# A New Moment-Based General-Relativistic Neutrino-Radiation Transport Code: Methods and First Applications to Neutron Star Mergers

**Authors:** David Radice, Sebastiano Bernuzzi, Albino Perego, Roland Haas  
**arXiv:** [2111.14858](https://arxiv.org/abs/2111.14858)  
**Date:** 2021-11-29  
**Summary page:** https://www.summarizepaper.com/en/paper/2111.14858v1/

## Key points

- Authors introduce a novel neutrino transport code for neutron star merger simulations in general relativity
- Code incorporates all orders in $v/c$ and retains all nonlinear neutrino-matter coupling terms
- Validity of the code rigorously tested through stringent assessments
- Full neutrino-matter coupling terms crucial for accurately capturing phenomena in relativistically moving media, especially in differentially rotating merger remnants
- Preliminary simulations demonstrate robustness of the scheme in simulating ab-initio mergers leading to black hole collapse and long-term neutron star remnants lasting up to ${\sim}70\,$ms
- Longest dynamical spacetime, 3D, general relativistic calculations with full neutrino transport conducted so far
- Comparative analyses performed on results obtained at varying resolutions and employing two distinct closures for the moment scheme show no significant evidence of out-of-thermodynamic equilibrium effects impacting postmerger dynamics or gravitational wave emission
- Average energies align well with theoretical expectations and previous simulations by other research groups using similar methodologies
- Exploration of dynamical properties and early wind ejecta characteristics using M1 closure compared to older neutrino treatment reveals higher proton fractions but modest differences in nucleosynthesis yields.

## Summary

In their work titled "A New Moment-Based General-Relativistic Neutrino-Radiation Transport Code: Methods and First Applications to Neutron Star Mergers," authors David Radice, Sebastiano Bernuzzi, Albino Perego, and Roland Haas introduce a novel neutrino transport code designed for neutron star merger simulations within the framework of general relativity. This  stands out as the first of its kind in the merger context to incorporate  at all orders in $v/c$, while also retaining all nonlinear neutrino-matter coupling terms. The validity of the code is rigorously tested through a series of stringent assessments. The study emphasizes the crucial role played by full neutrino-matter coupling terms in accurately capturing phenomena such as  in relativistically moving media, particularly evident in differentially rotating merger remnants. Through preliminary simulations, the authors demonstrate the robustness of their scheme in simulating ab-initio mergers leading to black hole collapse and long-term neutron star remnants lasting up to ${\sim}70\,$ms. Notably, these simulations represent the longest dynamical spacetime, 3D, general relativistic calculations with full neutrino transport conducted thus far. Comparative analyses are performed on results obtained at varying resolutions and employing two distinct closures for the moment scheme. The investigation reveals no significant evidence of out-of-thermodynamic equilibrium effects impacting postmerger dynamics or gravitational wave emission. Moreover,  and average energies align well with theoretical expectations and previous simulations conducted by other research groups utilizing similar methodologies. The authors further explore dynamical properties and early wind ejecta characteristics obtained using the M1 closure compared to their older neutrino treatment. While M1 results consistently exhibit higher proton fractions, resulting differences in nucleosynthesis yields are deemed modest. Overall, this study lays a solid foundation for future comprehensive investigations encompassing a broader spectrum of neutrino reactions, binary systems, and equations of state within the context of neutron star mergers.