Conference Summary of QNP2018

TL;DR

This paper summarizes the key topics, recent developments, and future prospects discussed at the QNP2018 conference, covering high-density quantum many-body systems, hadron physics, and experimental facilities.

Contribution

It provides an overview of the current status, recent advances, and interdisciplinary connections in hadron and nuclear physics as discussed at QNP2018.

Findings

Highlighting the rapid development of gravitational waves and nuclear physics

Advances in hadron interactions and nuclear structure with strangeness

Progress in lattice QCD and experimental facilities like EIC and GSI-FAIR

Abstract

This report is the summary of the Eighth International Conference on Quarks and Nuclear Physics (QNP2018). Hadron and nuclear physics is the field to investigate high-density quantum many-body systems bound by strong interactions. It is intended to clarify matter generation of universe and properties of quark-hadron many-body systems. The QNP is an international conference which covers a wide range of hadron and nuclear physics, including quark and gluon structure of hadrons, hadron spectroscopy, hadron interactions and nuclear structure, hot and cold dense matter, and experimental facilities. First, I introduce the current status of the hadron and nuclear physics field related to this conference. Next, the organization of the conference is explained, and a brief overview of major recent developments is discussed by selecting topics from discussions at the plenary sessions. They include rapidly-developing field of gravitational waves and nuclear physics, hadron interactions and nuclear structure with strangeness, lattice QCD, hadron spectroscopy, nucleon structure, heavy-ion physics, hadrons in nuclear medium, and experimental facilities of EIC, GSI-FAIR, JLab, J-PARC, Super-KEKB, and others. Nuclear physics is at a fortunate time to push various projects at these facilities. However, we should note that the projects need to be developed together with related studies in other fields such as gravitational physics, astrophysics, condensed-matter physics, particle physics, and fundamental quantum physics.

Full text

QNP2018 conference, Quarks, Hadrons, Nuclei, QCD

1 Introduction to nuclear physics at QNP2018

Hadron and nuclear physics is the field to investigate properties of many-body systems bound by strong interactions and to understand matter generation of universe starting from big bang, then developments to quark and gluon plasma, hadrons, nuclei, and further to neutron stars as illustrated in Fig. 1 [1]. The wide range of topics on the quantum-chromodynamics (QCD) phase diagram, such as quark-gluon plasma, dense stars, and hadrons in nuclear medium, are discussed in the QNP2018 session, “hot and cold dense matter”. Hereafter, the terminology “nuclear physics” is often used in this article by taking it as a broad field including hadron physics. Nuclear physics started as a field to study nuclear structure and reactions. The basic constituents of nuclei are protons, neutrons, and mesons which mediate the nuclear force. Nucleon-nucleon (NN) interactions have been determined by NN scattering measurements and deuteron properties. Nuclear structure and reactions have been investigated by using these basic NN interactions. Now, new types of nuclei are investigated by extending flavor degrees freedom to strange and charm. Especially, hypernuclear experiments are now in progress. We know that peculiar properties of nuclei, as a dense many-body system, often stem from the Pauli exclusion principle. For example, independent particle models work as good descriptions of nuclei in spite of the fact that the average nucleon separation is almost equal to the diameter of the nucleon. We are now stepping into nuclei partially without the exclusion principle by hadrons with strangeness and charm. Furthermore, there are significant progress recently from the lattice QCD side on baryon interactions. These topics were discussed in the session, “hadron interactions and nuclear structure”.

Hadron properties have been investigated by measuring global observables such as masses, decay widths, spins, and parities. In comparison with theoretical calculations on these quantities, several exotic-hadron candidates have been reported experimentally in the last decade. It is a fortunate time to investigate hadron-structure physics by extending our knowledge to exotic states beyond the original quark model of 1964 [2]. The topics of exotic hadrons and N∗ resonances are discussed in the session, “hadron spectroscopy”. On the other hand, parton structure of nucleon and nuclei has been studied in deep inelastic scattering of leptons and also hadron-hadron collisions. It is now coming to the stage of probing three-dimensional (3D) structure functions which contain information on form factors and parton distribution functions. The 3D tomography is crucial to clarify the origin of nucleon spin including partonic orbital-angular-momentum contributions and to open a new field of gravitational form factors of hadrons. These topics were discussed in the session, “quark and gluon structure of hadrons”.

