Recent measurements of the compact star XTE J1814-338, with a mass of $M=1.2_{-0.05}^{+0.05}\ M_{\odot}$ and a radius of $R=7_{-0.4}^{+0.4} \ {\rm Km}$ alongside those of HESS J1731-347, which has a mass of $M=0.77_{-0.17}^{+0.20}\ M_{\odot}$ and a radius of $R=10.4_{-0.78}^{+0.86} \ {\rm Km}$, provide compelling evidence for the potential existence of exotic matter in neutron star cores. These observations offer important insights into the equation of state of dense nuclear matter. Recently, Fornal and Grinstein, in order to overcame the discrepancy between the neutron lifetime measured in beam and bottle experiments, proposed the existence of neutron dark decay. In the present work, an effort is made to connect the interpretation of the above exotic compact objects with the possible existence of dark particles, assumed to be products of neutron dark decay. Our hypothesis offers an advantage over comparable proposals, as the coexistence of dark matter and hadronic matter within neutron stars emerges from an intrinsic mechanism, thereby obviating the need to invoke external merger-related processes. It is still unclear to what extent the proposed dark decay of the neutron is affected by the extreme environment within neutron stars. Within this framework, we examined the case in which a mechanism suppressing the dark neutron decay becomes operative at densities few times above nuclear saturation density. We found that the proposed alternative explanation accommodates the simultaneous existence of neutron dark decay while consistently predicting both the two solar mass limit and the presence of compact objects with subsolar masses.
We study the feasibility of detecting exotic cores in merging neutron stars with ground-based gravitational-wave detectors. We focus on models with a sharp nuclear/exotic matter interface, and assume a uniform distribution of neutron stars in the mass range $[1,2] M_\odot$. We find that the existence of exotic cores can be confirmed at the 70% confidence level with as few as several tens of detections. Likewise, with such a sample, we find that some models of exotic cores can be excluded {with high confidence}.
Explosive astrophysical systems - such as supernovae or compact star binary mergers - provide conditions where exotic degrees of freedom can be populated. Within the covariant density functional theory of nuclear matter we build several general purpose equations of state which, in addition to the baryonic octet, account for $Δ(1232)$ resonance states. The thermodynamic stability of $Δ$-admixed nuclear matter is investigated in the limiting case of vanishing temperature for charge fractions $Y_Q=0.01$ and $Y_Q=0.5$ and wide ranges of the coupling constants to the scalar and vector mesonic field…
It is shown the possibility of existence of metastable atoms of dineutroneum as a bound state of two neutrons and one neutrino. Such atoms can appear in a reaction of deuterons with free or quasi-free electrons. The estimation of mass, size and lifetime of dineutroneum atom is fulfilled.
How Neutron Star Observations Point Towards Exotic Matter: Existing Explanations and a Prospective Proposal
Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata（拉普拉塔国立大学天文与地球物理学院）；Consejo Nacional de Investigaciones Científicas y Técnicas（国家科学研究委员会）
The recent high precision observation of the most massive pulsar J1614-2230 with $(1.97\pm0.04)$ solar mass ($M_\odot$) was reported with a suggestion that many nuclear models which consider exotic particles in the core could be ruled out. However, many recent calculations could explain this star with various exotic particles, rendering the precise mass measurements insufficient to conclude on exotic cores. We examine the sensitivity of the radius of such a star to the details of its core. With our calculations and analysis, here we show that, for the most massive neutron star, with a precise …
Charge form factors corresponding to proton density distributions in exotic nuclei, such as $^{6,8}$He, $^{11}$Li, $^{17,19}$B and $^{14}$Be are calculated and compared. The results can be used as tests of various theoretical models for the exotic nuclei structure in possible future experiments using a colliding electron-exotic nucleus storage ring. The result of such a comparison would show the effect of the neutron halo or skin on the proton distributions in exotic nuclei.
We consider the effect of hard core repulsion in the baryon-baryon interaction at short distance to the properties of a neutron star. We obtain that, even with hyperons in the interior of a neutron star, the neutron star mass can be as large as $\sim 2 M_\odot$.
