SearcharxivSearch

arXiv subjects

B. K. Agrawal

Publications and source records attributed to B. K. Agrawal.

At least 19 recordsLinked to original sources

Conflation of Ensemble-Learned Nuclear Mass Models for Enhanced Precision

Ensemble learning algorithms, the gradient boosting and bagging regressors, are employed to correct the residuals of nuclear mass excess for a diverse set of six nuclear mass models. The weighted average of these corrected residuals reduces due to their partial cancellation, yielding a significant improvement in nuclear mass predictions. Our conflated model, which integrates ensemble learning and model averaging (ELMA), achieves a root mean square error of approximately 65 keV, well below the critical threshold of 100 keV, for the complete data set of Atomic Mass Evaluation (AME2020). The validity of ELMA is demonstrated through the evaluation of $Q$ values for $α$ decay, showing a marked decrease in deviations from experimental data relative to predictions from individual nuclear mass models. We have also compiled a table of nuclear mass excesses and binding energies for about 6,300 nuclei, which serves as a valuable resource for various nuclear physics applications and is publicly accessible via the ELMA web interface (https://ddnp.in).

nucl-th

Inferring the Equation of State from Neutron Star Observables via Machine Learning

We have conducted an extensive study using a diverse set of equations of state (EoSs) to uncover strong relationships between neutron star (NS) observables and the underlying EoS parameters using symbolic regression method. These EoS models, derived from a mix of agnostic and physics-based approaches, considered neutron stars composed of nucleons, hyperons, and other exotic degrees of freedom in beta equilibrium. The maximum mass of a NS is found to be strongly correlated with the pressure and baryon density at an energy density of approximately 800 MeV.fm$^{-3}$. We have also demonstrated that the EoS can be expressed as a function of radius and tidal deformability within the NS mass range 1-2$M_\odot$. These insights offer a promising and efficient framework to decode the dense matter EoS directly from the accurate knowledge of NS observables.

nucl-th

Calibrating global behaviour of equation of state by combining nuclear and astrophysics inputs in a machine learning approach

We implemented symbolic regression techniques to identify suitable analytical functions that map various properties of neutron stars (NSs), obtained by solving the Tolman-Oppenheimer-Volkoff (TOV) equations, to a few key parameters of the equation of state (EoS). These symbolic regression models (SRMs) are then employed to perform Bayesian inference with a comprehensive dataset from nuclear physics experiments and astrophysical observations. The posterior distributions of EoS parameters obtained from Bayesian inference using SRMs closely match those obtained directly from the solutions of TOV equations. Our SRM-based approach is approximately 100 times faster, enabling efficient Bayesian analyses across different combinations of data to explore their sensitivity to various EoS parameters within a reasonably short time.

nucl-th

Unraveling the global behavior of equation of state by explicit finite nuclei constraints

We obtain posterior distribution of equations of state (EOSs) across a broad range of density by imposing explicitly the constraints from precisely measured fundamental properties of finite nuclei, in combination with the experimental data from heavy-ion collisions and the astrophysical observations of radius, tidal deformability and minimum-maximum mass of neutron stars. The acquired EOSs exhibit a distinct global behavior compared to those usually obtained by imposing the finite nuclei constraints implicitly through empirical values of selected key parameters describing symmetric nuclear matter and symmetry energy in the vicinity of the saturation density. The explicit treatment of finite nuclei constraints yields softer EOSs at low densities which eventually become stiffer to meet the maximum mass criteria. The Kullback-Leibler divergence has been used to perform a quantitative comparison of the distributions of neutron star properties resulting from the EOSs obtained from implicit and explicit finite nuclei constraints.

nucl-th

Implications of comprehensive nuclear and astrophysics data on the equations of state of neutron star matter

The equations of state (EoSs) governing neutron star (NS) matter obtained for both non-relativistic and relativistic mean-field models are systematically confronted with a diverse set of terrestrial data and astrophysical observations within the Bayesian framework. The terrestrial data, spans from bulk properties of finite nuclei to the heavy-ion collisions, constrain the symmetric nuclear matter EoS and the symmetry energy up to twice the saturation density ($ρ_0$= 0.16 fm$^{-3}$). The astrophysical observations encompass the NS radius, the tidal deformability, and the lower bound on maximum mass. Three distinct posterior distributions of EoSs are generated by gradually updating the priors with different constraints: (i) only the maximum NS mass, (ii) incorporating additional terrestrial data, (iii) combining both the terrestrial data and astrophysical observations. These EoS distributions are then compared using the Kullback-Liebler divergence which highlights the significant constraints imposed on the EoSs by the currently available lower bound of NS maximum mass and terrestrial data. The remaining astrophysical observations marginally refine the EoS within the density range $\sim$ 2-3$ρ_0$. It is observed that the relativistic mean field model yields stiffer EoS around the saturation density, but predict smaller values of the speed of sound and proton fraction in the interior of massive stars.