Hadrons, nuclei, quark-gluon plasma, and neutron stars are bound systems of strong interactions, and the basic theory of strong interactions is known as QCD. All these matters should be described by the framework of QCD, namely by quark and gluon degrees of freedom, in principle. However, it is not straightforward to apply QCD to these topics in the whole kinematical region. At high energy reactions with large-momentum transfer, we know that the perturbative QCD can be applied because of the asymptotic freedom. At low energies, we rely on lattice QCD simulations; however, it is still very limited and it cannot be used at finite baryon densities. Therefore, it is useful and often the only method to employ descriptions of effective hadron and nuclear models in terms of hadron degrees of freedom [1]. An example is shown in Fig. 3 [3] to indicate a possible transition from hadron degrees of freedom to quark-gluon degrees of freedom by the exclusive reaction $\gamma+p\to\pi^{+}+n$ with the scattering angle of 90∘ in the center-of-mass frame. In hard exclusive hadron reactions, it is known that cross sections follow the constituent counting rule, namely the straight line in Fig. 3. Although the transition energy could change depending on reactions, nuclei are described by hadron degrees of freedom at low energies and by quark-gluon ones at high energies.

One of major fundamental nuclear-physics topics is to understand how hadron masses are generated because up- and down-quark masses are very small as illustrated in Fig. 3 [4, 5]. It is a similar topic to investigate the origin of nucleon spin. The nucleon spin is given by the matrix element of total angular momentum expressed by the quark-gluon energy-momentum tensor, whereas the hadron mass is expressed by the quark-gluon energy-momentum tensor. The scalar component of the matrix element of the energy-momentum tensor is the source of the gravity, namely the hadron mass, and the vector component indicates pressure and shear-forces inside the hadron [4, 5]. This hadron tomography is a fast developing field, and time has come to understand “how an apple falls in terms of quarks and gluons”.

2 QNP2018 organization

The QNP conferences are series of meetings previously held in Adelaide (2000), Jülich (2002), Bloomington (2004), Madrid (2006), Beijing (2009), Palaiseau (2012), and Valparaiso (2015). This QNP2018 was held at the Tsukuba International Congress Center in Tsukuba, Japan on November 13-17, 2018. At this conference, experimentalists and theorists discussed recent developments in nuclear physics, and the following topics were covered [6]:

             (A) Quark and gluon structure of hadrons,

             (B) Hadron spectroscopy,

             (C) Hadron interactions and nuclear structure,

             (D) Hot and cold dense matter.

There were also discussions on future facilities and presentations on fundamental physics experiments by using the neutron and nuclei. In addition to plenary sessions, parallel sessions were organized in these topics. A poster session was arranged. There are 24 talks in the plenary session, 128 (A:27, B:36, C:27, D:38) talks in the parallel, and 40 (A:6, B:9, C:7, D:16, other:2) posters.

We had 216 participants (students: 45, postdocs: 44, staffs: 127). There are one participant from Africa, 162 from Asia, 32 from Europe, 16 from North America, and 5 from South America:

             Africa 1 (South Africa: 1),

             Asia 162 (China: 18, India: 3, Indonesia: 4, Israeli: 4, Japan: 114, Korea: 12, Taiwan: 6,

                           Turkey: 1),

             Europa 32 (Austria: 3, Belgium: 1, France: 4, Germany: 11, Hungary: 1, Italy: 1, Poland: 1,

                             Portugal: 1, Russia: 5, Slovenia: 1, Spain: 2, UK: 1),

             North America 16 (Mexico: 2, USA: 14),

             South America 5 (Brazil: 2, Chile: 3).

