Masatoshi Sato, YITP, Kyoto University

On the Electromagnetic Response of Topological Superconductors

##### We resolve several puzzles related to the electromagnetic response of topological superconductors in 3+1 dimensions. In particular we show by an analytical calculation that the interface between a topological and normal superconductor does not exhibit any quantum Hall effect as long as time reversal invariance is preserved. We contrast this with the analogous case of a topological insulator to normal insulator interface. The difference is that in the topological insulator the electromagnetic vector potential couples to a vector current in a theory with a Dirac mass, while in the superconductor a pair of Weyl fermions are gapped by Majorana masses and the electromagnetic vector potential couples to their axial currents.

Hitoshi Murayama, UC Berkeley / Kavli IPMU

Some Exact Results in QCD-like and Chiral Gauge Theories

##### I present some exact non-perturbative results in QCD-like and chiral gauge theories. They are exact when supersymmetric gauge theories are perturbed by anomaly-mediated supersymmetry breaking (AMSB). Thanks to the UV-insensitivity of AMSB, SUSY results can be perturbed with no ambiguities even when applied to composite fields. I analytically derive chiral symmetry breaking in QCD-like theories. Our results for chiral gauge theories do not agree with what had been suggested by tumbling. We suggest alternative schemes of tumbling-like interpretations. We see no evidence that large SUSY breaking leads to phase transitions for the chiral symmetry breaking, perhaps protected by holomorphy.

Neill Warrington, University of Washington

[cancelled] Contour Deformations for Lattice Field Theory

##### Highly oscillatory path integrals are common in lattice field theory. They crop up as sign problems and as signal to noise problems and prevent Monte Carlo calculations of both lattice QCD at finite chemical potential and real-time dynamics. A general method for treating highly oscillatory path integrals has emerged in which the domain of integration of the path integral is deformed into a complexified field space. In this talk I will review this method, and I will discuss recent progress in machine learning manifolds for lattice QCD.

Gurtej Kanwar, University of Bern

Machine learning for ensemble generation in lattice field theory

##### Critical slowing down and topological freezing are key obstacles to progress in lattice QCD calculations of hadronic properties causing the cost of ensemble generation to severely diverge in the continuum limit. Recently, a class of machine learning techniques known as flow-based models has been successfully applied to produce exact sampling schemes that can circumvent critical slowing down and/or topological freezing in proof-of-principle applications. This talk summarizes these flow-based MCMC methods, including the incorporation of gauge and translational symmetries. I further discuss progress towards including the contributions of fermions, required for example to include dynamical quark contributions to flow-based sampling for lattice QCD.

Neil Turok, The University of Edinburgh

Towards the path integral for gravity

##### We show how Feynman?s path integral for quantum mechanical theories may be defined without a Wick rotation to imaginary time. Instead, we employ analytic continuation (and Cauchy?s theorem) in the complexified space of paths being integrated over. We outline a new existence proof for real time path integrals and describe physical applications, from nonrelativistic quantum mechanics to interference patterns in radio astronomy and caustics in Yang-Mills theories. Our target is gravity: I outline the remaining challenges.

Neil Turok, University of Edinburgh

Towards the path integral for gravity

##### We show how Feynman’s path integral for quantum mechanical theories may be defined without a Wick rotation to imaginary time. Instead, we employ analytic continuation (and Cauchy’s theorem) in the complexified space of paths being integrated over. We outline a new existence proof for real time path integrals and describe physical applications, from nonrelativistic quantum mechanics to interference patterns in radio astronomy and caustics in Yang-Mills theories. Our target is gravity: I outline the remaining challenges.

Heng Tong Ding, CCNU

[QCD theory Seminar] QCD phase structure in strong magnetic fields

##### The properties of strongly interacting matter in the external magnetic field have attracted many studies in recent years as strong magnetic fields appear in heavy-ion collisions, the early universe, and magnetars. Many novel and unexpected pheonmena have been found from lattice QCD studies, such as inverse magnetic catalysis, reduction of chiral transition temperature in strong magnetic fields, and a plasuible QCD critical end point in the plane of temperature and magnetic field. In this talk I will present the first latiice QCD study on the Gell-Mann-Oakes-Renner (GMOR) relation in the external magnetic field, and find that the GMOR relation can naturally reconcile the magnetic catalysis at zero temperature and reduction of chiral transition temperature in nonzero magnetic fields. I will further dicuss the change of the degrees of freedom and the strength of transition in the strong magnetic fields via fluctuations of net baryon number, electric charge, and strangeness, and propose certain observables to detect the existence of a magnetic field in the late stage of heavy ion collisions. The talk is based on arXiv:

2008.00493 and 2104.06843.

Sugumi Kanno, Kyusyu University

Indirect detection of gravitons through quantum entanglement

##### In this talk, I will present our recent work on indirect detection of gravitons. We consider an experiment that the entanglement between two macroscopic mirrors suspended at the end of an equal-arm interferometer is destroyed by the noise of gravitons through bremsstrahlung. By calculating the correlation function of the noise, we obtain the decoherence time from the decoherence functional. We estimate that the decoherence time induced by the noise of gravitons in squeezed states stemming from inflation is approximately 20 seconds for 40 km long arms and 40 kg mirrors. Our analysis shows that observation of the decoherence time of quantum entanglement has the potential to detect gravitons indirectly. This indirect detection of gravitons would give strong evidence of quantum gravity.

Shinichiro Akiyama, University of Tsukuba

Tensor renormalization group approach to (1+1)-dimensional Hubbard model

##### Tensor renormalization group (TRG) approach is a variant of the real-space renormalization group to evaluate the path integral in the thermodynamic limit, without resorting to any probabilistic interpretation for the given Boltzmann weight. Moreover, since the TRG can directly deal with the Grassmann variables, this approach can be formulated in the same manner for the systems with bosons, fermions, or both of them. These advantages of the TRG approach have been confirmed by the earlier studies of various lattice theories, which suggest that the TRG potentially enables us to investigate the parameter regimes where it is difficult to access with the standard stochastic numerical methods, such as the Monte Carlo simulation. In this talk, we explain recent our numerical study of the (1+1)-dimensional Hubbard model with the TRG approach. Our results of the critical chemical potential and the critical exponent ν are consistent with the exact solutions obtained by the Bethe Ansatz. This talk is based on arXiv:2105.00372.

Hajime Otsuka, KEK

理論センタープロジェクト「弦からヒッグズ／フレーバー」セミナー Sharpening the Boundaries Between Flux Landscape and Swampland by Tadpole Charge