We compute the energy loss of heavy quarkonia in high-temperature QCD plasmas and investigate the energy-loss effects on quarkonium suppression. Based on the effective vertex derived from the Bethe-Salpeter amplitude for quarkonium, the collisional and radiative energy loss are determined by quarkonium-gluon elastic scattering and the associated gluon-bremsstrahlung, respectively. In the energy regime $E<m_{\mathrm{{\rm Y}}}^2/T$ the collisional energy loss is dominant over the radiative one, and the total energy loss increases with the plasma temperature and the initial energy of quarkonium. Our numerical analysis indicates that the medium-induced energy loss of the $\mathrm{{\rm Y}}(1S$) results in stronger suppression at higher momentum, although the energy-loss effects are found to be small compared with the previous estimates of quarkonium dissociation in heavy-ion collisions.

Second-order susceptibilities ${\chi }_{ij}^{11}$ of baryon, electric, and strangeness, $B$, $Q$, and $S$, charges are calculated in the chiral mean field (CMF) model and compared with available lattice QCD data. The susceptibilities are sensitive to the short-range repulsive interactions between different hadron species, especially to the hardcore repulsion of hyperons. Decreasing the hyperons size, as compared with the size of the nonstrange baryons, does improve significantly the agreement of the CMF model results with the lattice QCD data. The electric-charge-dependent susceptibilities are sensitive to the short-range repulsive volume of mesons. The comparison with lattice QCD data suggests that strange baryons, nonstrange mesons, and strange mesons have significantly smaller excluded volumes than nonstrange baryons. The CMF model with these modified hadron volumes allows for a mainly hadronic description of the QCD susceptibilities significantly above the chiral pseudocritical temperature. This improved CMF model, which is based on the lattice QCD data, has been used to study the properties of both cold QCD matter and neutron-star matter. The phase structure in both cases is essentially unchanged, i.e., a chiral first-order phase transition occurs at low temperatures $(T_{\mathrm{CP}}\approx 17\mathrm{MeV})$, and hyperons survive deconfinement to higher densities than nonstrange hadrons. The neutron-star maximal mass remains close to $2.1M_{\odot }$ and the mass-radius diagram is only modified slightly due to the appearance of hyperons and is in agreement with astrophysical observations.

In the framework of the relativistic mean-field model with Thomas-Fermi approximation, we study the structures of low-density nuclear matter in a three-dimensional geometry with reflection symmetry. The numerical accuracy and efficiency are improved by expanding the mean fields according to fast cosine transformation and considering only one octant of the unit cell. The effect of finite cell size is treated carefully by searching for the optimum cell size. Typical pasta structures (droplet, rod, slab, tube, and bubble) arranged in various crystalline configurations are obtained for both fixed proton fractions and $\beta$ equilibration. It is found that the properties of droplets and bubbles are similar in body-centered cubic (bcc) and face-centered cubic (fcc) lattices, where the fcc lattice generally becomes more stable than bcc lattice as density increases. For the rod and tube phases, the honeycomb lattice is always more stable than the simple one. By introducing an $\omega \text{−}\rho$ cross coupling term, we further examine the pasta structures with a smaller slope of symmetry energy $L=41.34$ MeV, which predicts larger onset densities for core-crust transition and nonspherical nuclei. Such a variation due to the reduction of $L$ is expected to have impacts on various properties in neutron stars, supernova dynamics, and binary neutron star mergers.