The members of the international advisory committee are

S. Brodsky, W. Brooks, V. Burkert, W.-C. Chang, H. Enyo, A. Gal, H. Gao, M. Garcon, P. Giubellino, T. Hatsuda, B. Kopeliovich, T.-S. H. Lee, S. H. Lee, M. Lutz, Y.-G. Ma, G. Martinez, R. McKeown, L. McLerran, C. Meyer, A. K. Mohanty, T. Nagae, S. Nagamiya, T. Nakano, M. Oka, E. Oset, B. Pasquini, J.-C. Peng, B. Pire, J. Qiu, B. Sharkov, I. Strakovsky, M. Strikman, H. Tamura, U. Thoma, A. Thomas, R. Venugopalan, W. Weise, U. Wiedner, N. Xu, and B. Zou.

The local organizers are

A. Dote, Y. Goto, M. Harada, A. Hosaka, K. Itakura, H. Kamano, S. Kumano (co-chair), A. Monnai, O. Morimatsu, S. N. Nakamura, M. Naruki, H. Noumi, H. Ohnishi, K. Ozawa, H. Sako, F. Sakuma, S. Sawada (co-chair), H. Takahashi, T. Takahashi, K. Tanaka, and K. Tanida.

The conference is financially supported by APCTP, J-PARC/KEK, JSPS Grant on Innovative Area, RCNP at Osaka University, Tsukuba Tourism and Convention Association/Tsukuba City. We also have supports by JAEA, RIKEN and SOKENDAI.

3 Selected topics in the conference

All the talks cannot be summarized in this article, so that selected topics are explained briefly in the following from discussions at the plenary sessions [6].

3.1 Gravitational waves and nuclear physics

Neutron-star studies have a long history since the theoretical prediction in 1934 [7]; however, there are significant progress recently in this field motivated by the experimental observations on the neutron-star masses and gravitational waves, together with theoretical developments on baryon-baryon interactions and dense matters [8]. The experimental discovery of the gravitational waves in 2016 and subsequent developments of this gravitational-wave astrophysics have strong impact on nuclear physics. In particular, there was a first observation (GW170817) on gravitational waves and electromagnetic radiation in 2017 coming from the merger of two neutron stars [9].

Elements beyond the most stable iron are considered to be produced mainly by s (slow) and r (rapid) processes. The r-process elements used to be considered as produced in supernovae, which were not enough to explain all the existing elements. However, the neutron-star merger discovery paved the way for understanding the generation of the heavy elements. The r-processes can be inferred by observing the kilonovae, namely electromagnetic radiations by decays of heavy elements produced in the r-processes. The observations of neutron-star mergers seem to be consistent with theoretical kilonovae predictions. It suggests that neutron-star mergers should be one of major sources in creating heavy elements by the r-processes.

The neutron-star mergers also provide information on the equation of state (EOS) [8], which is the relation between the pressure and density, for neutron stars by tidal deformability. Various EOSs have been proposed theoretically in nuclear physics and they were tested by observing neutron-star masses and radii. The neutron-star mergers provide a constraint on the EOS because the neutron stars are distorted as they become closer, namely the merging process depends on the neutron-star radii and how easy they are stretched. This EOS affects gravitational-wave amplitude and frequency, so that it is constrained by the gravitational-wave observations. So far, the observations are consistent with theoretical models. Asides from the gravitational waves, we know that the EOS is significantly softened if hyperons exist in the neutron stars, so that hyperon-nucleon interactions should be investigated experimentally as mentioned in the following Sec.3.2, such as at Japan Proton Accelerator Research Complex (J-PARC), for providing precise information on building a realistic theoretical neutron-star model.

3.2 Hadron interactions and strangeness nuclear physics

The realm of nuclear physics can be expanded by extending the flavor degrees of freedom to strange and charm. Strangeness nuclear physics is investigated by hypernuclei and kaonic nuclei. The strange-quark mass is of the order of the QCD scale parameter $\Lambda_{QCD}$, which indicates that the strangeness is appropriate for probing QCD dynamics. Hyperons ($Y$) could exist in neutron stars, and information on $YN$ and $YY$ interactions is valuable for the neutron star EOS. In addition, there is no Pauli blocking for a hyperon in a nucleus, so that they are good probes of deep regions of nuclei. We started investigating dense many-body systems without a restriction of the exclusion principle.

