The Sachdev-Ye-Kitaev (SYK) model consists of Q fermions with an all-to-all random two-body hopping [1,2]. Its static state is a non-Fermi liquid with nonzero entropy at vanishing temperature, which is called the Sachdev-Ye (SY) state [3]. Since the SY state has been conjectured to be holographically dual to charged black holes with two-dimensional anti-de Sitter horizons, the SYK model has attracted much attention as a new theoretical tool for studying quantum gravity (see, e.g., Ref. [4]). In this work, we propose a possible route for realizing the SYK model experimentally with use of ultracold gases in optical lattices [5]. We also show how to measure out-of-time-order correlators of the SYK model, which may allow for capturing one of the most crucial characteristics of a black hole, namely maximally chaotic property.
[1] A. Y. Kitaev, “A simple model of quantum holograpy”, KITP string seminar and Entanglement 2015 program (Feb. 12, April 7, and May 27, 2015). http://online.kitp.ucsb.edu/online/entangled15/.
[2] S. Sachdev, Phys. Rev. X 5, 041025 (2015).
[3] S. Sachdev and J. Ye, Phys. Rev. Lett. 70, 3339 (1993).
[4] J. Maldacena and D. Stanford, Phys. Rev. D 94, 106002 (2016).
[5] I. Danshita, M. Hanada, and M. Tezuka, arXiv:1606.02454v2 [cond-mat.quant-gas], to appear in PTEP.

Parton distribution and correlation functions describe the relation between a hadron and the quarks and gluons (or partons) within it, and carry rich information on hadron’s partonic structure. They cannot be calculated by QCD perturbation theory, and have been extracted from experimental data of high energy scattering cross sections with the aid of QCD factorization. Parton distribution functions are the simplest of all correlation functions. Without them, we would not be able to understand the hard probes in hadronic collisions, including the Higgs discovery at the LHC. In this talk, I will discuss how can the QCD factorization help extract the same parton distribution and correlation functions from various “lattice cross sections” – hadron matrix elements that can be systematically calculated by using lattice QCD techniques. With the ab initio lattice QCD calculations, complementary to the experimental measurements, I argue that we are entering a new era of exploring hadron’s internal structure in terms of the dynamics of quarks and gluons.

In this talk, we begin by reviewing the basic of CFT correlation functions for operators with spins and the structures of their holographic duals: Spinning Witten Diagrams. I will then talk about so-called “Spinning Geodesic Witten Diagrams” (SGWDs), proposed to be the holographic dual configuration of spinning conformal partial waves, from the perspectives of CFT operator product expansions. To this end, we explicitly consider three point SGWDs which are natural building blocks of all possible four point SGWDs, discuss their gluing procedure through integration over spectral parameter, and this leads us to a direct identification with the integral representation of CFT conformal partial waves. If the time allows, I will also wrap up the seminar with some on-going work for construct so-called Mellin amplitudes for spinning fields. The contents of this talk will be based on work collaborated with En-Jui Kuo and Hideki Kyono.

It has been a long-standing problem to analytically prove confinement of pure Yang-Mills theory. One possible scenario is to start with the weakly-coupled region, obtain the trans-series expansion with respect to gauge coupling constant, and then continue back to the strongly-coupled region, with the help of mathematics of “resurgence”. The program has been pursued mostly on the geometry R^3 x S^1, however there the trans-series expansion could be spoiled by IR divergences. In this talk we discuss 4d SU(N) Yang-Mills theory on R×T^3, with twisted boundary condition eliminating IR zero modes. By connecting the 4d Yang-Mills theory to 2d CP^{N-1}-model, we find that the fractional instantons connecting N vacua of the CP^{N-1}-model dynamically restores center symmetry. This talk is based on arXiv:1704.05852, in collaboration with Kazuya Yonekura.

Thanks to the successful operations of Fermi-LAT and MAGIC telescopes, the emission mechanism of gamma-ray pulsars has been getting clearer in the last 10 years. The number of detecter pulsars has increased from less than 10 to more than 200, and those emission can be basically explained as the Curvature radiation from the Outer Gap. On the other hand, new problems have also been found. For example, very high energy emission from the Crab pulsar could not be explained by the Curvature radiation. In this seminar, I will talk about the basic models of gamma-ray pulsar and observational and theoretical progress in the last 10 years, together with the future prospect of CTA observations.

If the neutrino mass ordering turns out to be inverted, one may always reorder it to be normal. In this case the resulting pattern of the PMNS matrix looks strange and needs an explanation. Given the normal neutrino mass ordering, on the other hand, one is usually concerned about how small the effective Majorana mass term of a neutrinoless double-beta decay can be. In this seminar I will address both issues by presenting some new phenomenological observations, and discuss a possible connection between the mu-tau reflection symmetry breaking and the neutrino mass ordering, octant of theta(23) and CP violation.