Exotic and drip-line nuclei as well as nuclei immersed in a low density gas of neutrons in the outer crust of neutron stars are systematically investigated with respect to their neutron pairing properties. This is done using Skyrme density-functional and different pairing forces such as a density-dependent contact interaction and a separable form of a finite-range Gogny interaction. Hartree-Fock-Bogoliubov and BCS theories are compared. It is found that neutron pairing is reduced towards the drip line while overcast by strong shell effects. Furthermore resonances in the continuum can have an i…
Using HF+BCS method we study light nuclei with nuclear charge in the range $2 \leq Z \leq 8$ and lying near the neutron drip line. The HF method uses effective Skyrme forces and allows for axial deformations. We find that the neutron drip line forms stability peninsulas at $^{18}$He and $^{40}$C. These isotopes are found to be stable against one neutron emission and possess the highest known neutron to proton ratio in stable nuclei.
In this work, we study the effect of differential rotation, finite temperature and strangeness on the quasi stationary sequences of hyper massive neutron stars (HMNS). We generate constant rest mass sequences of differentially rotating and uniformly rotating stars. The nucleonic matter relevant to the star interior is described within the framework of the relativistic mean field model with the DD2 parameter set. We also consider the strange $Λ$ hyperons using the BHB$Λϕ$ equation of state (EoS). Additionally, we probe the behaviour of neutron stars (NS) with these compositions at different tem…
Nuclear superfludity in exotic nuclei close to the drip lines and in the inner crust matter of neutron stars have common features which can be treated with the same theoretical tools. In the first part of my lecture I discuss how two such tools, namely the HFB approach and the linear response theory can be used to describe the pairing correlations in weakly bound nuclei, in which the unbound part of the energy spectrum becomes important. Then, using the same models, I shall discuss how the nuclear superfluidity can affect the thermal properties of the inner crust of neutron stars.
A set of theoretical mass-radius relations for rigidly rotating neutron stars with exotic cores, obtained in various theories of dense matter, is reviewed. Two basic observational constraints are used: the largest measured rotation frequency (716 Hz) and the maximum measured mass ($2\;M_\odot$). Present status of measuring the radii of neutron stars is described. The theory of rigidly rotating stars in general relativity is reviewed and limitations of the slow rotation approximation are pointed out. Mass-radius relations for rotating neutron stars with hyperon and quark cores are illustrated u…
The excited states of Be-12, Be-14 and B-15 have been studied with a method of antisymmetrized molecular dynamics. In the predicted excited states we find novel molecule-like structures with very exotic clusters such as 6He+8He in Be-14 and 6He+9Li in B-15. The origin of the He-6 cluster development in the very neutron-rich nuclei is understood by the new-type correlation among 4 neutrons and 2 protons. In this paper we also present our recent challenge to study $sd$-shell nuclei. Shape coexistence problems in Ar-36 and Ca-40 are discussed.
We have investigated the implications of the neutron halo configuration, observed in the ground-state of some neutron-rich light nuclei, on neutron radiative transition processes. In particular, we have studied the influence of the neutron halo on the direct radiative capture (DRC) process. The energy dependence as well as the strength of E1 emission due to incident p-wave neutrons is strongly influenced by the halo configuration of the residual nucleus capturing state. We have compared the calculated 10Be(n,gamma)11Be DRC cross section with that derived from the experiment in the inverse kine…
The measurement of $1.97 \pm 0.04 M_{solar}$ for PSR J1614-2230 and $2.01 \pm 0.04M_{solar}$ for PSR J0348+0432 puts a strong constraint on the neutron star equation of state and its exotic composition at higher densities. In this paper, we investigate the possibility of exotic equation of state within the observational mass constraint of $2M_{solar}$ in the framework of relativistic mean field model with density-dependent couplings. We particularly study the effect of antikaon condensates in the presence of hyperons on the mass-radius relationship of the neutron star.
