In this work it is shown that the use of a hydrodynamical model of heavy-ion collisions which incorporates recent developments, together with updated photon emission rates, greatly improves agreement with both ALICE and PHENIX measurements of direct photons, supporting the idea that thermal photons are the dominant source of direct photon momentum anisotropy. The event-by-event hydrodynamical model uses the impact parameter dependent Glasma model (IP-Glasma) initial states and includes, for the first time, both shear and bulk viscosities, along with second-order couplings between the two viscosities. The effect of both shear and bulk viscosities on the photon rates is studied, and those transport coefficients are shown to have measurable consequences on the photon momentum anisotropy.

Multiplicity-correlation measurements provide insight into the dynamics of high-energy collisions. Models describing these collisions need these correlation measurements to tune the strengths of the underlying QCD processes which influence all observables. Detectors, however, often possess limited coverage or reduced efficiency that influence correlation measurements in obscure ways. In this paper, the effects of nonuniform detection acceptance and efficiency on the measurement of multiplicity correlations between two distinct detector regions (termed forward-backward correlations) are derived. An analysis method with such effects built in is developed and subsequently verified using different event generators. The resulting method accounts for acceptance and efficiency in a model-independent manner with high accuracy, thereby shedding light on the relative contributions of the underlying processes to particle production.

We show that the large difference in the measured lifetime for asymmetric fission of the highly excited ($T\approx 1.5\text{-MeV}$) $Z=120$ nucleus as measured by the atomic techniques (crystal blocking and x-ray methods) with those measured by the nuclear techniques (mass-angle distribution and prefission neutron multiplicity) cannot be due to the different sensitivities of the atomic and nuclear techniques in different time domains. The claim of formation of a superheavy $Z=120$ nucleus with a high fission barrier on the basis of an observed long fission time by the atomic techniques is in direct conflict with all other available measurements and calculations.

A Bogoliubov quasiparticle formulation of an equation-of-motion phonon method, suited for open-shell nuclei, is derived. Like its particle-hole version, it consists of deriving a set of equations of motions whose iterative solution generates an orthonormal basis of $n$-phonon states ($n=0,1,2,...$), built of quasiparticle Tamm-Dancoff phonons, which simplifies the solution of the eigenvalue problem. The method is applied to the open-shell neutron-rich ${}^{20}\text{O}$ for illustrative purposes. A Hartree-Fock-Bogoliubov canonical basis, derived from an intrinsic two-body optimized chiral Hamiltonian, is used to derive and solve the eigenvalue equations in a space encompassing a truncated two-phonon basis. The spurious admixtures induced by the violation of the particle number and the center-of-mass motion are eliminated to a large extent by a Gram-Schmidt orthogonalization procedure. The calculation takes into account the Pauli principle, is self-consistent, and is parameter free except for the energy cutoff used to truncate the two-phonon basis, which induces an increasing depression of the ground state through its strong coupling to the quasiparticle vacuum. Such a cutoff is fixed so as to reproduce the first $1^{-}$ level. The two-phonon states are shown to enhance the level density of the low-energy spectrum, consistently with the data, and to induce a fragmentation of the $E1$ strength which, while accounting for the very low $E1$ transitions, is not sufficient to reproduce the experimental cross section in the intermediate energy region. This and other discrepancies suggest the need of including the three-phonon states. These are also expected to offset the action of the two phonons on the quasiparticle vacuum and, therefore, free the calculation from any parameter.

Background: Studies of fission dynamics, based on nuclear energy density functionals, have shown that the coupling between shape and pairing degrees of freedom has a pronounced effect on the nonperturbative collective inertia and, therefore, on dynamic (least-action) spontaneous fission paths and half-lives.

Purpose: The aim is to analyze the effects of particle-number fluctuation degrees of freedom on symmetric and asymmetric spontaneous fission (SF) dynamics, and to compare the findings with the results of recent studies based on the self-consistent Hartree-Fock-Bogoliubov (HFB) method.

Methods: Collective potentials and nonperturbative cranking collective inertia tensors are calculated using the multidimensionally-constrained relativistic-mean-field (MDC-RMF) model. Pairing correlations are treated in the BCS approximation using a separable pairing force of finite range. Pairing fluctuations are included as a collective variable using a constraint on particle-number dispersion. Fission paths are determined with the dynamic programming method by minimizing the action in multidimensional collective spaces.

