We propose a new framework by means of the dineutron condensate (DC) wave function to describe the dineutron correlation, which is characterized by the spatially strong correlation of a spin-zero neutron-neutron pair, in neutron-rich nuclei with an active deformed core surrounded by valence neutrons. Using the DC wave function for a 2$α$+2n system, which corresponds to a toy model for the $^{10}$Be system, we investigate the neutron-neutron correlation around the $2α$ core and discuss the mechanism of the dineutron formation at the surface of finite nuclei. To investigate dineutron correlations in realistic nuclear systems, we superpose the antisymmetrized molecular dynamics (AMD) wave functions and the DC wave functions. Applying the AMD+DC method to $^{10}$Be, we show effects of the DC wave functions in the ground and excited $0^+$ states of $^{10}$Be and discuss the dineutron correlation in them.
We extend the method of antisymmetrized molecular dynamics to investigate dineutron correlation. We apply this method to $^{10}$Be as an example and investigate the motion of two neutrons around a largely deformed $^8$Be core by analyzing the two-neutron overlap function around the core. We show that the core structure plays an important role in dineutron formation and expansion from the core and that the present framework is effective for the studies of dineutron correlation.
We discuss the correlation of the spin-singlet two-neutron pair, the dineutron correlation, in the ground and excited $0^+$ states of $^8$He by using a method to describe the dineutron correlation in neutron-rich nuclei, the dineutron condensate wave function. The shell-model configuration and the dineutron configuration compete with each other in the ground state of $^8$He. We compare the ground state structure of $^8$He with that of $^6$He and suggest that the dineutron correlation is weaker in the ground state of $^8$He than $^6$He. We also suggest the possibility of the dineutron condensat…
Two-neutron correlation in the $^1S$ channel in quasi two-dimensional (2D) neutron systems at zero temperature is studied by means of the BCS theory with finite-range effective nuclear forces. The dineutron correlation in low density neutron systems confined in an infinite slab is investigated in a simplified model that neutron motion of one direction is frozen. When the slab is thin enough, two neutrons form a tightly bound dineutron with a small size in the quasi-2D system, and a Bose dineutron gas is found in low density limit. With increase of Fermi momentum, the neutron system changes fro…
We propose a simple schematic model for two-neutron halo nuclei. In this model, the two valence neutrons move in a one-dimensional mean field, interacting with each other via a density-dependent contact interaction. We first investigate the ground state properties, and demonstrate that the dineutron correlation can be realized with this simple model due to the admixture of even- and odd-parity single-particle states. We then solve the time-dependent two-particle Schrödinger equation under the influence of a time-dependent one-body external field, in order to discuss the effect of dineutron cor…
We study the spatial structure of four valence neutrons in the ground state of $^8$He and $^{18}$C nuclei using a core+4$n$ model. For this purpose, we employ a density-dependent contact interaction among the valence neutrons, and solve the five-body Hamiltonian in the Hartree-Fock-Bogoliubov (HFB) approximation. We show that two neutrons with the coupled spin of $S$=0 exhibit a strong dineutron correlation around the surface of these nuclei, whereas the correlation between the two dineutrons is much weaker. Our calculation indicates that the probability of the (1p$_{3/2})^4$ and [(1p$_{3/2})^…
We study the dinucleon (dineutron and diproton) correlation of the ground states of $^9$Li, $^{10}$Be, and $^{9,10}$C. We assume an $α+t$ core for $^9$Li, an $α+α$ core for $^{10}$Be and $^{10}$C, and an $α+^3$He core for $^9$C, and investigate the effect of core structure changes on the degree of dineutron formation and spatial expansion from the core. For $^9$Li, $t$ cluster breaking in the core significantly enhances the dineutron component inside the nuclei. Moreover, its component markedly depends on the strength of the spin-orbit interaction since a dineutron is fragile and dissociates r…
The nature of pairing correlations in neutron matter is re-examined. Working within the conventional approximation in which the $nn$ pairing interaction is provided by a realistic bare $nn$ potential fitted to scattering data, it is demonstrated that the standard BCS theory fails in regions of neutron number density where the pairing constant $λ$, depending crucially on density, has a non-BCS negative sign. We are led to propose a non-BCS scenario for pairing phenomena in neutron matter that involves the formation of a hidden dineutron state. In low-density neutron matter where the pairing con…
To discuss the dineutron correlation in the ground state, the quasi-free neutron knockout reaction on $^6$He is investigated.In the present work, the momentum distribution of the two emitted neutrons is calculated with the $α$~+~$n$~+~$n$ three-body model and a simple reaction model to assess the effects of the knockout process via the $^5$He resonance and the target dependence in the momentum distribution. A clear signature of the dineutron correlation can be obtained by choosing the kinematical condition so the process via the $^5$He resonance is excluded because the $^5$He resonance drastic…
We discuss the two-neutron emission decay of $^{26}$O nucleus from a viewpoint of dineutron correlations between the valence neutrons. We first discuss how the dineutron correlation is realized both in the coordinate space and in the momentum space by employing a three-body model. We then discuss the decay energy spectrum as well as the angular distribution of emitted neutrons. We show that an emission of two neutrons in back-to-back directions is enhanced, which we interpret as a clear manifestation of the dineutron correlation between the valence neutrons.
