The historic first detection of the binary neutron star merger GW170817 by the LIGO-Virgo collaboration has set a limit on the gravitational deformability of neutron stars. In contrast, radio observations of PSR $\mathrm{J}0740+6620$ find a very massive neutron star. Tension between the small deformability and the large maximum mass may suggest that the pressure rises rapidly with density and thus the speed of sound in dense matter is likely a large fraction of the speed of light. We use these observations and simple constant sound-speed model equations of state to set a lower bound on the maximum speed of sound in neutron stars. If the tidal deformability of a $1.4M_{\odot }$ neutron star is less than 600, as is suggested by subsequent analyses of GW170817, then we find that the sound speed in the cores of neutron stars is likely larger than the conformal limit of $c/\sqrt{3}$. Implications of this for our understanding of both hadronic and quark-gluon descriptions of dense matter are discussed.

Deexcitation $\gamma$ rays associated with an isomeric state of the ${}^{195}\mathrm{Os}$ were measured for the first time. The half-life of the isomeric state of the neutral atom ${}^{195\mathrm{m}}\mathrm{Os}$ was deduced. The isomers were produced in multinucleon transfer reactions between ${}^{136}\mathrm{Xe}$ beams and ${}^{198}\mathrm{Pt}$ targets, and were collected and separated by KEK Isotope Separation System. Analysis of $\gamma \text{−}\gamma$ correlation and comparison with previous measurement of the isomeric state using the ESR at GSI Darmstadt suggests a possible decay scheme of ${}^{195\mathrm{m}}\mathrm{Os}$.

A new model, based on the BCS approach, is especially designed to describe nuclear phenomena $(A,Z)\to (A,Z\pm 2)$ of double-charge exchange (DCE). Although it was proposed and applied in the particle-hole limit, by one of the authors [Krmpotić, Fizika B 14, 139 (2005)], it has not yet been applied within the BCS mean-field framework, nor has its ability to describe DCE processes been thoroughly explored. It is a natural extension of the $pn$-QRPA model, developed by Halbleib and Sorensen [Nucl. Phys. A 98, 542 (1967)] to describe the single $\beta$ decays $(A,Z)\to (A,Z\pm 1)$, to the DCE processes. As such, it exhibits several advantages over the $pn$-QRPA model when used in the evaluation of the double beta decay (DBD) rates. For instance, (i) the extreme sensitivity of the nuclear matrix elements (NMEs) on the model parametrization does not occur; (ii) it allows us to study the NMEs, not only for the ground state in daughter nuclei, as the $pn$-QRPA model does, but also for all final $0^{+}$ and $2^{+}$ states, accounting at the same time for their excitation energies and the corresponding DBD $Q$ values; (iii) together with the DBD-NMEs it also provides the energy spectra of Fermi and Gamow-Teller DCE transition strengths, as well as the locations of the corresponding resonances and their sum rules; (iv) the latter are relevant for both the DBD and the DCE reactions, since the underlying nuclear structure is the same; this correlation does not exist within the $pn$-QRPA model. As an example, detailed numerical calculations are presented for the $(A,Z)\to (A,Z+2)$ process in ${}^{48}\mathrm{Ca}\to {}^{48}\mathrm{Ti}$ and the $(A,Z)\to (A,Z-2)$ process in ${}^{96}\mathrm{Ru}\to {}^{96}\mathrm{Mo}$, involving all final $0^{+}$ states and $2^{+}$ states.

In this paper, a practical extrapolation method is proposed to predict nuclear masses which are not experimentally accessible. Our new method is based on the Garvey-Kelson mass relations and $\mathrm{J}\ddot{\mathrm{a}}\mathrm{necke}$ formulas. We demonstrate that the predictive power of this method is very competitive in comparison with other previous models in the market; the root-mean-squared deviations of our extrapolated results are typically below 200 keV for mass number $A$ larger than 120, if one predicts nuclear masses within four or five steps beyond the borders of the Atomic Mass Evaluation (AME) databases.