The investigation of fusion reactions involving light neutron-rich exotic nuclei is of paramount significance to understand nucleosynthesis in astrophysical scenarios. It is also estimated as a possible heat source to ignite ${}^{12}\mathrm{C}+{}^{12}\mathrm{C}$ reaction and production of x-ray superbursts from accreting neutron star. Recently, the fusion of neutron-rich ${}^{20}\mathrm{O}$ with ${}^{12}\mathrm{C}$ target has been studied with measurement of fusion cross-section $\left({\sigma }_{\text{fus}}\right)$. Bass model under predicts the ${\sigma }_{\text{fus}}$ and time-dependent Hartree-Fock model also fails to explain the experimental data. To explicate the same, the investigation of ${}^{20}\mathrm{O}+{}^{12}\mathrm{C}$ reaction at near barrier energies has been made within quantum mechanical fragmentation-based dynamical cluster-decay model (DCM). Within DCM, the fragmentation potential comprises temperature-dependent Coulomb, nuclear and centrifugal potentials, along with temperature-dependent binding energies (T.B.E.) calculated within the macroscopic approach of Davidson mass formula. Recently, we have explored the temperature-dependence of different nuclear properties and nuclear symmetry energy within microscopic relativistic mean-field (RMF) theory [M. Kaur et al., Nucl. Phys. A 1000, 121871 (2020)]. In the present work, we inculcate the microscopic T.B.E. from RMF theory within DCM and investigate the structure of fragmentation potential for ${}^{32}\mathrm{Si}^{*}$ formed in ${}^{20}\mathrm{O}+{}^{12}\mathrm{C}$ reaction, comparatively for macroscopic (mac) and microscopic (mic) T.B.E. obtained from Davidson mass formula and RMF theory, respectively. The structure and magnitude of fragmentation potential are found to change drastically/notably along with a change in energetically favored/minimized fragments for both choices of T.B.E. The $\alpha$ particles $({}^4\mathrm{He}$, ${}^5\mathrm{He})$ are favored at lower angular momenta in fragmentation profile for mic T.B.E. case only, which is in the agreement with predictions of statistical model results. This change in the nuclear structure embodied via fragmentation potential energy carries its imprints in the preformation probability $P_0$ of different fragments and affects the contribution of individual light-charged particle (LCP) channel in the ${\sigma }_{\text{fus}}$. A comparison of the relative cross-section of different LCP channels toward ${\sigma }_{\text{fus}}$ is quite different for both cases of T.B.E. The cross-section of ${}^2\mathrm{H}$ and ${}^4\mathrm{He}$ LCP channels is relatively enhanced for mic T.B.E. compared to mac T.B.E. Among different LCP channels, the ${}^5\mathrm{He}$ channel is the major contributor in ${\sigma }_{\text{fus}}$, which is in line with the results of the statistical EVAPOR model. The DCM-calculated ${\sigma }_{\text{fus}}$ is in agreement with the experimental data.

The origin of weakly bound objects like clusters and hypernuclei, observed in heavy-ion collisions, is of theoretical and experimental interest. It is in the focus of the experiments at the BNL Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) since it is not evident how such weakly bound objects can survive in an environment whose hadronic decay products point to a temperature of the order of 150 MeV. It is also one of the key research topics in the future facilities of FAIR and NICA, which are under construction in Darmstadt (Germany) and Dubna (Russia), respectively. The first results on the cluster dynamics within the model-independent cluster recognition library “phase-space minimum spanning tree” (psMST) applied to different transport approaches: PHQMD, PHSD, SMASH, and UrQMD are presented here. The psMST is based on the “minimum spanning tree” (MST) algorithm for the cluster recognition which exploits correlations in coordinate space, and it is extended to correlations of baryons in the clusters in momentum space. The sensitivity of the cluster formation on the microscopic realization of the $n$-body dynamics and on the potential interactions in heavy-ion collisions is shown.

Background: The Efimov effect is a universal phenomenon in physics whereby three-body systems are stabilized via the interaction of an unbound two-body subsystems. A hypothetical state in ${}^{12}\mathrm{C}$ at 7.458-MeV excitation energy, comprising a loose structure of three $\alpha$ particles in mutual two-body resonance, has been suggested in the literature to correspond to an Efimov state in nuclear physics. The existence of such a state has not been demonstrated experimentally.

Purpose: Using a combination of $\gamma$ spectroscopy, charged-particle spectroscopy, and astrophysical rate calculations allowing for strict limits on the existence of such a state to been established here.