One of the major projects at J-PARC is on strangeness nuclear physics by using secondary kaon beams [10, 11, 12, 13]. I explain some of major J-PARC results so far. The first achievement is the discovery of the charge-symmetry breaking (CSB) in hypernuclei [11]. The charge symmetry has been taken as granted in discussing gross nuclear phenomena. In ordinary nuclei without strangeness, the CSB was observed, for example, as the binding-energy difference between 3H and 3He. At J-PARC, the energy spacing between the spin-doublet states of ${}^{4}_{\Lambda}$He($1^{+}$, $0^{+}$) was determined to be $1406\pm 2\pm 2$ keV, which is a clear indication of the CSB in comparison with ${}^{4}_{\Lambda}$H($1^{+}$, $0^{+}$). This result sheds light on a new aspect of hyperon interactions with $\Sigma\Lambda$ mixture as the origin of the CSB phenomena in hypernuclei. Second, from the measurement of the binding energy ($B_{\Xi^{-}}=4.38\pm 0.25$ MeV) in the $\Xi^{-}$-14N system [12], it became clear that $\Xi N$ interaction is attractive. It opens a new filed of hypernuclear physics with strangeness $-2$ beyond strangeness $-1$. Hypernuclei have interesting characteristics in experimental observables. For example, shell structure of each $\Lambda$ state ($s_{\Lambda}$, $p_{\Lambda}$, and so on) in nuclei should be clearly seen at J-PARC by ($\pi$,$K^{+}$) reactions, much clearly than observed in a KEK-PS experiment. Third, there are experimental progress on new hadronic systems with an antikaon such as $K^{-}pp$ system [13]. Since an antikaon and a nucleon could form the bound state as $\Lambda(1405)$, kaonic nuclei could exist. At this stage, there are large variations in theoretical and experimental binding energies and decay widths. However, we expect that future experimental measurements and theoretical efforts will clarify this issue. J-PARC strangeness nuclear physics also has a strong impact on neutron-star EOS. In the near future, the high-momentum beamline will be ready, so that the J-PARC physics scope will be extended to different nuclear-physics topics. There is an approved experiment on charmed baryons, so that charmed-nucleus projects will be possible in future at J-PARC.

Next, short-range nucleon-nucleon correlations have been investigated in nuclei at the Relativistic Heavy Ion Collider (RHIC) and Thomas Jefferson National Accelerator Facility (JLab) by the reactions $(p,2pN)$ and $(e,e^{\prime}pN)$, respectively [14]. The two nucleons are emitted on opposite sides in the final state, so that these nucleons are considered to be close together in the initial state, and the process is sensitive to short-range $NN$ interactions. It used to be mysterious to find much stronger correlation for $pn$ than $pp$ and $nn$ ones in 2006 [15]; however, later studies found that it originates from the isospin-dependent tensor force. Such isospin dependence affects neutron-star studies because protons could exist in the stars with a certain fraction [16]. The short-range correlations are also investigated in the inclusive electron scattering $A(e,e^{\prime})X$ at very large $x$ ($>1$), where $x$ is the Bjorken variable. Especially, the nuclear modification ($\sigma_{{}^{56}\text{Fe}}/\sigma_{\text{D}}$, where $D$ is deuteron) plateau was observed at $x=1.4-1.9$ for the iron to indicate the two-nucleon correlations [14]. In future, we will step into the three-nucleon correlation studies above $x=2$. It is also possible to investigate such three-nucleon correlations at hadron facilities. The two- and three-nucleon correlation studies at high momenta beyond the Fermi momentum are valuable for studying dense matters such as the neutron stars.

