Nuclear Theory·nucl-th

Tuesday·February 19, 2019

12 papers—8 primary·4 cross-listed

01
An in-medium chiral power counting for nuclear matter and some applications
J.A. Oller🇪🇸
We review on a chiral power counting for in-medium chiral perturbation theory with nucleons and pions as explicit degrees of freedom coupled to external sources. It allows for a systematic expansion including both local and pion-mediated inter-nucleon interactions. One can identify from this power counting classes of non-perturbative diagrams that require resummation. A non-perturbative method based on Unitary Chiral Perturbation Theory was also developed for performing those resummations. This power counting and non-perturbative techniques were firstly applied to calculate the pion self-energy, the pion-decay constants and the quark condensate in nuclear matter up-to-and-including next-to-leading order (NLO) contributions. The cancellation of the contributions at NLO to the pion self-energy and decay constants from in-medium nucleon-nucleon (NN) interactions was derived. Some NLO NN contributions survive for the quark condensate due to the quark-mass dependence of the pion mass. Next, we discuss the calculation of the energy density in the nuclear medium by employing the in-medium NN scattering amplitudes. For symmetric and neutron matter it reproduces in good agreement, and without fine tuning, calculations from realistic NN potentials with a model for the three-nucleon interaction. These results are applied to derive the equation of state (EOS) for neutron stars and obtain an upper limit for a neutron mass slightly above 2 solar masses. Our results also fulfill other constraints from the detection of the gravitational waves in the event GW170817, like the upper bound on the maximal mass of a neutron star and the allowed interval for the radius of a 1.4-solar-mass neutron star. The knowledge of the neutron-matter EOS is also employed to give an upper bound of the gravitational constant within the strong gravitational field of a 2 solar-mass neutron star.
nucl-thhep-phJ.Phys.G(2019)·7 citations
02
Non-Empirical Interactions for the Nuclear Shell Model: An Update
S. Ragnar Stroberg · Scott K. Bogner · Heiko Hergert · Jason D. Holt
The nuclear shell model has been perhaps the most important conceptual and computational paradigm for the understanding of the structure of atomic nuclei. While the shell model has been predominantly used in a phenomenological context, there have been efforts stretching back over a half century to derive shell model parameters based on a realistic interaction between nucleons. More recently, several ab initio many-body methods---in particular many-body perturbation theory, the no-core shell model, the in-medium similarity renormalization group, and coupled cluster theory---have developed the capability to provide effective shell model Hamiltonians. We provide an update on the status of these methods and investigate the connections between them and potential strengths and weaknesses, with a particular focus on the in-medium similarity renormalization group approach. Three-body forces are demonstrated to be an important ingredient in understanding the modifications needed in phenomenological treatments. We then review some applications of these methods to comparisons with recent experimental measurements, and conclude with some remaining challenges in ab initio shell model theory.
nucl-thAnn.Rev.Nucl.Part.Sci.(2019)·295 citations
03
Impact of Glasma on heavy quark observables in nucleus-nucleus collisions at LHC
Yifeng Sun🇮🇹 · Gabriele Coci🇮🇹 · Santosh Kumar Das🇮🇳 · Salvatore Plumari🇮🇹 · Marco Ruggieri🇨🇳 · Vincenzo Greco🇮🇹
In the pre-thermal equilibrium stage of relativistic heavy-ion collisions, a strong quasi-classical transverse gluon field emerges at about $τ_{0} ≃ 0.1 fm/c$ and evolves together with their longitudinal counterparts according to the classical Yang-Mills (CYM) equations. Recently it has been shown that these fields induce a diffusion of charm quarks in momentum space resulting in a tilt of their spectrum without a significant drag. We find that in nucleus-nucleus collisions at LHC such a novel dynamics of charm quarks leads to an initial enhancement of the nuclear modification factor ( $R_{AA}$ ) at $p_{T}$ larger than 2 GeV $/ c$ contrary to the standard lore. Moreover, the same dynamics leads to a larger final elliptic flow ( $v_{2}$ ) inducing a relation between $R_{AA}$ and $v_{2}$ that is quite close to the experimental measurements. Our study also shows that such an initial pre-thermal stage is unlikely to be described in terms of a standard drag and diffusion dynamics, because even if one tune such coefficients to reproduce the same $R_{AA} (p_{T})$ this would imply a significantly smaller $v_{2}$ .
nucl-thPLB(2019)·79 citations
04
How nuclear jets form and disintegrate into clusters in heavy-ion collisions
P. Napolitani🇫🇷 · M. Colonna🇮🇹
The most extreme deformations that can be explored in heavy-ion collisions at Fermi-energies are collimated flows of nuclear matter which recall jet dynamics. From microphysics to the cosmological scale, jets are rather common topologies. In nuclear physics, pioneering works focused on the breakup of these structures, resulting into early nuclear-fission models in analogy to the droplet formation in viscous liquids; such view became emblematic to explain surface-energy effects and surface instability by analogy with the Rayleigh instability. Through a dynamical approach based on the Boltzmann-Langevin equation, well adapted to out-of-equilibrium conditions, we explored the possibility that nuclear jets could arise in heavy-ion collisions from different conditions than those leading to fission or neck fragmentation, and that they can breakup from mechanisms that are almost unrelated to cohesive properties.
nucl-thNuovo Cim.C(2019)·1 citation
