Session 1WL: Computational Nuclear Physics I

What can we learn from large-scale MCSM calculations?

I shall overview recent results from Monte Carlo Shell Model (MCSM) calculations. We are performing large-scale MCSM calculations in two ways. One of them is MCSM ab-initio calculations for light nuclei. The JISP16 and others Hamiltonians are taken, and the structure of light nuclei is studied including intrinsic density. Abe will discuss in detail on those results before this presentation. I then present a brief summary of them. In this talk, I will focus on usual MCSM calculations on heavier exotic nuclei, with particular interest to nuclear shapes. In our recent studies, we have shown that exotic Ni isotopes manifest various intriguing features. One of them is shape coexistence of spherical, oblate and prolate shapes at low excitation energies. The prolate band is nearly super deformed, and comes down as low as below 2 MeV. The appearance of this band is related the Type II Shell Evolution which is another visible consequence of the tensor force. This phenomena can be generalized to Dual Quantum Liquid picture. If time allows, Zr and other nuclei will be discussed. Recent MCSM calculation enables us to see shape fluctuations of various components of a given shell-model eigenstate. The pairing interaction produces fluctuations around the same shape, while the shape itself fluctuates to form eigenstates. Such dynamical fluctuation includes those seen in doubly magic 56,68,78Ni as well as gamma-soft nuclei.   

Quantum Monte Carlo calculations of neutron and nuclear matter

Recent advances in experiments of the symmetry energy of nuclear matter and in neutron star observations yield important new insights on the equation of state of neutron matter at nuclear densities. In this regime the EOS of neutron matter plays a critical role in determining the mass-radius relationship for neutron stars. We show how microscopic calculations of neutron matter, based on realistic two- and three-nucleon forces, make clear predictions for the relation between the isospin-asymmetry energy of nuclear matter and its density dependence, and the maximum mass and radius for a neutron star. We will also discuss the recent extension of the Auxiliary Field Diffusion Monte Carlo method to study the equation of state of nuclear matter using two-body nucleon interactions. The equation of state of isospin-asymmetric nuclear matter will also be discussed.   

Hadron interactions and exotic hadrons from lattice QCD

One of the interesting subjects in hadron physics is to look for the multiquark configurations. One of candidates is the H-dibaryon (udsuds), and the possibility of the bound H-dibaryon has been recently studied from lattice QCD. We also extend the HAL QCD method to define potentials on the lattice between baryons to meson-meson systems including charm quarks to search for the bound tetraquark Tcc (ud $\bar{c}$ $\bar{c}$) and Tcs (ud $\bar{c}$ $\bar{s}$). In the presentation, after reviewing the HAL QCD method, we report the results on the H-dibaryon, the tetraquark Tcc (ud $\bar{c}$ $\bar{c}$) and Tcs (ud $\bar{c}$ $\bar{s}$), where we have employed the relativistic heavy quark action to treat the charm quark dynamics with pion masses, m$_{\pi} =$ 410, 570, 700 MeV.   

Recent Breakthrough in Lattice QCD for Hadron Structure

I present a first direct lattice-QCD calculation of the Bjorken-x dependence of hadron structure functions. The method is based on the observation that while in the rest frame of the proton, the parton distributions correspond to light-cone correlations, in the infinite-momentum frame, the same distributions correspond to time-independent space correlations. An effective field theory approach taking the infinite-momentum limit can be established, yielding matching conditions to relate finite-momentum lattice-QCD data to the light-cone distribution. This improves on the traditional lattice approach which relies on an operator product expansion to access limited information about the lowest moments of the distributions. In this talk, I present a pioneering study of the nucleon quark density, helicity and transversity distributions, and pion distribution amplitude.   

Merger of binary neutron stars in numerical relativity

The merger of binary neutron stars is one of most promising sources of gravitational waves. It is also a promising candidate for the central engine of short-hard gamma-ray bursts and a source of the strong transient electromagnetic signal that could be the counterpart of gravitational-wave signals. Numerical relativity is probably the unique tool for theoretically exploring the merger process, and now, it is powerful enough to provide us a wide variety of aspects of the binary-neutron-star merger. In this talk, I will summarize our current understanding of the entire merger event that is obtained by a large-scale numerical-relativity simulations. In particular, I focus on the relation between the neutron-star equation of state and gravitational waves emitted during the late inspiral and merger phase, and observable electromagnetic signal that is likely to be emitted by the dynamical ejecta through r-process nucleosynthesis.   

Improved microphysics in neutron star merger simulations

Neutron star mergers are expected to be among the main sources of gravitational waves detectable by the Advance LIGO/VIRGO/KAGRA detector network. In many cases, these mergers are also likely to power bright electromagnetic transients, including short gamma-ray bursts and ``kilonovae,'' the optical/infrared emission due to the radioactive decay of neutron rich elements in material unbound by the merger. These EM counterparts can provide important information on the environment in which the merger takes place and the nature of the binary, and their detection could shed a light on the origin of short gamma-ray bursts and of r-process elements. Numerical simulations of neutron star mergers using general relativistic codes are required to understand the merger dynamics, the impact of the equation of state of the neutron star on the gravitational wave signal, and the potential of a given binary to power electromagnetic counterparts to that signal. Until recently, however, general relativistic codes used very simple models for the neutron star - often a simple gamma-law equation of state without any additional microphysics. Although sufficient to model the gravitational wave signal before merger, this cannot be used to follow the post-merger evolution of the system, or even some aspects of the disruption of the neutron star. To do so, nuclear-theory based equations of state with temperature and composition dependence have to be used, and the effects of neutrinos and magnetic fields should be taken into account. In this talk, I will discuss current efforts to include more advanced microphysics in general relativistic simulations, what we can do so far, and what the remaining computational challenges are. I will also show how existing numerical simulations have helped us constrain the outcome of neutron star mergers, and what remains to be done in order to extract as much information as possible from upcoming gravitational wave and electromagnetic observations.