Method: Using the combined data sets from two recent experiments, one with the TexAT (Texas Active Target) TPC (Time Projection Chamber) to measure $\alpha$ decay and the other with Gammasphere to measure $\gamma$ decay of states in ${}^{12}\mathrm{C}$ populated by ${}^{12}\mathrm{N}$ and ${}^{12}\mathrm{B}\beta$ decay, respectively, we achieve high sensitivity to states in close proximity to the $\alpha$ threshold in ${}^{12}\mathrm{C}$.

Results: No evidence of a state at 7.458 MeV is seen in either data set. Using a likelihood method, the 95% confidence limit $\gamma$-decay branching ratio is determined as a function of the $\beta$-decay feeding strength relative to the Hoyle state. In parallel, calculations of the $3\alpha$ reaction rate show the inclusion of the Efimov corresponds to a large increase in the reaction rate around $5\times {10}^7$ K.

Conclusion: From decay spectroscopy—at the 95% confidence limit, the Efimov state cannot exist at 7.458 MeV with any $\gamma$-decay branching ratio unless the $\beta$ strength is less than 0.7% of the Hoyle state. This limit is evaluated for a range of different excitation energies and the results are not favorable for existence of the hypothetical Efimov state in ${}^{12}\mathrm{C}$. Furthermore, the $3\alpha$ reaction rate with the inclusion of a state between 7.43 and 7.53 MeV exceeds the rate required for stars to undergo the red giant phase.

I study the properties of a spin-down neutron impurity immersed in a low-density free Fermi gas of spin-up neutrons. In particular, I analyze its energy ($E_{\downarrow }$), effective mass ($m_{\downarrow }^{*}$), and quasiparticle residue ($Z_{\downarrow }$). Results are compared with those of state-of-the-art quantum Monte Carlo calculations of the attractive Fermi polaron realized in ultracold atomic gases experiments, and with those of previous studies of the neutron polaron. Calculations are performed within the Brueckner-Hartree-Fock approach using the chiral two-body nucleon-nucleon interaction of Entem and Machleidt at ${\mathrm{N}}^3\mathrm{LO}$ with a 500 MeV cut-off and the Argonne V18 phenomenological potential. Only contributions from the ${}^1{S}_0$ partial wave, which is the dominant one in the low-density region considered, are included. Contributions from three-nucleon forces are expected to be irrelevant at these densities and, therefore, are neglected in the calculation. It is shown that for Fermi momenta between $\approx 0.25$ and $\approx 0.45{\mathrm{fm}}^{-1}$ the energy, effective mass, and quasiparticle residue of the impurity vary only slightly, respectively, in the ranges $-0.604E_F<E_{\downarrow }<-0.635E_F$, $1.300m<m_{\downarrow }^{*}<1.085m$, and $0.741<Z_{\downarrow }<0.836$ in the case of the chiral interaction, and $-0.621E_F<E_{\downarrow }<-0.643E_F$, $1.310m<m_{\downarrow }^{*}<1.089m$, and $0.739<Z_{\downarrow }<0.832$ when using the Argonne V18 potential. These results are compatible with those derived from ultracold atoms and show that a spin-down neutron impurity in a free Fermi gas of spin-up neutrons with a Fermi momentum in the range $0.25\lesssim k_F\lesssim 0.45{\mathrm{fm}}^{-1}$ exhibits properties very similar to those of an attractive Fermi polaron in the unitary limit.

The effects of the Skyrme tensor force on the natural-parity $0^{+}$, $2^{+}$, and $3^{-}$ states in ${}^{16}\mathrm{O}$ and ${}^{40}\mathrm{Ca}$ are studied with the subtracted second random-phase approximation (SSRPA) adopting the Skyrme energy density functional. The strength distribution of these normal parity states are calculated with and without tensor interactions. The tensor force produces a small effect on $0^{+}$ and $2^{+}$ states in the random-phase approximation (RPA). However, our study shows that the tensor force has a significant effect on these states due to the coupling between one particle-one hole (1p-1h) and two particle-twp hole (2p-2h) model space in SSRPA; i.e., it increases strength for the low-lying $0^{+}$ and $2^{+}$ states below 10 MeV and shifts the main peak of $2^{+}$ transition downwards by about 1 to 2 MeV. For the negative-parity $3^{-}$ state, the tensor force shifts the lowest state downwards obviously, which helps the prediction of experimental data. The effect of the tensor force on the $3^{-}$ state is even visible in the 1p-1h RPA model space. We introduce two different parameter sets of tensor forces to clarify the role of the triplet-odd tensor term on the low-lying states. The microscopic structures of the lowest $0^{+}$, $2^{+}$, and $3^{-}$ states are examined looking at their 1p-1h and 2p-2h mixing amplitudes.

