ANR Project RELANSE (2024-2028): Relativistic Lagrangians for finite nuclei and dense matter.

Rationale:

The project RELANSE will consolidate the unified description of finite nuclei and neutron stars (NS) by developing innovative relativistic modeling. Matter in the inner core of NS explores densities from ~2 (for the lightest NS) up to ~8 (for the heaviest ones) times the saturation energy-density of finite nuclei (ρ_sat~3.10¹⁴ g cm^-3). In this regime, the sound speed becomes comparable to the speed of light in vacuum. Some modeling even predicts matter to become acausal, violating a general principle of special relativity. To solve this issue, as well as to have a robust approach describing matter in a relativistic regime, the present project will develop relativistic descriptions of nuclear interaction for neutron stars and finite nuclei.
The conceptual beauty of cEDF is to connect the saturation mechanism at the origin of the existence of finite nuclei and the spin-orbit interaction, see for instance [Chanfray2020], at the origin of the magic numbers and element abundance in the universe to the properties of two strong scalar and vector fields. In our project, nuclear data will be employed to calibrate new cEDF models, as well as first-principle results, such as Lattice-QCD, the quark model and the vector dominance model. Our methodology is the following: Armed with constraints from first-principle and from finite nuclei, we will develop innovative relativistic approaches, which will be used to perform robust predictions for neutron star properties.
While relativistic Lagrangians have existed since the first models by Walecka, almost 50 years ago, new conceptual ideas have emerged recently, as well as new data from exotic beam facilities, contributing to the discovery of about 20 new isotopes every year, for about 20 years. Relativistic Hartree-Fock approaches were blocked by their loose reproduction of finite nuclei ground state [Bouyssy1987]. This obstacle has been overcome with the introduction of density-dependent coupling constants [Long2006], which are also used in relativistic Hartree approaches. The chiral feature of the nuclear interaction originating from the spontaneous symmetry breaking of QCD at low-density, where QCD is non-perturbative, is one of the most recent advances in the understanding of the emergence of the residual nuclear interaction. It has created a new avenue in the understanding of nuclear forces with the chiral effective field theory being one of the most visible new concepts leading to progresses in the description of nuclear matter at low-density and in finite nuclei. The breakdown density of this low-energy theory is estimated to be ~1-2ρ_sat. At larger densities, the chiral field shall be considered together with relativistic approaches. It is however not clear how chiral dynamics is impacted by medium effects, if it should be introduced alone [Drews2013], or if it combines with other features of QCD, e.g., the internal quark sub-structure of nucleons included in the QMC model [Guichon2018], along the lines originally proposed in Refs.[Chanfray2007,Chanfray2011], see also a recent review [Chanfray2023]. The present project will investigate these different scenarios and their consequences for dense matter properties, employing Bayesian statistics as in Ref.[Somasundaram2022]. We will evaluate the impact of experimental data uncertainties into the predictions for dense matter in NS and for finite nuclei. Consequences for the general description of nuclear structure, from involving quantum liquid states, clusters states, and more dynamical phenomenon, such as alpha decay, will also be investigated. The covariant formalism is known to be a successful tool to describe all these phenomena [Ebran2012, Mercier2021].
Astrophysical data will also be employed to select among various dense matter modeling, including the onset of new degrees of freedom above ρ_sat (Delta resonances, hyperons, or quarks). Our methodology consists in exploring various equations of state (EoS) including these new degrees of freedom, see for instance [Massot2012, Margueron2021]. In this project we will use Bayesian statistics in order to evaluate the goodness of these various scenarios (Bayes factors). This methodology requires data to evaluate the likelihood probability, see for instance Ref.[Margueron2018], which, in addition to experimental nuclear data, will also include astrophysical observations such as:

Radio astronomy, which is continuously pushing up the lowest bound of the maximal NS mass, will be compared to the solution of the TOV equations using our EoS.
NICER observatory has released masses and radii for two millisecond pulsars, and at least 2 other measurements are expected for the near future. We will solve TOV equations to compare our cEDF with these data.
Binary NS (BNS) mergers produce gravitational waves (GW) detected by the LIGO-Virgo collaboration since 2017. We will compare the tidal deformability from BNS with the one predicted by our cEDF. New data are also expected from O4 and O5 observing runs.
GW170817, AT2017gfo, GRB170817A are also one of the very few examples of the young multi-messenger astronomy. We will employ empirical relations deduced from numerical relativity simulations to relate our EoS to these data.

Team:

The project is a consortium coordinated by J. Margueron and composed by: G. Chanfray (IP2I Lyon), J.-P. Ebran (CEA DAM), H. Hansen (IP2I Lyon) and E. Khan (IJCLab Orsay). Two additionnal post-docs will be located at IP2I Lyon and IJCLab Orsay.

Computational features:

The description of finite nuclei will employ and adapt the codes developed at IJCLab while the description of dense matter will be based on a python library developed at IP2I (open-source). Skills in python programming object will be required, as well as Fortran95 or C++.

Management of the project:

The project will be managed through work-packages (WP):

WP1: Management and coordination
WP2: Development of new Lagrangians
WP3: Modeling of physical systems (finite nuclei and neutron stars) and comparison to data
WP4: Results, outreach and open access and open sources

References:

[Bouyssy1987] A. Bouyssy, J.-F. Mathiot, N. Van Giai, Phys. Rev. C 36, 380 (1987).
[Chanfray2007] G. Chanfray and M.Ericson, Phys. Rev. C 75, 015206 (2007).
[Chanfray2011] G. Chanfray and M.Ericson, Phys. Rev. C 83, 015204 (2011).
[Chanfray2020] G. Chanfray and J. Margueron, Phys. Rev. C 102, 024331 (2020).
[Chanfray2023] G. Chanfray, M. Ericson, M. Martini, Universe 9 (2023) 7, 316 e-Print: 2307.03484 [nucl-th]
[Drews2013] M. Drews, T. Hell, B. Klein, W. Weise, Phys. Rev. D 88, 096011 (2013).
[Ebran2012] J.P. Ebran, E. Khan, T. Niksic and D. Vretenar, Nature 487, 341 (2012).
[Guichon2018] P.A.M. Guichon, J.R. Stone and A.W. Thomas, Prog. Part. Nuc. Phys. 100, 262 (2018).
[Long2006] W. Long, N. Van Giai, J. Meng, Phys. Lett. B 640, 150 (2006).
[Margueron 2018] J. Margueron, R. Hoffmann Casali, and F. Gulminelli, Phys. Rev. C 97, 025805 (2018); 97, 025806 (2018).
[Margueron2021] J. Margueron, H. Hansen, P. Proust, G. Chanfray, Phys. Rev. C 104, 055803 (2021).
[Massot 2012] E. Massot, J. Margueron, G. Chanfray, Europhys. Lett. 97, 39002 (2012).
[Somasundaram2022] R. Somasundaram, J. Margueron, H. Hansen, G. Chanfray, Eur. Phys. J. A 58, 84 (2022).
[Mercier2021] F. Mercier, J. Zhao, J.-P. Ebran, E. Khan, T. Niksic and D. Vretenar, Phys. Rev. Lett. 127, 012501 (2021).