nucl-th

Direct mapping of tidal deformability to the iso-scalar and iso-vector nuclear matter parameters

Background: The equations of state (EoSs) which determine the properties of neutron stars (NSs) are often characterized by the iso-scalar and iso-vector nuclear matter parameters (NMPs). Recent attempts to relate the radius and tidal deformability of a NS to the individual NMPs have been inconclusive. These properties display strong correlations with the pressure of NS matter which depends on several NMPs. The knowledge of minimal NMPs that determine the NS properties will be necessary to address any connection between NS properties (e.g., tidal deformability) and that of finite nuclei. Purpose: To identify the important NMPs required to describe the tidal deformability of neutron star for astrophysically relevant range of their gravitational masses (1.2 -- 1.8 M$_\odot$) as encountered in the binary neutron star merger events. Method: We construct a large set of EoSs using four iso-scalar and five iso-vector NMPs. These EOSs are employed to perform a systematic analysis to isolate the NMPs that predominantly determine the tidal deformability, over a wide range of NS mass. The tidal deformability is then directly mapped to these NMPs. Results: The tidal deformability of the NS with mass 1.2-1.8 M$_\odot$ can be determined within 10$\%$ directly in terms of four nuclear matter parameters, namely, the incompressibility $K_0$ and skewness $Q_0$ of symmetric nuclear matter, and the slope $L_0$ and curvature parameter $K_{\rm sym,0}$ of symmetry energy. Conclusion: A function that quickly estimates the value of tidal deformability in terms of minimal nuclear matter parameters is developed. Our method can also be extended to other NS observables.

nucl-th

Indispensability of cross-shell contributions in neutron resonance spacing

Spin and parity dependent nuclear level densities (NLDs) are obtained for configuration interaction shell model using a numerically efficient spectral distribution method. The calculations are performed for $^{24}$Na, $^{25,26,27}$Mg nuclei using full $sd$-$pf$ model space that incorporates the cross-shell excitations from $sd$ to $pf$-shell. The NLDs so obtained are then employed to determine the s-wave neutron resonance spacing (D$_0$) which is one of the crucial inputs for the predictions of astrophysical reaction rates. Though the considered nuclei are not neutron-rich, the contributions from cross-shell excitations to $pf$-shell are indispensable to explain the experimental data for D$_0$ which otherwise are significantly overestimated.

nucl-th

Exploring the Macroscopic Properties and Nonradial Oscillations of Proto-Neutron Stars: Effects of Temperature, Entropy, and Lepton Fraction

Neutron stars (NSs) have traditionally been viewed as cold, zero-temperature entities. However, recent progress in computational methods and theoretical modelling has opened up the exploration of finite temperature effects, marking a novel research frontier. This study examines Proto-Neutron Stars (PNSs) using the BigApple parameter set to investigate their macroscopic properties. Two approaches are employed: one with constant temperatures (10-50 MeV) and the other fixing entropy per baryon (S) at predefined levels (S = 1 and S = 2). Notably, S remains constant with increasing baryon density due to electron-positron pair formation at finite temperatures. Analysis of PNS mass-radius profiles, considering neutrino trapping and temperature effects, reveals flattened curves and expanded radii with increasing temperature, resulting in slightly higher masses compared to zero temperature. The influence of lepton fraction ($Y_l$) on maximum PNS mass is explored, indicating that higher $Y_l$ values lead to a softer Equation of State (EoS), reducing maximum mass and increasing the canonical radius ($R_{1.4}$). Further investigation of a constant entropy EoS demonstrates that higher entropy is associated with increased maximum PNS masses and flatter mass-radius curves. Central temperature versus maximum mass relationships suggest a correlation between NS mass and temperature. Lastly, we investigate the behaviour of $f$-mode frequencies in PNS. It reveals that the frequency of these modes decreases with increasing entropy and temperature, reflecting complex thermodynamic interactions within the stars.

nucl-th

Establishing connection between neutron star properties and nuclear matter parameters through a comprehensive multivariate analysis