3.3 Lattice QCD

Lattice QCD studies became increasingly important in nuclear physics with the progress of supercomputer power and theoretical developments since it is the only possible way to calculate nonperturbative quantities directly from QCD, although actual applications are still limited to the zero baryon density phenomena. The lattice studies are in a good situation in hadron spectroscopy, baryon-baryon interactions, and quark-hadron matters at the zero density [17].

Recently, we obtained more measurements on heavy-quark hadrons, and the lattice QCD is successful in explaining their spectra below hadronic thresholds. Above the thresholds, simulations of scattering are necessary. Recently, significant efforts have been made in lattice QCD to understand the resonances of shallow bound states and also states, which decay strongly, above the thresholds. For nuclear physics, the baryon-baryon interaction studies are valuable. So far, the lattice studies have been limited to confirming existing phenomenological studies on nucleon-nucleon interactions determined by scattering experiments and deuteron properties. However, the lattice simulations started to produce predictions recently on phenomena, for which experimental information does not exist. In this sense, lattice QCD is becoming a new stage where its results could be ahead of phenomenological theory models and even beyond experimental works, such as predictions on the bound states of $p\Omega^{-}$ and $\Omega^{-}\Omega^{-}$ systems. Since we know that QCD is the fundamental theory for strong interactions, even experimental measurements may not be necessary if QCD were precisely solved by the lattice simulation including finite densities.

The fast-developing area of lattice QCD in hadron physics is on an application to parton distribution functions (PDFs) [17]. Since they are defined by lightcone-separated field correlators, it was not possible to calculate the Bjorken-$x$ dependent PDFs. However, we may study quasi-PDFs with an equal-time separation so that it becomes possible to calculate the quasi-PDFs by lattice QCD. The quasi-PDFs agree with the PDFs in the infinite momentum limit $p_{z}\to\infty$, which is very challenging numerically. There are already numerical results on some PDFs. This project is valuable particularly for the PDFs, where there is little experimental information.

3.4 Hadron spectroscopy

Hadron spectroscopy has been investigated since 1960’s. However, the last decade is an especially appropriate time to extend basic quark-model descriptions in the sense that there are many reports on exotic-hadron candidates [18]. The name “exotic hadron” has been used for a hadron with a different internal configuration from $q\bar{q}$ and $qqq$ in the basic quark model. Experimental efforts have been made to find deviations from theoretical calculations of phenomenological models which assume the $q\bar{q}$ and $qqq$ configurations. On the other hand, because the quark number is not a conserved quantity in QCD, we should think what the exotic hadrons really mean. For example, since valence-quark numbers are related to the conserved quantities such as charges and baryon numbers, and since constituent quark numbers are counted in hard exclusive reactions according to perturbative QCD, there may be a way to use high-energy hadron reactions to find internal structure of exotic hadron candidates [3]. The 3D tomography technique in Sec. 3.5 could be also used in future for clarifying the internal configuration.

For understanding confinement in QCD, excited states of the nucleon (N∗) have been investigated. The N∗ program at electron accelerator facilities, such as JLab, has two major projects. One is to measure the N∗ spectrum and decay widths systematically, and the other is to study transition form factors from the nucleon to the excited states, for understanding low-energy QCD and confinement. There are significant improvements for N∗ states above the mass of 1700 MeV from the PDG (Particle Data Group)-2012 version by confirming them as 4-star (certain existence) states. In the last several years, there were discovery reports especially from BaBar, BES, Belle, and LHCb for exotic-hadron candidates in heavy hadron systems called $X$, $Y$, and $Z$ states with charm and bottom. Such exotic candidates are, for example, $X(3872)$, $Y(4260,4360,4660)$, $Z_{c}(3900,4020,4200,4430)$, $Z_{b}(10610,10650)$, and $P_{c}(4380,4450)$ [18]. These states could be multiquark states, hadron molecules, and/or mixture of these states. Kinematical cusp effects should be also considered in discussing the exotic-hadron candidates. There are also recent progress on heavy-quark hadrons, namely excited charmed baryons of $\Omega_{c}$, doubly-charmed baryons $\Xi_{cc}$, other doubly-heavy-quark baryons, and bottom baryons $\Xi_{b}$ and $\Sigma_{b}$. There are theoretical predictions on doubly-heavy tetraquark hadrons. On the other hand, diquark degrees of freedom could become important in excited baryon spectra, and it could be seen in baryons with a heavy quark like charm or bottom quark [19].

