セミナー 2022年

Shoichiro Tsutsui, RIKEN

[QCD Theory Seminar] Recent progress on complex Langevin simulations of QCD at finite density

Online (Zoom)
The complex Langevin method (CLM) is one of the promising approaches to overcome the sign problem in conventional Monte Carlo simulations and is expected to reveal the phase diagram of QCD in the high-density region. On the other hand, the CLM is reliable only when the probability distribution of the drift term falls off exponentially or faster. Whether the CLM is reliable or not in the high-density region is still an open problem. In this talk, I will present recent results on the physics of QCD at finite density, based on the reliability of the CLM. I will also discuss possible setups to realize color superconductivity for future CLM simulations.

Sinya Aoki, YITP, Kyoto University

Conserved non-Noether charge in general relativity: Physical definition vs. Noether's 2nd theorem

Online (Zoom) https://kds.kek.jp/event/41513/
In this talk, we make a close comparison of a covariant definition of an energy/entropy in general relativity, recently proposed by a collaboration including the present authors, with existing definitions of energies such as the one from the pseudo-tensor and the quasi-local energy. We show that existing definitions of energies in general relativity are conserved charges from the Noether’s 2nd theorem for the general coordinate transformation, whose conservations are merely identities implied by the local symmetry and always hold without using equations of motion. Thus none of existing definitions in general relativity reflects the dynamical properties of the system, need for a physical definition of an energy. In contrast, our new definition of the energy/entropy in general relativity is generically a conserved non-Noether charge and gives physically sensible results for various cases such as the black hole mass, the gravitational collapse, and the expanding universe, while existing definitions sometimes lead to unphysical ones including zero and infinity. We conclude that our proposal is more physical than existing definitions of energies. Our proposal makes it possible to define almost uniquely the covariant and conserved energy/entropy in general relativity, which brings some implications to future investigations.

Shane O'Sullivan, Dublin City University

[JpDe Joint Seminar] Radio polarimetry and Cosmic Magnetism

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In this talk, I will introduce the techniques of radio polarimetry and Faraday rotation for the study of cosmic magnetic fields. Faraday rotation is a birefringent effect caused by magnetised plasma along the line of sight, which we measure using the frequency-dependent rotation of the plane of linearly polarized light from radio galaxies (i.e. synchrotron emission). Radio galaxies can be observed throughout the majority of the history of the Universe and are thus excellent beacons for measuring the properties of the cosmic web and their evolution with cosmic time. In particular, I will highlight recent results from the Low Frequency Array (LOFAR) radio telescope. LOFAR is the world’s premier low-frequency radio telescope, providing exceptional RM precision, in addition to unrivalled angular resolution, sensitivity and image fidelity, which facilitates the reliable identification of the host galaxy through comparison with optical and infrared data (from which one can then determine the redshift). Our recent work shows how these capabilities are allowing us to transform our understanding of cosmic magnetic fields and are providing a new way to study the properties of filaments and voids of the cosmic web in general.

Kai Murai, The University of Tokyo

SU(N)-natural inflation

Online (Zoom)
We study SU(N) gauge fields that couple to the inflaton through the Chern-Simons term. In this talk, I will shortly review the dynamics of SU(2) gauge fields during inflation and provide a general procedure to construct homogeneous, isotropic, and attractor solutions of SU(N) gauge fields during inflation. As specific examples, we construct the stable solutions for N=3 and 4 and numerically confirm that they are complete and attractor. I will also discuss the linear perturbations in our model. It is straightforward to apply our procedure to the other simple Lie groups.

Yuya Tanizaki, YITP

[QCD theory Seminar] Semiclassical description of confinement via center vortices and anomaly-preserving $T^2$ compactifications

Online (Zoom)
Confinement of 4d gauge theories is usually the strong-coupling problem, and we would like to develop its semiclassical understanding based on the idea of volume independence, or adiabatic continuity. We conjecture that the strong-coupling regime of many 4d gauge theories is continuously connected to the weak-coupling theories on $¥mathbb{R}^2¥times T^2$ on small $T^2$ with the nontrivial ‘t Hooft flux. We explicitly confirm the fractional theta periodicity for pure YM and chiral Lagrangian for QCD can be derived in this small $T^2$ regime. We also uncover why this is possible in view of anomaly-preservation of this compactification.

Masanori Tanaka, Osaka University

Sphaleron decoupling condition in extended Higgs models

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Electroweak baryogenesis is a promising scenario to explain the origin of the baryon asymmetry of the Universe. In order to realize the scenario, the electroweak phase transition should be strongly first order. The sphaleron decoupling condition is the criterion for the strongly first-order electroweak phase transition. In this talk, we discuss the sphaleron decoupling condition in extended Higgs models. We show that upper bounds on masses of additional Higgs bosons can be obtained by utilizing the sphaleron decoupling condition and the unitarity bound. This talk is based on the papers [arXiv: 2005.05250, 2201.04791].

Yuhma Asano, Tsukuba University

Classical solutions of pre-gravity theory in the IKKT matrix model

Online (Zoom) https://kds.kek.jp/event/41254/
IKKT matrix model, proposed as a non-perturbative formulation of type IIB superstring theory, has a rich structure even at a classical level. While the degrees of freedom of strings are considered to be implemented in the matrices of the theory, their exact interpretation has not been fully understood yet. In this talk, I will review the interpretation of the matrices in which the gravitational degrees of freedom appear as vielbeins of the Weitzenboeck connection in a nearly commutative limit (so called the semi-classical limit). In the interpretation, Einstein’s equations appear as a one-loop effect while the classical equations of motion of the matrix model can be interpreted as “pre-gravity.” I will explain the pre-gravity theory and show its gravitational solutions.

Mattia Bruno, CERN

New perspectives on Amplitudes from Eucliean Correlators

Online (Zoom)
The Monte Carlo methods used in Lattice QCD simulations rely on the rotation of the path integral to Euclidean metric. Unfortunately, the limited knowledge of correlation functions on finite subsets of points prevents a direct analytic continuation to Minkowski signature. In their seminal publication of 1990, Maiani and Testa showed that physical amplitudes away from threshold cannot be directly extracted, ie without an inverse problem, from Euclidean correlators, due to off-shell contaminations. In this presentation, I revisit and extend their original work, and explore the connection with recent developments on the inverse problem in Lattice QCD.

Shan-Ming Ruan, YITP Kyoto University

On the Page curve under final state projection

Online (Zoom) https://kds.kek.jp/event/40712/
The black hole singularity plays a crucial role in formulating Hawking’s information paradox. The global spacetime analysis may be reconciled with unitarity by imposing a final state boundary condition on the spacelike singularity. Motivated by the final state proposal, we explore the effect on final state projection in two-dimensional conformal field theories in our recent paper arXiv:2112.08433. We calculate the time evolution under postselection by employing the real part of pseudo-entropy to estimate the amount of quantum entanglement averaged over histories between the initial and final states. We find that this quantity possesses a Page curve-like behavior.

Paul Lasky, Monash University

[JpDe Joint Seminar] What's next in gravitational-wave astronomy?

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The LIGO and Virgo interferometers have observed almost 100 gravitational-wave signals from collisions of black holes and neutron stars. Meanwhile, pulsar timing arrays around the globe are beginning to see interesting signals that may be attributed to a gravitational-wave stochastic background, potentially from supermassive black hole binaries throughout the Universe, or even primordial gravitational waves from the very early Universe. In this talk, I will review the state of both ground-based gravitational-wave astronomy and pulsar timing arrays. I will also discuss our ability to use current and future gravitational-wave observations to test Einstein’s general theory of relativity in the ultra-strong field regime.

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