Results: The dynamics of spontaneous fission of ${}^{264}\mathrm{Fm}$ and ${}^{250}\mathrm{Fm}$ are explored. Fission paths, action integrals, and corresponding half-lives computed in the three-dimensional collective space of shape and pairing coordinates, using the relativistic functional DD-PC1 and a separable pairing force of finite range, are compared with results obtained without pairing fluctuations. Results for ${}^{264}\mathrm{Fm}$ are also discussed in relation with those recently obtained using the HFB model.

Conclusions: The inclusion of pairing correlations in the space of collective coordinates favors axially symmetric shapes along the dynamic path of the fissioning system, amplifies pairing as the path traverses the fission barriers, significantly reduces the action integral, and shortens the corresponding SF half-life.

Transport simulations are very valuable for extracting physics information from heavy-ion-collision experiments. With the emergence of many different transport codes in recent years, it becomes important to estimate their robustness in extracting physics information from experiments. We report on the results of a transport-code-comparison project. Eighteen commonly used transport codes were included in this comparison: nine Boltzmann-Uehling-Uhlenbeck-type codes and nine quantum-molecular-dynamics-type codes. These codes have been asked to simulate $\mathrm{Au}+\mathrm{Au}$ collisions using the same physics input for mean fields and for in-medium nucleon-nucleon cross sections, as well as the same impact parameter, the similar initialization setup, and other calculational parameters at $100A$ and $400A$ MeV incident energy. Among the codes we compare one-body observables such as rapidity and transverse flow distributions. We also monitor nonobservables such as the initialization of the internal states of colliding nuclei and their stability, the collision rates, and the Pauli blocking. We find that not completely identical initializations may have contributed partly to different evolutions. Different strategies to determine the collision probabilities and to enforce the Pauli blocking also produce considerably different results. There is a substantial spread in the predictions for the observables, which is much smaller at the higher incident energy. We quantify the uncertainties in the collective flow resulting from the simulation alone as about 30% at $100A$ MeV and 13% at $400A$ MeV, respectively. We propose further steps within the code comparison project to test the different aspects of transport simulations in a box calculation of infinite nuclear matter. This should, in particular, improve the robustness of transport model predictions at lower incident energies, where abundant amounts of data are available.

We propose to model hadronization of parton showers in QCD jets through a hybrid approach involving quark recombination and string fragmentation. This is achieved by allowing gluons at the end of the perturbative shower evolution to undergo a nonperturbative splitting into quark and antiquark pairs, then applying a Monte Carlo version of instantaneous quark recombination, and finally subjecting remnant quarks (those which have not found a recombination partner) to Lund string fragmentation. When applied to parton showers from the pythia Monte Carlo event generator, the final hadron spectra from our calculation compare quite well to pythia jets that have been hadronized with the default Lund string fragmentation. Our new approach opens up the possibility to generalize hadronization to jets embedded in a quark gluon plasma.

This work is a continuation of our effort [S. Mallik, S. Das Gupta, and G. Chaudhuri, Phys. Rev. C 91, 034616 (2015)] to examine if signatures of a phase transition can be extracted from transport model calculations of heavy ion collisions at intermediate energy. A signature of first-order phase transition is the appearance of a bimodal distribution in $P_m\left(k\right)$ in finite systems. Here $P_m\left(k\right)$ is the probability that the maximum of the multiplicity distribution occurs at mass number $k$. Using a well-known model for event generation [Botzmann-Uehling-Uhlenbeck (BUU) plus fluctuation], we study two cases of central collision: mass 40 on mass 40 and mass 120 on mass 120. Bimodality is seen in both the cases. The results are quite similar to those obtained in statistical model calculations. An intriguing feature is seen. We observe that at the energy where bimodality occurs, other phase-transition-like signatures appear. There are breaks in certain first-order derivatives. We then examine if such breaks appear in standard BUU calculations without fluctuations. They do. The implication is interesting. If first-order phase transition occurs, it may be possible to recognize that from ordinary BUU calculations. Probably the reason this has not been seen already is because this aspect was not investigated before.