We present model results for the two-halo-neutron correlation functions, $C_{nn}$, for the dissociation process of light exotic nuclei modelled as two neutrons and a core. A minimum is predicted for $C_{nn}$ as a function of the relative momentum of the two neutrons, $p_{nn}$, due to the coherence of the neutrons in the halo and final state interaction. Studying the systems $^{14}$Be, $^{11}$Li and $^{6}$He within this model, we show that the numerical asymptotic limit, $C_{nn}\to 1$, occurs only for $p_{nn}\gtrsim$ 400 MeV$/c$, while such limit is reached for much lower values of $p_{nn}$ in …
The series of Uranium isotopes with $N=154 \sim 172$ around the magic number N=162/164 are identified to be thermally fissile. The thermal neutron fission of a typical representative $^{249}$U of this region amenable to synthesis in the radioactive ion beam facilities is considered here. Semiempirical study of fission barrier height and width shows this nucleus to be infinitely stable against spontaneous fission due to increase in barrier width arising out of excess neutrons. Calculation of probability of fragment mass yields and microscopic study in relativistic mean field theory, show this n…
The multinucleon transfer reaction in the collisions of $^{40}$Ca+$^{124}$Sn at $E_{\textrm{c.m.}}=128.5$ MeV is investigated by using the improved quantum molecular dynamics model. The measured angular distributions and isotopic distributions of the products are reproduced reasonably well by the calculations. The multinucleon transfer reactions of $^{40}$Ca+$^{112}$Sn, $^{58}$Ni+$^{112}$Sn, $^{106}$Cd+$^{112}$Sn, and $^{48}$Ca+$^{112}$Sn are also studied. It shows that the combinations of neutron-deficient projectile and target are advantageous to produce the exotic neutron-deficient nuclei n…
We study the correlations between two neutrons in borromian nuclei $^{11}$Li and $^6$He by using a three-body model with a density-dependent contact two-body interaction. It is shown that the two neutrons show a compact bound feature at the nuclear surface due to the mixing of single particle states of different parity. We study the Coulomb breakup cross sections of $^{11}$Li and $^6$He using the same three-body model. We show that the concentration of the B(E1) strength near the threshold can be well reproduced with this model as a typical nature of the halo nuclei. The energy distributions o…
We present a simple model for electron transport in a possible layer of exotic nuclear clusters (in the so called nuclear pasta layer) between the crust and liquid core of a strongly magnetized neutron star. The electron transport there can be strongly anisotropic and gyrotropic. The anisotropy is produced by different electron effective collision frequencies along and across local symmetry axis in domains of exotic ordered nuclear clusters and by complicated effects of the magnetic field. We also calculate averaged kinetic coefficients in case local domains are freely oriented. Possible appli…
We study the correlation between the thickness of the neutron skin in finite nuclei and the nuclear symmetry energy for isotopic chains of even-even Ni, Sn, and Pb nuclei in the framework of the deformed self-consistent mean-field Skyrme HF+BCS method. The symmetry energy, the neutron pressure and the asymmetric compressibility in finite nuclei are calculated within the coherent density fluctuation model using the symmetry energy as a function of density within the Brueckner energy-density functional. The mass dependence of the nuclear symmetry energy and the neutron skin thickness are also st…
The exotic phenomenon of two-neutron halos and 2n-radioactivity are explored in the neutron-rich $^{40,42,44}$Mg by employing various variants of the relativistic mean-field approach. The extended tail of spatial density distributions including the enhanced neutron radii and skin thickness, pairing correlations, single-particle spectrum and wave functions predict $^{40,42,44}$Mg to be strong candidates for deformed neutron halos. Weakening of magicity at N$=$28 plays a significant role in the existence of a weakly bound halo in $^{40}$Mg which is currently the heaviest isotope of Mg accessible…
The presence of exotic states of matter in neutron stars (NSs) is currently an open issue in physics. The appearance of muons, kaons, hyperons, and other exotic particles in the inner regions of the NS, favored by energetic considerations, is considered to be an effective mechanism to soften the equation of state (EoS). In the so-called two-families scenario, the softening of the EoS allows for NSs characterized by very small radii, which become unstable and convert into a quark stars (QSs). In the process of conversion of a NS into a QS material can be ablated by neutrinos from the surface of…
The evolution of the PDR strength with the neutron excess is investigated in Sn isotopic and N=82 isotonic chains with regard to its possible connection with the neutron skin thickness. For this purpose a recently proposed method incorporating both HFB and multi-phonon QPM theory is applied. Analysis of the corresponding neutron and proton dipole transition densities is presented.