We study the properties of the strongly coupled quark-gluon plasma with a multistage model of heavy-ion collisions that combines the ${\mathrm{T}}_{\mathrm{R}}\mathrm{ENTo}$ initial condition ansatz, free-streaming, viscous relativistic hydrodynamics, and a relativistic hadronic transport. A model-to-data comparison with Bayesian inference is performed, revisiting assumptions made in previous studies. The role of parameter priors is studied in light of their importance for the interpretation of results. We emphasize the use of closure tests to perform extensive validation of the analysis workflow before comparison with observations. Our study combines measurements from the Large Hadron Collider (LHC) and the Relativistic Heavy Ion Collider (RHIC), achieving a good simultaneous description of a wide range of hadronic observables from both colliders. The selected experimental data provide reasonable constraints on the shear and the bulk viscosities of the quark-gluon plasma at $T\approx$ 150–250 MeV, but their constraining power degrades at higher temperatures, $T\gtrsim 250$ MeV. Furthermore, these viscosity constraints are found to depend significantly on how viscous corrections are handled in the transition from hydrodynamics to the hadronic transport. Several other model parameters, including the free-streaming time, show similar model sensitivity, while the initial condition parameters associated with the ${\mathrm{T}}_{\mathrm{R}}\mathrm{ENTo}$ ansatz are quite robust against variations of the particlization prescription. We also report on the sensitivity of individual observables to the various model parameters. Finally, Bayesian model selection is used to quantitatively compare the agreement with measurements for different sets of model assumptions, including different particlization models and different choices for which parameters are allowed to vary between RHIC and LHC energies.

We conjecture and verify a set of relations between global parameters of hot and fast-rotating compact stars which do not depend on the equation of state, including a relation connecting the masses of the mass-shedding (Kepler) and static configurations. We apply these relations to the GW170817 event by adopting the scenario in which a hypermassive compact star remnant formed in a merger evolves into a supramassive compact star that collapses into a black hole once the stability line for such stars is crossed. We deduce an upper limit on the maximum mass of static, cold neutron stars $2.{15}_{-0.17}^{+0.18}\le M_{\mathrm{TOV}}^{★}/M_{\odot }\le 2.{24}_{-0.44}^{+0.45}$ for the typical range of entropy per baryon, $2\le S/A\le 3$, and electron fraction $Y_e=0.1$ characterizing the hot hypermassive star. Our result implies that accounting for the finite temperature of the merger remnant relaxes previously derived constraints on the value of the maximum mass of a cold, static compact star.

Discovering unexplored high-spin states in neutron-rich nuclei can open up a new direction to study band structure and the associated shell structure in isospin-asymmetric many-body systems. However, experimental reach has so far been limited to neutron-deficient or stable nuclei which are preferentially produced in fusion reactions used in such studies. Here, we report the first $\gamma$-ray spectroscopy with fusion reactions using a reaccelerated rare-isotope beam of ${}^{45}\mathrm{K}$ performed at the ReA3 facility of the National Superconducting Cyclotron Laboratory. Using particle and $\gamma$-ray coincidence techniques, three new higher-lying states around 6 MeV and five new $\gamma$-ray transitions were identified for ${}^{46}\mathrm{Ca}$, suggesting three independent band structures formed from different particle-hole configurations. The rotational-like band built on the $0_2^{+}$ state is established up to the tentatively assigned $6_2^{+}$ state. New results are compared to large-scale shell model calculations, confirming the validity of the effective interaction describing particle-hole excitations across the $Z=20$ and $N=28$ shell gaps in the vicinity of doubly magic ${}^{48}\mathrm{Ca}$.

