Tuesday, May 23, 2006

Hep-th papers on Tuesday

Let me summarize each of the 24 hep-th papers that appeared last night.

Freed, Moore, and Segal argue that the electric and magnetic fluxes on a spacetime manifold can't be measured simultaneously because of a version of the uncertainty principle. That should not be unexpected. Only the electric potential or the magnetic potential may be chosen as a fundamental degree of freedom which makes their fluxes well-defined for a given configuration: the fluxes of the dual field are then undetermined.

Moreover, the authors argue that the uncertainty principle only applies to torsion classes. One of the implications is that the K-theory class of the Ramond-Ramond fields can't be physically measured. There also exists a less mathematically formal version of that paper even though I am a bit confused which of the two is addressed to the physicists. The insights have special consequences for self-dual form fields; they're connected with Pontrjagin dual groups.

Ben Craps offers the written version of his CERN lectures about big-bang-like singularities in string theory. He reviews the role of the winding modes, the Milne orbifold, attempts to use the AdS/CFT correspondence, and the matrix big bang.

Dasgupta, Grisaru, Gwyn, Katz, Knauf, and Tatar re-derive the metric describing certain wrapped fivebranes (based on functions among which "cot" is the most complex one). Also, they write down certain new non-Kähler geometries, and offer you a table that relates the non-Kähler deformations of certain backgrounds to other effects related by dualities, something that Anke Knauf has shown us some time ago.

Washington Taylor has written a mostly non-technical review of string field theory for a book. While it does not annoy the reader with too many formulae, it contains many references, including the references on newest developments, and may be viewed as a state-of-the-art summary of the subject.

Donagi, Reinbacher, and Yau take one of the simplest Calabi-Yau manifolds, namely the quintic hypersurface, define heterotic string theory on them, but consider very general configurations of the gauge field (the bundle). Using similar diagrams that were important for the recent heterotic standard model papers, they derive things as complicated as the Yukawa couplings of these backgrounds.

Bambah, Mahajan, and Mukku propose the non-Abelian version of an unusual kind of unification that one of the authors proposed in the Abelian case. What unification do we mean? It is a unification of Maxwell's theory with fluid dynamics, speculated to be relevant for the quark-gluon plasma. Well, I personally see no unification in that paper. What I see is some theory that contains the Yang-Mills field (previously Maxwell) together with an additional scalar field and a vector field, and they are coupled in some strange way. The Republican Party and the Democratic Party exist in the same country and interact, but that still does not make them unified. As you can see, it seems that the entertainment value of this paper exceeds most others (but not all of them, as you will see below).

Damiano Anselmi offers a proof (?) that general relativity is a renormalizable theory, if defined as "acausal gravity". The first sentences of the body of the paper indicate that the acausality is far from being the worst thing in this proposal: the first sentence argues that "the necessity to quantize gravity is a debatable issue". Because it seems likely that these initial sentences play an important role and because I personally consider a non-quantized gravity within the quantum world to be an obvious inconsistency, I can't recommend you to read the full paper.

Svrček and Witten study the axion decay constant "F" in various scenarios in string theory, including M-theory on G2 manifolds, intersecting braneworlds, heterotic M-theory, and others. Recall that the axions are useful to explain the strong CP-problem i.e. the smallness of the QCD theta-angle that couples to the instanton number. Also, the axions could be relevant to describe various potentially exciting recent experiments. However, astrophysical bounds tell you that the axions that couple to visible matter should not be too heavy. They should be lighter than the GUT scale or so. The authors find that this is naturally achieved only if the visible matter lives on shrunk cycles.

Buividovich and Kuvshinov investigate gauge theories - such as QCD - using the method of the random walk. Define the open Wilson line from a given point in spacetime to the point "(t,0,0,0)". You will get a point in the gauge group manifold, and you may ask what is the probability distribution to obtain a certain particular element of the group. The heat kernel equation on the group manifold becomes relevant.

W.F. Kao starts by saying that recently there is a growing interest in the Kantowski-Sachs (KS) universe - a term that I've never heard of - based on some purely higher-derivative gravity (?). While the phrase "pure higher-derivative gravity" may sound insulting, it does not seem to play much role in the paper. On the other hand, the KS universe seems to be nothing else than the FRW universe with an extra dependence on the coordinate "theta", making it anisotropic. The growing interest in this geometry is proved by three papers from 1999, 2003, 2004. This KS Universe is supposed to be relevant for inflation. Because I don't know where to put this paper in my understanding of the world, I can't tell you more.