We have attempted to mitigate the challenge of connecting the neutron star (NS) properties with the nuclear matter parameters that describe equations of state (EoSs). The efforts to correlate various neutron star properties with individual nuclear matter parameters have been inconclusive. A Principal Component Analysis is employed as a tool to uncover the connection between multiple nuclear matter parameters and the tidal deformability as well as the radius of neutron stars within the mass range of $1.2-1.8M_\odot$. The essential EOSs for neutron star matter at low densities have been derived using both uncorrelated uniform distributions and minimally constrained joint posterior distributions of nuclear matter parameters. For higher densities ($ρ> 0.32$fm$^{-3}$), the EOSs have been established through a suitable parameterization of the speed of sound, which consistently maintains causality and gradually approaches the conformal limit. Our analysis reveals that in order to account for over 90\% of the variability in NS properties, it is crucial to consider two or more principal components, emphasizing the significance of employing multivariate analysis. To explain the variability in tidal deformability needs a greater number of principal components compared to those for the radius at a given NS mass. The contributions from iso-vector nuclear matter parameters to the tidal deformability and radius of NS decrease by $\sim$ 25\% with the increase in mass of NS from 1.2$M_\odot$ to 1.8$M_\odot$.

nucl-th

Relativistic Mean Field Model parameterizations in the light of GW170817, GW190814, and PSR J0740 + 6620

Three parameterizations DOPS1, DOPS2, and DOPS3 (named after the Department of Physics Shimla) of the Relativistic Mean Field (RMF) model have been proposed with the inclusion of all possible self and mixed interactions between the scalar-isoscalar (σ), vector-isoscalar (ω) and vector-isovector (ρ) mesons up to quartic order. The generated parameter sets are in harmony with the finite and bulk nuclear matter properties. A set of Equations of State (EOSs) composed of pure hadronic (nucleonic) matter and nucleonic with quark matter (hybrid EOSs) for superdense hadron-quark matter in β-equilibrium is obtained. The quark matter phase is calculated by using the three-flavor Nambu-Jona-Lasinio (NJL) model. The maximum mass of a non-rotating neutron star with DOPS1 parameterization is found to be around 2.6 M$\odot$ for the pure nucleonic matter which satisfies the recent gravitational wave analysis of GW190814 Abbott et al.,(2020) with possible maximum mass constraint indicating that the secondary component of GW190814 could be a non-rotating heaviest neutron star composed of pure nucleonic matter. EOSs computed with the DOPS2 and DOPS3 parameterizations satisfy the X-Ray observational data and the recent observations of GW170817 maximum mass constraint of a stable non-rotating neutron star in the range 2.01 \pm 0.04 - 2.16 \pm 0.03 M\odot and also in good agreement with constraints on mass and radius measurement for PSR J0740+6620 (NICER) Riley et al., L27 (2021)}, Miller et al., (2021). The hybrid EOSs obtained with the NJL model also satisfy astrophysical constraints on the maximum mass of a neutron star from PSR J1614-2230 and Demorest et al., (2010) .We also present the results for dimensionless tidal deformability, $Λ$ which are consistent with the waveform models analysis of GW170817.

nucl-th

Observational constraint from the heaviest pulsar PSR J0952-0607 on the equation of state of dense matter in relativistic mean field model

In the present work, we constrain the equation of the state of dense matter in the context of heaviest observed neutron star mass M$_{max}$ = 2.35$\pm 0.17$ M$_{\odot}$ for the black widow pulsar PSR J0952-0607. We propose three interactions HPU1, HPU2 and HPU3 (named after Himachal Pradesh University)for the relativistic mean field model which include different combinations of non-linear, self and cross-couplings among isoscalar-scalar $σ$, isoscalar-vector $ω$ and isovector-vector $ρ$ meson fields up to the quartic order. These interactions are in harmony with the finite nuclei and bulk nuclear matter properties. The equations of state computed by using newly generated interactions for the $β$-equilibrated nucleonic matter satisfy the heaviest observed neutron star mass M$_{max}$ = 2.35$\pm 0.17$ M$_{\odot}$ for the black widow pulsar PSR J0952-0607. The results for the radius ($R_{1.4}$) and dimensionless tidal deformability (${Λ_{1.4}}$) corresponding to the canonical mass are also presented and agree well with the GW170817 event and astrophysical observations. The radius of $2.08M_{\odot}$ neutron star mass is predicted to be in the range $R_{2.08}$ = 12.98 -13.09 Km which also satisfies the NICER observations by Miller et al. (2021) and Riley et al.(2021). A covariance analysis is also performed to assess the theoretical uncertainties of model parameters and to determine their correlations with nuclear matter observables.

nucl-th

Systematic analysis of the impacts of symmetry energy parameters on neutron star properties