The experimental level structure of 134Sb is compared with the results of a shell-model calculation in which the two-body matrix elements of the effective interaction have been derived from the CD-Bonn nucleon-nucleon potential. The experimental data, including the very low-lying first-excited 1- state, are remarkably well reproduced by the theory. The results of this paper complement those of our previous studies on 135Sb and 134Sn, showing that our effective interaction is well suited to describe 132Sn neighbors beyond N=82. The various terms which contribute to the matrix elements of the pr…
In this theoretical study, we establish a correlation between the neutron skin thickness and the nuclear symmetry energy for the even$-$even isotopes of Fe, Ni, Zn, Ge, Se and Kr within the framework of the axially deformed self-consistent relativistic mean field for the non-linear NL3$^*$ and density-dependent DD-ME1 interactions. The coherent density functional method is used to formulate the symmetry energy, the neutron pressure and the curvature of finite nuclei as a function of the nuclear radius. We have performed broad studies for the mass dependence on the symmetry energy in terms of t…
Multi-messenger observations of binary neutron star mergers provide a unique opportunity to constrain the dense-matter equation of state. Although it is known from quantum chromodynamics that hadronic matter will undergo a phase transition to exotic forms of matter, e.g., quark matter, the onset density of such a phase transition cannot be computed from first principles. Hence, it remains an open question if such phase transitions occur inside isolated neutron stars or during binary neutron star mergers, or if they appear at even higher densities that are not realized in the Cosmos. In this ar…
We present the first application of the newly developed EKK theory of the effective nucleon-nucleon interaction to shell-model studies of exotic nuclei, including those where conventional approaches with fitted interactions encounter difficulties. This EKK theory enables us to derive the interaction suitable for several major shells ($sd$+$pf$ in this work). By using such an effective interaction obtained from the Entem-Machleidt QCD-based $χ\mathrm{N}^3\mathrm{LO}$ interaction and the Fujita-Miyazawa three-body force, the energies, E2 properties and spectroscopic factors of low-lying states o…
The bottom part of the neutron star crust is investigated using the Skyrme-Hartree-Fock approach with the Coulomb interaction treated beyond the Wigner-Seitz approximation. A variety of nuclear phases is found to coexist in this region. Their stability and relative energies are governed by the Coulomb, surface and shell energies. We have also found that a substantial contribution is coming from the spin-orbit interaction.
Parameters of nuclear density distributions are derived from least-squares fits to strong interaction observables in exotic atoms. Global analyses of antiprotonic and pionic atoms show reasonably good agreement between the two types of probes regarding the average behaviour of root-mean-square radii of the neutron distributions. Apparent conflict regarding the shape of the neutron distribution is attributed to different radial sensitivities of these two probes.
The low-lying dipole and quadrupole states in neutron rich nuclei, are studied within the fully self-consistent relativistic quasiparticle random-phase approximation (RQRPA), formulated in the canonical basis of the Relativistic Hartree-Bogoliubov model (RHB), which is extended to include the density dependent interactions. In heavier nuclei, the low-lying E1 excited state is identified as a pygmy dipole resonance (PDR), i.e. as a collective mode of excess neutrons oscillating against a proton-neutron core. Isotopic dependence of the PDR is characterized by a crossing between the PDR and one-n…
This thesis focuses on a variety of active research topics, such as nuclear matter, neutron stars, and phase transition within the framework of the RMF model. We use the previously successful effective field theory-driven Relativistic Mean Field (RMF) and density-dependent RMF (DD-RMF)formalisms for analyzing hadron matter to examine the infinite nuclear matter and neutron stars. The presence of exotic phases such as quarks has been investigated using the MIT Bag model and its variants, such as the vBag model, at various bag constants. The other exotic phases, such as hyperons, have also been …
The isotopic dependence of the complete fusion (capture) cross section is analyzed in the reactions $^{130,132,134,136,138,140,142,144,146,148,150}$Xe+$^{48}$Ca with stable and radioactive beams. It is shown for the first time that the very neutron-rich nuclei $^{186-191}$W can be reached with relatively large cross sections by complete fusion reactions with radioactive ion beams at incident energies near the Coulomb barrier. A comparison between the complete fusion and fragmentation reactions for the production of neutron-rich W and neutron-deficient Rn isotopes is performed.
