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## Current Works by Akira Ohnishi and His Collaborators

This page is created by Dr. Isse, and has been maintained by the members in Hokkaido University. AO thanks these collaborators.

We are studying nuclei in the universe, where nuclear matter is heated, compressed or expanded ! Under these conditions, we may have various degrees of freedom such as fragment-nucleon mixture, strangeness, hadron resonances/strings, and QGP.

### Strangeness Nuclear Physics and Supernova Matter EOS

H. Matsumiya, K. Tsubakihara, H. Maekawa, C. Ishizuka, A. Ohnishi,
Kousuke Sumiyoshi (Numazu College of Technology),
Shoichi Yamada (Waseda Univ.),
Hideyuki Suzuki (Tokyo Univ. of Science)

• A chiral symmetric relativistic mean field model with logarithmic sigma potential

We develop a chiral symmetric relativistic mean field model with logarithmic sigma potential derived in the strong coupling limit of the lattice QCD. We find that both of the nuclear matter and finite nuclei are well described in the present model. The normal vacuum is found to have global stability at zero and finite baryon densities, and an equation of state with moderate stiffness (K ~ 280 MeV) is obtained. The binding energies and charge radii of Z closed even-even nuclei are well reproduced in a wide mass range from C to Pb isotopes, except for the underestimates of binding energies in several jj closed nuclei.

• Tables of Hyperonic Matter Equation of State for Core-Collapse Supernovae
C. Ishizuka, A. Ohnishi, K. Tsubakihara, K. Sumiyoshi, S. Yamada
J. Phys. G (2008), to appear [arXiv:0802.2318] (pdf, src).

We present sets of equation of state (EOS) of nuclear matter including hyperons using an SUf3 extended relativistic mean field (RMF) model with a wide coverage of density, temperature, and charge fraction for numerical simulations of core collapse supernovae. Coupling constants of Σ and Ξ hyperons with the σ meson are determined to fit the hyperon potential depths in nuclear matter, UΣ0) ~ +30 MeV and UΞ0) ~ -15 MeV, which are suggested from recent analyses of hyperon production reactions. At low densities, the EOS of uniform matter is connected with the EOS by Shen et al., in which formation of finite nuclei is included in the Thomas-Fermi approximation. In the present EOS, the maximum mass of neutron stars decreases from 2.17 Msun (Neμ) to 1.63 Msun (NYeμ) when hyperons are included. In a spherical, adiabatic collapse of a 15Msun star by the hydrodynamics without neutrino transfer, hyperon effects are found to be small, since the temperature and density do not reach the region of hyperon mixture, where the hyperon fraction is above 1 \% (T > 40 MeV or &rhoB > 0.4 fm-3).

• Cascade hypernuclear production spectra at J-PARC
Hideki Maekawa, Kohsuke Tsubakihara, Hiroshi Matsumiya and Akira Ohnishi
[arXiv:0704.3929] (pdf, src)

We predict cascade hypernuclear production spectra expected in the forthcoming J-PARC experiment. In the Green's function method of the distorted wave impulse wave approximation with the local optimal Fermi averaging t-matrix, we can describe the Ξ- production spectra in the continuum and bound state region reasonably well. Predictions to the high resonlution spectra at J-PARC suggest that we should observe Ξ- bound state peak structure in (K-,K+) spectra in light nuclear targets such as 12C and 27Al.

• Hypernuclei and hyperonic matter in a chiral SU(3) symmetric RMF model

We develop a chiral SU(3) RMF model for octet baryons, as an extension of the recently developed chiral SU(2) RMF model with logarithmic sigma potential. For Σ-meson coupling, strong repulsion(SR) and weak repulsion(WR) cases are examined in existing atomic shift data of Σ-. In both of these cases, we need an attractive pocket of a few MeV depth around nuclear surface.

• Analysis of (π+-,K+) and (K-,K+) hypernuclear production spectra in distorted wave impulse approximation

We study the hyperon-nucleus potential with distorted wave impulse wave approximation (DWIA) using Green's function method. In order to include the nucleon and hyperon potential effects in Fermi averaging, we introduce the local optimal momentum approximation of target nucleons. We can describe the quasi free Λ, Σ and Ξ production spectra in a better way than in the standard Fermi averaged t-matrix treatments.

### High Energy Heavy-Ion Collisions

R. Mizukawa, M. Isse, A. Ohnishi, N. Otuka,
Pradip Kumar Sahu (Inst. of Phys., Bhubaneswar, India),
S.C.Phatak (Inst. of Phys., Bhubaneswar, India),
Yasushi Nara(Frankfurt Univ.),
Tetsufumi Hirano (Univ. of Tokyo)
• Mean-Field Effects on Collective Flows in High-Energy Heavy-Ion Collisions from AGS to SPS energies

Collective flows in heavy-ion collisions from AGS ((2-11)A GeV) to SPS ((40,158)A GeV) energies are investigated in a nonequilibrium transport model with nuclear mean-field (MF). Sideward <px>, directed v1, and elliptic flows v2 are systematically studied with different assumptions on the nuclear equation of state (EOS). We find that momentum dependence in the nuclear MF is important for the understanding of the proton collective flows at AGS and SPS energies. Calculated results with momentum dependent MF qualitatively reproduce the experimental data of proton sideward, directed, and elliptic flows in a incident energy range of (2-158)A GeV.