The dineutron correlations and the crossover from superfluidity of neutron Cooper pairs in the $^1S_0$ pairing channel to Bose-Einstein condensation (BEC) of dineutron pairs in both symmetric and neutron matter are studied within the relativistic Hartree-Bogoliubov theory, with the effective interaction PK1 of the relativistic mean-field approach in the particle-hole channel and the finite-range Gogny force in the particle-particle channel. The influence of the pairing strength on the behaviors of dineutron correlations is investigated. It is found that the neutron pairing gaps at the Fermi su…
The ground and excited states of $^{8}$He were investigated with a method of antisymmetrized molecular dynamics(AMD). We adopted effective nuclear interactions which systematically reproduce the binding energies of $^4$He, $^6$He and $^8$He. The ground state of $^8$He has both the $j$-$j$ coupling feature($p_{3/2}$ closure) and the $L$-$S$ coupling feature($^4$He$+2n+2n$) with a slight tail of dineutron at the long distance region. The theoretical results give an indication of the $0^+_2$ state with dineutron gas-like structure. The dineutron structure, $^4$He+$2n$+$2n$, of this state is simil…
We investigate how strong a hypothetical 1S0 bound state of two neutrons would affect different observables in the neutron-deuteron reactions. To that aim we extend our momentum space scheme of solving three-nucleon Faddeev equations to incorporate in addition to the deuteron also the 1S0 dineutron bound state. We discuss effects induced by dineutron on the angular distribution of the neutron-deuteron elastic scattering and cross sections of the deuteron breakup. A comparison to the available data for neutron-deuteron total cross sections and elastic scattering angular distributions cannot dec…
We investigated the development and breaking of the dineutron correlation in $^{10}$Be by analyzing the elastic and inelastic scatterings with a framework combing the microscopic structure and reaction models. For studying the structure, the $^{10}$Be nucleus was constructed under the assumption of a four-body ($α+ α+ n + n$) cluster model. In this work, we focused on the change in the inner structure for the 0$_1^+$, 2$_1^+$, and 2$_2^+$ states when the strength of the spin-orbit interaction is varied. The inner structure, including various physical quantities such as energy, radius, and tran…
We investigate the dineutron in the $2^+_1$ state of $^6$He via analysis of its decay mode by using the complex scaling method. In this letter, we propose the cross section for the resonant state to distinguish the resonant contributions from the nonresonant ones. As the results, it is found that the shoulder peak appears in the cross section for the resonant state as a function of $\varepsilon_{n\text{-}n}$. Furthermore, we show that the $S$ = 0 component of the cross section, where $S$ is the total spin of the valence two neutrons, has a peak around the shoulder peak, which comes from the di…
Background. Knockout reactions with proton targets provide an invaluable tool to access the properties of two-neutron halo nuclei. Recently, experimental results for the average opening angle as a function of the intrinsic neutron momentum in 11Li have shown a localization of dineutron correlations on the nucleus surface. Purpose. Study the model dependence and the effect of distortion and absorption on the opening angle distributions to assess the reliability of this observable to extract properties of Borromean two-neutron halo nuclei. Method. A quasifree sudden model is used to describe the…
We investigate the sensitivity of the three-nucleon system to changes in the neutron-neutron scattering length to next-to-leading order in the pionless effective field theory, focusing on the the triton-3He binding energy difference and neutron-deuteron elastic scattering. Due to the appearance of an electromagnetic three-body counterterm at this order, the triton-3He binding energy difference remains consistent with the experimental value even for large positive neutron-neutron scattering lengths while the elastic neutron-deuteron scattering phase shifts are insensitive. We conclude that a bo…
We present a reaction model to describe the fast removal of the $α$-particle core in $^8$He nucleus with eventual emission of four neutrons. The obtained four neutron energy distributions allows to explain the sharp low energy peak observed by studying the missing mass spectra of four neutrons in [Nature Vol. 606, p. 678], as a consequence of dineutron-dineutron correlations.
