Friday, June 23, 2006

Strings 2006 Day 3 and 4

Paul Cook, a grad student from Caltech, and I have reviewed the first three days' talks here. There may be mistakes because many of these areas are not directly linked to our research so if we've made errors then please tell us. Also, if people have questions about what certain words or concepts mean then please ask. It's excellent for me to find out such things and formulating a response should help me to understand these ideas in greater depth.

As noted in the comments in the previous post it's a real shame that the talks have not been filmed as a large proportion of the string community is not here to see what's been going on. As this is my first strings conference I have many impressions which I will talk about when the lectures are all out of the way and things have calmed down a little here. I've now been going to stringy talks for the last three weeks with only two days break so I'm feeling slightly jaded. I'll also talk about, though hopefully not wallow in, these sentiments at some point.


On Tuesday morning Jonathan Feng was reviewing dark matter candidates and in particular the possibility of detecting warm dark matter in the form of gravitinos or axinos. The reason that these particles must be warm is to prevent the clumping of matter which is seen in cold dark matter simulations and the lack of structure in hot dark matter scenarios. Most interestingly from this talk was the possibility of detecting dark matter in collider experiments (LHC or ILC) even if the lightest superpartner is only gravitionally coupled. This is because charged supersymmetric particles which are stable for a matter of seconds or months can be trapped before they decay into gravitinos. In his paper there are links to possible experimental scenarios for this detection.

Tohru Eguchi described how the distribution of Calabi Yau monifolds is peaked around singular points in the Calabi-Yau moduli space, but also that the number of distinct Calabi-Yau manifolds near each singularity in the moduli space is finite. The "discrete" nature of the distribution of physical manifolds follows from considerations of flux quantisation.

Robert Dijkgraaf spoke about constructing gauge theories on a compact Calabi Yau. He spoke about a factorisation method for the moduli space of Calabi Yaus whereby the two-cycles and three-cycles are split into separate sections of the space. Somehow the moduli space of two-cycles was embedded in that of three-cycles (such that two cycles are constructed by contraction of a single direction in the latter).

Eva Silverstein spoke on "Black Holes as Catalytic Vacuum Converters". She first outlined a simple scenario where the space is in a metastable ground state where supersymmetry is dynamically broken. Consider a black hole in this space. As it Hawking radiates (but not necessarily to an extremal state), the horizon will increasingly "push" the moduli toward the black hole's preffered attractor point. This could quite generically cause the moduli to cross into a basin of attraction of a lower energy vacuum. If this happens in a sufficiently large area of space near the black hole, this could catalyze a change of vacuum of the whole universe, in a time far less than it might take to tunnel. She then outlined a specific scenario, where heavy mass particles are Hawking radiated from an evaporating black hole (note that begin radiated means that this scenario is NOT dependent on what initially formed the black hole). If a sufficient density of these particles is reached, this would cause a vacuum transition into a vacuum where these particles are light. Phenomenologically, this puts constraints on the sorts of black holes and/or vacuum structure in our universe, as any evaporating black hole could perform this catalysation.

Ashok Sen
spoke about the calculation of entropy of four dimensional black holes in higher derivative corrected gravity. This is a further application of his technique of calculating the entropy and moduli solely as a Legendre transform of the integral of the Lagrangian over the S^(D-2) of the horizon. These black hole solutions are not necessarily supersymmetric.

Herman Verlinde spoke about his bottom-up approach to string phenomenology. In this approach he separates the high energy closed string dynamics from the gauge theory described by open strings by studying the theory of a D-brane in a throat of a Calabi Yau. The structure of the throat determines the quiver diagram describing the gauge theory and in particular the cone over a del Pezzo-8 singularity gives the minimal supersymmetric standard model with several extra U(1) factors and extra generations of Higgs. It appears that the resolution of these extra U(1)s may be understood (My memory on this point is not great I'm afraid).

Things get a little more hazy now and so we shan't be able to review all of the talks in as much detail. Again it would be great if anyone who has taken notes from the talks would like to provide a review.

David Tong
spoke in his usual enthusiastic style, giving a review of his work on quantum vortex strings. He started by giving an overview of the possible solitonic objects that one can find in an N=2 supersymmetric gauge theory when more and more degrees of freedom are added and allowed to develop vevs. This simple gauge structure allows a huge number of interesting configurations of domain walls connected by vortex strings with monopoles threaded on the strings etc. The fascinating point about these theories is that when you study the quantum dynamics of the vortex string, given by a two dimensional sigma model, you find that the BPS spectrum of states exactly models that from the four dimensional theory. David was in danger of being savaged by Witten during the questions but escaped with what sounded like convincing rebuttals.


Few details from Brain Greene's talk which was presented by Koenraad Schalm. The punchline was that stringy effects in the CMB spectrum would add oscillatory behaviour which should be detectable in the next generation of microwave telescopes.

Mina Aganagic reviewed how the holomorphic anomaly of the topological string partition function can be understood from the point of view of symplectic transformations of the cycles in the Calabi Yau. In the "complex polarisation", the topological string partition function is modular under monodromies (a subgroup of the symplectic group) generated by moving around loci in the moduli space where a cycle in the Calabi Yau shrinks; however it suffers from the holomorphic anomaly. On the other hand, in "real polarisation", the partition function has no holomorphic anomoly, but suffers from a failure of modular invariance. The holomorphic ambiguity in complex polarisation can be identified with the failure of modular invariance in real polarisation. She then showed how (as has previously been demonstrated using complex polarisation) one can find the topological string partition function recursively in a worldsheet genus expansion, using the failure of modular invariance to pick up terms from lower-genus boundaries of the moduli space, at any given genus.

Sarah Shandera (standing in last-minute for Henry Tye) spoke about specific string-theory motivated observables in cosmology. In particular, she outlined how in the context of D3 - anti-D3 brane annihilation (a useful scenario as it provides both inflation and then a simple mechanism for reheating at the end of inflation), one can get sufficient inflation even when the inflaton does not seem to obey the slow-roll condition. In her model, the DBI action necessarily implies a "speed limit" on the D3 brane as it falls down a Calabi Yau throat toward an anti-D3 brane, despite a steeper potential. This model implies an effective field theory with a particularly unusual kinetic term (from a field theory perspective). As a result it makes very definite predictions, which furthermore will potential be observable within the next few years. The work included some great actual numbers for cosmologists to work with -- something which hopefully the rest of the string theory community will be able to emulate soon!

Andy Strominger and Shiraz Minwalla's Thursday talks were reported on in the previous post.

We're going to leave it at that for now but hope to be able to talk further about the other lectures in the next couple of days.


PlatoHagel said...


Just to note that the picture in your index is forcing the index down to the bottom, or, the pictures in your articles might be to big. Standard sizes for this kind of format usually are about 420 in width. Your index definitely alot smaller.

Wonderful reporting. well done

Unknown said...

Hi Plato, thank you. I think that this is just down to me playing around too much with the template file. I'll attempt to sort it out at some point.

I'm glad you found the reports worthwhile.

All the best,


Anonymous said...

Are you making this up? Either way, I must protest at the complete lack of italics for this portion of the blog. How else are we font-o-philes to tell physics gabble from non-phyics prose? Eh? Errr rrright.

Unknown said...

C.G - Yes, italics went out the window when physics became the majority of the posts. I presumed that the quick-witted amongst you would be able to tell reasonably fast that you had even less idea about what I was going on about than normal.