## Tuesday, June 01, 2010

### A generous hep-th Tuesday

On Tuesdays, the arXiv brings us papers that were submitted on Mondays - but also during the weekends. So the number of papers is higher than it is on the other days.

 arXiv.org

The absolute number of interesting papers is higher, too. But I often have the feeling that even the percentage of the interesting papers is higher on Tuesdays. ;-) It would be interesting to evaluate this conjecture statistically. If that's true, a possible explanation could be that better physicists - or those who have just found something neat - often work during the weekends while the worse ones don't.

Today, there are 18 hep-th papers. About 16 of them are related to strings, supersymmetry, or extra dimensions compactified in the stringy way - well, mostly strings. Most of them are interesting and long, too. The average is close to 35 pages.

Phenomenology
Heterotic models

Ambroso and Ovrut discuss heterotic E8 x E8 compactifications where B-L is broken by some right-handed neutrino vevs. These models have three generations and predict specific (and accessible yet viable) patterns of superpartner and Higgs masses and a realistic cosmology that they describe in detail. See Javier's blog for many more words about this line of heterotic research.

F-theory bottom-up models

Chen and Chung construct F-theory bottom-up models with an E8 singularity and a flux breaking the gauge group to SU(5) x U(1) - the flipped SU(5) models.

Chen, Knapp, Kreuzer, and Mayrhofer identify SO(10) F-theory bottom-up models from blowups of Fano manifolds. The CY4 compactification is a complete intersection of hypersurfaces and they want the SO(10) group to be broken to a flipped SU(5), too - which reduces the models to the set of the previous paper.

Stringy cosmology

Vanchurin says some slightly unusual things about cosmic strings. He believes that the backreaction induces some parameters such as the "coherence length" and the "number of directions". The latter grows logarithmically with time at late times. These were actually his "non-scaling loops" and in this paper, he combines these non-scaling loops with the "scaling loops" to see how the total string length of the network evolves in time.

SUSY cosmology

Badziak says that racetrack inflation may work while matter F-terms are responsible for the SUSY breaking. He compares the (reduced) fine-tuning that is needed with the other racetrack inflation models, and he compares the importance and interaction strength of the inflaton vs the matter fields in different stages.

Stringy observables and mathematics
Worldvolume dynamics

Donagi and Wijnholt discuss M5-brane instantons or, equivalently (by duality), D3-brane instantons (with nontrivial dilaton-axion background) in F-theory that can be reduced to Maldacena-Strominger-Witten (MSW) CFTs in 1+1 dimensions because of their compactification on a real 4-cycle. The resulting MSW theory is studied by heterotic methods to obtain the effects of the instantons.

Kurt Lechner claims that five-branes in the SO(32) heterotic string theory develop k-anomalies (the action is not BRST-invariant and the variation is proportional to some k; the BRST-symmetry renamed as k-symmetry kind of replaces an infinite extension of SUSY in this esoteric formalism) at one loop. They're supposed to be cured by a quantum deformation of the torsion constraint. This solution strengthens the idea that the fivebranes may be described by a sigma-model at low energies. Recall that gauge and other "normal" anomalies always cancel in consistent string theory compactifications.

Calabi-Yaus and PDEs for a single function

Chris M. Hull, Ulf Lindstrom, Martin Rocek, Rikard von Unge, and Maxim Zabzine look at generalized Kähler geometries. Generically, all fields - the metric, dilaton, and H-field - are calculable from a single function K, the Kähler potential, which obeys a non-linear equation that generalizes the so-called Monge-Ampere equation.

Moonshine

Miranda Cheng generalizes the calculations of eliptic genus behind the M24 Mathieu non-monstrous K3 moonshine to the case of twisted partition sums. Everything still seems to work (i.e. reduce to irreps of M24) and her paper may include some special understanding why this new moonshine works.

Simple cubic open-string vertices

Dimitri Polyakov looks at the 3-point interaction vertices involving a graviton and two spin-s open string fields. The Weyl tensor appears for s=3, as expected. However, I think that the main conclusion, namely that 2s-2 derivatives (or more) are needed in the interaction term, is trivial (because it follows from group theory of the Lorentz group) and has been known since the very birth of string theory (expand the Veneziano amplitude etc.).