The evolution of electromagnetic transitions along isotope chains is of particular importance for the nuclear structure and dynamics, as well as for the r-process nucleosynthesis. Recent measurement of inelastic proton scattering on even-even ${}^{112-124}\mathrm{Sn}$ isotopes provides novel insight into the isotopic dependence of E1 and M1 strength distributions. We investigate M1 transitions in even-even ${}^{100-140}\mathrm{Sn}$ isotopes from a theoretical perspective, based on the relativistic nuclear energy density functional. The M1 transition strength distribution is characterized by an interplay between single- and double-peak structures that can be understood from the evolution of single-particle states, their occupations governed by the pairing correlations, and two-quasiparticle transitions involved. It is shown that the discrepancy between model calculations and experiments for the M1 transition strength is considerably more reduced than previously known, and the quenching of the $g$ factors for the free nucleons needed to reproduce the experimental data on M1 transition strength amounts $g_{\mathrm{eff}}/g_{\mathrm{free}}=0.80–0.93$. Because some of the M1 strength above the neutron threshold may be missing in the inelastic proton scattering measurement, further experimental studies are required to confirm if only small modifications of the bare $g$ factors are actually needed when applied in finite nuclei.

Background: Recently, the researches of cluster structure have attracted considerable attention. Many microscopic structure models have been applied to describe the cluster configurations.

Purpose: To better analyze the cluster structure and associate with the experimental observations, comparative studies are carried out in this paper by combining the nuclear structure model with the electron scattering theory.

Method: The density distributions for candidate nuclei of normal and cluster states are obtained from the deformed relativistic Hartree-Bogoliubov (RHB) model. Using some moderate approximations, the corresponding Coulomb form factors $|F_C{\left(q\right)|}^2$ and inclusive cross sections for different configurations are calculated by the distorted wave Born approximation (DWBA) and coherent density fluctuation model (CDFM), respectively.

Results: Comparing the $|F_C{\left(q\right)|}^2$ and inclusive cross sections of different configurations, the effects of nuclear cluster structure can be revealed, due to the differences of nuclear charge radii $R_C$ in coordinate space and the discrepancies of Fermi momentum $k_F$ in momentum space.

Conclusions: Results illustrate that the cluster structure can be reflected from the elastic and inclusive electron scattering. The studies conducted in this paper provide a new approach to analyze the cluster configurations, and are also helpful to guide future electron scattering experiments.

In this work, we present a prediction of the positron-proton and electron-proton elastic scattering cross-section ratio $R_{e^{+}{e}^{-}}$ based on a new phenomenological parametrization of the hard two-photon-exchange (TPE) corrections to electron-proton elastic scattering cross section ${\sigma }_R$. The TPE parametrization proposed in this work is rather simple, as it only requires the use of suitable global fits of both the Rosenbluth magnetic ${\tilde{G}}_M$, and the true magnetic $G_M$ form factors, or alternatively, the electric form factors ${\tilde{G}}_E$ and $G_E$. We compare our results to recent extractions, and world's data on $R_{e^{+}{e}^{-}}$ with emphasis mainly on the kinematics range of the recent direct measurements from the CLAS, VEPP-3, and OLYMPUS experiments. With the proper choice of ${\tilde{G}}_M$ and $G_M$ parametrizations, and while our results are in generally good agreement with $R_{e^{+}{e}^{-}}$ measurements taken at high $\varepsilon$ points and for all $Q^2$ range, they agree remarkably well, within the error bands of the predictions, with $R_{e^{+}{e}^{-}}$ measurements taken at low-$\varepsilon$ points for $Q^2=1.0$ and $1.5{(\mathrm{GeV}/\mathrm{c})}^2$, where emphasis should be placed for evidence of hard TPE correction. Finally, we believe that the assumption that hard TPE corrections could account for the discrepancy on the proton's form factors ratio ${\mu }_p{G}_E/G_M$ is still an open question as more measurements of $R_{e^{+}{e}^{-}}$ are clearly needed for $Q^2>2.1{(\mathrm{GeV}/\mathrm{c})}^2$, in the region where the discrepancy is significant.