• Hadron-string cascade versus hydrodynamics in Cu+Cu collisions at √sNN=200 GeV

Single particle spectra as well as elliptic flow in Cu+Cu collisions at √sNN=200 GeV are investigated within a hadronic cascade model and an ideal hydrodynamic model. Pseudorapidity distribution and transverse momentum spectra for charged hadrons are surprisingly comparable between these two models. However, a large deviation is predicted for the elliptic flow. The forthcoming experimental data will clarify the transport and thermalization aspects of matter produced in Cu+Cu collisions.

• Elliptic Flow in a Hadron String Cascade Model at 130 GeV Energy

We present the analysis of elliptic flow at s1/2=130 A GeV energy in a hadron-string cascade model. We find that the final hadronic yields are qualitatively described. The elliptic flow v2 is reasonably well described at low transverse momentum (pT < 1 GeV/c) in mid-central collisions. However, this model is found to underestimate v2 at high pT or in peripheral collisions.

• Jet-Fluid String Formation and Decay in High-Energy Heavy-Ion Collisions
M. Isse, T. Hirano, R. Mizukawa, A. Ohnishi, K. Yoshino, Y. Nara
Int. J. Mod. Phys. E, to appear (QM06 Proceedings) [arXiv:nucl-th/0702068] (pdf, src).

We propose a new hadronization mechanism, jet-fluid string (JFS) formation and decay, to understand observables in intermediate to high-pT regions comprehensively. In the JFS model, hard partons produced in jet lose their energy in traversing the QGP fluid, which is described by fully three-dimensional hydrodynamic simulations. When a jet parton escapes from the QGP fluid, it picks up a partner parton from a fluid and forms a color singlet string, then it decays to hadrons. We find that high-pT v2 values in JFS are about two times larger than in the independent fragmentation model.

### Strong Coupling Limit Lattice QCD

• Phase diagram at finite temperature and quark density in the strong coupling limit of lattice QCD for color SU(3)

We study the phase diagram of quark matter at finite temperature (T) and finite chemical potential (μ) in the strong coupling limit of lattice QCD for color SU(3). We derive an analytical expression of the effective free energy as a function of T and μ, including baryon effects. The finite temperature effects are evaluated by integrating over the temporal link variable exactly in the Polyakov gauge with anti-periodic boundary condition for fermions. The obtained phase diagram shows the first order phase transition at low temperatures and the second order phase transition at high temperatures separated by the tri-critical point in the chiral limit. Baryon has effects to reduce the effective free energy and to extend the hadron phase to a larger μ direction at low temperatures.

• Meson mass spectrum at finite temperature and density in the strong coupling limit of lattice QCD for color SU(3)

We investigate the meson mass spectrum in the strong coupling limit of lattice QCD with one species of staggered fermion for the SU(Nc) color gauge group, including N_c=3. We analytically derive meson masses as functions of temperature and chemical potential via chiral condensates. We show that meson masses quickly decrease to zero when the chemical potential or the temperature approaches to the critical value. • Phase diagram evolution at finite coupling in strong coupling lattice QCD We investigate the chiral phase transition in the strong coupling lattice QCD at finite temperature and density with finite coupling effects. We adopt one species of staggered fermion, and develop an analytic formulation based on strong coupling and cluster expansions. We derive the effective potential as a function of two order parameters, the chiral condensate sigma and the quark number density $\rho_q$, in a self-consistent treatment of the next-to-leading order (NLO) effective action terms. NLO contributions lead to modifications of quark mass, chemical potential and the quark wave function renormalization factor. While the ratio mu_c(T=0)/Tc(mu=0) is too small in the strong coupling limit, it is found to increase as beta=2Nc/g^2 increases. The critical point is found to move in the lower T direction as beta increases. Since the vector interaction induced by $\rho_q$ is shown to grow as beta, the present trend is consistent with the results in Nambu-Jona-Lasinio models. The interplay between two order parameters leads to the existence of partially chiral restored matter, where effective chemical potential is automatically adjusted to the quark excitation energy. Moved to YITP, 2008.04.08: Ohnishi
update 2007.04.21 :Ohnishi
update 2007.02.03 :Ohnishi
update 2006.03.17 :Ohnishi
update 2005.04.11 :Isse
update 2004.08.20 :Isse
Open 2003.07.18 : Isse
Ohnishi Group
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