量子アニーリングは、量子効果を利用して組み合わせ最適化問題(＝複雑な相互作用を持つイジング模型の基底状態探索）を解くための枠組みである。本講演では、その基本的な考え方と定式化やハードウェアでの実装状況を説明するとともに，最近急速に研究が活性化しているnon-stoquasticハミルトニアンの問題について解説する。

Weak values were introduced as outcomes of weak measurements performed on ensembles of pre- and post-selected quantum systems, i.e., a conditional expectation value. I will argue that a weak value of an observable is a robust property of a single pre- and post-selected quantum system rather than a statistical property. During an infinitesimal time a system with a given weak value affected other systems as if it had been in an eigenstate with eigenvalue equal to the weak value. The difference between the weak value and the expectation value has been demonstrated experimentally on the example of photon polarization

We discuss the sign problem based on our recent two papers [1,2].
The sign problem is one of the grand challenges in theoretical physics, and it is also a problem relevant to recent heavy-ion collision experiments. In heavy-ion collisions at colliding eneriges of $¥sqrt{s_{NN}}=5-20$ GeV, recent data seem to suggest the existence of the first order QCD phase transition, while the existence of massive neutron stars implies that the EOS of dense matter needs to be stiff enough. It is desired to obtain the QCD phase boundary and EOS in the first-principles method, Monte Carlo simulations of lattice QCD, but the sign problem in finite density QCD prevents us to obtain precise results.
Recently developed two methods, the complex Langevin method (CLM) and the Lefschetz thimble method (LTM), are promising. In these methods, real variables are extended to complex, integration path/regions are modified from the original path, and can solve or at least circumvent the sign problem. Still, we have problems when singular points are located near the original integration path.
In the first part, we discuss the Lefschetz thimbles in the NJL model with vector interaction [1].
The NJL model has been utilized to discuss the QCD phase diagram, but it is not easy to apply CLM and LTM. Even in the mean field treatment of the NJL model, we have many singular points coming from the fermion determinant. Furthermore, when we introduce the vector interaction, another sign problem arises from the Wick rotation of the temporal component of the auxiliary vector field (auxiliary sign problem). We have found that these two problems can be in principle solved in LTM.
It is possible to obtain thimbles by requiring that the momentum integration path is chosen to be on the same Riemann sheet along the flow trajectories.
The latter problem can be handled by complexifying the vector field. In practice, however, the flow trajectories stop after some time near the fixed points.
In the second part, we propose a new method, the path optimization method (POM), to search for the integration path [2].
Since the flow equation in LTM blows up at some point in most of the actions, it is favorable to obtain the new integration path without solving the flow equation. One of the natural ideas is to apply the variational method. We first give the trial function which parametrize the integration path, and optimize the trial function by minimizing the weight cancellation (or by minimizing the cost function). We apply POM to a gaussian model [3]. We have found that the optimized path agrees with the thimbles around the fixed points, the local maxima of the Boltzmann weights, and the analytic results are well reproduced even in the parameter region where the sign problem is very severe and CLM is found to fail. Thus POM seems to be another promising tool for the sign problem. In the presentation, we also show some more recent results of POM.
[1] Lefschetz thimbles in fermionic effective models with repulsive vector-field, Yuto Mori, Kouji Kashiwa, Akira Ohnishi, arXiv:1705.03646 [hep-lat].
[2] Toward solving the sign problem with path optimization method, Yuto Mori, Kouji Kashiwa, Akira Ohnishi, arXiv:1705.05605 [hep-lat].
[3] New Insights into the Problem with a Singular Drift Term in the Complex Langevin Method, J. Nishimura and S. Shimasaki, Phys. Rev. D92, 011501 (2015), arXiv:1504.08359.

The standard paradigm of collisionless cold dark matter is in tension with measurements on large scales. In particular, the best fit values of the Hubble rate H0 and the matter density perturbation sigma8 inferred from the cosmic microwave background seem inconsistent with the results from direct measurements. We show that both problems can be solved in a framework in which dark matter consists of two distinct components, a dominant component and a subdominant component. The primary component is cold and collisionless. The secondary component is also cold, but interacts strongly with dark radiation, which itself forms a tightly coupled fluid. The growth of density perturbations in the subdominant component is inhibited by dark acoustic oscillations due to its coupling to the dark radiation, solving the σ8 problem, while the presence of tightly coupled dark radiation ameliorates the H0 problem. The subdominant component of dark matter and dark radiation continue to remain in thermal equilibrium until late times, inhibiting the formation of a dark disk. We present an example of a simple model that naturally realizes this scenario in which both constituents of dark matter are thermal WIMPs. Our scenario can be tested by future stage-IV experiments designed to probe the CMB and large scale structure.