The impacts of various symmetry energy parameters on the properties of neutron stars (NSs) have been recently investigated, and the outcomes are at variance, as summarized in Table III of Phys. Rev. D 106, 063005 (2022). We have systematically analyzed the correlations of slope and curvature parameters of symmetry energy at the saturation density ($ρ_0=0.16 \text{fm}^{-3}$) with the tidal deformability and stellar radius of non-spinning neutron stars in the mass range of $1.2 - 1.6 M_\odot$ using a large set of minimally constrained equations of state (EoSs). The EoSs at low densities correspond to the nucleonic matter and are constrained by empirical ranges of a few low-order nuclear matter parameters from the finite nuclei data and the pure neutron matter EoS from chiral effective field theory. The EoSs at high densities ($ρ> 1.5 - 2ρ_0$) are obtained by a parametric form for the speed of sound that satisfies the causality condition. Several factors affecting the correlations between the NS properties and the individual symmetry energy parameters usually encountered in the literature are considered. These correlations are quite sensitive to the choice of the distributions of symmetry energy parameters and their interdependence. But, variations of NS properties with the pressure of $β-$ equilibrated matter at twice the saturation density remain quite robust which maybe due to the fact that the pressure depends on the combination of multiple nuclear matter parameters that describe the symmetric nuclear matter as well as the density dependence of the symmetry energy. Our results are practically insensitive to the behavior of EoS at high densities.

nucl-th

Relativistic approach for the determination of nuclear and neutron star properties in consideration of PREX-II results

The bulk properties of nuclear matter and neutron stars with the newly generated relativistic interaction DBHP are investigated which provides an opportunity to modify the coupling parameters keeping in view the finite nuclei, nuclear matter, PREX-II data for neutron skin thickness in $^{208}$Pb and astrophysical constraints. The relativistic interaction has been generated by including all possible self and mixed interactions between $σ$, $ω$, and $ρ$-meson up to the quartic order satisfying the naturalness behavior of parameters. A covariance analysis is performed to assess the statistical uncertainties on the model parameters and observables of interest along with correlations amongst them. We obtained a value of neutron skin thickness for $^{208}$Pb nucleus $Δr_{np}$ = 0.24 $\pm$ 0.02 fm. The maximum gravitational mass of neutron star and radius corresponding to the canonical mass ($R_{1.4}$) come out to be 2.03 $\pm$ 0.04 M$\odot$ and 13.39 $\pm$ 0.41 km respectively. The dimensionless tidal deformability, $Λ$ for a neutron star is also analyzed.

nucl-th

CREX- and PREX-II-motivated relativistic interactions and their implications for the bulk properties of nuclear matter and neutron stars

We investigate the implications of parity-violating electron scattering experiment on neutron skin thickness of $^{48}$Ca (CREX) and $^{208}$Pb (PREX-II) data on the bulk properties of finite nuclei, nuclear matter, and neutron stars. The neutron skin thickness from the CREX and PREX-II data is employed to constrain the parameters of relativistic mean field models which includes different non-linear, self and cross-couplings among isoscalar-scalar $σ$, isoscalar-vector $ω$, isovector-scalar $δ$ and isovector-vector $ρ$ meson fields up to the quartic order. Three parametrizations of RMF model are proposed by fitting CREX, PREX-II and both CREX as well as PREX-II data to assess their implications. A covariance analysis is performed to assess the theoretical uncertainties of model parameters and nuclear matter observables along with correlations among them. The RMF model parametrization obtained with the CREX data acquires much smaller value of symmetry energy (J= 28.97$\pm$ 0.99 MeV), its slope parameter (L= 30.61$\pm 6.74$ MeV) in comparison to those obtained with PREX-II data. The neutron star properties are studied by employing the equations of state (EoSs) composed of nucleons and leptons in $β$ equilibrium.