We have measured the azimuthal anisotropy of neutrons emitted from the ${}^{56}\mathrm{Fe}(\gamma ,n){}^{55}\mathrm{Fe}$ reaction with a linearly polarized $\gamma$-ray beam generated by laser Compton scattering at NewSUBARU. Neutron yields at the polar angle of ${90}^{\circ }$ have been measured as a function of the azimuthal angle $\varphi$ between the detector and the linear polarization plane of the $\gamma$-ray beam. The azimuthal anisotropy of neutrons measured at $\varphi =0^{\circ }$, ${10}^{\circ }$, ${25}^{\circ }$, ${45}^{\circ }$, ${60}^{\circ }$, ${70}^{\circ }$, and ${90}^{\circ }$ has been well reproduced using a theoretically predicted function of $a+bcos\left(2\varphi \right)$.

Background: For the deeply bound one-nucleon removal at intermediate energies using a ${}^9\text{Be}$ or ${}^{12}\text{C}$ target, a strong reduction of cross section was observed relative to the prediction of eikonal theoretical model. The large disagreement has not been explained and the systematic trend is inconsistent with results from transfer reactions. The recently observed asymmetric parallel momentum distribution of the knockout residue indicates the significant dissipative core-target interaction in the knockout reaction with a composite target, implying new reaction mechanisms beyond the eikonal reaction descriptions.

Purpose: To investigate the reaction mechanism for deeply bound nucleon removal at intermediate energies.

Method: Neutron removal from ${}^{14}\text{O}$ using a ${}^{12}\text{C}$ target at 60 MeV/nucleon was performed. Nucleon knockout cross sections were measured. The unbound excited states of ${}^{13}\text{O}$ were reconstructed by using the invariant mass method with the residues and the associated decay protons measured in coincidence. The measured cross sections are compared with an intra-nuclear cascade (INC) prediction.

Results: The measured cross section of (${}^{14}\text{O}$, ${}^{11}\text{C}$) is 60(9) mb, which is 3.5 times larger than that of (${}^{14}\text{O}$, ${}^{13}\text{O}$) channel. This $2pn$-removal cross section is consistent with INC prediction, which is 66 mb with the main contribution being non-direct reaction processes. On the other hand, the upper limit of the cross section for one-neutron removal from ${}^{14}\text{O}$ followed by proton evaporation is 4.6(20) mb, integrated up to 6 MeV above the proton separation energy of ${}^{13}\text{O}$. The calculated total cross section for such reaction processes by the INC model is 2.5 mb, which is within the measured upper limit.

Conclusions: The data provide the first constraint on the role of core excitation and evaporation processes in deeply bound nucleon removal from asymmetric nuclei. The experiment results suggest that non-direct reaction processes, which are not considered in the eikonal model, play an important role in deeply bound nucleon removal from asymmetric nuclei at intermediate energies.

We report on the mass measurements of several neutron-rich $\mathrm{Rb}$ and $\mathrm{Sr}$ isotopes in the $A\approx 100$ region with the TITAN Penning-trap mass spectrometer. By using highly charged ions in the charge state $q=10+$, the masses of ${}^{98,99}\mathrm{Rb}$ and ${}^{98–100}\mathrm{Sr}$ have been determined with a precision of 6–12 keV, making their uncertainty negligible for $r$-process nucleosynthesis network calculations. The mass of ${}^{101}\mathrm{Sr}$ has been determined directly for the first time with a precision eight times higher than the previous indirect measurement and a deviation of $3\sigma$ when compared to the Atomic Mass Evaluation. We also confirm the mass of ${}^{100}\mathrm{Rb}$ from a previous measurement. Furthermore, our data indicate the existence of a low-lying isomer with $80\mathrm{keV}$ excitation energy in ${}^{98}\mathrm{Rb}$. We show that our updated mass values lead to minor changes in the $r$ process by calculating fractional abundances in the $A\approx 100$ region of the nuclear chart.

