Nuclear Theory·nucl-th

Tuesday·August 15, 2017

11 papers—8 primary·3 cross-listed

01
Dynamical evolution of the spectator systems produced in ultra-relativistic heavy-ion collisions
K. Mazurek🇵🇱 · A. Szczurek🇵🇱 · C. Schmitt🇫🇷 · P.N. Nadtochy🇷🇺
In peripheral heavy-ion collisions at ultra-relativistic energies, usually only parts of the colliding nuclei effectively interact with each other. In the overlapping zone, a fireball or quark-gluon plasma is produced. The excitation energy of the heavy remnant can range from a few tens to several hundreds of MeV, depending on the impact parameter. The decay of these excited spectators is investigated in this work for the first time within a dynamical approach based on the multi-dimensional stochastic Langevin equation. The potential of this exploratory work to understand the connection between electromagnetic fields generated by the heavy spectators and measured pion distributions is discussed.
nucl-thhep-phPRC(2018)·16 citations
02
Theoretical investigation of two-particle two-hole effect on spin-isospin excitations through charge-exchange reactions
Tokuro Fukui · Futoshi Minato
Coherent one-particle one-hole (1p1h) excitations have given us effective insights into general nuclear excitations. However, the two-particle two-hole (2p2h) excitation beyond 1p1h is now recognized as critical for the proper description of experimental data of various nuclear responses. The spin-flip charge-exchange reactions $^{48} Ca (p, n)^{48} Sc$ are investigated to clarify the role of the 2p2h effect on their cross sections. The Fermi transition of $^{48} Ca$ via the $(p, n)$ reaction is also investigated in order to demonstrate our framework. The transition density is calculated microscopically with the second Tamm-Dancoff approximation, and the distorted-wave Born approximation is employed to describe the reaction process. A phenomenological one-range Gaussian interaction is used to prepare the form factor. For the Fermi transition, our approach describes the experimental behavior of the cross section better than the Lane model, which is the conventional method. For spin-flip excitations including the GT transition, the 2p2h effect decreases the magnitude of the cross section and does not change the shape of the angular distribution. The $Δ l = 2$ transition of the present reaction is found to play a negligible role. The 2p2h effect will not change the angular-distributed cross section of spin-flip responses. This is because the transition density of the Gamow-Teller response, the leading contribution to the cross section, is not significantly varied by the 2p2h effect.
nucl-thnucl-exPRC(2017)·4 citations
03
Elementary excitations in homogeneous superfluid neutron star matter: role of the neutron-proton coupling
Marcello Baldo (INFN, Sez. Catania, Italy)🇮🇹 · Camille Ducoin (IPNL, Lyon, France)🇫🇷
The thermal evolution of Neutron Stars is affected by the elementary excitations that characterize the stellar matter. In particular, the low-energy excitations, with a spectrum linear in momentum, can play a major role in the emission and propagation of neutrinos. In this paper, we focus on the elementary modes in the region of proton superfluidity, where the neutron component is expected to have a very small or zero pairing gap. We study the overall spectral functions of protons, neutrons and electrons on the basis of the Coulomb and nuclear interactions. This study is performed in the framework of the Random Phase Approximation, generalized in order to describe the response of a superfluid system. The formalism we use ensures that the Generalized Ward's Identities are satisfied. Despite their relative small fraction, the protons turn out to modify the neutron spectral function as a consequence of the nuclear neutron-proton interaction. This effect is particularly evident at the lower density, just below the crust for a density close to the saturation value, while at increasing density the neutrons and the protons are mainly decoupled. The proton spectral function is characterized by a pseudo-Goldstone mode below $2Δ$ , twice the pairing gap, and a pair-breaking mode above $2Δ$ . The latter merges in the sound mode of the normal phase at higher momenta. The neutron spectral function develops a collective sound mode only at the higher density. The electrons have a strong screening effect on the proton-proton interaction at the lower momenta, and decouple from the protons at higher momenta.
nucl-thPRC(2017)·3 citations
04
Interdependence of different symmetry energy elements
C. Mondal · B. K. Agrawal · J. N. De · S. K. Samaddar · M. Centelles · X. Viñas