This paper presents the $\beta$-decay feeding intensity distribution and Gamow-Teller transition strength distribution of ${}^{71,73}\mathrm{Ni}$. These quantities were measured using the technique of total absorption spectroscopy at the National Superconducting Cyclotron Laboratory with the Summing NaI(Tl) detector. These measurements provide sensitive constraints to theoretical models used to predict $\beta$-decay properties far from stability for astrophysical applications. Specifically, for the astrophysical $r$ process, the majority of the involved nuclei are not accessible by current facilities, and the nuclear input is mainly provided by theory. The present work reports on two neutron-rich nickel isotopes in the region where the weak $r$ process is expected to be relevant in stellar nucleosynthesis. The experimental results are compared to two theoretical models, namely the shell model and the quasiparticle random-phase approximation, to help further refine theoretical calculations and aid in future $r$-process studies.

Background: A recent sensitivity study has shown that the ${}^{35}\mathrm{K}(p,\gamma ){}^{36}\mathrm{Ca}$ reaction is one of the ten $(p,\gamma )$ reaction rates that could significantly impact the shape of the calculated x-ray burst light curve. Its reaction rate used up to now in type I x-ray burst calculations was estimated using an old measurement for the mass of ${}^{36}\mathrm{Ca}$ and theoretical predictions for the partial decay widths of the first $2^{+}$ resonance with arbitrary uncertainties.

Purpose: In this work, we propose to reinvestigate the ${}^{35}\mathrm{K}(p,\gamma ){}^{36}\mathrm{Ca}$ reaction rate, as well as related uncertainties, by determining the energies and decay branching ratios of ${}^{36}\mathrm{Ca}$ levels, within the Gamow window of x-ray bursts, in the 0.5 to 2 GK temperature range.

Method: These properties were studied by means of the one-neutron pickup transfer reaction ${}^{37}\mathrm{Ca}(p,d){}^{36}\mathrm{Ca}$ in inverse kinematics using a radioactive beam of ${}^{37}\mathrm{Ca}$ at 48 MeV ${\mathrm{nucleon}}^{-1}$. The experiment was performed at the GANIL facility using the liquid hydrogen target CRYPTA, the MUST2 charged particle detector array for the detection of the light charged particles, and a zero degree detection system for the outgoing heavy recoil nuclei.

Results: The atomic mass of ${}^{36}\mathrm{Ca}$ is confirmed and new resonances have been proposed together with their proton decay branching ratios. This spectroscopic information, used in combination with very recent theoretical predictions for the $\gamma$-decay width, were used to calculate the ${}^{35}\mathrm{K}(p,\gamma ){}^{36}\mathrm{Ca}$ reaction rate. The recommended rate of the present work was obtained within a uncertainty factor of 2 at $1\sigma$. This is consistent with the previous estimate in the x-ray burst temperature range. A large increase of the reaction rate was found at higher temperatures due to two newly discovered resonances.

Conclusions: The ${}^{35}\mathrm{K}(p,\gamma ){}^{36}\mathrm{Ca}$ thermonuclear reaction rate is now well constrained by the present work in a broad range of temperatures covering those relevant to type I x-ray bursts. Our results show that the ${}^{35}\mathrm{K}(p,\gamma ){}^{36}\mathrm{Ca}$ reaction does not affect the shape of the x-ray burst light curve, and that it can be removed from the list of the few influential proton radiative captures reactions having a strong impact on the light curve.

Despite their astrophysical relevance, nuclear pasta phases are relatively unstudied at high temperatures. We present molecular dynamics simulations of symmetric nuclear matter with several topologies of “lasagna” at a range of temperatures to study the pasta-uniform transition. Using the Minkowski functionals, we quantify trends in the occupied volume, surface area, mean breadth, and Euler characteristic. The amplitude of surface displacements of the pasta increase with temperature which produce short-lived topological defects such as holes and filaments near melting, resulting in power laws for increasing surface curvature with temperature. We calculate the static structure factor and report the shear viscosity and thermal conductivity of pasta, finding that the shear viscosity is minimized at the melting temperature. These results may have implications for the thermoelastic properties of nuclear pasta and finite-temperature corrections to the equation of state at pasta densities.