Non-commutativity from boundary conditions

Hazra dedicated his or her thesis to parents and grand-parents. It seems to be a review of Seiberg-Witten's picture of non-commutativity in string theory. The non-commutativity is argued to be an inevitable consequence of the boundary conditions. The detailed flavor of the statements seems to differ from the Seiberg-Witten lore but I am convinced that all small factual deviations have to be errors in this context so I kind of doubt that there can be something that is both new and valid (and important) in the thesis.

Asymptotic darkness vs asymptotic safety

Steven B. Giddings, Maximilian Schmidt-Sommerfeld, and Jeppe R. Andersen look at trans-Planckian scattering which is claimed to be de facto equivalent in any theory of gravity or supergravity; unitarity plays a role and the processes may be reduced to low-energy physics. The processes also imply that the question whether gravity is "asymptotically safe" (as in Weinberg's models) is ill-defined. The paper is another way of seeing that new, non-QFT processes are needed in the strong gravity regime to restore unitarity.

Matrix models

Szabo and Tierz show that the whole partition sums encoding the Donaldson-Thomas invariants of local toric Calabi-Yau manifolds without divisors may be decoded from the values of the Schur measure for particular arguments. Complicated links to other measures, lattice hierarchies, Chern-Simons theory, vicious walkers, and many other things will surely energize many readers - but probably among specialized experts only. :-)

Alexandrov, who may be one of them, is aware that the matrix integrals - partition sums of 0+0-dimensional field theories with matrices, if you wish - are generating functions for the number of partitions (i.e. Young diagrams) of various kinds. He uses some facts about the higher Casimirs and generalizes this problem to a wider class, still finding that there are logarithmic or exponential terms in the corresponding matrix potential.

Conceptual research of holography
De Sitter holography

Xi Dong, Bart Horn, Eva Silverstein, and Gonzalo Torroba propose a new competitor of the dS/CFT correspondence: a dS/dS correspondence which is not quite holographic (although it's derived through a CFT intermediate step). The dS/dS duality relates de Sitter theories in two dimensionalities that differ by one; the lower-dimensional one has to include a pair of large-N matter sectors, too. There are lots of order-of-magnitude estimates of the entropy and other things. It's interesting but I don't believe that these theories may be as valid or as useful as the AdS/CFT.

Mir Ali, Frenny Ruiz, Carlos Saint-Victor, and Justin F. Vazquez-Poritz send an open string to a wormhole in AdS, described by a pair of CFTs. The open string looks like a pair of charges in the same CFT or two charges in two different CFTs, depending on the location of the string. The transition point has a pair of charges in both CFTs. And depending on the distance between the charges, the force between them is Coulomb-like or confining. Speedy strings create a gap and a transmission of energy from the front to the rear side of the string. The qualitative statements are kind of obvious.

Non-stringy papers
Lots of Poisson brackets

Josef Klusoň looks at the Higgs mechanism for gravity via the Hamiltonian methods. I don't understand what's the result and I don't really believe that the physics may be consistent at the quantum level, or may be consistent with the real world. Moreover, I am confused by the fact that the paper by Arkani-Hamed, Schwartz, and Georgi is ignored (not cited) even though I believe it says nearly everything one can say about the spontaneous breaking of the diffeomorphisms and the resulting phenomena of massive gravity such as the van Dam-Veltman-Zakharov discontinuity. There are many wrong, naive "surfer-dude-like" papers about the methods to "Higgs the gravity" in the literature but I won't read this one in detail to decide whether Josef's paper is another one.

Massless particle via Rube-Goldberg

Alba, Crater, Lusanna seem to be describing the motion of a massless particle in the ordinary Minkowski space using the least readable and the least transparent formalism that is possible. I don't understand the point or the results of the paper and I don't believe that there can be any new yet valid physics coming from this unnecessarily obscure formalism. They also try to make the helicity Grassmann-valued. I don't understand what it could mean - unless they talk about a supersymmetric theory of a kind. I think that it's even more true than in the case of the previous paper, these authors are lost in the formalism and they don't "physically" know what they're doing and whether it's possible, new, justifiable, related to any other insights or observations, or otherwise interesting. Such a lack of awareness of the broader physical "big picture" makes it way too easy to go off the right tracks.