The quantum-phase-transitional behavior of the spherical shell-model mean field plus the geometric quadrupole-quadrupole and standard pairing model within a single-$j$ shell is analyzed in detail. Various quantities, such as low-lying energy levels, some typical energy ratios, the overlaps of the excited states with those of the corresponding limiting cases, $B\left(E2\right)$ values and electric quadrupole moments of some low-lying states and their ratios, as functions of the control parameter of the model in a $j=15/2$ shell are calculated as an example, in which only a crossover occurs due to the Pauli exclusion. The results show that there are noticeable changes within the crossover region of the rotation-like to the pair-excitation (superconducting) phase transition, especially in the half-filling case. As an application to realistic nuclear systems, a chain of isotones ${}^{212}\mathrm{Rn}\text{−}{}^{213}\mathrm{Fr}\text{−}{}^{214}\mathrm{Ra}\text{−}{}^{215}\mathrm{Ac}$ is chosen to be described by the model with valence protons in the $1h_{9/2}$ shell. As far as the low-lying energy levels, the experimentally observed $B\left(E2\right)$ values, and the electric quadrupole moment within the yrast band are concerned, these nuclei seem fitted reasonably well. The results indicate that these nuclei are all within the rotation-like to the pair-excitation phase transition near the crossover point.

We have calculated the temperature dependence of shear $\eta$ and bulk $\zeta$ viscosities of quark matter due to quark-meson fluctuations. The quark thermal width originating from quantum fluctuations of quark-$\pi$ and quark-$\sigma$ loops at finite temperature is calculated with the formalism of real-time thermal field theory. Temperature-dependent constituent-quark and meson masses and quark-meson couplings are obtained in the Nambu–Jona-Lasinio model. We found a nontrivial influence of the temperature-dependent masses and couplings on the Landau-cut structure of the quark self-energy. Our results for the ratios $\eta /s$ and $\zeta /s$, where $s$ is the entropy density (also determined in the Nambu–Jona-Lasinio model in the quasiparticle approximation), are in fair agreement with results of the literature obtained from different models and techniques. In particular, our result for $\eta /s$ has a minimum very close to the quantum lower bound, $\eta /s=1/4\pi$.

The coupled-channels Lagrangian approach underlying the Giessen model (GiM) is extended to describe the $\pi N\to \pi N,2\pi N$ scattering in the resonance energy region. As a feasibility study we investigate single- and double-pion production up to the second resonance region. The $2\pi N$ production has been significantly improved by using the isobar approximation with $\sigma N$ and $\pi \mathrm{\Delta }\left(1232\right)$ in the intermediate state. The three-body unitarity is maintained up to an interference pattern between the isobar subchannels. The scattering amplitudes are obtained as a solution of the Bethe-Salpeter equation in the $K$-matrix approximation. As a first application we perform a partial-wave analysis of the $\pi N\to \pi N,{\pi }^0{\pi }^{0}N$ reactions in the Roper resonance region. We obtain $R_{\sigma N}\left(1440\right)={27}_{-9}^{+4}\%$ and $R_{\pi \mathrm{\Delta }}\left(1440\right)={12}_{-3}^{+5}\%$ for the $\sigma N$ and $\pi \mathrm{\Delta }\left(1232\right)$ decay branching ratios of $N^{*}\left(1440\right)$, respectively. The extracted $\pi N$ inelasticities and reaction amplitudes are consistent with the results from other groups.

During the Big Bang, ${}^6\mathrm{Li}$ was synthesized via the ${}^2\mathrm{H}(\alpha ,\gamma ){}^6\mathrm{Li}$ reaction. After almost 25 years of the failed attempts to measure the ${}^2\mathrm{H}(\alpha ,\gamma ){}^6\mathrm{Li}$ reaction in the laboratory at Big Bang energies, just recently the LUNA Collaboration presented the first successful measurements at two different Big Bang energies [Anders et al., Phys. Rev. Lett. 113, 042501 (2014)]. In this paper we will discuss how to improve the accuracy of the direct experiment. To this end the photon's angular distribution is calculated in the potential model. It contains contributions from electric dipole and quadrupole transitions and their interference, which dramatically changes the photon's angular distribution. The calculated distributions at different Big Bang energies have a single peak at $\sim {50}^{\circ }$. These calculations provide the best kinematic conditions to measure the ${}^2\mathrm{H}(\alpha ,\gamma ){}^6\mathrm{Li}$ reaction. The expressions for the total cross section and astrophysical factor are also derived by integrating the differential cross section over the photon's solid angle. The LUNA data are in excellent agreement with our calculations using a potential approach combined with a well established asymptotic normalization coefficient for ${}^6\mathrm{Li}\to \alpha +d$. Comparisons of the available experimental data for the $S_{24}$ astrophysical factor and different calculations are presented. The Big Bang lithium isotopic ratio ${}^6\mathrm{Li}/{}^7\mathrm{Li}=(1.5\pm 0.3)\times {10}^{-5}$ following from the LUNA data and the present analysis are discussed in the context of the disagreement between the observational data and the standard Big Bang model, which constitutes the second lithium problem.

