We present the results of an elliptic flow, $v_2$, analysis of $\mathrm{Cu}+\mathrm{Cu}$ collisions recorded with the solenoidal tracker detector (STAR) at the BNL Relativistic Heavy Ion Collider at $\sqrt{s_{\mathit{NN}}}=62.4$ and $200$ GeV. Elliptic flow as a function of transverse momentum, $v_2$($p_T$), is reported for different collision centralities for charged hadrons $h^{\pm }$ and strangeness-ontaining hadrons $K_S^0$, $\Lambda$, $\Xi$, and $\varphi$ in the midrapidity region $\left|\eta \right|<1.0$. Significant reduction in systematic uncertainty of the measurement due to nonflow effects has been achieved by correlating particles at midrapidity, $\left|\eta \right|<1.0$, with those at forward rapidity, $2.5<\left|\eta \right|<4.0$. We also present azimuthal correlations in $p+p$ collisions at $\sqrt{s}=200$ GeV to help in estimating nonflow effects. To study the system-size dependence of elliptic flow, we present a detailed comparison with previously published results from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. We observe that $v_2$($p_T$) of strange hadrons has similar scaling properties as were first observed in $\mathrm{Au}+\mathrm{Au}$ collisions, that is, (i) at low transverse momenta, $p_T<2\hspace{0.5em}\mathrm{GeV}/c$, $v_2$ scales with transverse kinetic energy, $m_T-m$, and (ii) at intermediate $p_T$, $2<p_T<4\hspace{0.5em}\mathrm{GeV}/c$, it scales with the number of constituent quarks, $n_q$. We have found that ideal hydrodynamic calculations fail to reproduce the centrality dependence of $v_2$($p_T$) for $K_S^0$ and $\Lambda$. Eccentricity scaled $v_2$ values, $v_2/\varepsilon$, are larger in more central collisions, suggesting stronger collective flow develops in more central collisions. The comparison with $\mathrm{Au}+\mathrm{Au}$ collisions, which go further in density, shows that $v_2/\varepsilon$ depends on the system size, that is, the number of participants $N_{\mathrm{part}}$. This indicates that the ideal hydrodynamic limit is not reached in $\mathrm{Cu}+\mathrm{Cu}$ collisions, presumably because the assumption of thermalization is not attained.

Reduced hindrance factors for $K$-forbidden $E2$ transitions from two- and three-quasiparticle isomers in the $A\approx 180$ deformed region are compared. Evidence is presented that when all three quasiparticles are of the same nucleon type (proton or neutron) there is a strong dependence of the $E2$ reduced hindrance factor on the isomer excitation energy.

We report results from tests of ${}^{83}\mathrm{Kr}$${}^m$ as a calibration source in liquid argon and liquid neon. ${}^{83}\mathrm{Kr}$${}^m$ atoms are produced in the decay of ${}^{83}\mathrm{Rb}$, and a clear ${}^{83}\mathrm{Kr}$${}^m$ scintillation peak at $41.5$ keV appears in both liquids when filling our detector through zeolite coated with ${}^{83}\mathrm{Rb}$. Based on this scintillation peak, we observe 6.0 photoelectrons/keV in liquid argon with a resolution of $8.2\%$ ($\sigma /E$) and $3.0$ photoelectrons/keV in liquid neon with a resolution of $19\%$ ($\sigma /E$). The observed peak intensity subsequently decays with the ${}^{83}\mathrm{Kr}$${}^m$ half-life after stopping the fill, and we find evidence that the spatial location of ${}^{83}\mathrm{Kr}$${}^m$ atoms in the chamber can be resolved. ${}^{83}\mathrm{Kr}$${}^m$ will be a useful calibration source for liquid argon, neon dark matter, and solar neutrino detectors.

Based on the extended optical model with the double folding potential, in which the polarization potential is decomposed into direct reaction (DR) and fusion parts, simultaneous ${\chi }^2$ analyses are performed of elastic scattering and fusion cross-section data for the ${}^9\mathrm{Be}+{}^{28}\mathrm{Si}$, ${}^{144}\mathrm{Sm}$, and ${}^{208}\mathrm{Pb}$ systems at near-Coulomb-barrier energies. The polarization potentials thus determined are found to reveal an interesting target mass number dependence reflecting the experimental observation that the fusion cross section becomes larger than the DR cross section as the target changes from ${}^{208}\mathrm{Pb}$ to ${}^{28}\mathrm{Si}$.