05
Mean field and two-body nuclear effects in inclusive electron scattering on argon, carbon and titanium: the superscaling approach
M. B. Barbaro🇮🇹 · J. A. Caballero🇪🇸 · A. De Pace🇮🇹 · T. W. Donnelly🇺🇸 · R. González-Jiménez🇪🇸 · G. D. Megias🇪🇸
We compare the predictions of the SuSAv2 model including two-particle two-hole meson-exchange currents with the recent JLab data for inclusive electron scattering on three different targets (C, Ar and Ti). The agreement is very good over the full energy spectrum, with some discrepancy seen only in the deep inelastic region. The 2p2h response, peaked in the dip region between the quasielastic and $Δ$ -resonance peak, is essential to reproduce the data. We also analyze the $k_{F}$ (Fermi momentum) dependence of the data in terms of scaling of second kind, showing that the 2p2h response scales very differently from the quasielastic one, in full accord with what is predicted by the model. The results represent a valuable test of the applicability of the model to neutrino scattering processes on different nuclei.
nucl-thhep-phPRC(2019)·25 citations
06
Nuclear jets in heavy-ion collisions recall a stream of sand
P. Napolitani🇫🇷 · M. Colonna🇮🇹
Head-on collisions between nuclei of different size at Fermi energies may give rise to extremely deformed dynamical regimes and patterns. Those latter, may suddenly turn into a stream of nuclear clusters, resembling collimated jets. Because the underlying instabilities are inadequately described by usual modelling approaches based on equilibrium approximations, this mechanism resulted rather unnoticed, even though it should be frequently registered in experiments. We employ the Boltzmann-Langevin equation to specifically address out-of-equilibrium conditions and handle dynamical fluctuations. An interesting interplay between surface and volume instabilities is discussed for the first time. Stable and rather regular patterns of streaming clusters arise from these conditions. Counterintuitively, we find that these clustered structures are not triggered by cohesive forces and they recall the granular flow of a stream of dry sand.
nucl-thPLB(2019)·6 citations
07
Microscopic description of pair transfer between two superfluid systems (II): a quantum mixing of Time-Dependent Hartree-Fock Bogolyubov trajectories
D. Regnier🇫🇷 · D. Lacroix🇫🇷
While superfluidity is accurately grasped with a state that explicitly breaks the particle number symmetry, a precise description of phenomena like the particle transfer during heavy-ion reactions can only be achieved by considering systems with good particle numbers. We investigate the possibility to restore particle number in many-body dynamical problems by mixing-up several Time-Dependent Hartree-Fock Bogolyubov (TDHFB) trajectories. In our approach, each trajectory is independent from the others and the quantum mixing between trajectories is deduced from a variational principle. The associated theory can be seen as a simplified version of the Multi-Configuration TDHFB (MC-TDHFB) theory. Its accuracy to tackle the problem of symmetry restoration in dynamical problems is illustrated for the case of two superfluid systems that exchange particles during a short time. In Ref. [Phys. Rev. C 97, 034627 (2018)], using a schematic model where two systems initially described by a pairing Hamiltonians are coupled during a short contact time, it was demonstrated that statistical mixing of TDHFB trajectories can only qualitatively describe the transfer process and that a fully quantum treatment is mandatory. We show here that the present MC-TDHFB approach gives an excellent agreement with the exact solution when the two superfluids are the same (symmetric case) or different (asymmetric case) and from weak to strong interaction strength. Finally, we discuss the benefits and bottleneck of this method in view of its application to realistic systems.
nucl-thPRC(2019)·23 citations
08
States of the $^{12}$ C Nucleus in the Toroidal Configuration
Cheuk-Yin Wong · Andrzej Staszczak
The $^{12}$ C nucleus with $N$ =6 and $Z$ =6 is a doubly closed-shell nucleus in a toroidal potential. In the description of the ground state and the Hoyle state of $^{12}$ C in the resonating group method or the generator coordinate method, a superposition of the orientations of Wheeler's triangular cluster on the cluster plane would naturally generate an intrinsic toroidal density. A toroidal state also has a probability amplitude to overlap with a 3-alpha cluster, which is the dominant decay mode for the Hoyle state. For these reasons, we study a toroidal description of the states of $^{12}$ C in the toroidal configuration both phenomenologically and microscopically. A toroidal $^{12}$ C nucleus distinguishes itself by toroidal particle-hole multiplet excitations between one toroidal single-particle shell to another. From such a signature and experimental data, we find phenomenologically that the Hoyle state and many of its higher excited states may be tentatively attributed to those of the $^{12}$ C nucleus in a toroidal configuration. We then study the $^{12}$ C system from a microscopic mean-field approximation using variational wave functions. We find that the equidensity surfaces of the $^{12}$ C ground state exhibit a dense toroidal core immersed in lower-density oblate spheroids in the surface region. Furthermore, there are prominent toroidal features of the equidensity surfaces for the state at the Hoyle excitation energy, at which previous cluster model calculations indicate the presence of a 3-alpha cluster state. A toroidal coexistence model therefore may emerge to suggest the possibility that the physical Hoyle state may have probability amplitudes to be in the toroidal configuration and the 3-alpha cluster configuration.
nucl-thastro-ph.SRnucl-ex1 citation

Affiliations

first authorsco-authorsvia INSPIRE