Gokce Basar

Resurgence theory, ghost-instantons, and analytic continuation of path integrals

A bstractA general quantum mechanical or quantum field theoretical system in the path integral formulation has both real and complex saddles (instantons and ghost-instantons). Resurgent asymptotic analysis implies that both types of saddles contribute to physical observables, even if the complex saddles are not on the integration path i.e., the associated Stokes multipliers are zero. We show explicitly that instanton-anti-instanton and ghost-anti-ghost saddles both affect the expansion around the perturbative vacuum. We study a self-dual model in which the analytic continuation of the partition function to negative values of coupling constant gives a pathological exponential growth, but a homotopically independent combination of integration cycles (Lefschetz thimbles) results in a sensible theory. These two choices of the integration cycles are tied with a quantum phase transition. The general set of ideas in our construction may provide new insights into non-perturbative QFT, string theory, quantum gravity, and the theory of quantum phase transitions.

Triangle anomaly in Weyl semimetals

Weyl semimetals possess massless chiral quasi-particles, and are thus aected by the triangle anomalies. We discuss the features of the chiral magnetic and chiral vortical eects specic to Weyl semimetals, and then propose three novel phenomena caused by the triangle anomalies in this material: 1) anomaly cooling; 2) charge transport by soliton waves as described by the Burgers’ equation, and 3) the shift of the BKT phase transition of superuid vortices coupled to Weyl fermions. In addition, we establish the conditions under which the chiral magnetic current exists in real materials.

Sign problem and Monte Carlo calculations beyond Lefschetz thimbles

A bstractWe point out that Monte Carlo simulations of theories with severe sign problems can be profitably performed over manifolds in complex space different from the one with fixed imaginary part of the action (“Lefschetz thimble”). We describe a family of such manifolds that interpolate between the tangent space at one critical point (where the sign problem is milder compared to the real plane but in some cases still severe) and the union of relevant thimbles (where the sign problem is mild but a multimodal distribution function complicates the Monte Carlo sampling). We exemplify this approach using a simple 0+1 dimensional fermion model previously used on sign problem studies and show that it can solve the model for some parameter values where a solution using Lefschetz thimbles was elusive.

Volume independence in the large N limit and an emergent fermionic symmetry.

Large-N volume independence in circle-compactified QCD with adjoint Weyl fermions implies the absence of any phase transitions as the radius is dialed to arbitrarily small values. This class of theories are believed to possess a Hagedorn density of hadronic states. It turns out that these properties are in apparent tension with each other, because a Hagedorn density of states typically implies a phase transition at some finite radius. This tension is resolved if there are degeneracies between the spectra of bosonic and fermionic states, as happens in the Nf = 1 supersymmetric case. Resolution of the tension for Nf > 1 then suggests the emergence of a fermionic symmetry at large N , where there is no supersymmetry. We can escape the Coleman-Mandula theorem since the N = ∞ theory is free, with a trivial S-matrix. We show an example of such a spectral degeneracy in a non-supersymmetric toy example which has a Hagedorn spectrum.

Scaling relation between pA and AA collisions

We compare the flow-like correlations in high multiplicity proton-nucleus (

Monte Carlo algorithm for simulating fermions on Lefschetz thimbles

p+A

Hydrodynamics, resurgence, and transasymptotics

) and nucleus-nucleus (

Monte Carlo calculations of the finite density Thirring model

A+A

Magneto-sonoluminescence and its signatures in photon and dilepton production in relativistic heavy ion collisions

) collisions. At fixed multiplicity, the correlations in these two colliding systems are strikingly similar, although the system size is smaller in

Chiral and Gravitational Anomalies on Fermi Surfaces

p+A