The structure of neutron-rich nuclei beyond ${}^{78}\mathrm{Ni}$ was studied using postaccelerated radioactive beams of ${}^{83,84,85}\mathrm{Ga}$ utilizing $\beta \hspace{0.5em}\gamma$ and $\beta -n\hspace{0.5em}\gamma$ spectroscopy. Our data, when combined with energy level systematics, suggests a possible new spherical subshell closure at $N=58$ is created by the nearly degenerated $\nu 3s_{1/2}$ and $\nu 2d_{5/2}$ orbitals being well separated from other orbitals above $N=50$. The near degeneracy of these states could be evidenced by isomerism in this region. The energies of the $2_1^{+}$ and proposed $4_1^{+}$ states observed in $N=52$ ${}^{84}\mathrm{Ge}$ are interpreted as an indication of a possible weakening of the doubly magic ${}^{78}\mathrm{Ni}$ core for nuclei beyond $N=50$. The experimental evidence is supported by spherical HFB calculations using the ${\mathrm{SkO}}_T$ functional involving the tensor term in the nucleon-nucleon interaction.

The spherical to deformed $\gamma$-unstable shape transition in nuclei is discussed in terms of the sextic oscillator as a $\gamma$-independent potential in the Bohr Hamiltonian. The wave functions, energy eigenvalues, and electric quadrupole and monopole transition rates are calculated in closed analytical form for the lowest-lying energy levels. It is shown that the locus of critical points for the spherical to deformed $\gamma$-unstable shape phase transition corresponds to a parabola in the parameter space of the model. The ratios of energy eigenvalues and electromagnetic transition probabilities are constant along this parabola. It is thus possible to associate parameter-free benchmark values to the ratios of relevant observables at the critical point of the transition that can be compared to experimental data. In addition, systematic studies of the shape evolution in isotope chains can be performed within the model. As an application, the model parameters are fitted to the energy spectra of the chains of even-even Ru, Pd, and Cd isotopes and the electric quadrupole transition probabilities are calculated. It is found that ${}^{104}\mathrm{Ru}$, ${}^{102}\mathrm{Pd}$, and ${}^{106,108}\mathrm{Cd}$ nuclei, which are usually considered to be good candidates for the $E$(5) symmetry, lie rather close to the critical parabola that separates the spherical and deformed $\gamma$-unstable domains. The isotope ${}^{116}\mathrm{Cd}$ is proposed as a new candidate for a similar critical-point nucleus.

The excited states of ${}^{148}\mathrm{Ho}$ and ${}^{149}\mathrm{Ho}$ isotopes are studied using $\gamma$-ray and electron spectroscopy in off-beam and in-beam modes following ${}^{112,114}\mathrm{Sn}$(${}^{40}\mathrm{Ar}$,$\mathit{xnyp}$) reactions. Experiments include measurements of single $\gamma$-rays and conversion electron spectra as well as $\gamma$-$\gamma$, electron-$\gamma$, $\gamma -t$, and $\gamma -\gamma -t$ coincidences with the use of the OSIRIS-II 12-HPGe array and conversion electron spectrometer. Based on the present results, the level schemes of ${}^{148}\mathrm{Ho}$ and ${}^{149}\mathrm{Ho}$ are revised and significantly extended, up to about 4 and 5 MeV of excitation energy, respectively. Spin and parity of $5^{-}$ are assigned to the $9.59$-s isomer in ${}^{148}\mathrm{Ho}$ based on conversion electron results. Previously unobserved $\gamma$ rays feeding the ${10}^{+}$ isomer in ${}^{148}\mathrm{Ho}$ and the $27/2^{-}$ isomer in ${}^{149}\mathrm{Ho}$ nuclei are proposed. Shell-model calculations are performed. Possible core-excited states in ${}^{149}\mathrm{Ho}$ are discussed.

We study the relation between neutron removal cross section (${\sigma }_{-N}$) and neutron skin thickness for finite neutron-rich nuclei using the statistical abrasion ablation model. Different sizes of neutron skin are obtained by adjusting the diffuseness parameter of neutrons in the Fermi distribution. It is demonstrated that there is a good linear correlation between ${\sigma }_{-N}$ and the neutron skin thickness for neutron-rich nuclei. Further analysis suggests that the relative increase of neutron removal cross section could be used as a quantitative measure for neutron skin thickness in neutron-rich nuclei.