Accurate mass measurements of neutron-rich iron and cobalt isotopes ${}^{67}\mathrm{Fe}$ and ${}^{69,70}\mathrm{Co}$ have been realized with the JYFLTRAP double Penning-trap mass spectrometer. With novel ion-manipulation techniques, the masses of the ${}^{69,70}\mathrm{Co}$ ground states and the $1/2^{-}$ isomer in ${}^{69}\mathrm{Co}$ have been extracted for the first time. The measurements remove ambiguities in the previous mass values and yield a smoother trend on the mass surface, extending it beyond $N=40$. The moderate $N=40$ subshell gap has been found to weaken below ${}^{68}\mathrm{Ni}$, a region known for shape coexistence and increased collectivity. The excitation energy for the $1/2^{-}$ intruder state in ${}^{69}\mathrm{Co}$ has been determined for the first time and is compared to large-scale shell-model calculations. The new mass values also reduce significantly mass-related uncertainties for the astrophysical rapid neutron-capture process calculations.

High spin states of ${}^{37}\mathrm{Ar}$, populated through the ${}^{27}\mathrm{Al}({}^{12}\mathrm{C},np){}^{37}\mathrm{Ar}$ reaction with a 40 MeV ${}^{12}\mathrm{C}$ beam, were studied using the Indian National Gamma Array (INGA) facility. The existing level scheme has been extended up to 10.5 MeV by adding some new levels and transitions. The spins and parities of the new levels were assigned from $R_{\mathrm{DCO}}$, $R_{\mathrm{ADO}}$, and linear polarization measurements. The spins and parities of the existing levels also were modified or confirmed in the present experiment. The multipole mixing ratios ($\delta$) for most of the transitions were measured and compared with the earlier measurements wherever available. Large basis shell model calculations with different particle restrictions in $sd$ and $pf$ orbitals were performed to understand the microscopic origin of these levels. A simple two-level mixing calculation was also performed to extract the amount of multiparticle multihole configuration mixing for a few levels.

Rotational structures have been measured using the Jurogam II and GAMMASPHERE arrays at low spin following the ${}^{155}\mathrm{Gd}(\alpha ,2n){}^{157}\mathrm{Dy}$ and ${}^{148}\mathrm{Nd}({}^{12}\mathrm{C},5n){}^{155}\mathrm{Dy}$ reactions at 25 and 65 MeV, respectively. We report high-$K$ bands, which are conjectured to be the first candidates of a $K^{\pi }=2^{+}\gamma$ vibrational band, built on the $\left[505\right]11/2^{-}$ neutron orbital, in both odd-$A{}^{155,157}\mathrm{Dy}$ isotopes. The coupling of the first excited $K=0^{+}$ states or the so-called β vibrational bands at 661 and 676 keV in ${}^{154}\mathrm{Dy}$ and ${}^{156}\mathrm{Dy}$ to the $\left[505\right]11/2^{-}$ orbital, to produce a $K^{\pi }=11/2^{-}$ band, was not observed in both ${}^{155}\mathrm{Dy}$ and ${}^{157}\mathrm{Dy}$, respectively. The implication of these findings on the interpretation of the first excited $0^{+}$ states in the core nuclei ${}^{154}\mathrm{Dy}$ and ${}^{156}\mathrm{Dy}$ are also discussed.

Level structures of ${}^{212}\mathrm{Rn}$ have been studied by in-beam $\gamma$-ray spectroscopic methods using the ${}^{209}\mathrm{Bi}({}^6\mathrm{Li},3n){}^{212}\mathrm{Rn}$ reaction at beam energies of 28, 30, and 34 MeV. A number of new nonyrast states based on $\pi h_{9/2}^4$ and $\pi h_{9/2}^3{f}_{7/2}$ configurations have been identified. A $3^{(-)}$ collective state is also proposed at 2121 keV, which is most likely formed by mixing the octupole vibration with the $3^{-}$ member of the $\pi h_{9/2}^3{i}_{13/2}$ multiplet. The level scheme is compared with large-scale shell model calculations and discussed in terms of excitations of valence protons and without contributions from the ${}^{208}\mathrm{Pb}$ core. An overall excellent agreement is obtained for states that can be described in this model space.

