Probing Matter at the Extreme Conditions with Neutron Stars

Project Information

ProMatEx-NS

Grant agreement ID: 101109652

DOI

10.3030/101109652

Project terminated on 7 November 2024

EC signature date 15 March 2023

Start date 1 September 2023

End date 31 August 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 165 312,96

Coordinated by

UNIVERSIDAD DE SALAMANCA
Spain

Periodic Reporting for period 1 - ProMatEx-NS (Probing Matter at the Extreme Conditions with Neutron Stars)

Reporting period: 2023-09-01 to 2025-08-31

Summary of the context and overall objectives of the project

In the project, ProMatEx-NS, we have studied the behavior of dense nuclear matter inside neutron stars. The mass of neutron stars can be as high as twice the mass of the sun. But, their size is quite small, about 10-15 km in radius. Therefore, neutron stars are compact and extremely gravitating objects. These stars are born as the aftermath of the core-collapse supernova explosions of massive stars, more than ten times the mass of the sun. The interior of neutron stars provides an environment where the density becomes more than normal nuclear matter density. The constituents of this type of matter are still unknown. Since this type of condition cannot be created in terrestrial laboratories, neutron stars can be excellent laboratories for understanding fundamental physical laws of particle interactions and gravity.

The nature of nuclear interactions and the composition of matter at neutron star cores are still open problems and subject to intense research. During the last decade, new astrophysical observations and nuclear experiments renewed interest in these topics. Our objective in this project is to study different aspects of neutron stars: from the crust to the core. We test our understanding of nuclear physics and develop a robust framework for the modelling of neutron star interior using the latest observational data and the results from nuclear experiments relevant to the composition of the neutron star core. As an application, we further consider the neutron star observations as indirect dark matter detection experiments and test theories of particle physics beyond the standard model.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

In this project, we have worked specifically on constructing a relativistic metamodel of dense nuclear matter that can incorporate uncertainties on nuclear matter properties coming from both terrestrial experiments, first principle calculations, and astrophysical observations. We have tested several energy density functionals as candidate metamodel to characterize neutron star properties.
We have published two preprints (arXiv:2408.15220 arXiv:2409.11558) on the constraints on the neutron star density functionals from different astrophysical observations. These preprints are currently under peer review.

Then we have used two relativistic density functionals used widely in the nuclear physics literature to understand their applicability toward incorporating experimental uncertainties of the nuclear matter parameters and their application to neutron star structure. We have also investigated the possibility of strange quarks appearing inside neutron stars in the form of hyperons and how one can include new particle species like hyperons into the metamodel. We are in process of writing two manuscripts, one on the model comparison and the other on the inclusion of hyperons.

For the part of the project concerning the application of neutron stars as dark matter detectors, we have done preliminary studies on the capture rates of dark matter particles inside neutron stars. We are currently doing calculations on the dependence of the total amount of captured of dark matter particles inside the star on the possible variation of neutron star internal structures predicted by our relativistic metamodel from the first part of the project.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The relativistic density functional explored in the project provides greater flexibility in the isovector sector that has not been explored before. We have compared and expanded previous models to focus on the effect of isovector incompressibility on the equation of state of a neutron star. One of the most fascinating results of this approach is to explore a large variation of proton fraction inside the neutron stars which was not found in the previous literature.

Even with the inclusion of hyperons, we have expanded previous approaches of incorporating the baryon octet. We have found that the standard way of determining hyperon coupling with the SU(6) quark counting scheme is too restrictive. When we used a metamodelling approach to the hyperon couplings, we have found a larger variation of the hyperon fraction is possible.

These findings have important consequences for neutron star cooling simulations and the interpretation of observed data.