We calculate the binding energies for multi-nucleon bound states with the nuclear mass number less than or equal to 4 in 2+1 flavor QCD at the lattice spacing of a = 0.09 fm employing a relatively heavy quark mass corresponding to m_pi = 0.51 GeV. To distinguish a bound state from attractive scattering states, we investigate the volume dependence of the energy shift between the ground state and the state of free nucleons by changing the spatial extent of the lattice from 2.9 fm to 5.8 fm. We conclude that ^4He, ^3He, deuteron and dineutron are bound at m_pi = 0.51 GeV. We compare their binding…
We discuss a decay of unbound nuclei beyond the neutron drip-line using a three-body model with a core nucleus and two valence neutrons. We particularly discuss the role of dineutron correlation between the valence neutrons in the two-neutron emission from the ground state of $^{26}$O and $^{10}$He nuclei. Our calculations clearly indicate that the emission of the two neutrons in the back-to-back direction is enhanced due to the dineutron correlation.
We discuss the energy and angular distributions of two emitted neutrons from the dipole excitation of typical weakly-bound Borromean nuclei, $^{11}$Li and $^6$He. To this end, we use a three-body model with a density dependent contact interaction between the valence neutrons. Our calculation indicates that the energy distributions for the valence neutrons are considerably different between the two nuclei, although they show similar strong dineutron correlations in the ground state to each other. This different behaviour of the energy distribution primarily reflects the interaction between the …
Cluster structures of excited $^{10}$Be states are investigated with a hybrid model of dineutron condensate wave functions and $^6$He+$α$ cluster wave functions. Two kinds of cluster states are theoretically suggested a few MeV above the $α+α+n+n$ threshold energy. They have quite distinct cluster structure; ones have gas-like structures of $α+α$+dineutron, and the others have $^6$He+$α$ with extremely extended an $α$ cluster. Although these cluster states have not been confirmed experimentally yet, theoretically suggested properties of these states such as monopole transition strengths and $α…
Within the complex-energy configuration interaction framework, we study correlations of valence neutrons to explain the behavior of charge radii in the neutron halo nuclei $^{6,8}$He. We find that the experimentally observed decrease of the charge radius between $^6$He and $^8$He is caused by a subtle interplay between three effects: dineutron correlations, a spin-orbit contribution to the charge radius, and a core swelling effect. We demonstrate that two-neutron angular correlations in the $2^+_1$ resonance of $^6$He differ markedly from the ground-state correlations in $^{6,8}$He.
Multi-neutron correlations in light nuclei via ab-initio lattice simulations
Forschungszentrum Jülich（于利希研究中心）；King Fahd University of Petroleum and Minerals (KFUPM)（法赫德国王石油与矿物大学）；Gaziantep Islam Science and Technology University（加济安泰普伊斯兰科学与技术大学）；Universität Bonn（波恩大学）；Beihang University（北京航空航天大学）
We study the spatial correlation of a particle-hole pair in the isovector channel in $^{56}$Co and $^{40}$K nuclei. To this end, we employ the Hartree-Fock+Tamm-Dancoff approximation with the Skyrme interaction. We find a large concentration of the two-body density at positions where the neutron particle and the proton hole states locate on the opposite side to each other with respect to the core nucleus. This feature originates from a repulsive nature of the isovector residual interaction, which is in stark contrast to the dineutron correlation with an attractive pairing interaction between t…
This is the second of two papers which investigate cold dilute neutron matter on the lattice using pionless effective field theory. In the unitary limit, where the effective range is zero and scattering length is infinite, simple scaling relations relate thermodynamic functions at different temperatures. When the second virial coefficient is properly tuned, we find that the lattice results obey these scaling relations. We compute the energy per particle, pressure, spin susceptibility, dineutron correlation function, and an upper bound for the superfluid critical temperature.
The particle unbound $^{26}$O nucleus is located outside the neutron drip line, and spontaneously decays by emitting two neutrons with a relatively long life time due to the centrifugal barrier. We study the decay of this nucleus with a three-body model assuming an inert $^{24}$O core and two valence neutrons. We first point out the importance of the neutron-neutron final state interaction in the observed decay energy spectrum. We also show that the energy and and angular distributions for the two emitted neutrons manifest a clear evidence for the strong neutron-neutron correlation in the thre…
Impact of Effective Nucleon Mass and Multineutron States on the Equation of State for Core-Collapse Supernovae
The University of Tokyo（东京大学）；Kanto Gakuin University（关东学院大学）；RIKEN（理化学研究所）；National Institute of Technology, Numazu College（国立名古屋工业学校沼津高等专门学校）；Nankai University（南开大学）；Tokyo University of Science（东京科学大学）
Background: Magnetic dipole (M1) excitation is the leading mode of nuclear excitation by the magnetic field, which couples unnatural-parity states. Since the M1 excitation occurs mainly for open-shell nuclei, the nuclear pairing effect is expected to play a role. As expected from the form of operator, this mode may provide the information on the spin-related properties, including the spin component of dineutron and diproton correlations. In general, the sum rule for M1 transition strength has not been derived yet. Purpose: To investigate the M1 excitation of the systems with two valence nucleo…
The effects of correlations on the bulk properties of nuclei are investigated in large model spaces including up to 21 single-particle orbits. The evaluation of the single-particle Green function is made feasible by means of the BAGEL approximation. The spectral function for single-nucleon pick-up and removal is investigated for the nuclei $^{16}O$ and $^{40}Ca$ . Special attention is paid to the effects produced by correlations on the calculated ground state properties of closed shell nuclei. It is observed that correlations beyond the Brueckner Hartree Fock approximation tend to improve the …
A method to investigate possible loosely bound dibaryon states is suggested. The momentum dependence of a two-pion correlation function is shown to depend on the (non)existence of a bound state.
