Nuclear Theory

We calculate the lifetime of the hypertriton as function of the Λ separation energy B Λ in an effective field theory with Λ and deuteron degrees of freedom. We also consider the impact of new measurements of the weak decay parameter of the Λ . While the sensitivity of the total width to B Λ is small, the partial widths for decays into individual final states and the experimentally measured ratio R= Γ 3 He /( Γ 3 He + Γ pd ) show a strong dependence. For the standard value B Λ =(0.13±0.05) MeV, we find R=0.37±0.05 , which is in good agreement with past experimental studies and theoretical calculations. For the recent STAR value B Λ =(0.41±0.12±0.11) MeV, we obtain R=0.57±0.11 .

We study the composition of nuclear matter at sub-saturation densities, non-zero temperatures, and isospin asymmetry, under the conditions characteristic of binary neutron star mergers, stellar collapse, and low-energy heavy-ion collisions. The composition includes light clusters with mass number A≤4 , a heavy nucleus ($\isotope[56]{Fe}$), the Δ -resonances, the isotriplet of pions, as well as the Λ hyperon. The nucleonic mean-fields are computed from a zero-range density functional, whereas the pion-nucleon interactions are treated to leading order in chiral perturbation theory. We show that with increasing temperature and/or density the composition of matter shifts from light-cluster to heavy baryon dominated one, the transition taking place nearly independent of the magnitude of the isospin. Our findings highlight the importance of simultaneous treatment of light clusters and heavy baryons in the astrophysical and heavy-ion physics contexts.

The abundances of light clusters within a formalism that considers in-medium effects are calculated using several relativistic mean-field models, with both density-dependent and density-independent couplings. Clusters are introduced as new quasiparticles, with a modified coupling to the scalar meson field. A comparison with experimental data from heavy ion collisions allows settling the model dependence of the results and the determination of the couplings of the light clusters to the meson fields. We find that extra experimental constraints at higher density are needed to convincingly pin down the density associated to the melting of clusters in the dense nuclear medium. The role of neutron rich clusters, such as 6 He, in asymmetric matter is discussed.

The yields of light elements ( Z=1,2 ) obtained from spontaneous ternary fission of 252 Cf are treated within a nonequilibrium approach, and the contribution of unstable nuclei and excited bound states is taken into account. These light cluster yields may be used to probe dense matter, and to infer in-medium corrections. Continuum correlations are calculated from scattering phase shifts using the Beth-Uhlenbeck formula, and the effect of medium modification is estimated. The relevant distribution is reconstructed from the measured yields of isotopes. This describes the state of the nucleon system at scission and cluster formation, using only three Lagrange parameters which are the nonequilibrium counterparts of the temperature and chemical potentials, as defined in thermodynamic equilibrium. We concluded that a simple nuclear statistical equilibrium model neglecting continuum correlations and medium effects is not able to describe the measured distribution of H and He isotopes. Moreover, the freeze-out concept may serve as an important ingredient to the nonequilibrium approach using the relevant statistical operator concept.

We combine equation of state of dense matter up to twice nuclear saturation density ( n sat =0.16 fm −3 ) obtained using chiral effective field theory ( χ EFT), and recent observations of neutron stars to gain insights about the high-density matter encountered in their cores. A key element in our study is the recent Bayesian analysis of correlated EFT truncation errors based on order-by-order calculations up to next-to-next-to-next-to-leading order in the χ EFT expansion. We refine the bounds on the maximum mass imposed by causality at high densities, and provide stringent limits on the maximum and minimum radii of ∼1.4 M ⊙ and ∼2.0 M ⊙ stars. Including χ EFT predictions from n sat to 2 n sat reduces the permitted ranges of the radius of a 1.4 M ⊙ star, R 1.4 , by ∼3.5km . If observations indicate R 1.4 <11.2km , our study implies that either the squared speed of sound c 2 s >1/2 for densities above 2 n sat , or that χ EFT breaks down below 2 n sat . We also comment on the nature of the secondary compact object in GW190814 with mass ≃2.6 M ⊙ , and discuss the implications of massive neutron stars >2.1 M ⊙ (2.6 M ⊙ ) in future radio and gravitational-wave searches. Some form of strongly interacting matter with c 2 s >0.35(0.55) must be realized in the cores of such massive neutron stars. In the absence of phase transitions below 2 n sat , the small tidal deformability inferred from GW170817 lends support for the relatively small pressure predicted by χ EFT for the baryon density n B in the range 1−2 n sat . Together they imply that the rapid stiffening required to support a high maximum mass should occur only when n B ≳1.5−1.8 n sat .

In this paper, we perform a linear stability analysis of Israel-Stewart theory around a global equilibrium state, including the effects of shear-stress tensor, net-baryon diffusion current and diffusion-viscous coupling. We find all the relevant modes of this theory and derive necessary conditions that these modes must satisfy in order to be stable and subluminal. With these conditions, we then derive constraints for the shear and diffusion relaxation times and the transport coefficients related to diffusion-viscous coupling.

