The Delta-isobar masquerade: intrahadronic phase transitions and their quark-mimicking signatures in neutron stars

Abstract

We investigate the conditions under which $\Delta (1232)$ isobars trigger a first-order phase transition within purely hadronic neutron-star matter, using the SW4L relativistic mean-field parametrization. For scalar-vector coupling differences $0.15\lesssim x_{\sigma \Delta }-x_{\omega \Delta }\lesssim 0.2$ and $x_{\sigma \Delta }\gtrsim 1.3$, the onset of ${\Delta }^{-}$ resonances produces a van der Waals-like instability driven by a self-amplifying feedback in the scalar meson sector, in which the ${\Delta }^{-}$ particle fraction acts as the order parameter of a Landau-type transition. A Maxwell construction yields a sharp density discontinuity at baryon densities $n_b\sim (1.3$-$2)n_0$, separating a $\Delta$-free outer core from a $\Delta$-rich inner core. The resulting neutron-star sequences satisfy all current multimessenger constraints: maximum masses $M_{\max }\approx 2.15$-$2.25M_{\odot }$, radii $R_{1.4}\approx 11$-$12$ km, and tidal deformabilities ${\Lambda }_{1.4}\approx 190$-$480$, compatible with NICER observations and GW170817. We compute, for the first time for a $\Delta$-induced interface, the $\ell =2$ composition $g$-mode eigenfrequencies, obtaining ${\nu }_g\sim 400$-$1100$ Hz with gravitational-wave damping times ${\tau }_g\sim 10^3$-$10^9$ s. These frequencies overlap quantitatively with those predicted for hadron-quark phase-transition interfaces, demonstrating that the mass-radius ``knee'', reduced tidal deformability, and $g$-mode spectrum conventionally regarded as signatures of quark deconfinement can be reproduced by a purely intrahadronic mechanism. This extends the masquerade problem from static observables to the domain of gravitational-wave asteroseismology, implying that a future detection of a discontinuity $g$-mode alone would not suffice to identify quark matter in neutron-star cores.

Comments: 15 pages, 4 figures