3.5 Nucleon structure

The project on unpolarized structure functions of the nucleon is becoming one of precision-physics fields because of accurate determination of the PDFs and theoretical efforts on higher-order perturbative-QCD corrections [5]. It is the basics for finding a new hadron-physics phenomenon and a signature beyond the standard model in any high-energy hadron reactions, especially at the Large Hadron Collider (LHC). On the other hand, the issue on the origin of nucleon spin has not been clarified yet [5, 20, 21]. Theoretical efforts had been done on how to decompose the nucleon spin into quark and gluon spin and orbital-angular-momentum (OAM) contributions in a color-gauge invariant way. This theoretical issue was settled down recently [21]. The remaining work is to determine gluon-spin and partonic OAM effects in experiments. Especially, in order to determine the OAM contributions, we need to investigate three-dimensional (3D) structure functions including transverse structure. This field is called hadron tomography, whereas one-dimensional structure functions, expressed by the longitudinal-momentum fraction $x$, have been mainly investigated.

There are three types in the 3D structure functions: generalized parton distributions (GPDs), generalized distribution amplitudes (GDAs), and transverse-momentum-dependent parton distributions (TMDs) [5, 20]. The second moments of the GPDs are related to the total angular momentum carried by partons, so that they are key quantities to solve the nucleon spin puzzle. The GPDs contain the PDFs and transverse form factors. There are experimental projects by JLab and CERN-COMPASS and in future by EIC. The GDAs are $s$-$t$ crossed quantities of the GPDs, and they are studied by KEKB and in future by GSI-FAIR (Gesellschaft für Schwerionenforschung -Facility for Antiproton and Ion Research). The GPDs and GDAs are related to hadronic matrix elements of energy-momentum tensor of quarks and gluons, and they are expressed by gravitational form factors. Therefore, this field is related to the origin of hadron masses and also pressure/shear-force in hadrons in terms of quark and gluon degrees of freedom.

The TMDs are or will be investigated by COMPASS, Fermilab, GSI-FAIR, LHC, JLab, and RHIC. There is an interesting development recently on explicit phenomenon of color flow in the TMD studies. The color flow is considered in structure functions to satisfy the color gauge invariance and it is called gauge link in lattice QCD. In the TMDs, the color flow should exist in transverse directions in addition to the longitudinal one, so that the TMDs could change a sign depending on processes. Recently, this color-flow effect was found by observing the TMDs in COMPASS and RHIC experiments. It could open a new field on explicit color phenomena in hadron physics. The field is also connected to color versions of quantum entanglement and the Aharonov-Bohm effect [5, 22].

3.6 Hot and dense quark-hadron matters

Soon after the big bang, the universe was filled with hot and dense particles, which were dominated by quarks and gluons, and then they formed hadrons and nuclei as illustrated in Fig. 1. High-energy heavy-ion collision experiments are intended to investigate QCD at finite density and temperature by laboratory experiments [23]. High-energy experiments are going on at LHC and RHIC. The initial state of high-energy heavy-ion collisions is described by dense gluon systems called color glass condensate. Then, these two Lorentz-contracted gluon sheets pass through each other to produce longitudinal electric and magnetic fields, and this matter at the pre-equilibrium stage is called glasma. Subsequently, the system reaches a thermodynamic equilibrium as quark-gluon plasma, which is described by hydrodynamics. Finally, hadronization occurs and then hadrons are observed by experiments. Theorists have been working a comprehensive framework based on transport theory for describing the reaction from the initial to the final state.