Nuclear fusion reactions of D-D are examined in an environment comprised of high density cold fuel embedded in metal lattices in which a small fuel portion is activated by hot neutrons. Such an environment provides for enhanced screening of the Coulomb barrier due to conduction and shell electrons of the metal lattice, or by plasma induced by ionizing radiation (γ quanta). We show that neutrons are far more efficient than energetic charged particles, such as light particles ($e^{-},e^{+}$) or heavy particles ($p,d,\alpha$) in transferring kinetic energy to fuel nuclei (D) to initiate fusion processes. It is well known that screening increases the probability of tunneling through the Coulomb barrier. Electron screening also significantly increases the probability of large vs small angle Coulomb scattering of the reacting nuclei to enable subsequent nuclear reactions via tunneling. This probability is incorporated into the astrophysical factor $S\left(E\right)$. Aspects of screening effects to enable calculation of nuclear reaction rates are also evaluated, including Coulomb scattering and localized heating of the cold fuel, primary D-D reactions, and subsequent reactions with both the fuel and the lattice nuclei. The effect of screening for enhancement of the total nuclear reaction rate is a function of multiple parameters including fuel temperature and the relative scattering probability between the fuel and lattice metal nuclei. Screening also significantly increases the probability of interaction between hot fuel and lattice nuclei increasing the likelilhood of Oppenheimer-Phillips processes opening a potential route to reaction multiplication. We demonstrate that the screened Coulomb potential of the target ion is determined by the nonlinear Vlasov potential and not by the Debye potential. In general, the effect of screening becomes important at low kinetic energy of the projectile. We examine the range of applicability of both the analytical and asymptotic expressions for the well-known electron screening lattice potential energy $U_e$, which is valid only for $E\gg U_e$ ($E$ is the energy in the center of mass reference frame). We demonstrate that for $E\le U_e$, a direct calculation of Gamow factor for screened Coulomb potential is required to avoid unreasonably high values of the enhancement factor $f\left(E\right)$ by the analytical—and more so by the asymptotic—formulas.

$d$-D nuclear fusion events were observed in an electron-screened, deuterated metal lattice by reacting cold deuterons with hot deuterons $\left(d^{*}\right)$ produced by elastically scattered neutrons originating from bremsstrahlung photodissociation (where “d” and “D” denote ${}^2\mathrm{H}$). Exposure of deuterated materials ($\mathrm{Er}{\mathrm{D}}_3$ and $\mathrm{Ti}{\mathrm{D}}_2$) to photon energies in the range of 2.5–2.9 MeV resulted in photodissociation neutrons that were below 400 keV and also the 2.45-MeV neutrons consistent with ${}^2\mathrm{H}(d,n){}^3\mathrm{He}$ fusion. Additionally, neutron energies of approximately 4 and 5 MeV for $\mathrm{Ti}{\mathrm{D}}_2$ and $\mathrm{Er}{\mathrm{D}}_3$ were measured, consistent with either boosted neutrons from kinetically heated deuterons or Oppenheimer-Phillips stripping reactions in the highly screened environment. Neutron spectroscopy was conducted using calibrated lead-shielded liquid (EJ-309) and plastic (stilbene) scintillator detectors. The data support the theoretical analysis in a companion paper, predicting fusion reactions and subsequent reactions in the highly screened environment.

The global polarization of the $\mathrm{\Lambda }$ and $\overline{\mathrm{\Lambda }}$ hyperons is measured for Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ and 5.02 TeV recorded with the ALICE at the Large Hadron Collider (LHC). The results are reported differentially as a function of collision centrality and hyperon's transverse momentum ($p_T$) for the range of centrality 5–50%, $0.5<p_T<5$ GeV/$c$, and rapidity $\left|y\right|<0.5$. The hyperon global polarization averaged for Pb-Pb collisions at $\sqrt{s_{NN}}=2.76$ and 5.02 TeV is found to be consistent with zero, $\langle P_H\rangle (\%)\approx 0.01\pm 0.06\text{(stat.)}\pm 0.03\text{(syst.)}$ in the collision centrality range 15–50%, where the largest signal is expected. The results are compatible with expectations based on an extrapolation from measurements at lower collision energies at the Relativistic Heavy Ion Collider, hydrodynamical model calculations, and empirical estimates based on collision energy dependence of directed flow, all of which predict the global polarization values at LHC energies of the order of $0.01\%$.

