This paper shows that combining laboratory nuclear data with theory and astrophysical observations makes a fast nucleon cooling channel — the electronic direct Urca process — very unlikely in neutron stars of about 1.4 solar masses or less. The authors report that, within their nucleon‑only model, the probability that direct Urca turns on at or below 1.4 M☉ falls from roughly 20% (when using only chiral effective field theory and astrophysical constraints) to below 1% once laboratory measurements are included.

The researchers performed a Bayesian analysis of the dense matter equation of state (EoS). They brought together three kinds of input: (1) a multidimensional posterior of nuclear matter parameters extracted from many nuclear structure measurements, (2) isospin‑transport data from INDRA‑FAZIA heavy‑ion experiments, and (3) theoretical calculations from chiral effective field theory plus astrophysical observations. To avoid uncontrolled guesses at very high density, they used a flexible, “asymptotically causal” metamodel that lets low‑order nuclear parameters vary freely while remaining well behaved at high density.

At a high level, the key quantity is the symmetry energy — how the energy of nuclear matter changes when there are more neutrons than protons. The symmetry energy controls the proton fraction in a neutron star core. The heavy‑ion data and the nuclear structure observables constrain the symmetry energy in the density range probed by experiments. The authors evaluated the INDRA‑FAZIA likelihood directly along each sampled symmetry‑energy curve and propagated the full correlated distribution of nuclear matter parameters through the metamodel to predict proton fractions in stars.

Why this matters: the direct Urca process is a rapid neutrino emission mechanism that requires a sufficiently large central proton fraction. If direct Urca operates, a neutron star cools much faster than if only slower neutrino processes act. By sharply reducing the probability that nucleonic direct Urca is allowed in low‑mass stars, the new analysis narrows the set of explanations for observed cooling behavior and gives tighter guidance about the proton content of typical neutron star cores.