Seminars

title: “Humanly traversable wormholes”

Title: Partition functions of the tensionless string

Abstract: I discuss string theory on AdS3xS3xT4 in the tensionless limit,with one unit of NS-NS flux. This theory is conjectured to be dual to the symmetric product orbifold CFT. I show how to compute the full string partition function on various locally AdS3 backgrounds, such as thermal AdS3, the BTZ black and conical defects,and find that it is independent of the actual background, but only depends on the boundary geometry. Consequently, reproducing the boundary partition function does not require a sum over bulk geometries,but rather agrees with the string partition function on any bulk geometry with the appropriate boundary. I argue that the same mechanism shows the factorization of the bulk partition function when geometries with disconnected boundaries are considered.

Title:

Merging Swampland Conjectures using strings and membranes

Abstract:
In this talk I will discuss new relations that appear among the Swampland Conjectures when analysing the properties of low codimension BPS objects, in particular strings and membranes in N=1 4D EFTs. I will review the state of the art of the Swamplandprogram and revisit the most important conjectures, namely the Weak Gravity Conjecture (WGC) and the Swampland Distance Conjecture (SDC). Then I will describe how to apply the WGC to low codimension BPS objects by interpreting the 4d backreaction of these objectsas a classical RG flow of their couplings. Constraints on the UV charges and tensions get then translated to constraints on the axionic kinetic terms and scalar potential of the EFT, which uncovers new relations among the Swampland Conjectures. In particular,we show that the WGC for strings implies the SDC, by using that infinite distance limits can be realised as RG flow endpoints of fundamental axionic strings. Similarly, WGC-saturating membranes generate a scalar potential satisfying the de Sitter Conjecture.

Title:Reggeized scattering, entanglement and chaos using AdS/CFT

ABSTRACT:The eikonalized parton-parton scattering amplitude at large energy and 

large impact parameter, is dominated by the exchange of a hyperbolic surface in  

walled AdS. Its analytical continuation yields a worldsheet instanton that is 

at the origin of the Reggeization of the amplitude and a thermal-like 

quantum entropy. I will  construct the entangled density matrix following from the

 exchanged surface, and show that its von-Neumann entanglement entropy 

coincides with its thermal-like entropy.  The ratio of the entanglement entropy 

to the transverse growth of the exchanged surface is similar to the Bekenstein 

entropy ratio  for a black-hole, with a natural definition of saturation 

and the on-set of chaos in high energy collisions.  The largest  eigenvalues 

of the entangled density matrix obey a cascade equation in rapidity, 

reminiscent of non-linear QCD evolution of wee-dipoles  

at low-x and weak coupling.  We suggest that the largest eigenvalues  

describe the probability distributions of wee-quanta at low-x 

and strong coupling that maybe measurable at present and future  hadron-hadron 

and lepton- hadron colliders.

Title: Landau diagrams in AdS and S-matrices from conformal correlators

Abstract: Quantum field theories in AdS generate conformalcorrelation functions on the boundary, and in the limit where AdS is nearly flat one should be able to extract an S-matrix from such correlators. We discuss a particularly simple position-space procedure to do so. It features a direct map from boundary positionsto (on-shell) momenta and thereby relates cross ratios to Mandelstam invariants. This recipe succeeds in several examples, includes the momentum-conserving delta functions, and can be shown to imply the two proposals based on Mellin space and on the OPE data.Interestingly the procedure does not always work: the Landau singularities of a Feynman diagram are shown to be part of larger regions, to be called `bad regions’, where the flat-space limit of the Witten diagram diverges. To capture these divergences we introducethe notion of Landau diagrams in AdS. As in flat space, these describe on-shell particles propagating over large distances in a complexified space, with a form of momentum conservation holding at each bulk vertex. As an application we recover the anomalousthreshold of the four-point triangle diagram at the boundary of a bad region.

Title: Boundary conditions for a free scalar field

Abstract:

In the first part of the talk, I will consider the problem of classifying the conformal boundary conditions (b.c.) for the free scalar CFT. We will see how to tackle this problem using conformal bootstrap techniques. In the case of 4d bulk, I will show ournumerical results, which feature a “kink” that indicates the existence of a non-trivial “interacting” b.c., besides the obvious Neumann and Dirichlet “free” b.c.’s.

In the second part of the talk, I will discuss the RG flow of a 4d free scalar field coupled to large N vector models on the boundary. The (classically marginal) coupling involves the boundary mode of the scalar and the O(N) single-trace scalar operator. Iwill explain that the resulting RG connects two fixed points with bulk and boundary decoupled, and the local 3d sector is the free vector model in one end, and the critical one in the other end. This same RG admits two weak-strong dual descriptions, one fromthe free end, and one from the critical end.