In 2013, the BESIII collaboration in China and the Belle collaboration in Japan reported the observations of four-quark states. In this seminar, the constituent quark model and the diquark are introduced. Some structure and dynamics of the baryons, four-quark states and pentaquark states in terms of diquark are reviewed. The candidates of four-quark states and pentaquark states are given and some explanations in the normal quark model are given. In particular, the diquark cluster is examined in the strong decays of baryons with one heavy quark in the 3P0 model.

Possible existence of η' meson-nucleus bound states (η'-mesic nuclei) has recently attracted both theoretical and experimental interests, since their properties such as binding energies and widths would reflect nature of axial U(1) anomaly and chiral symmetry breaking in Quantum Chromodynamics (QCD). We started experimental programs to investigate η'-mesic nuclei at GSI Heavy Ion Research Center (Germany). The first experiment was performed in 2014 by using a 2.5 GeV proton beam to produce η'-mesic nuclei via the ¹²C(p,d)η’x¹¹C reaction. The excitation energy of ¹¹C was obtained by measuring the momentum of the outgoing deuteron with the high-resolution spectrometer FRS. Despite the achieved high energy resolution and statistical sensitivity, no peak structure indicating the formation of η’-mesic nuclei was observed. Thus, upper limit for the formation cross section was determined and a constraint on the η-nucleus interaction was deduced.
Presently, we are preparing for a future experiment with further improved experimental sensitivity. The key of this new experiment is simultaneous measurements of the forward outgoing deuteron from the ¹²C(p,d)η’x¹¹C reaction and decay particles (protons) from η’-mesic nuclei. This semi-exclusive measurement will allow us to efficiently select signal events and thus suppress a large amount of physical background processes. The experiment is feasible by combining the FRS (or Super-FRS) spectrometer at GSI (FAIR) in Germany with a large acceptance detector system (WASA) surrounding a reaction target. Various developments are ongoing to realize this new experimental setup by the end of 2020 and to start production runs in 2021.
In this seminar, we will present the results of the first experiment and discuss the proposed future experiment. We will also introduce ongoing activities and developments to integrate the WASA detector system to FRS at GSI.

Near the heavy hadron threshold, many exotic states have been observed in the experiments, while those structures are not explained by the standard hadron picture. Those exotics are expected to be a multiquark state such as the hadronic molecule and compact multiquark. Especially, near the hadron threshold, hadronic molecules as a hadron composite could be generated dynamically. However, the hadron interaction which is an important ingredient has not been understood well. In this talk, we study the hidden-charm meson-baryon molecules, which can be compared with the pentaquark Pc reported by the recent Large Hadron Collider beauty (LHCb) experiment. Then, we introduce the one pion exchange potential (OPEP) as a long range interaction, and the short range interaction induced by coupling to the compact five-quark states. The OPEP is one of the basic interactions in the nuclear physics which includes the tensor term producing an attraction, and is enhanced by the heavy quark symmetry in the heavy quark sector. The short range interaction is given by coupling of molecules and compact five-quark states, where the spin structure is important to determine the relative strength for channels. By solving the coupled-channel Schrodinger equations, the energy spectra and the role of the interactions are discussed.

The usual Casimir effect is a zero-point energy shift induced by two parallel plates, which is a perturbative phenomenon in QED (particularly, photon fields). On the other hand, in the QCD vacuum including quarks and gluons (and also hadrons), the Casimir effect should be affected by the nonperturbative properties such as the spontaneous chiral symmety breaking, color confinement, and instantons. Recently, similar phenomena in the pure Yang-Mills theory were observed from lattice simulations [1,2,3], so that the "QCD Casimir effect" would also attract much attention in the future. In this talk I will discuss our recent studies [4,5] about the Casimir effects for chiral partners in hadrons such as nucleons and D mesons.
[1] M.N. Chernodub, V.A. Goy, A.V. Molochkov, and H.H. Nguyen, Phys.Rev.Lett.121, 191601 (2018).
[2] M.N. Chernodub, V.A. Goy, and A.V. Molochkov, Phys.Rev.D99, 074021 (2019).
[3] M. Kitazawa, S. Mogliacci, I. Kolbé, and W.A. Horowitz, Phys.Rev.D99, 094507 (2019).
[4] T. Ishikawa, K. Nakayama, and K. Suzuki, Phys.Rev.D99, 054010 (2019).
[5] T. Ishikawa, K. Nakayama, D. Suenaga, and K. Suzuki, Phys.Rev.D100, 034016 (2019).

格子QCD、強い相互作用を記述する量子色力学 (QCD) を有限体積かつ有限格子間隔aの格子上に表現するこの手法は、モンテカルロ法による数値計算と結びつくことによって第一原理からハドロンの質量スペクトルを再現するまでに至った。近年では「ヤン-ミルズ勾配流」の応用による格子エネルギー運動量テンソル（EMT）の構成法の発見によって、クォークグルーオンプラズマの粘性係数のような重要な物理量を格子QCDによって導出することも可能となった。しかし、特に有限温度の格子EMT計算において、連続理論と格子理論のズレである「離散化誤差」が計算結果に与える影響は未知数である。本セミナーではゼロ温度で考案したツリーレベル離散化誤差の抑制法を有限温度に対し拡張、実際に格子EMTを用いた熱力学量計算へと適用することによって、離散化誤差が結果に及ぼす影響について議論したい。