Scientific Series

Gravitational lensing is one of the primary investigation tools of all current and future wide field surveys. In this talk I will review its current status (with the Kilo Degree Survey (KiDS)) and show what unique cosmological information it gives us. Lensing is not limited to a, low redshift, dark universe probe, it can also be used as a tool to probe baryons and nicely work in synergy with baryonic probes (e.g. CMB, Xray, tSZ, HI). I will show some of the work in progress to help constraining Active Galactic Nuclei feedback

Dark matter is all around us, however its particle physics nature is still mysterious. Searches for dark matter have largely focused on candidates with weak scale interactions to the standard model particles, the so-called WIMP paradigm. However, the parameter space of the WIMP paradigm is becoming increasingly constrained by both the LHC and direct detection experiments. Another possibility that is well motivated both theoretically and observationally is to have a dark sector connected with the visible one via a mediator whose coupling to the visible one is tiny. In this talk, based on my own work, I will discuss models in this direction and how to search for signals from the dark sector using various experiments including particle accelerators, dark matter direct detection experiments, cosmic ray experiments, and cosmic microwave background observation. 

How can we quantify the entanglement between subsystems when we only have access to incomplete information about them and their environment?‎ Existing approaches (such as Rényi entropies) can only detect the short-range entanglement across a boundary between a subsystem and its surroundings, and then only if the whole system is pure. These methods cannot detect the long-range entanglement between two subsystems embedded in a larger system. There is a natural choice of entanglement measure for this situation, called the entanglement negativity, which can do this and cope with mixed states as well. However it is defined in terms of the full density matrix, which we generally won't have access to.

I will begin this talk with a brief overview ‎of some replica trick-based eigenspectrum reconstruction methods, and their various strengths and limitations. Then I will show how to modify these to find the moments of the partially transposed density matrix. Once those numbers have been obtained, it is possible to modify the earlier eigenspectrum reconstruction methods to obtain lower and upper bounds for the entanglement negativity.

Addendum: An audience member pointed out that the adjective "quasi-topological" already has a meaning and it's something different from the subject of this talk. So with hindsight it should have been called `quasi-conformal quantum computing'

We will discuss some expectations regarding properties of N=1 SCFTs in four dimensions obtained by compactifying (1,0) theories in six dimensions on a Riemann surface. We will illustrate in detail how these properties come about in the special case of compactifications of two M5 branes probing Z_2 singularity.  In particular, we will obtain a large class of strongly coupled  N=1 theories in four dimensions obtained in such compactifications. We will derive some of their robust properties, such as anomalies and supersymmetric indices.

In the framework of the ordinary seesaw model with right-handed neutrinos (and nothing else) we show that the total lepton number violating decay of the Higgs doublet into a right-handed neutrino and a standard model lepton can successfully account for the baryon asymmetry of the Universe. This is possible thanks to thermal effects shortly before the sphalerons decouple.

The AdS/Ricci-flat (AdS/RF) correspondence is a map between families of asymptotically locally AdS solutions on a torus and families of asymptotically flat spacetimes on a sphere. In this talk I will discuss how to relax these restrictions for linearized perturbations around solutions connected via the original AdS/RF correspondence.
To this end we perform a Kaluza-Klein (KK)  reduction, keeping all (massive) KK modes,  of AdS on torus and of Minkowski on a sphere.  We show that in the limit of large dimension of the compact manifolds (torus and sphere), the AdS/RF correspondence maps individual KK modes from one side to the other.
When the dimension is finite, the correspondence maps single modes to infinite superpositions of modes. One may further take appropriate limits so that there is either no torus (AdS side) or no sphere (Minkowski side) to map perturbations of solutions that possess no symmetry, thus completely relaxing the original restrictions.
This correspondence should allow us to develop a detailed holographic dictionary for asymptotically flat spacetimes.

The origin and composition of 85% of the matter in the universe is completely unknown. Among several viable options, Weakly Interacting Massive Particles (WIMPs) are motivated dark matter candidates that can be tested by different and complementary search strategies. Crucially, different searches probe WIMP couplings at different energy scales, and such a separation of scales has striking consequences in connecting different experimental probes. This motivates the development of theoretical tools to properly connect the different energy scales involved in constraining WIMP models. I will introduce these tools and I will illustrate with several examples how crucial the inclusion of these effects in WIMP searches is.

In this talk, we will discuss an open Gromov-Witten invariant on hyperKahler surfaces, including K3 surfaces and certain Hitchin moduli spaces. The invariant is defined via the Lagrangian Floer theory and satisfy the Kontsevich-Soibelman wall-crossing formula and are expect to recover the generalized Donaldson-Thomas invariants studied in the work of Gaiotto-Moore-Neitzke.

Collisions at the Large Hadron Collider (LHC) are dominated by jets, collimated sprays of particles that arise from quantum chromodynamics (QCD) at high energies.  With the remarkable performance of the ATLAS and CMS detectors, jets can now be characterized not just by their overall direction and energy but also by their substructure.  In this talk, I highlight the increasingly important role that jet substructure is playing in searches for dark matter and other new physics at the LHC, especially when exploring extreme kinematic regimes involving large Lorentz boosts.

I also explain how innovative theoretical studies of jet substructure have taught us surprising lessons about QCD, revealing new probes of hot dense matter and universal features of gauge theories.

In the past few years we have witnessed a flurry of activity in the field of topological phases of matter. An outstanding theme in the field is the study of the interplay between geometry and topology of many-body wave functions, which has attracted the attention of condensed matter and high-energy physicists. In this talk, I will present the quantum field-theoretic descriptions of the fascinating novel phenomena emergent from intertwined geometry and topology, which are vividly exemplified by the geometric responses in fractional quantum Hall systems. For the strict topological limit, where only the global topology of space matters, the fractional quantum Hall systems are characterized by their universal properties such as fractional quantum Hall conductivity and a degeneracy on surfaces with the topology of a torus. Quite surprisingly, these topological fluids also couple to the geometry and have universal responses to the adiabatic deformations of the background geometry. These responses are given by a Wen-Zee term, Hall viscosity term, and gravitational Chern-Simons term. Through the field-theoretic approaches of the topological fluids, I will for the first time show how to derive all the universal geometric responses. To account for the coupling to the background geometry, I show that the concept of “flux attachment” must be modified in the curved space and use it to derive the responses from Chern-Simons theories for all the known fractional quantum Hall states. I also apply these results to the theory of the anisotropic quantum Hall systems, where the geometric responses play a central role in understanding their universal physics.

We derive in the framework of soft collinear effective field theory (SCET) a Lagrangian describing the t-channel exchange of Glauber quarks, which are incorporated through fermionic potential operators in the effective theory. The Wilson line structure of the operators, which is derived from matching calculations and the symmetries of the effective theory, describe additional soft and collinear emissions from a fermionic t-channel exchange in the forward scattering limit to all orders. Lightcone singularities in the fermionic potential are regulated using a rapidity regulator, whose corresponding renormalization group evolution gives rise to the quark reggeization at amplitude level.

Many-body entanglement can lead to exotic phases of matter beyond conventional symmetry breaking paradigm. Those exotic phases may contain fractionalized quasiparticles and emergent gauge fields. In this talk,  I will focus on a wide class of long-range entangled phases—quantum spin liquid. In quantum spin liquids, the spins are entangled in some intricate fashion giving rise to interesting physics such as emergent topological field theory and QED3 theory. I will show in detail how such exotic physics can emerge in simple spin systems.