Relations between the nuclear symmetry energy coefficient and its density derivatives are derived. The relations hold for a class of interactions with quadratic momentum dependence and a power-law density dependence. The structural connection between the different symmetry energy elements as obtained seems to be followed by almost all reasonable nuclear energy density functionals, both relativistic and non-relativistic, suggesting a universality in the correlation structure. This, coupled with known values of some well-accepted constants related to nuclear matter, helps in constraining values of different density derivatives of the nuclear symmetry energy shedding light on the isovector part of the nuclear interaction.
nucl-thPRC(2017)·76 citations
05
Microscopic study of induced fission dynamics of $^{226}$ Th with covariant energy density functionals
H. Tao · J. Zhao · Z. P. Li · T. Niksic · D. Vretenar
Static and dynamic aspects of the fission process of $^{226}$ Th are analyzed in a self-consistent framework based on relativistic energy density functionals. Constrained relativistic mean-field (RMF) calculations in the collective space of axially symmetric quadrupole and octupole deformations, based on the energy density functional PC-PK1 and a $δ$ -force pairing, are performed to determine the potential energy surface of the fissioning nucleus, the scission line, the single-nucleon wave functions, energies and occupation probabilities, as functions of deformation parameters. Induced fission dynamics is described using the time-dependent generator coordinate method in the Gaussian overlap approximation. A collective Schrödinger equation, determined entirely by the microscopic single-nucleon degrees of freedom, propagates adiabatically in time the initial wave packet built by boosting the ground-state solution of the collective Hamiltonian for $^{226}$ Th. The position of the scission line and the microscopic input for the collective Hamiltonian are analyzed as functions of the strength of the pairing interaction. The effect of static pairing correlations on the pre-neutron emission charge yields and total kinetic energy of fission fragments is examined in comparison with available data, and the distribution of fission fragments is analyzed for different values of the initial excitation energy.
nucl-thPRC(2017)·73 citations
06
Models, measurements, and effective field theory: proton capture on Beryllium-7 at next-to-leading order
Xilin Zhang · Kenneth M. Nollett · Daniel R. Phillips
We employ an effective field theory (EFT) that exploits the separation of scales in the p-wave halo nucleus $^{8} B$ to describe the process $^{7} Be (p, γ)^{8} B$ up to a center-of-mass energy of 500 keV. The calculation, for which we develop the lagrangian and power counting, is carried out up to next-to-leading order (NLO) in the EFT expansion. The power counting we adopt implies that Coulomb interactions must be included to all orders in $α_{em}$ . We do this via EFT Feynman diagrams computed in time-ordered perturbation theory, and so recover existing quantum-mechanical technology such as the two-potential formalism for the treatment of the Coulomb-nuclear interference. Meanwhile the strong interactions and the E1 operator are dealt with via EFT expansions in powers of momenta, with a breakdown scale set by the size of the $^{7}$ Be core, $Λ \approx 70$ MeV. Up to NLO the relevant physics in the different channels that enter the radiative capture reaction is encoded in ten different EFT couplings. The result is a model-independent parametrization for the reaction amplitude in the energy regime of interest. To show the connection to previous results we fix the EFT couplings using results from a number of potential model and microscopic calculations in the literature. Each of these models corresponds to a particular point in the space of EFTs. The EFT structure therefore provides a very general way to quantify the model uncertainty in calculations of $^{7} Be (p, γ)^{8} B$ . We also demonstrate that the only N $^{2}$ LO corrections in $^{7} Be (p, γ)^{8} B$ come from an inelasticity that is practically of N $^{3}$ LO size in the energy range of interest, and so the truncation error in our calculation is effectively N $^{3}$ LO. We also discuss the relation of our extrapolated $S (0)$ to the previous standard evaluation.
nucl-thastro-ph.SRnucl-exPRC(2018)·19 citations
07
Quasi-elastic neutrino charged-current scattering off $^{12}$ C: effects of the meson exchange currents and large nucleon axial mass
A.V. Butkevich🇷🇺 · S.V. Luchuk🇷🇺
The quasi-elastic scattering of muon neutrino and electrons on a carbon target are analyzed using the relativistic distorted-wave impulse approximation (RDWIA). We also evaluate the contribution of the two-particle and two-hole meson exchange current ( $2 p - 2 h$ MEC) to electroweak response functions. The nuclear model dependence of the (anti)neutrino cross sections is studied within the RDWIA+MEC approach and RDWIA model with the large nucleon axial mass. It is shown that the results for the squared momentum transfer distribution $d σ / d Q^{2}$ and for invariant mass of the final hadronic system distribution $d σ / d W$ obtained within these models are substantially different.
