties of a "quantum field theory" on graphs - which should probably be called "quantum mechanics" only, using the standard jargon. The author calculates physical properties including scattering amplitudes and conductance.

Orientable objects?

D. Gitman and A. Shelepin discuss "orientable objects" associated with some fields on the Poincaré group. I don't understand their point and the meaning of their "objects" but frankly speaking, I tend to doubt that dozens of pages that seem to be struggling with some elementary facts about the Poincaré group and spinors contains something really new. If I am wrong, some of their unexpected conclusions sound rather sharp - for example, you need ten quantum numbers to describe an orientable object. ;-)

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