セミナー 2022年

Yasuaki Hikida, YITP Kyoto University

dS_3/CFT_2 correspondence

Online (Zoom) https://kds.kek.jp/event/40653/
In order to understand quantum gravity on cosmological backgrounds, dS/CFT correspondence is expected to play important roles. However, it has not been well understood yet compared with AdS/CFT correspondence. One of the reasons is that currently very few concrete examples are available and only generic arguments can be made. Recently, we provided a concrete proposal of dS_3/CFT_2 correspondence by making use of Chern-Simons description of (higher spin) gravity. In this talk, we will explain our proposal and supporting arguments by examining partition functions, correlation functions and so on.

Hiromasa Takaura, KEK theory center


Online (Zoom)
We recently proposed new collider experiments using a μ+ beam, utilizing the technology developed for the muon g-2 experiment at J-PARC to realize low emittance. We consider the experiment where the μ+ beam accelerated to 1 TeV collides with the TRISTAN energy (30 GeV) electron beam in the storage ring with the circumference of 3 km, the same size as the TRISTAN ring. The center-of-mass energy amounts to √s=346 GeV. Estimating possible luminosity, we find that the μ+e- collider can be a good Higgs boson factory and precision measurements of Higgs couplings are possible. We also consider a μ+μ+ collider with center-of-mass energy √s=2 TeV using the same ring. As a first study of new physics search, we discuss SUSY particle production processes in these colliders.
arXiv: 2201.06664
Y. Hamada, R. Kitano, R. Matsudo, H. Takaura, M. Yoshida

Masaki Oshikawa, ISSP University of Tokyo

Resolving the Berezinskii-Kosterlitz-Thouless transition in the two-dimensional XY model with tensor-network-based level spectroscopy

Online (Zoom) https://kds.kek.jp/event/41067/
The Berezinskii-Kosterlitz-Thouless (BKT) transition was historically the first example of topological phase transitions. Here we re-investigate the BKT transition in the 2D classical XY model, combining the Tensor Network Renormalization (TNR) and the Level Spectroscopy method based on the finite-size scaling of the Conformal Field Theory. By systematically analyzing the spectrum of the transfer matrix of the systems of various moderate sizes which can be accurately handled with a finite bond dimension, we determine the critical point removing the logarithmic corrections. This improves the accuracy by an order of magnitude over previous studies including those utilizing TNR. Our analysis also gives a visualization of the celebrated Kosterlitz Renormalization Group flow based on the numerical data.

Masakiyo Kitazawa, Osaka University

[QCD Theory Seminar] Search for phase transitions in dense QCD in heavy-ion collisions

Online (Zoom)
QCD is believed to have various phase transitions at nonzero density and temperature. In this talk, I will discuss two topics related to their experimental searches in relativistic heavy-ion collisions. The first topic is concerned with a search for the QCD critical point using fluctuation observables. I will overview recent theoretical and experimental progress in this field especially focusing on the non-Gaussian fluctuations of conserved charges. I will then discuss a more ambitious subject to search for color superconductivity. Near the phase boundary of color superconducting phases, diquark fluctuations are enhanced as the soft mode of the phase transition. We show that these fluctuations cause an anomalous enhancement of the dilepton production rate at low invariant-mass region through the analysis of the photon self-energy including diquark fluctuations. We argue that this enhancement can be used for an experimental signal of the precursor of color superconductivity.

Hiromasa Watanabe, Tsukuba University

Confinement/deconfinement transition in the D0-brane matrix model -- A signature of M-theory?

Online (Zoom) https://kds.kek.jp/event/40652/
We study the confinement/deconfinement transition in the D0-brane matrix model (the BFSS matrix model) and its one-parameter deformation (the BMN matrix model) numerically by lattice Monte Carlo simulations. Our results confirm general expectations from the dual string/M-theory picture for strong coupling. In particular, we observe the confined phase in the BFSS matrix model, which seems a nontrivial consequence of the M-theory picture. We suggest that these models provide us with an ideal framework to study the Schwarzschild black hole, M-theory, and furthermore, the parameter region of the phase transition between type IIA superstring theory and M-theory. This talk is based on the paper arXiv:2110.01312. In addition, the connection between string theory and M-theory in the context of the BFSS matrix model will be overviewed.

Kiyoharu Kawana, Seoul National University

Multi-critical point principle and its Phenomenology

Online (Zoom) https://kds.kek.jp/event/40651/
Multi-critical point principle (MPP) is an interesting theoretical possibility, which claims that coupling constants of low energy effective theory are tuned to the point where the vacuum structure or history of the Universe changes drastically. In this talk, I will try to explain the basic concepts and ideas of the MPP without going into the details of underlying physics. Then, I will present a few phenomenological applications of this principle, including our recent works (arXiv:2107.10720 and paper in preparation).

Yuuki Hayashi, Tohoku University

Renormalon subtraction using Fourier transform - towards precise QCD calculation

Online (Zoom)
Perturbative QCD gives divergent series due to renormalons, and theoretical predictions using such series are essentially ambiguous. By subtracting renormalons from the Wilson coefficients in the framework of the operator product expansion (OPE), we can achieve a precise calculation of the QCD effect. We propose a method for renormalon subtraction with systematic approximation accuracy by using the Fourier transform. It utilizes the properties of the Fourier transform, and the Wilson coefficient with the renormalons removed is presented as a one-parameter integral whose integrand has suppressed (or vanished) renormalons. In this talk, I will show an application to B and D meson masses as one of the first analyses. By subtracting the ambiguities of $O(¥Lambda_{QCD})$ and $O(¥Lambda_{QCD}^2/m)$, the non-perturbative parameters of HQET are determined with high accuracy. The results are consistent with theoretical expectations, and improvements in convergence and scale dependence are confirmed.

Raul Briceno, Old Dominion University

Long-range processes in QCD

Online (Zoom)
A rich variety of phenomena in the Standard Model and its extensions manifest in long-range processes involving bound states of quantum chromodynamics (QCD), namely hadrons. These are processes where intermediate hadronic states propagate over a long distance, between electroweak interactions. Examples include virtual Compton scattering and double-beta decay. Such processes are at the cusp of what can be systematically studied given two challenges. First, these reactions involve hadrons, and as a result one must use a non-perturbative tool to access their amplitudes. Currently lattice QCD is the only systematically improvable way we have for doing just this. Second, lattice QCD is defined in a finite, Euclidean spacetime. This introduces its own specific challenges, time in purely imaginary in lattice QCD, and by truncating the space one looses the notion of asymptotic states.
In this talk I explain how these issues can all be resolved systematically for a relatively large kinematic region. In presenting the necessary formalism for doing this, I will summarize recent progress in lattice QCD in order to argue that the community is up to the challenge.

Massimo D'Elia, University of Pisa

[QCD Theory Seminar] Updates on the QCD Phase Diagram in a Magnetic Field

Online (Zoom)
I will review results regarding the QCD Phase Diagram in the presence of a magnetic background field, based on lattice QCD simulations. In particular, I will discuss recent numerical evidence that the thermal QCD crossover turns into a first order transition for large enough magnetic fields. The critical endpoint is found to be located between eB = 4 GeV^2 and 9 GeV^2, with the corresponding critical temperature ranging between 100 and 60 MeV.

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