The aim of the present study is to make for the first time in the literature a systematic and quantitative assessment of the evaluation of the imaginary part of the optical potential calculated within the folding model and its consequences on the localization of surface reactions. Comparing theoretical and experimental reaction cross sections, for some light projectiles on a 9 Be target, it has recently been shown that a single-folded s.f. (light-) nucleus- 9 Be imaginary optical potential is more accurate than a double-folded d.f. optical potential. Within the eikonal formalism for the cross sections and phase shifts, the single-folded potential was obtained using a n- 9 Be phenomenological optical potential and microscopic projectile densities. This paper is a follow-up in which we systematically study a series of different light and medium-mass projectile induced reactions on 9 Be. Our results confirm that the s.f. cross sections are larger than the d.f. cross sections and the effect increases with the projectile mass. Furthermore the strong absorption radius parameter extracted from the S matrices calculated with the s.f. has a stable value r s =1.3 - 1.4 fm for all projectile masses in the range of incident energies 40-100AMeV. This indicates that a clear geometrical separation can be made between the region of surface reactions, the region of strong absorption into other channels and the region of weak nuclear interaction. The d.f. results are instead much scattered and the separation between surface reactions and other channels does not seem to be consistent. Excellent agreement with recent experimental results confirms the validity of our approach.

We investigate the possible existence of anomalous mass defects in the low mass region of stellar sequences of strange stars. We employ the nonperturbative equation of state derived in the framework of the Field Correlator Method to describe the hydrostatic equilibrium of the strange matter. The large distance static Q Q ¯ potential V 1 and the gluon condensate G 2 are the main parameters of the model. We use the surface gravitational redshift measurements as a probe to determine the ratio (P/E ) C at the center of strange stars. For V 1 =0 and $G_2\gappr0.035\,{\rm GeV}^4$\,, we show that (P/E ) C ≃0.262 and the corresponding redshift z S ≃0.47 are limiting values, at the maximum mass of the highest mass stellar sequence. As a direct application of our study, we try to determine the values of V 1 and G 2 from astrophysical observations of the compact star 1E\,1207.4-5209. Due to the uncertainties in the surface redshift determination, we made two attempts to obtain the model parameters. Our findings show that V 1 =0.44±0.10 \,GeV and G 2 =0.008±0.001 GeV 4 \, in the first attempt and V 1 =0.43±0.085 \,GeV and G 2 =0.0093±0.00092 GeV 4 in the second attempt. These values of V 1 and G 2 are in good agreement with the lattice and QCD sum rules calculations, and show an evident consistency of the strange star hypothesis of the compact star 1E\,1207.4-5209. The corresponding ratios (P/E ) C are 0.073 +0.029 +0.024 and 0.087±0.028 \,, respectively. As a consequence of the high values of V 1 and G 2 , the respective anomalous mass defects of 1E\,1207.4-5209 are | Δ 2 M|≃2.56× 10 53 \,erg\, and | Δ 2 M|≃2.94× 10 53 \,erg\,.

We review the properties of neutron matter in the low-density regime. In particular, we revise its ground state energy and the superfluid neutron pairing gap, and analyze their evolution from the weak to the strong coupling regime. The calculations of the energy and the pairing gap are performed, respectively, within the Brueckner--Hartree--Fock approach of nuclear matter and the BCS theory using the chiral nucleon-nucleon interaction of Entem and Machleidt at N 3 LO and the Argonne V18 phenomenological potential. Results for the energy are also shown for a simple Gaussian potential with a strength and range adjusted to reproduce the 1 S 0 neutron-neutron scattering length and effective range. Our results are compared with those of quantum Monte Carlo calculations for neutron matter and cold atoms. The Tan contact parameter in neutron matter is also calculated finding a reasonable agreement with experimental data with ultra-cold atoms only at very low densities. We find that low-density neutron matter exhibits a behavior close to that of a Fermi gas at the unitary limit, although, this limit is actually never reached. We also review the properties (energy, effective mass and quasiparticle residue) of a spin-down neutron impurity immersed in a low-density free Fermi gas of spin-up neutrons already studied by the author in a recent work where it was shown that these properties are very close to those of an attractive Fermi polaron in the unitary limit.

Significant transition strength in light α -conjugate nuclei at low energy, typically below 10 MeV, has been observed in many experiments. In this work the isoscalar low-energy response of N=Z nuclei is explored using the Finite Amplitude Method (FAM) based on the microscopic framework of nuclear energy density functionals. Depending on the multipolarity of the excitation and the equilibrium deformation of a particular isotope, the low-energy strength functions display prominent peaks that can be attributed to vibration of cluster structures: α + 12 C+ α and α + 16 O in 20 Ne, 12 C+ 12 C in 24 Mg, 4 α + 12 C in 28 Si, etc. Such cluster excitations are favored in light nuclei with large deformation.