Usual Woods-Saxon single-particle levels with BCS pairing are not able to reproduce the experimental occupation probabilities of the proton and neutron levels $1p_{3/2}$, $1p_{1/2}$, $0f_{5/2}$, and $0g_{9/2}$ in the double-$\beta$ decay system ${}^{76}\mathrm{Ge}$ to ${}^{76}\mathrm{Se}$. Shifting down the $0g_{9/2}$ level by hand can explain the data, but it is not satisfactory. Here it is shown that a self-consistent Hartree-$\mathrm{Fock}+\mathrm{BCS}$ approach with experimental deformations for ${}^{76}\mathrm{Ge}$ and ${}^{76}\mathrm{Se}$ may decisively improve the agreement with the recent data on occupation probabilities by J. P. Schiffer et al. [Phys. Rev. Lett. 100, 112501 (2008)] and B. P. Kay et al. [Phys. Rev. C 79, 021301(R) (2009)]. Best agreement with available data on ${}^{76}\mathrm{Ge}$ and ${}^{76}\mathrm{Se}$, as well as on neighbor isotopes, is obtained when the spin-orbit strength for neutrons is allowed to be larger than that for protons. The two-neutrino double-$\beta$ decay matrix element is also shown to agree with the data.

Following the procedure introduced by Israel and Stewart, we expand the entropy current up to the third order in the shear stress tensor ${\pi }^{\alpha \beta }$ and derive a novel third-order evolution equation for ${\pi }^{\alpha \beta }$. This equation is solved for the one-dimensional Bjorken boost-invariant expansion. The scaling solutions for various values of the shear viscosity to the entropy density ratio $\eta /s$ are shown to be in very good agreement with those obtained from kinetic transport calculations. For the pressure isotropy starting with 1 at ${\tau }_0=0.4$ fm/$c$, the third-order corrections to Israel-Stewart theory are approximately $10\%$ for $\eta /s=0.2$ and more than a factor of 2 for $\eta /s=3$. We also estimate all higher-order corrections to Israel-Stewart theory and demonstrate their importance in describing highly viscous matters.

The Landau parameters of Skyrme interactions in the spin and spin-isospin channels are studied using various Skyrme effective interactions with and without tensor correlations. We focus on the role of the tensor terms on the spin and spin-isospin instabilities that can occur in nuclear matter above saturation density. We point out that these instabilities are realized in nuclear matter at the critical density of about two times the saturation density for all the adopted parameter sets. The critical density is shown to be very much dependent not only on the choice of the Skyrme parameter set, but also on the inclusion of the tensor terms.

Taking into account more accurately the isospin dependence of nucleon-nucleon interactions in the in-medium many-body force term of the Gogny effective interaction, new expressions for the single-nucleon potential and the symmetry energy are derived. Effects of both the spin (isospin) and the density dependence of nuclear effective interactions on the symmetry potential and the symmetry energy are examined. It is shown that they both play a crucial role in determining the symmetry potential and the symmetry energy at suprasaturation densities. The improved single-nucleon potential will be useful for more accurate simulation of nuclear reactions induced by rare-isotope beams within transport models.

Dynamical dipole $\gamma$-ray emission in heavy-ion collisions is explored in the framework of the quantum molecular dynamics model. The studies are focused on systems of ${}^{40}\mathrm{Ca}$ bombarding ${}^{48}\mathrm{Ca}$ and its isotopes at different incident energies and impact parameters. Yields of $\gamma$ rays are calculated and the centroid energy and dynamical dipole emission width of the $\gamma$ spectra are extracted to investigate the properties of $\gamma$ emission. In addition, sensitivities of dynamical dipole $\gamma$-ray emission to the isospin and the symmetry energy coefficient of the equation of state are studied. The results show that detailed study of dynamical dipole $\gamma$ radiation can provide information on the equation of state and the symmetry energy around the normal nuclear density.

$\gamma$-ray decays from excited states in ${}^{30}\mathrm{Ne}$ and inclusive and exclusive cross sections were measured in the ${}^9\mathrm{Be}$(${}^{32}\mathrm{Mg}$,${}^{30}\mathrm{Ne}$$+\gamma$)$X$ two-proton knockout reaction at incident beam energies of $99.7$ and $86.7$ MeV/nucleon. The measured cross section is suppressed compared to calculations and is indicative of a reduced overlap of initial and final state wave functions in ${}^{32}\mathrm{Mg}$ and ${}^{30}\mathrm{Ne}$. We interpret this reduction as due to large $4p4h$ amplitudes present in the ${}^{30}\mathrm{Ne}$ ground state wave function, but not ${}^{32}\mathrm{Mg}$. Such large intruder components are predicted to help stabilize the heavier fluorine isotopes against neutron emission.