There was an important discovery at RHIC to find a nearly-perfect fluid for the quark-gluon plasma by observing the elliptic flow in comparison to hydrodynamical calculations [23]. However, recent experimental measurements indicate that the elliptic flow exists even for proton-proton (pp) and proton-nucleus (pA) collisions. Therefore, we need to understand the phenomena first for reactions of small systems such as pp and pA, and then the AA collisions may be reinvestigated. Next, since the largest magnetic field in nature is created in heavy-ion reactions, it is interesting to investigate topics associated with it. Quarks have spin, and their chiral magnetic effects are now under serious investigations. The effects should be observed as azimuthal correlations of produced particles in the final state. The topic of the chiral magnetic effects is fast developing as an interdisciplinary field such as with neutrino spin in neutron stars for supernova and with Weyl semimetals in condensed-matter physics.

Hadron masses in nuclear medium have been investigated to find the origin of hadron masses [23]. The up- and down-quark masses are much smaller than hadron masses, so that we need to find how the major part of the hadron masses is generated. The traditional idea is due to chiral symmetry breaking. An order parameter of the symmetry breading is the quark condensate $\langle q\bar{q}\rangle$. Since it is not an observable, meson-mass shifts, which are connected to the condensate, are experimentally investigated in nuclear medium. Experimental studies have been done at CERN-SPS, COSY, ELSA, GSI-HADES, JLab, KEK-PS, MAMI, RHIC, and SPring-8. In order to find the chiral-symmetry breaking from experimental measurements, we need to have transport-model calculations to compare with experimental observables. We also would like to have consistent measurements on mass shifts and widths among experimental groups. This topic should be investigated by the future experimental projects at CERN-SPS, GSI-FAIR, HIAF (High-Intensity Heavy Ion Accelerator Facility), J-PARC, and NICA (Nuclotron-based Ion Collider fAcility).

3.7 New experimental facilities

I briefly explain some of recent and future facilities on nuclear physics related to QNP2018. It should be noted that the following is not a complete list. For example, COMPASS, HIAF, NICA, and some other facilities are not mentioned.

3.7.1 J-PARC

The J-PARC is a high-intensity accelerator of the 1 MW range at relatively high energies 3-30 GeV [24]. Material and life-science experiments are done by using neutrons and muons produced by the 3-GeV proton beam. Nuclear and particle physics are investigated with secondary beams as well as the primary-proton beam. The relevant nuclear-physics experiments for the QNP2018 are done in the hadron experimental facility, where secondary beams such as pions, kaons, and antiprotons as well as the primary 30-GeV proton beam are available. The hadron experiments are intended to study new forms of hadronic and nuclear systems by extending flavor degrees of freedom to strange. The high-momentum beamline will be ready soon, so the J-PARC project will be extended to other hadron topics such as hadron properties in nuclear medium, charmed baryons, and nucleon structure. There is also a proposal on heavy-ion acceleration to study quark-hadron matters. The first priority of future hadron physics projects is to extend the current hall for 105 m in length for full operation of hadron and nuclear physics projects.

3.7.2 JLab

In 2017, the JLab completed the upgrade of its Continuous Electron Beam Accelerator Facility to achieve the beam energy of 12 GeV [25], the construction of the new end station Hall D, and new detector equipments. The project includes a wide range of topics, including the unpolarized and polarized 3D quark distributions in the nucleons and nuclei, as well as the longitudinal PDFs, at medium and large $x$ by deeply exclusive and semi-inclusive electron-scattering processes. The GPD studies will establish 3D picture of the nucleon including the transverse coordinates, and they are connected to the gravitational form factors. The TMD studies could allow us to step into the color-flow physics. The color confinement will be investigated by hadron spectroscopy and electromagnetic form factors of nucleons and nucleon resonances. Physics beyond the standard model will be investigated by parity-violating electron scattering. The JLab will play a leading role in hadron physics at medium and high energies in the next decade.