Laamara, Drissi, and Saidi want to relate topological string theory on the conifold with the fractional quantum hall effect (FQHE). Well, there are already quite many things that are claimed to be equivalent to topological string theory on the conifold. The links between FQHE and fluids of D-branes (with their characteristic non-commutativity) were investigated by Susskind, Hellerman, Polychronakos, and others years ago, but the conifold seems to be a novelty introduced by the present authors.

Justin David and Ashoke Sen prove the formula for the generating function for the degeneracies of dyons of the CHL string - the kind of formulae that Davide Gaiotto knows very well. They map the system to the D1-D5 system and use, among other tools, various properties of the modular functions.

Delduc and Ivanov explain the origin of some dualities in supersymmetric quantum mechanics. They're dualities between theories with the same number of fermions and different numbers of bosons. The equivalence is shown by gauging additional superfields.

Gherghetta and Giedt realize the Randall-Sundrum models in terms of type IIB string theory on "AdS5 times T11" where "T11" is the base of the conifold, equipped with probe D7-branes. The tip is governed by the Klebanov-Strassler theory; the following region by the Klebanov-Witten regime; and the rest is connected to a compact Calabi-Yau manifold. The additional D7-branes provide you with bulk gauge fields - something very popular among the RS phenomenologists. They also claim to have some purely stringy natural predictions for physics of these models but I have not found what they are.

R.P. Malik studies QED using the BRST formalism written in terms of superfields. Not only that I have never heard of the "horizontality condition", but the BRST formalism itself seems like an overkill to me if it is applied to QED, especially in this BRST superfield formalism.

Lee and Yee construct new 7-manifolds "X" that can be multiplied by "AdS4" to give you new solutions of M-theory. These seven-dimensional manifolds - in fact, "tri-Sasakian" manifolds - are obtained from twelve-real-dimensional hyperKähler manifolds. Four dimensions are lost by a method I have not understood, and the last fifth dimension disappears by quotienting by a U(1). They also say something about the dual CFT3 theory, for example things that follow from their calculated volume of the seven-manifold.

Neznamov only offers a PDF file with a few pages that only seem to contain some trivial - but not necessarily correct - manipulation with the Dirac gamma matrices. His following PDF paper is a bit longer but even more entertaining. He argues that the Standard Model fermions may be massive even without Yukawa couplings by generalizing - more precisely, by screwing - the covariant derivative. Neznamov's covariant derivative has the standard time-component, but the spatial component has an extra term "inverse nabla times squared mass". Lorentz invariance, unitarity, gauge invariance, mathematics, and common sense must be sacrificed for more noble goals.

Hortacsu and Taskin provide more than the PDF file. They argue that in some model, a composite spin 1 particle must interact although the spin 1/2 partons don't interact. I am not sure what they exactly claim and why but it sounds like an attempt to violate the Weinberg-Witten theorem - although the latter is not even cited.

Janos Polonyi wants to study the emergence of the classical limit in QED. Well, yes, I don't quite follow what is the open question that is being answered. At any rate, to answer "this" question, the author defines some kind of generalized density matrix, to make the quantum theory look more classical. Because I don't believe that anything from the quantum character of quantum field theory has to be sacrificed to understand the classical limit properly, I doubt that one can learn something new and true from that paper.

Edward Shuryak discusses AdS/QCD. He claims that he was the first discoverer of the statement that confinement in the field theory should be described as an explicitly added potential in the AdS bulk that depends on the holographic dimension and increases as you go away from the UV. This picture, recently promoted by Karch et al., is certainly a natural and qualitatively correct dual parameterization of the same effect except that I don't know how this follows from string theory, which makes it hard to imagine that a new precise understanding of physics of confinement.

Berenstein and Cotta look at "emergent geometry" within the AdS/CFT context. It is an orbifold of a previous paper by David Berenstein. David's conceptually intriguing goal is to construct the bulk geometry from certain partons - particles that live on the moduli space of D3-branes in flat space - that can be identified within the gauge theory. These partons effectively repel each other which makes them arrange themselves into the right geometry, assuming that you believe the statistical mechanical kind of reasoning. It is a sort of cute heuristic description of the origin of the compact part of the geometry. It would be even more interesting if one could derive something about the moduli space of possible geometries from this starting point.

Mintz, Farina, Maia Neto, and Rodrigues look at particle production (controlled by the Bogoliubov coefficients) of bosons in a 1+1-dimensional theory with a boundary where the boundary conditions interpolate between the Neumann ones and the Dirichlet ones.

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