The generic properties of compact objects made of two different fluids of dark matter are studied in a scale invariant approach. We investigate compact objects with a core-shell structure, where the two fluids are separated, and with mixed dark matter components, where both dark matter fluids are immersed within each other. The constellations considered are combinations of incompressible fluids, free and interacting Fermi gases, and equations of state with a vacuum term, i.e. self-bound dark matter. We find novel features in the mass-radius relations for combined dark matter compact objects wh…
The occurrence of a bubble, due to an inversion of s$_{1/2}$ state with the state usually located above, is investigated. Proton bubbles in neutron-rich Argon isotopes are optimal candidates. Pairing effects which can play against the bubble formation are evaluated. They cannot prevent bubble formation in very neutron-rich argon isotopes such as $^{68}$Ar. This pleads for a measurement of the charge density of neutron-rich argon isotopes in the forthcoming years, with the advent of electron scattering experiments in next generation exotic beam facilities such as FAIR or RIBF.
[Background] Reactions with stable beams have demonstrated a strong interplay between nuclear structure and fusion. Exotic beam facilities open new perspectives to understand the impact of neutron skin, large isospin, and weak binding energies on fusion. Microscopic theories of fusion are required to guide future experiments. [Purpose] To investigate new effects of exotic structures and dynamics in near-barrier fusion with exotic nuclei. [Method] Microscopic approaches based on the Hartree-Fock (HF) mean-field theory are used for studying fusion barriers in $^{40-54}$Ca+$^{116}$Sn reactions fo…
The ground-state properties of neutron-rich exotic Na and Mg isotopes with even numbers of neutrons, N, are studied up to driplines. The shell-model calculations with an ab initio effective nucleon-nucleon interaction reported in [Tsunoda, Otsuka, Takayanagi et al., Nature 587, 66 (2020)] are extended to observables such as magnetic dipole and electric quadrupole moments, and charge and matter radii. Good agreements with experimental data are found, and predictions are shown up to driplines. A prescription to extract the deformation parameters for the eigenstates of Monte Carlo Shell Model is …
A generalized formula is provided, to calculate the M$_n$/M$_p$ ratio of the multipole transition matrix elements, in the framework of the so-called phenomenological analysis. It takes into account the possible difference between the neutron and proton radii and diffuseness, which can occur, especially in exotic nuclei. The validity domain of the original Bernstein formula is discussed, in the case of the proton scattering probe at few tens of MeV. The largest discrepancies are obtained for very neutron-rich nuclei (N/Z$\gtrsim$1.6) or when the electromagnetic deformation parameter is larger t…
If neutrons are progressively added to a normal nucleus, the Pauli principle forces them into states of higher momentum. When the core becomes neutron-saturated, the nucleus expels most of the wavefunction of the last neutrons outside to form a halo, which because of its large size can have lower momentum. It is an open question how nature stabilizes such a fragile system and provides the glue needed to bind the halo neutrons to the core. Here we show that this problem is similar to that of the instability of the normal state of an electron system at zero temperature solved by Cooper, solution…
We present a review of recent works on clustering phenomena in unstable nuclei studied by antisymmetrized molecular dynamics (AMD). The AMD studies in these decades have uncovered novel types of clustering phenomena brought about by the excess neutrons. Among them, this review focuses on the molecule-like structure of unstable nuclei. One of the earliest discussions on the clustering in unstable nuclei was made for neutron-rich Be and B isotopes. AMD calculations predicted that the ground state clustering is enhanced or reduced depending on the number of excess neutrons. Today, the experiments…
Recent experimental observation of magicity in $^{78}$Ni has infused the interest to examine the persistence of the magic character across the N$=$50 shell gap in extremely neutron rich exotic nucleus $^{78}$Ni in ground as well as excited states. A systematic study of Ni isotopes and N$=$50 isotones in ground state is performed within the microscopic framework of relativistic mean-field (RMF) and the triaxially deformed Nilson Strutinsky model (NSM). Ground state density distributions, charge form factors, radii, separation energies, pairing energies, single particle energies and the shell co…
Neutron stars feature extremely high magnetic fields with deduced field strengths of $10^{15}$ G in the case of magnetars and potentially much higher values inside of the star. In this context we consider the appearance of $ρ^-$ meson condensation taking into account the effect of the magnetic field. The results show that, depending on parameters, such a condensation in magnetized neutron stars might (just) occur.