nucl-th

Neutron Skin Thickness Dependence of Astrophysical $S$-factor

Background: The density dependence of nuclear symmetry energy is crucial in determining several properties of finite nuclei to the neutron stars with mass $\sim$ 1.4 $M_\odot$. The values of neutron skin thickness, isovector giant dipole resonances energies and various nuclear reaction cross-sections in asymmetric nuclei have been utilized to determine the slope of symmetry energy ($L_0$) at the saturation density. Recent PREX-II and CREX measurements of neutron skin thickness in $^{208}$Pb and $^{48}$Ca nuclei yield very different values of $L_0$ which overlap marginally within 90$\%$ confidence interval. Purpose: Our objective is to demonstrate the role of symmetry energy on the sub-barrier fusion cross-section and the astrophysical $S$-factor for asymmetric nuclei. Method: The nucleus nucleus potentials are generated using the double folding model (DFM) for three different nucleon-nucleon interactions. These DFM potentials are used for the calculation of the sub-barrier fusion cross-section and the astrophysical $S$-factor. The nucleon densities required for DFM potentials are generated from different families of non-relativistic and relativistic mean-field models which correspond to a wide range of neutron skin thickness or $L_0$. Results: We have calculated the sub-barrier fusion cross-section for several asymmetric nuclei involving O, Ca, Ni, and Sn isotopes. The results are presented for the barrier parameters, cross-section, and astrophysical $S$-factor for $^{54}$Ca+$^{54}$Ca and $^{124}$Sn+$^{124}$Sn as a function of neutron skin thickness. Conclusions: The cross-section for the neutron-rich nuclei show a strong dependence on the behavior of symmetry energy or the neutron skin thickness. The increase in skin thickness lowers the height of the barrier as well as its width which enhances the values of the $S$-factor by more than an order of magnitude.

nucl-th

Effect of isovector scalar meson on equation of state of dense matter within relativistic mean field model

The effects of the isovector-scalar $δ$-meson field on the properties of finite nuclei, infinite nuclear matter and neutron stars are investigated within the Relativistic Mean Field (RMF) model which includes non-linear couplings. Several parameter sets (SRV's) are generated to asses the influence of $δ$-meson on the properties of neutron star. These parametrizations correspond to different values of coupling constant of $δ$-meson to the nucleons with remaining ones calibrated to yield finite nuclei and infinite nuclear matter properties consistent with the available experimental data. It is observed that to fit the properties of finite nuclei and infinite nuclear matter, a stronger coupling between isovector-vector $ρ$ meson and nucleons is required in the presence of $δ$ field. Furthermore, the $δ$-meson is found to affect the radius of canonical neutron star significantly. The value of dimensionless tidal deformability, $Λ$ for the canonical neutron star also satisfies the constraints from the waveform models analysis of GW170817 binary neutron star merger event. A covariance analysis is performed to estimate the statistical uncertainties of the model parameters as well as correlations among the model parameters and different observables of interest.

nucl-th

Nearly model-independent constraints on dense matter equation of state in a Bayesian approach

We apply Bayesian approach to construct a large number of minimally constrained equations of state (EOSs) and study their correlations with a few selected properties of a neutron star (NS). Our set of minimal constraints includes a few basic properties of saturated nuclear matter and low-density pure neutron matter EOS which is obtained from a precise next-to-next-to-next-to-leading-order (N$^{3}$LO) calculation in chiral effective field theory. The tidal deformability and radius of NS with mass $1-2 M_\odot$ are found to be strongly correlated with the pressure of $β$-equilibrated matter at densities higher than the saturation density ($ρ_0 = 0.16$ fm$^{-3}$) in a nearly model-independent manner. These correlations are employed to parametrize the pressure for $β$-equilibrated matter, around 2$ρ_0$, as a function of neutron star mass and the corresponding tidal deformability. The maximum mass of neutron star is also found to be strongly correlated with the pressure of $β$-equilibrated matter at densities $\sim 4.5ρ_0$.

nucl-th

Inferring the nuclear symmetry energy at supra saturation density from neutrino cooling

An ambitious goal of the astrophysical community is not only to constrain the equation of state (EOS) of neutron star (NS) matter by confronting it with astrophysics observations, but ultimately also to infer the NS composition. Nevertheless, the composition of the NS core is likely to remain uncertain unless we have an accurate determination of the nuclear symmetry energy at supra saturation density ($ρ>ρ_0$). We investigate how the nucleonic direct Urca (dUrca) processes can be used as an effective probe to constraint the high density nuclear symmetry energy. A large number of minimally constrained EOSs has been constructed by applying a Bayesian approach to study the correlations of the symmetry energy at different densities with a few selected properties of a NS. The nuclear symmetry energy above the baryon density 0.5 fm$^{-3}$ ($\sim 3 ρ_0$) is found to be strongly correlated with NS mass at which the onset of nucleonic dUrca neutrino cooling takes place in the core. This allows us to constrain the high density behavior of nuclear symmetry energy within narrow bounds. {The pure neutron matter pressure constraint from chiral effective field theory rules out the onset of nucleonic dUrca in stars with a mass $\lesssim$ 1.4 $M_\odot$.} The onset of dUrca inside 1.6 M$_\odot$ to 1.8 M$_\odot$ NS implies a slope of the symmetry energy $L$ at $\sim 2.5~ρ_0$, respectively, between 54 and 48 MeV.

nucl-th