Nuclear Theory

In the current work an equation of state model with a first-order phase transition for astrophysical applications is presented. The model is based on a two-phase approach for quark-hadron phase transitions, which leads by construction to a first-order phase transition. The resulting model has already been successfully used in several astrophysical applications, such as cold neutron stars, core-collapse supernova explosions and binary neutron star mergers. Main goal of this work is to present the details of the model, discuss certain features and eventually publish it in a tabulated form for further use.

It is now well established that jet modification is a multistage effect; hence a single model alone cannot describe all facets of jet modification. The JETSCAPE framework is a multistage framework that uses several modules to simulate different stages of jet propagation through the QGP medium. These simulations require a set of parameters to ensure a smooth transition between stages. We fine tune these parameters to successfully describe a variety of observables, such as the nuclear modification factors of leading hadrons and jets, jet shape, and jet fragmentation function. Photons can be produced in the hard scattering or as radiation from quarks inside jets. In this work, we study photon-jet transverse momentum imbalance and azimuthal correlation for both p−p and Pb−Pb collision systems. All the photons produced in each event, including the photons from hard scattering, radiation from the parton shower, and radiation from hadronization are considered with an isolation cut to directly compare with experimental data. The simulations are conducted using the same set of tuned parameters as used for the jet analysis. No new parameters are introduced or tuned. We demonstrate a significantly improved agreement with photons from Pb−Pb collisions compared to prior efforts. This work provides an independent, parameter free verification of the multistage evolution framework.

In this paper, we continue our phenomenological studies of heavy-ion collisions using 3+1d anisotropic hydrodynamics (aHydro). In previous works, we compared quasiparticle aHydro (aHydroQP) with ALICE 2.76 TeV Pb-Pb and RHIC 200 GeV Au-Au collision results. At both energies, the agreement was quite good between aHydroQP and the experimental data for many observables. In this work, we present comparisons of the Hanbury Brown--Twiss (HBT) radii and their ratios determined using pi+ pi+ pairs produced in 200 GeV Au-Au collisions. We first present comparisons with STAR results for the HBT radii and their ratios. We then present comparisons with PHENIX results for the HBT radii and their ratios. In both cases, we find reasonable agreement between aHydroQP predictions and available experimental results for the ratios of HBT radii. At the level of the radii themselves, in some cases quantitative differences on the order of 10-20% remain, which deserve further study.

Based on the isospin dependent transport model IBUU, the pion production and its absorption are thoroughly studied in the central collision of Au+Au at a beam energy of 400 MeV/nucleon. It is found that the pions are firstly produced by the hard Δ decay at the average density around 1.75 ρ 0 , whereas about 18\% of them are absorbed absolutely in the subsequent inelastic collisions. For the free pions observed, more than half of them have been scattered for one or more times before they are free from matter. And the more scattering numbers of pions, the higher the momentum they possess. These pions, due to longer time of their existence in high density nuclear matter, carry more information about the symmetry energy of the nuclear matter at high densities.

Nucleon-nucleon interactions, both bare and effective, play an important role in our understanding of the non-perturbative strong interaction, as well as nuclear structure and reactions. In recent years, tremendous efforts have been seen in the lattice QCD community to derive nucleon-nucleon interactions from first principles. Because of the daunting computing resources needed, most of such simulations were still performed with larger than physical light quark masses. In the present work, employing the recently proposed covariant chiral effective field theory (ChEFT), we study the light quark mass dependence of the nucleon-nucleon interaction extracted by the HALQCD group. It is shown that the pion-full version of the ChEFT can describe the lattice QCD data with m π =469 MeV and their experimental counterpart reasonably well, while the pion-less version can describe the lattice QCD data with m π =672,837,1015,1171 MeV, for both the 1 S 0 and 3 S 1 - 3 D 1 channels. The slightly better description of the single channel than the triplet channel indicates that higher order studies are necessary for the latter. Our results confirmed previous studies that the nucleon-nucleon interaction becomes more attractive for both the singlet and triplet channels as the pion mass decreases towards its physical value. It is shown that the virtual bound state in the 1 S 0 channel remains virtual down to the chiral limit, while the deuteron only appears for a pion mass smaller than about 400 MeV. It seems that proper chiral extrapolations of nucleon-nucleon interaction are possible for pion masses smaller than 500 MeV, similar to the mesonic and one-baryon sectors.