We demonstrate that the observation of neutron stars with masses greater than one solar mass places severe demands on any exotic neutron decay mode that could explain the discrepancy between beam and bottle measurements of the neutron lifetime. If the neutron can decay to a stable, feebly-interacting dark fermion, the maximum possible mass of a neutron star is 0.7 solar masses, while all well-measured neutron star masses exceed one solar mass. The survival of $2 M_\odot$ neutron stars therefore indicates that any explanation beyond the Standard Model for the neutron lifetime puzzle requires da…
Journal of Combinatorial Theory, Series B, 98(1):173-225, 2008n exotic nuclei are studied in the framework of a fully self-consistent relativistic continuum random phase approximation (RCRPA). In this method the contribution of the continuum spectrum to nuclear excitations is treated exactly by the single particle Green's function. Different from the cases in stable nuclei, there are strong low-energy excitations in neutron-rich nuclei and proton-rich nuclei. The neutron or proton excess pushes the centroid of the strength function to lower energies and increases the fragmentation of the stren…
Recent proposals have suggested that a previously unknown decay mode of the neutron into a dark matter particle could solve the long lasting measurement problem of the neutron decay width. We show that, if the dark particle in neutron decay is the major component of the dark matter in the universe, this proposal is in disagreement with modern astro-physical data concerning neutron star masses.
Recent studies have provided new experimental information on the very neutron-rich nucleus 135Sb. We have performed a shell-model calculation for this nucleus using a realistic effective interaction derived from the CD-Bonn nucleon-nucleon potential. This gives a very good description of the observed properties of 135Sb. We show that the anomalously low position of the first excited state, J=5/2+, and the strongly hindered M1 transition to the ground state have their origin in the effective neutron-proton interaction.
Theoretical predictions and experimental discoveries for neutron-rich, short-lived nuclei far from stability indicate that the familiar concept of nucleonic shell structure should be considered as less robust than previously thought. The notion of single-particle motion in exotic nuclei is reviewed with a particular focus on three aspects: (i) variations of nuclear mean field with neutron excess due to tensor interactions; (ii) importance of many-body correlations; and (iii) influence of open channels on properties of weakly bound and unbound nuclear states.
The excited states of Be-12, Be-14 and B-15 were studied by an antisymmetrized molecular dynamics method. The theoretical results reproduced the energy levels of recently measured excited states of Be-12, and also predicted rotational bands with innovative clustering structures in Be-12, Be-14 and B-15. Clustering states with new exotic clusters (He-6, He-8 and Li-9) were theoretically suggested. One new aspect in very neutron-rich nuclei is a 6-nucleon correlation among 4 neutrons and 2 protons, which plays an important role in the formation of He-6 clusters during clustering: 8He + 6He of Be…
Low-energy strength is predicted for the isoscalar monopole response of neutron-rich Ni isotopes, in calculations performed using the microscopic Skyrme HF+RPA and relativistic RHB+RQRPA models. Both models, although based on different energy density functionals, predict the occurrence of pronounced monopole states in the energy region between 10 MeV and 15 MeV, well separated from the isoscalar GMR. The analysis of transition densities and corresponding particle-hole configurations shows that these states represent almost pure neutron single hole-particle excitations. Even though their locati…
We call "projectile fragmentation" of neutron halo nuclei the elastic breakup (diffraction) reaction, when the observable studied is the neutron-core relative energy spectrum. This observable has been measured in relation to the Coulomb breakup on heavy target and recently also on light targets. Such data enlighten the effect of the neutron final state interaction with the core of origin. Projectile fragmentation is studied here by a time dependent model for the excitation of a nucleon from a bound state to a continuum resonant state in a neutron-core complex potential which acts as a final st…
The BCS and HFB theories which can accommodate the pairing correlations in the ground states of atomic nuclei are presented. As an application of the pairing theories, we investigate the spatial extension of weakly bound Ne and C isotopes by taking into account the pairing correlation with the Hartree-Fock-Bogoliubov (HFB) method and a 3-body model, respectively. We show that the odd-even staggering in the reaction cross sections of $^{30,31,32}$Ne and $^{14,15,16}$C are successfully reproduced, and thus the staggering can be attributed to the unique role of pairing correlations in nuclei far …