nucl-thPRC(2018)·14 citations
08
Particle-phonon coupling effects within theory of finite Fermi systems
E.E. Saperstein · S.V. Tolokonnikov
Recent results of the study of the particle-phonon coupling (PC) effects in odd magic and semi-magic nuclei within the self-consistent theory of finite Fermi systems are reviewed. In addition to the usual pole diagrams, the non-pole ones are considered. Their contributions are often of a crucial importance. PC corrections to the single-particle energies for $^{40}$ Ca and $^{208}$ Pb are presented. The quadrupole moments of odd In and Sb isotopes, the odd-proton neighbors of even Sn isotopes, are presented also with accounting for the PC corrections. At last, recently announced problem of extremely high values charge radii of heavy Ca isotopes is solved in terms of a consistent consideration of the PC effects. In all the cases, rather good description of the data is obtained.
nucl-thAIP Conf.Proc.(2017)·1 citation
09
Revisiting the Hybrid Quantum Monte Carlo Method for Hubbard and Electron-Phonon Models
Stefan Beyl · Florian Goth · Fakher F. Assaad
A unique feature of the hybrid quantum Monte Carlo (HQMC) method is the potential to simulate negative sign free lattice fermion models with subcubic scaling in system size. Here we will revisit the algorithm for various models. We will show that for the Hubbard model the HQMC suffers from ergodicity issues and unbounded forces in the effective action. Solutions to these issues can be found in terms of a complexification of the auxiliary fields. This implementation of the HQMC that does not attempt to regularize the fermionic matrix so as to circumvent the aforementioned singularities does not outperform single spin flip determinantal methods with cubic scaling. On the other hand we will argue that there is a set of models for which the HQMC is very efficient. This class is characterized by effective actions free of singularities. Using the Majorana representation, we show that models such as the Su-Schrieffer-Heeger Hamiltonian at half filling and on a bipartite lattice belong to this class. For this specific model sub-cubic scaling is achieved.
↳ cond-mat.str-elhep-latnucl-thPRB(2018)·65 citations
10
Probing mass-radius relation of protoneutron stars from gravitational-wave asteroseismology
Hajime Sotani🇯🇵 · Takami Kuroda🇨🇭 · Tomoya Takiwaki🇯🇵 · Kei Kotake🇯🇵
The gravitational-wave (GW) asteroseismology is a powerful technique for extracting interior information of compact objects. In this work, we focus on spacetime modes, the so-called $w$ -modes, of GWs emitted from a proto-neutron star (PNS) in the postbounce phase of core-collapse supernovae. Using results from recent three-dimensional supernova models, we study how to infer the properties of the PNS based on a quasi-normal mode analysis in the context of the GW asteroseismology. We find that the $w_{1}$ -mode frequency multiplied by the PNS radius is expressed as a linear function with respect to the ratio of the PNS mass to the PNS radius. This relation is insensitive to the nuclear equation of state (EOS) employed in this work. Combining with another universal relation of the $f$ -mode oscillations, we point out that the time dependent mass-radius relation of the PNS can be obtained by observing both the $f$ - and $w_{1}$ -mode GWs simultaneously. Our results suggest that the simultaneous detection of the two modes could provide a new probe into finite-temperature nuclear EOS that predominantly determines the PNS evolution.
↳ astro-ph.HEnucl-thPRD(2017)·66 citations
11
Strangeon Matter in a Liquid Drop Model
Zheng Wang🇨🇳 · Jiguang Lu🇨🇳 · Renxin Xu🇨🇳
The liquid drop model of 2-flavored ( $u$ and $d$ ) nucleus is well known and successful, analogically, a similar drop model for 3-flavored ( $u$ , $d$ and $s$ ) nucleus is developed. A 3-flavored nucleus conjectured could be stable only if its baryon number is lager than a critical one, $A_{c}$ , in which strangeons are the constituent as an analogy of nucleons for nucleus. We try to model strangeon matter in a sense of phenomenological liquid drop, with two free parameters: the mass per bayron of a strangeon in vacuum, $M$ , and potential deep between strangeons, $ϵ$ . It is found that, for $M \sim$ GeV and $ϵ \sim 100$ MeV, strangeon matter could be stable and its critical number could be as low as $A_{c} = 300$ .
↳ astro-ph.HEnucl-thJPS Conf.Proc.(2018)·1 citation

Affiliations

first authorsco-authorsvia INSPIRE