Δ(1232) isobars and weak-interaction bulk viscosity in neutron-star merger matter
Abstract
Binary neutron star mergers probe nuclear matter at densities of a few times nuclear saturation density and temperatures of tens of MeV, conditions under which heavier baryons such as the Δ(1232) isobars may appear. Previous calculations of the bulk viscosity of merger matter have considered nucleonic matter, with and without muons, and hyperonic matter, but not Δ isobars. In this work, we extend these calculations to neutrino-transparent matter in which Δ isobars are present in strong equilibrium. We derive the relativistic matrix elements for the charged-current Δ→ N and Δ→Δ' transitions, evaluate all electronic and muonic Urca processes over the full phase space using the nucleon width approximation, and include the in-medium Δ→ Nπ width with Pauli blocking. The bulk viscosity is obtained from the coupled linear response of the electron and muon fractions about the finite-temperature flavor equilibrium. We find that the Δ- takes over the negative charge from the leptons, which raises the proton fraction and opens the nucleonic direct Urca process. Up to about 3~MeV this change in composition, rather than the Δ reactions themselves, controls flavor equilibration. As a consequence, the resonant maximum of the bulk viscosity moves to lower temperatures for all oscillation frequencies we consider, and kHz density oscillations near resonance are damped several times faster than in nucleonic matter. We also show that, for nucleonic matter, the choice of the equilibrium state about which the rates are linearized shifts the resonance temperature by up to a few MeV at high density.
Related
- Detectability of bulk viscosity effects on post-merger gravitational wave signals from binary neutron star mergers
- Andreev Beta Conversion and Bulk Viscosity in Superfluid Neutron-Star Matter
- Deconfinement Confined to the Heaviest Neutron Stars: Strange Quark Matter from a Flavor-Dressed Contact Interaction
- Δ(1232) isobars and neutrino opacities in hot dense matter
- A first-principles binary neutron star merger model of GW170817, GRB170817A, and AT2017gfo
BibTeX
@misc{kumar2026δ1232,
title = {{$Δ(1232)$ isobars and weak-interaction bulk viscosity in neutron-star merger matter}},
author = {Bharat Kumar},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.25224},
eprint = {2610.25224},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.25224},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Δ(1232) isobars and weak-interaction bulk viscosity in neutron-star merger matter
AU - Bharat Kumar
PY - 2026
DA - 2026/10/18
PB - arXiv
DO - 10.48550/arxiv.2610.25224
UR - https://arxiv.org/abs/2610.25224
AB - Binary neutron star mergers probe nuclear matter at densities of a few times nuclear saturation density and temperatures of tens of MeV, conditions under which heavier baryons such as the Δ(1232) isobars may appear. Previous calculations of the bulk viscosity of merger matter have considered nucleonic matter, with and without muons, and hyperonic matter, but not Δ isobars. In this work, we extend these calculations to neutrino-transparent matter in which Δ isobars are present in strong equilibrium. We derive the relativistic matrix elements for the charged-current Δ→ N and Δ→Δ' transitions, evaluate all electronic and muonic Urca processes over the full phase space using the nucleon width approximation, and include the in-medium Δ→ Nπ width with Pauli blocking. The bulk viscosity is obtained from the coupled linear response of the electron and muon fractions about the finite-temperature flavor equilibrium. We find that the Δ^- takes over the negative charge from the leptons, which raises the proton fraction and opens the nucleonic direct Urca process. Up to about 3~MeV this change in composition, rather than the Δ reactions themselves, controls flavor equilibration. As a consequence, the resonant maximum of the bulk viscosity moves to lower temperatures for all oscillation frequencies we consider, and kHz density oscillations near resonance are damped several times faster than in nucleonic matter. We also show that, for nucleonic matter, the choice of the equilibrium state about which the rates are linearized shifts the resonance temperature by up to a few MeV at high density.
ER -