Understanding quasielastic electron and neutrino scattering from nuclei has taken on new urgency with current and planned neutrino oscillation experiments, and with electron scattering experiments measuring specific final states, such as those involving nucleon pairs in “back-to-back” configurations. Accurate many-body methods are available for calculating the response of light ($A\le 12$) nuclei to electromagnetic and weak probes, but they are computationally intensive and only applicable to the inclusive response. In the present work we introduce a novel approach, based on realistic models of nuclear interactions and currents, to evaluate the short-time (high-energy) inclusive and exclusive response of nuclei. The approach accounts reliably for crucial two-nucleon dynamics, including correlations and currents, and provides information on back-to-back nucleons observed in electron and neutrino scattering experiments. We demonstrate that in the quasielastic regime and at moderate momentum transfers both initial- and final-state correlations and two-nucleon currents are important for a quantitatively successful description of the inclusive response and final-state nucleons. Finally, the approach can be extended to include relativistic—kinematical and dynamical—effects, at least approximately in the two-nucleon sector, and to describe the response in the resonance-excitation region.

The quaternary fission of ${}^{244–254}\mathrm{Cf}$ isotope with two $\alpha$ particles as the middle fragments in collinear configuration has been studied by taking the interacting barrier as the sum of Coulomb and nuclear proximity potential. The most favorable quaternary fission path is the one that has a high Q value and a minimum driving potential with respect to the mass and charge asymmetries. The favorable fragment combinations are obtained from the cold reaction valley plot and then calculating the yield for charge minimized fragments. For ${}^{244,248,254}\mathrm{Cf}$ isotopes, the highest yield is predicted for the isotope of Sb as one fragment, ${}^{134}\mathrm{Sb}$ ($Z=51$, $N=83$), ${}^{130}\mathrm{Sb}$ ($Z=51$, $N=79$), ${}^{132}\mathrm{Sb}$ ($Z=51$, $N=81$), respectively, whereas for ${}^{246}\mathrm{Cf}$ isotope, fragments with isotope of I as one fragment ${}^{134}\mathrm{I}$ ($Z=53$, $N=81$) possesses the highest yield. For ${}^{250}\mathrm{Cf}$ isotope, fragments with isotope of In as one fragment, ${}^{132}\mathrm{In}$ ($Z=49$, $N=83$) possesses the highest yield. In the case of ${}^{252}\mathrm{Cf}$ isotope, the highest yield is for the fragments with Te as one fragment, ${}^{132}\mathrm{Te}$ ($Z=52$, $N=80$). These findings confirm the role of doubly magic or near doubly magic nucleus in quaternary fission, which supports the conclusion by Poenaru et al. [Nucl. Phys. A 747, 182 (2005)]. The deformation and orientation of fragments has also been taken into account for the two $\alpha$ accompanied quaternary fission, and it has been found that in addition to closed shell effect, ground-state deformation also plays an important role in determining the isotopic yield in the quaternary fission process. We hope that our study on isotopic yield in quaternary fission of even-even ${}^{244–254}\mathrm{Cf}$ isotope will be a guide for future experiments.

Background: Neutrinos in the low-energy regime provide a gateway to a wealth of interesting physics. While plenty of literature exists on detailing the calculation and measurement of total reaction strengths, relatively little attention is paid to the measurement and modeling of the final lepton through differential cross sections at low energies, despite the experimental importance.

Purpose: We calculate differential cross sections for low-energy neutrino-nucleus scattering. We examine the role played by forbidden transitions in these distributions and how this differs across different energies and nuclear target masses. Attention is also paid to predictions for typical experimental neutrino spectra.

Method: The differential cross sections are calculated within a continuum random-phase approximation framework, which allows us to include collective excitations induced by long-range correlations. The Coulomb interaction of the final lepton in charged current events is treated in an effective way.