Correlations in the nuclear wave-function beyond the mean-field or Hartree-Fock approximation are very important to describe basic properties of nuclear structure. Various approaches to account for such correlations are described and compared to each other. This includes the hole-line expansion, the coupled cluster or ``exponential S'' approach, the self-consistent evaluation of Greens functions, variational approaches using correlated basis functions and recent developments employing quantum Monte-Carlo techniques. Details of these correlations are explored and their sensitivity to the underl…
Background$\colon$ The $^{29}$F system is located at the lower-N boundary of the "island of inversion" and is an exotic, weakly bound system. Little is known about this system beyond its two-neutron separation energy ($S_{2n}$) with large uncertainties. A similar situation is found for the low-lying spectrum of its unbound binary subsystem $^{28}$F. Purpose$\colon$ To investigate the configuration mixing, matter radius and neutron-neutron correlations in the ground state of $^{29}$F within a three-body model, exploring the possibility of $^{29}$F to be a two-neutron halo nucleus. Method$\colon…
Using quantum molecular dynamics simulations, we investigate the formation of fragments in symmetric reactions between beam energies of E=30AMeV and 600AMeV. After a comparison with existing data we investigate some observables relevant to tackle equilibration: dsigma/dErat, the double differential cross section dsigma/pt.dpz.dpt,... Apart maybe from very energetic E>400AMeV and very central reactions, none of our simulations gives evidence that the system passes through a state of equilibrium. Later, we address the production mechanisms and find that, whatever the energy, nucleons finally ent…
We investigate femtoscopic $ππ$ correlations using the UrQMD approach combined with a correlation afterburner. The dependence of $ππ$ correlations on the charged particle multiplicity and formation time in p+p collisions at $\sqrt{s_{NN}}$= 7 TeV is explored and compared to present ALICE data. The data allows to constrain the formation time in the string fragmentation to $τ_f \leq 0.8$ fm/c.
We investigate the residual effect of lambda-proton strong interactions in pp correlations with one proton from lambda decays. It is found that the residual correlation is about 10% of the lambda-proton correlation strength, and has a broad distribution centered around q~40 MeV/c. The residual correlation cannot explain the observed structure on the tail of the recently measured pp correlation function in central Pb+Pb collisions by NA49 at the SPS.
Separating entanglement at emission from femtoscopic correlations
Istituto Nazionale di Fisica Nucleare, Sezione di Catania, Dipartimento di Fisica “Ettore Majorana”, Università di Catania（意大利国家核物理研究所，卡塔尼亚分部，费雷托·马约拉纳物理系，卡塔尼亚大学）
$\textbf{Background:}$ The deuteron shows the essential features of short-range correlations found in all nuclei. Experimental observables related to short-range correlations are connected with the high-momentum components of one- and two-body momentum distributions. An intuitive understanding of short-range correlations is provided by the suppression of the two-body density in coordinate space at small distances. $\textbf{Purpose:}$ The Wigner function provides a quasi-probability distribution in phase-space that allows to investigate short-range correlations as a function of distance and rel…
We investigate strong-coupling effects in a three-component atomic Fermi gas. It is a promising candidate for simulating quantum chromodynamics (QCD), and furthermore, the emergence of various phenomena such as color superfluidity and Efimov effect are anticipated in this system. In this paper, we study the effects of two-body and three-body correlations by means of the many-body $T$-matrix approximation (TMA) as well as the Skorniakov-Ter-Martirosian (STM) equation with medium corrections. We investigate the effects of finite temperature and chemical potential on the trimer binding energy at …
We investigate the correlations between the quark spin and orbital angular momentum inside the nucleon. Similarly to the Ji relation, we show that these correlations can be expressed in terms of specific moments of measurable parton distributions. This provides a whole new piece of information about the partonic structure of the nucleon.
Identical particle correlations at fixed multiplicity are considered by means of quantum canonical ensemble of finite systems. We calculate one-particle momentum spectra and two-particle Bose-Einstein correlation functions in the ideal gas by using a recurrence relation for the partition function. Within such a model we investigate the validity of the thermal Wick's theorem and its applicability for decomposition of the two-particle distribution function. The dependence of the Bose-Einstein correlation parameters on the average momentum of the particle pair is also investigated. Specifically, …