The shape and collectivity of ${}^{106}\mathrm{Cd}$ was investigated via a sub-barrier-energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory Re-accelerator facility using the JANUS setup. Transition matrix elements between low-lying states were found to agree with adopted values, and information on the shape and collectivity of higher-lying states was extracted for the first time. Locally optimized large-scale shell-model calculations were found to describe well the $B\left(E2\right)$ transition strengths but failed to reproduce the spectroscopic quadrupole moments $Q_s$. An analysis of the $E2$ rotational invariants and the normalized quadrupole moment $q_s$ indicates that this may be due to a significant degree of triaxiality in ${}^{106}\mathrm{Cd}$ which is not captured by the present shell-model calculations. Analogous calculations for the Fe isotopes (two protons below the $Z=28$ magic number) reveal the critical role of high-$j$ neutron configurations for the description of quadrupole moments in the heavy Fe and Cd isotopes (two protons below magic $Z=50$), but this effect is insufficient to explain the shape of ${}^{106}\mathrm{Cd}$, posing a puzzle for the understanding of nuclear structure towards $N=50$.

Within an isospin- and momentum-dependent transport model, we investigate the necessity of self-consistent calculations for the electromagnetic field in probing the nuclear symmetry energy using pion observables in heavy-ion collisions at intermediate energies. To this end, we perform the ${}^{96}\mathrm{Ru}+{}^{96}\mathrm{Ru}$ collisions at 400 MeV/nucleon with two calculation scenarios for the electromagnetic field including the self-consistent calculation and the most used Liénard-Wiechert formula; the latter is a simplified version of the complete Liénard-Wiechert formula in that it neglects the radiation field for practical calculations in heavy-ion collisions at intermediate and/or relativistic energies. As a comparison, we also consider the static Coulomb field formula for calculations of the electromagnetic field in heavy-ion collisions. It is shown that the most used simplified Liénard-Wiechert formula is not enough for the electromagnetic field calculation because the absent radiation field in this formula also affects significantly the charged pions as well as their ${\pi }^{-}/{\pi }^{+}$ ratio. Moreover, we also examine effects of the electromagnetic field in these scenarios on the double ${\pi }^{-}/{\pi }^{+}$ ratio of two isobar reaction systems ${}^{96}\mathrm{Ru}+{}^{96}\mathrm{Ru}$ and ${}^{96}\mathrm{Zr}+{}^{96}\mathrm{Zr}$ at 400 MeV/nucleon. It is shown that the double ${\pi }^{-}/{\pi }^{+}$ ratio of two reactions tends to be less affected by the electromagnetic field calculation scenario and thus can still be an effective probe of the nuclear symmetry energy in heavy-ion collisions. Therefore, according to these findings, it is suggested that the self-consistent calculation for the electromagnetic field should be carefully taken into account when using the pion observables to probe the nuclear symmetry energy in heavy-ion collisions.

At temperatures below 0.1 GK, the ${}^{19}\mathrm{F}(p,\gamma ){}^{20}\mathrm{Ne}$ reaction is the only breakout path out of the CNO cycle. Experimental studies of this reaction are challenging from a technical perspective due to copious $\gamma$-ray background from the far stronger ${}^{19}\mathrm{F}(p,\alpha ){}^{16}\mathrm{O}$ reaction channel. Here, we present the first inverse kinematics study of the ${}^{19}\mathrm{F}(p,\gamma ){}^{20}\mathrm{Ne}$ reaction, in which we measure the strength of the 323-keV resonance. We find a strength value of $\omega \gamma =3.3_{-0.9}^{+1.1}$ meV, which is a factor of two larger than the most recent previous study. The discrepancy is likely the result of a direct to ground state transition which previous studies were not sensitive to. We also observe the transition to the first $2^{-}$ state, which has not been observed for this resonance in previous studies. A new thermonuclear reaction rate is calculated and compared with the literature.