3.7.3 Super-KEKB

The Super-KEKB factory is an electron-positron collider with the center-of-mass energy of 10.6 GeV and the peak luminosity $8\times 10^{35}\text{cm}^{-2}\text{s}^{-1}$, which is 40 times larger than the KEKB collider [26]. It was completed in 2018. Its major motivation is to study CP violation and flavor physics to find new physics beyond the standard model. However, many new announcements of KEKB have been on discoveries of new hadrons. In fact, the $X(3872)$ article has the highest citations of 1571 in all the papers published by the Belle collaboration of KEKB, so that it is the most famous discovery of KEKB so far. The KEKB has significant contributions not only in the discoveries of exotic-hadron candidates named $X$, $Y$, and $Z$ but also in precision determination of fragmentation functions. The Super-KEKB facility will produce measurements with much better precisions, and we expect to have more discoveries and precision measurements on exotic hadrons and fragmentation functions. In addition, the GDAs are $s$-$t$ crossed quantities of the GPDs, and they are investigated by the two-photon processes at KEKB. Such studies lead to the understanding of 3D structure of hadrons and their gravitational form factors [4].

3.7.4 GSI-FAIR

The GSI-FAIR is a multipurpose facility with four experimental collaborations: APPA (Atomic, Plasma Physics and Applications), CBM (Compressed Baryonic Matter), NUSTAR (Nuclear Structure, Astrophysics and Reaction), and PANDA (Antiproton Annihilation at Darmstadt) [27]. The APPA is for fundamental investigations on material sciences and biophysics including medical applications. The CBM is for understanding dense and hot nuclear matter at lower temperature and higher baryon density than RHIC and LHC. The NUSTAR is for investigating exotic nuclei and heavy elements far off stability to understand nucleosynthesis. The PANDA is for hadron structure and dynamics with antiproton beams. The FAIR will provide beams of ions and antiprotons. The existing facility including SIS18 has been upgraded to serve as injector for FAIR, and it is considered as the FAIR phase-0 program. The main component of FAIR is the SIS100 ring accelerator, and it will accelerate protons up to 29 GeV (4 GeV at SIS18) and heavy ions up to 11 GeV/u (1 GeV/u). The facility is now under construction and it is expected to be completed in 2025.

3.7.5 EIC

An Electron-Ion Collider (EIC) is a major future nuclear-physics program in US. There are other EIC projects at CERN and in China, but I focus on the US facility in the following. There are two site possibilities at BNL and JLab. The project is in progress for completing it in the middle of 2020’s. The c.m. energies span 22$-$63 GeV (15$-$40 GeV) or 45$-$141 GeV (32$-$90 GeV) for polarized $ep$ ($eA$) collisions [28]. It will extend the current kinematical range to smaller $x$, as small as $10^{-4}$, with higher $Q^{2}$. The physics motivation is to understand how quarks and gluons make up nearly all of the visible matter in the universe. We expect that it will be realized by investigating 3D tomography of nucleons and nuclei, solving the proton spin puzzle, finding color-glass condensate, and studies of quark and gluon confinement. The EIC was included in the 2015 Nuclear Science Advisory Committee (NSAC) Long Range Plan. In 2018, a report by the National Academies of Sciences, Engineering, and Medicine positively endorsed the EIC proposal. We expect that it will be the leading facility from the middle of 2020’s to clarify basic issues of hadron physics.

4 Conclusion

Nuclear physics is the field of investigating matter generation of universe and properties of quark-hadron many-body systems as ultimate materials. We have excellent opportunities now to enhance our activities, not only because of major experimental facilities: BES, COMPASS, EIC, Fermilab, GSI-FAIR, HIAF, KEKB, JLab, J-PARC, LHC, NICA, RHIC, and so on, but also relations with other developing fields, such as gravitational waves, quantum computation/entanglement, high-energy cosmic rays, neutrino physics, and others. The hadron-nuclear physics needs to be developed together with neighboring fields, especially condensed-matter physics, particle physics, and astrophysics. There are 89 young students and postdocs among the total participant number of 216 at this QNP2018 conference, which suggests a bright future of our field.

Acknowledgments

The author thanks K. Kyutoku and A. Monnai for useful comments in writing this article.