March 2021

Date: March 8

Time: 19:00 IST (UTC + 5:30)

Speaker: Chong Wang (Perimeter Institute of Theoretical Physics)

Title: Monopoles in Dirac spin liquids: from band topology to competing orders

• Abstract: The interplay of symmetry and topology has been at the forefront of recent progress in quantum matter. In this talk I will discuss an unexpected connection between band topology and competing orders in a quantum magnet. The key player is the two-dimensional Dirac spin liquid (DSL), which at low energies is described by an emergent Quantum Electrodynamics with massless Dirac fermions (a.k.a. spinons) coupled to a U(1) gauge field. A long-standing open question concerns the symmetry properties of the magnetic monopoles, an important class of critical degrees of freedom. I will show that the monopole properties can be determined from the topology of the underlying spinon band structure. In particular, the lattice momentum and angular momentum of monopoles can be determined from the charge (or Wannier) centers of the corresponding spinon insulators. I will then discuss the consequences of the monopole properties, such as the stability of the DSL on different lattices, universal (experimental and numerical) signatures of DSL, and competing symmetry-breaking phases near the DSL state.

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Date: March 11

Time: 17:30 IST (UTC + 5:30)

Speaker: Raju Venugopalan (BNL)

Title: Classicalization and unitarization of wee partons in QCD and gravity: the CGC-Black Hole correspondence

• Abstract: The seminal work of Lipatov pointed to remarkable similarities between QCD and Gravity amplitudes in the Regge limit. We argue that this correspondence likely extends further to saturated parton states in the two theories and note that key features of these states appear to be universal at maximal occupancy. We discuss some of the intriguing consequences of this universality and the potential insight it offers into the dynamics of the two theories at high energies.

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Date: March 15

Time: 17:30 IST (UTC + 5:30)

Speaker: Pawel Caputa (Warsaw University)

Title: Path Integral Optimization from CFT to AdS

• Abstract: I will talk about the program of extracting holographic geometries from path-integrals in conformal field theories. I will review basic examples and discuss the notion of path integral complexity. In the second part, I will present recent works on path integral optimization from Hartle-Hawking wave functions in AdS/CFT.

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Date: March 18

Time: 17:30 IST (UTC + 5:30)

Speaker: Aleksi Vuorinen (University of Helsinki)

Title: Quark Matter: From Hard Thermal Loops to Neutron-Star Cores

• Abstract: Confirming or ruling out the existence of deconfined quark matter inside neutron stars is one of the most prominent open problems in nuclear astrophysics. While the ultimate goal continues to be the observation of a smoking gun signal directly indicating the presence or creation of quark matter, a more indirect approach to the problem has lately become feasible: by combining ab-initio theoretical results for the microscopic properties of dense QCD matter with the latest astrophysical measurements of neutron star properties, it is possible to build stringent model-independent constraints for the material properties of neutron-star matter at different densities. In my talk, will describe some recent developments in this line of work, covering both perturbative QCD calculations aimed at extending the weak-coupling expansion of the pressure of high-density quark matter, and studies concentrating on the question, whether matter in the cores of the heaviest stable neutron stars has characteristics closer to those of quark or nuclear matter.

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Date: March 22

Time: 17:30 IST (UTC + 5:30)

Speaker: Boris Pioline (LPTHE -- Paris VI)

Title: Attractor indices, brane tilings and crystals

• Abstract: In type II strings compactified on a Calabi-Yau threefold $X$, the Donaldson-Thomas (DT) invariants $\Omega(\gamma,z)$ counting BPS black holes have an intricate dependence on the moduli $z$, due to wall-crossing phenomena. When $X$ is a toric threefold, these indices are related to the DT invariants of a quiver with potential with superpotential, encoded by a brane tiling. I will present a conjecture for the DT invariants for all dimension vectors $d$ in a certain chamber $z_*(d)$ known as the attractor (or self-stability) chamber. In short, "DT invariants all vanish, except when they are known not to." In combination with the attractor flow tree formulae, this conjecture provides an algorithmic way of computing the DT invariants $\Omega(\gamma,z)$ for any $\gamma,z$. The conjecture is supported by a large number of checks, including a successful comparison with the Vafa-Witten invariants of a Fano surface $S$ when $X$ is the total space of the canonical bundle $K_S$, and with the counting of molten crystals for framed DT invariants in the non-commutative chamber. Based on works with G. Beaujard, J. Manschot and S. Mozgovoy, arXiv:2004.14466 and 2012.14358

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Date: March 25

Time: 17:00 IST (UTC + 5:30)

Speaker: Tomas Brauner (University of Stavanger)

Title: Effective field theory for spontaneously broken symmetries

• Abstract: The low-energy physics of matter is, as a rule, dominated by the collective motion of its constituents. It exhibits universal behavior that can be largely attributed to spontaneous breaking of global symmetries of space, time and whatever internal degrees of freedom are present. I will start the talk with a review of the construction of effective field theories for broken symmetries. The second part of the talk will then focus on some recent applications, including topological interactions of Nambu-Goldstone bosons and scattering amplitudes of Nambu-Goldstone bosons in the long-wavelength limit.

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Date: March 29

Time: 17:00 IST (UTC + 5:30)

Speaker: Dimitrios Giataganas (University of Athens)

Title: Stochastic Neural Networks as Thermodynamic Physical Systems

• Abstract: Neural Networks have been recently used as a very effective tool for the study and prediction of data in various fields of physics. Despite this enormous success, a complete theoretical understanding about why these techniques are so successful is still missing. A possible starting point for a theoretical treatment suggests that deep learning is a form of coarse graining. On the other hand, in theoretical physics the notion of coarse graining is characterize by the renormalization group flow. We discuss the possibility of an underlying fundamental relation between certain machine learning methods and the renormalization group flow in theoretical physics. To do so we will be introducing the restricted Boltzmann machines and their training methods on lattice spin models. While the stochastic neural networks have no direct knowledge about the Hamiltonian and the interactions of the physical models, we show that they identify spontaneously the phase transitions of the lattice spin models. We discuss this behavior in the context of Renormalization Group Flow and thermodynamics.

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