Results: Kinematic distributions are calculated for ${}^{16}\mathrm{O}$, ${}^{40}\mathrm{Ar}$, and ${}^{208}\mathrm{Pb}$. The ${}^{40}\mathrm{Ar}$ model results are compared for charged current (CC) (${\nu }_e,e^{-}$) reactions to events generated by the Modeling of Argon Reaction Low-energy Yields (MARLEY) event generator [S. Gardiner, Ph.D. thesis, University of California, Davis (2018)], with noticeable discrepancies.

Conclusion: Forbidden transitions have a marked effect on the kinematic distributions of the final lepton at low-energy kinematics, such as for decay-at-rest neutrinos or for a Fermi-Dirac spectrum at low temperature. This could introduce biases in experimental analyses. Backward scattering is noticeably more prominent than with MARLEY.

The ${}^{13}\mathrm{C}(\alpha ,n){}^{16}\mathrm{O}$ reaction is the main source of neutrons for the $s$ and $i$ processes in asymptotic giant branch stars and carbon-enhanced metal-poor stars, respectively. The reaction rate over the relevant temperature range from 0.1 to 0.3 GK translates into a center-of-mass energy range of 150 to 540 keV. Current measurements extend down to 300 keV, still requiring an extrapolation of the cross section. At these low energies, the high-energy tail of a $1/2^{+}$ state near the reaction threshold makes a significant contribution to the cross section, but its amplitude is still highly uncertain. In this paper the uncertainties associated with the low-energy cross-section extrapolation are investigated, in particular the sensitivity to the energy, width, and Coulomb renormalized asymptotic normalization coefficient of the near-threshold resonance. Recently it has been suggested that the energy of the near-threshold level may have a large impact on the extrapolation, but this is not found to be the case compared with the other sources of uncertainty.

We probe nuclear multipole resonances in the framework of the random-phase approximation by using the interaction obtained from the chiral effective field theory. The three-nucleon force is included in a form of the in-medium two-nucleon interaction, which was derived from the chiral three-nucleon force. The isoscalar monopole, isoscalar dipole, isovector dipole, and isoscalar quadrupole resonances of the closed-shell ${}^{56,68,78}\mathrm{Ni}$ have been investigated. The calculations reasonably reproduce the experimental multipole resonances of ${}^{56,68}\mathrm{Ni}$, and well describe the pygmy dipole resonance and dipole polarizability measured in ${}^{68}\mathrm{Ni}$. The multipole resonances of ${}^{78}\mathrm{Ni}$, including pygmy dipole resonance and dipole polarizability, are predicted. The detailed effects of the tensor force and three-body force are analyzed by dissecting the chiral interaction. We find that in general the tensor force effect on electric giant resonances is not as significant as the effect from the three-body force, although the tensor force provides more than half of the binding energy. The effect from three-body force is strong in light nuclei. Particularly, three-body force is crucial for the formation of the pygmy resonance in calculations.

We calculate transport coefficients of the quark-gluon plasma (QGP) within the dynamical quasiparticle model (DQPM) by explicitly computing the parton interaction rates as a function of temperature $T$ and baryon chemical potential ${\mu }_B$ on the basis of the DQPM couplings and partonic propagators. The latter are extracted from lattice QCD by matching the equation of state, entropy density and energy density at ${\mu }_B=0$. For baryon chemical potentials $0\le {\mu }_B\le 500\mathrm{MeV}$ we employ a scaling ansatz for the effective coupling which was shown before to lead to thermodynamic consistent results in this range. We compute the ratio of the shear and bulk viscosities to the entropy density, i.e., $\eta /s$ and $\zeta /s$, the electric conductivity ${\sigma }_0/T$, as well as the baryon diffusion coefficient ${\kappa }_B$ and compare to related approaches from the literature. We find that the ratios $\eta /s$ and $\zeta /s$ as well as ${\sigma }_0/T$ are in accord with the results from lattice QCD at ${\mu }_B=0$ and only weakly depend on the ratio $T/T_c\left({\mu }_B\right)$ where $T_c\left({\mu }_B\right)$ denotes the critical temperature at finite baryon chemical potential.

Background: Heavy-ion induced fusion reaction provides the opportunity for synthesizing heavy and superheavy elements in laboratories. In general, the process can be divided in two successive dynamical evolutions. First, a compact dinuclear configuration is formed through capture and then either it decays via quasifission or it equilibrates to a compound nucleus.