The neutrino emissivity from the neutron branch of the modified Urca process is calculated. The nuclear correlation effects are taken into account by employing the correlation functions extracted from the lowest-order constrained variational (LOCV) method applied to asymmetric nuclear matter. Two-body nucleon interaction is modeled by a realistic Argonne AV18 potential. In order to get consistency with semiempirical saturation parameters of nuclear matter and the existence of $2M_{\odot }$ pulsars, we add a phenomenological Urbana UIX three-body potential to the nucleon Hamiltonian and apply a newly formulated version of the LOCV method that allows for three-body nucleon interactions. We find that at fixed temperature neutrino emissivity is a (weakly) decreasing function of density, due to quenching of the contribution from tensor correlations with increasing density. This is in variance with all previous works. We also find that three-body forces allow for the opening of the direct Urca process at nucleon density $0.3{\mathrm{fm}}^{-3}$.

A relationship between the Coulomb inclusive break-up probability and the radiative capture reaction rate for weakly bound three-body systems is established. This direct link provides a robust procedure to estimate the reaction rate for nuclei of astrophysical interest by measuring inclusive break-up processes at different energies and angles. This might be an advantageous alternative to the determination of reaction rates from the measurement of $B\left(E1\right)$ distributions through exclusive Coulomb break-up experiments. In addition, it provides a reference to assess the validity of different theoretical approaches that have been used to calculate reaction rates. The procedure is applied to ${}^{11}\mathrm{Li}$ $\left({}^9\mathrm{Li}+n+n\right)$ and ${}^6\mathrm{He}$ $\left({}^4\mathrm{He}+n+n\right)$ three-body systems for which some data exist.

We extend the microscopic particle-rotor model for hypernuclear low-lying states by including the derivative and tensor coupling terms in the point-coupling nucleon $\mathrm{\Lambda }$-particle $\left(N\mathrm{\Lambda }\right)$ interaction. Taking ${}_{\mathrm{\Lambda }}{}^{13}\mathrm{C}$ as an example, we show that a good overall description for excitation spectra is achieved with four sets of effective $N\mathrm{\Lambda }$ interactions. We find that the $\mathrm{\Lambda }$ hyperon binding energy decreases monotonically with increasing strengths of the high-order interaction terms. In particular, the tensor coupling term decreases the energy splitting between the first $1/2^{-}$ and $3/2^{-}$ states and increases the energy splitting between the first $3/2^{+}$ and $5/2^{+}$ states in ${}_{\mathrm{\Lambda }}{}^{13}\mathrm{C}$.

The structure of the low-lying states in the odd- and even-mass $A\approx 100$ Ru isotopes is studied in the framework of two algebraic models. The even-mass Ru nuclei are first described within the interacting boson model 1 (IBM-1). The output of these calculations was then used to calculate the odd-$A$ isotopes within the interacting boson-fermion model 1 (IBFM-1), where a coupling of the odd neutron to the even-even core is considered. The level energies and transition probabilities calculated in the present work are tested against the experimental data. One-nucleon transfer spectroscopic factors as well as electromagnetic moments were also calculated for the odd-$A$ Ru and compared to the experimental values. The transitional character of the isotopes is studied. Most of the low-lying positive-parity states in the odd-$A$ Ru nuclei below 2 MeV are interpreted on the basis of $\nu d_{5/2}$ and $\nu g_{7/2}$ configurations. The role of the $\nu s_{1/2}$ orbital in the nuclear structure of the odd-mass Ru nuclei at low energies is also studied. The negative-parity states are interpreted as $\nu h_{11/2}$ excitations coupled to the core. The evolution of the IBM-1 and IBFM-1 parameters is discussed.