We investigate the many-body level density of a gas of noninteracting fermions. We determine its behavior as a function of the temperature and the number of particles. As the temperature increases, and beyond the usual Sommerfeld expansion that describes the degenerate gas behavior, corrections due to a finite number of particles lead to Gumbel-like contributions. We discuss connections with the partition problem in number theory, extreme value statistics, and differences with respect to the Bose gas.

We discuss the effect of surface energy coefficients as well as nuclear surface diffuseness in the proximity potential and ultimately in the fusion of heavy-ions. Here we employ different versions of surface energy coefficients. Our analysis reveals that these technical parameters can influence the fusion barriers by a significant amount. A best set of these parameters is also given that explains the experimental data nicely.

Correlations between the behavior of the nuclear symmetry energy, the neutron skins, and the percentage of energy-weighted sum rule (EWSR) exhausted by the pygmy dipole resonance (PDR) in ${}^{68}\mathrm{Ni}$ and ${}^{132}\mathrm{Sn}$ are investigated by using different random phase approximation (RPA) models for the dipole response, based on a representative set of Skyrme effective forces plus meson-exchange effective Lagrangians. A comparison with the experimental data has allowed us to constrain the value of the derivative of the symmetry energy at saturation. The neutron skin radius is deduced under this constraint.

The dynamical nucleus-nucleus potentials for fusion reactions ${}^{40}\mathrm{Ca}+{}^{40}\mathrm{Ca}$, ${}^{48}\mathrm{Ca}+{}^{208}\mathrm{Pb}$, and ${}^{126}\mathrm{Sn}+{}^{130}\mathrm{Te}$ are studied with the improved quantum molecular dynamics model together with the extended Thomas-Fermi approximation for the kinetic energies of nuclei. The obtained fusion barrier for ${}^{40}\mathrm{Ca}+{}^{40}\mathrm{Ca}$ is in good agreement with the extracted fusion barrier from the measured fusion excitation function, and the depths of the fusion pockets are close to the results of time-dependent Hartree-Fock calculations. The energy dependence of the fusion barrier is also investigated. The fusion pocket becomes shallow for a heavy fusion system and almost disappears for heavy nearly symmetric systems, and the obtained potential at short distances is higher than the adiabatic potential.

We present a measurement of ${\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ photonuclear production in ultraperipheral $\mathrm{Au}-\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV from the STAR experiment. The ${\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ final states are observed at low transverse momentum and are accompanied by mutual nuclear excitation of the beam particles. The strong enhancement of the production cross section at low transverse momentum is consistent with coherent photoproduction. The ${\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ invariant mass spectrum of the coherent events exhibits a broad peak around $1540\pm 40$ $\mathrm{MeV}/c^2$ with a width of $570\pm 60$ $\mathrm{MeV}/c^2$, in agreement with the photoproduction data for the ${\rho }^0(1700)$. We do not observe a corresponding peak in the ${\pi }^{+}{\pi }^{-}$ final state and measure an upper limit for the ratio of the branching fractions of the ${\rho }^0(1700)$ to ${\pi }^{+}{\pi }^{-}$ and ${\pi }^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$ of $2.5\%$ at $90\%$ confidence level. The ratio of ${\rho }^0(1700)$ and ${\rho }^0(770)$ coherent production cross sections is measured to be $13.4\pm {0.8}_{\mathrm{stat}.}\pm {4.4}_{\mathrm{syst}.}\%$.

The ${}^{17}\mathrm{O}$($p,\gamma$)${}^{18}\mathrm{F}$ reaction sensitively influences hydrogen burning nucleosynthesis in a number of stellar sites, including classical novae. These thermonuclear explosions, taking place in close binary star systems, produce peak temperatures in the range of $T=100–400$ MK. Recent results indicate that the thermonuclear rates for this reaction in this particular temperature range are dominated by the direct capture process. We report on the measurement of the ${}^{17}\mathrm{O}$($p,\gamma$)${}^{18}\mathrm{F}$ cross section between the narrow resonances at $E_r^{\mathrm{lab}}=193$ and $519$ keV, where the $S$ factor is expected to vary smoothly with energy. We extract the direct capture contribution from the total cross section and demonstrate that earlier data are inconsistent with our results.