Purpose: In this paper, we present a systematic study of nuclear capture process to disentangle the significances of collective angular momentum, target-projectile mass asymmetry, and dissipative forces. Partial contributions from different angular momenta are analyzed for three different reaction channels with ${}^{16}\mathrm{O}, {}^{48}\mathrm{Ca}$, and ${}^{50}\mathrm{Ti}$ projectiles on the ${}^{208}\mathrm{Pb}$ target.

Method: A four-dimensional Langevin dynamical framework is developed to simulate the time evolution of two colliding nuclei starting from a well separated configuration until a captured composite is formed. The driving potential and the dissipative forces are estimated using the double-folding procedure and the surface friction model, respectively.

Results: Irrespective of target-projectile mass asymmetry, the dynamics is found to be strongly influenced by the collective angular momentum when it reaches beyond a critical value depending on the beam energy. Effects are more prominent for ${}^{48}\mathrm{Ca}$ and ${}^{50}\mathrm{Ti}$ as these systems populate higher angular momenta. Nuclear dissipation is shown to be strongly correlated with the angular momentum.

Conclusions: A deeper understanding of the nuclear dynamics in heavy-ion induced capture process is presented. The importances of target-projectile mass asymmetry, collective angular momentum, and nuclear dissipation are decoupled. This study may provide a better guidance in designing fusion experiments.

Background: The phenomenon of fusion hindrance may have important consequences on the nuclear processes occurring in astrophysical scenarios, if it is a general behavior of heavy-ion fusion at extreme subbarrier energies, including reactions involving lighter systems, e.g., reactions in the carbon and oxygen burning stages of heavy stars. The hindrance is generally identified by the observation of a maximum of the $S$ factor vs energy. Whether there is an $S$-factor maximum at very low energies for systems with a positive fusion $Q$ value is an experimentally challenging question.

Purpose: Our aim has been to search for evidence of fusion hindrance in ${}^{12}\mathrm{C}+{}^{24}\mathrm{Mg}$ which is a medium-light system with positive $Q$ value for fusion, besides the heavier cases where hindrance is recognized to be a general phenomenon. ${}^{12}\mathrm{C}+{}^{24}\mathrm{Mg}$ is very close to the ${}^{16}\mathrm{O}+{}^{16}\mathrm{O}$ and ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}$ systems that are important for the late evolution of heavy stars.

Methods: The experiment has been performed in inverse kinematics using the ${}^{24}\mathrm{Mg}$ beam from the XTU Tandem accelerator of LNL in the energy range 26–52 MeV with an intensity of 4–8 pnA. The targets were ${}^{12}\mathrm{C}$ evaporations $50\mu \mathrm{g}/{\mathrm{cm}}^2$ thick, isotopically enriched to $99.9\%$. The fusion-evaporation residues were detected at small angles by a $E-\mathrm{\Delta }E$-ToF detector telescope following an electrostatic beam deflector.

Results: Previous measurements of fusion cross section for ${}^{12}\mathrm{C}+{}^{24}\mathrm{Mg}$ were limited to above-barrier energies. In the present experiment the excitation function has been extended down to $\simeq 15\mu \mathrm{b}$ and it appears that the $S$ factor develops a clear maximum vs energy, indicating the presence of hindrance. This is the first convincing evidence of an $S$ factor maximum in a medium-light system with a positive fusion $Q$ value. These results have been fitted following a recently suggested method and a detailed analysis within the coupled-channels model that has been performed using a Woods-Saxon potential and including the ground state rotational band of ${}^{24}\mathrm{Mg}$. The coupled-channels calculations give a good account of the data near and above the barrier but overpredict the cross sections at very low energies.

Conclusions: The hindrance phenomenon is clearly observed in ${}^{12}\mathrm{C}+{}^{24}\mathrm{Mg}$, and its energy threshold is in reasonable agreement with the systematics observed for several medium-light systems. The fusion cross sections at the hindrance threshold show that the highest value (${\sigma }_s=1.6\mathrm{mb}$) is indeed found for this system. Therefore it may even be possible to extend the measurements further down in energy to better establish the position of the $S$-factor maximum.