We calculate the T -matrices of elastic pion-nucleon ( πN ) scattering up to fourth order in SU(3) heavy baryon chiral perturbation theory. The pertinent low-energy constants are determined by fitting to πN phase shifts below 200 MeV pion laboratory momentum in the physical region. The scattering lengths and scattering volumes are extracted from the chiral amplitudes, and turn out to be in good agreement with those of other approaches and the available experimental values. We also discuss the subthreshold parameters and the related issues. On the basis of the various phase shifts, the threshold parameters and the subthreshold parameters, the convergence of the chiral expansion is analyzed in detail. The calculation provides the possibility to consider explicitly more complex processes involving strangeness.

We calculate the longitudinal structure function of the deuteron up through next-to-next-to-leading order in the framework of pionless effective field theory. We use these results to compute the two-photon polarizability contribution to Lamb shift in muonic deuterium, which can be utilized to extract the nuclear charge radius of the deuteron. We present analytical expressions order-by-order for the relevant transition matrix elements and the longitudinal structure function, and we give numerical results for the corresponding contributions to the Lamb shift. We also discuss the impact of relativistic and other higher-order effects. We find agreement with previous calculations and explain the accuracy of our calculation.

Upon investigating whether the variation of the antineutron-nucleus annihilation cross-sections at very low energies satisfy Bethe-Landau's power law of ? ann (p)??/ p α behavior as a function of the antineutron momentum p , we uncover unexpected regular oscillatory structures in the low antineutron energy region from 0.001 to 10 MeV, with small amplitudes and narrow periodicity in the logarithm of the antineutron energies, for large- A nuclei such as Pb and Ag. Subsequent semiclassical analyses of the S matrices reveal that these oscillations are pocket resonances that arise from quasi-bound states inside the pocket and the interference between the waves reflecting inside the optical potential pockets with those from beyond the potential barriers, implicit in the nuclear Ramsauer effect. They are the continuation of bound states in the continuum. Experimental observations of these pocket resonances will provide vital information on the properties of the optical model potentials and the nature of the antineutron annihilation process.

The production cross sections of heaviest isotopes of superheavy nuclei with charge numbers 112--118 are predicted in the xn --, pxn --, and αxn --evaporation channels of the 48 Ca-induced complete fusion reactions for future experiments. The estimates of synthesis capabilities are based on a uniform and consistent set of input nuclear data. Nuclear masses, deformations, shell corrections, fission barriers, and decay energies are calculated within the macroscopic-microscopic approach for even-even, odd-Z, and odd-N nuclei. For odd systems, the blocking procedure is used. To find, the ground states via minimization and saddle points using Immersion Water flow technique, multidimensional deformation spaces, containing non-axially are used. As shown, current calculations based on a new set of mass and barriers, agree very well with experimentally known cross-sections, especially in the 3n --evaporation channel. The dependencies of these predictions on the mass/fission barriers tables and fusion models are discussed. A way is shown to produce directly unknown superheavy isotopes in the 1n -- or 2n --evaporation channels. The synthesis of new superheavy isotopes unattainable in reactions with emission of neutrons is proposed in the promising channels with emission of protons ( σ pxn ≃10−200 fb) and alphas ( σ αxn ≃5−500 fb).

Recently, a hint for dibaryon NΔ( D 21 ) was observed at WASA-AT-COSY with a mass about 30±10 MeV below the NΔ threshold. It has a relatively small binding energy compared with the d ∗ (2380) and a width close to the width of the Δ baryon, which suggests that it may be a dibaryon in a molecular state picture. In this work, we study the possible S -wave molecular states from the NΔ interaction within the quasipotential Bethe-Salpeter equation approach. The interaction is described by exchanging π , ρ , and ω mesons. With reasonable parameters, a D 21 bound state can be produced from the interaction. The results also suggest that there may exist two more possible D 12 and D 22 states with smaller binding energies. The π exchange is found to play the most important role to bind two baryons to form the molecular states. An experimental search for possible NΔ( D 12 ) and NΔ( D 22 ) states will be helpful for understanding the hint of the dibaryon NΔ( D 21 ) .