Neutron Dark Decay and Exotic Compact Objects

TL;DR

The paper investigates whether neutron dark decay can explain exotic compact objects and reconcile subsolar masses with the 2$M_\odot$ NS constraint. It builds a composite EOS by combining APR hadronic matter with a self-interacting fermionic DM sector and DM–neutron couplings, then solves the TOV equations to generate mass–radius sequences. The study finds that a density-dependent suppression of the decay can produce configurations matching HESS J1731-347 while still allowing $M_{\rm max} > 2\,M_\odot$, and that explaining XTE J1814-338 may require additional parameterization or a two-branch scenario. These results imply that neutron dark decay could be a viable mechanism for generating exotic NS phenomenology and place constraints on the strength of DM self-interactions and DM–baryon couplings inside neutron-star cores.

Abstract

Recent measurements of the compact star XTE J1814-338, with a mass of $M=1.2_{-0.05}^{+0.05}\ M_{\odot}$ and a radius of $R=7_{-0.4}^{+0.4} \ {\rm Km}$ alongside those of HESS J1731-347, which has a mass of $M=0.77_{-0.17}^{+0.20}\ M_{\odot}$ and a radius of $R=10.4_{-0.78}^{+0.86} \ {\rm Km}$, provide compelling evidence for the potential existence of exotic matter in neutron star cores. These observations offer important insights into the equation of state of dense nuclear matter. Recently, Fornal and Grinstein, in order to overcame the discrepancy between the neutron lifetime measured in beam and bottle experiments, proposed the existence of neutron dark decay. In the present work, an effort is made to connect the interpretation of the above exotic compact objects with the possible existence of dark particles, assumed to be products of neutron dark decay. Our hypothesis offers an advantage over comparable proposals, as the coexistence of dark matter and hadronic matter within neutron stars emerges from an intrinsic mechanism, thereby obviating the need to invoke external merger-related processes. It is still unclear to what extent the proposed dark decay of the neutron is affected by the extreme environment within neutron stars. Within this framework, we examined the case in which a mechanism suppressing the dark neutron decay becomes operative at densities few times above nuclear saturation density. We found that the proposed alternative explanation accommodates the simultaneous existence of neutron dark decay while consistently predicting both the two solar mass limit and the presence of compact objects with subsolar masses.

Figures (5)

• Figure 1: (a) The ${\rm M-R}$ diagram (for the cases of the ordinary neutron star matter (Ordinary NS) and for the dark matter-neutron matter admixture (Compact object)). Dark matter domination refers to the mass range in which the dark particle fractions exceeds that of neutron fractions in the center of the compact object. (b) The number density fractions and (c) the mass fraction $f_{ \chi}$ for the self-interaction cases $z_{\chi}=130,220,1500$ MeV (from the top to the bottom). The observational constraints for the HESS J1731-347 HESS-2023 and for the XTE J1814-338 Kini-2024Baglio-2013 are also indicated.
• Figure 2: The same as the Fig. (\ref{['zx-only']}) for $z_{\chi n}=10, 20, 30,40$ MeV (from the top to the bottom).
• Figure 3: The same as the Fig. (\ref{['zx-only']}) for a) $z_{\chi}=150$ MeV and $z_{\chi n}=150$ MeV, b) $z_{\chi}=200$ MeV and $z_{\chi n}=150$ MeV, c) $z_{\chi}=220$ MeV and $z_{\chi n}=200$ MeV, d) $z_{\chi}=250$ MeV and $z_{\chi n}=180$ MeV, e) $z_{\chi}=350$ MeV and $z_{\chi n}=300$ MeV (from the top to the bottom).
• Figure 4: a) The M-R dependence and b) the dark matter mass fraction $f_{ \chi}$ for the case $z_{\chi}=200$ MeV and for various values of the cutoff density, in units of nuclear saturation density $n_s$ where $n_s=0.16$ fm$^{-3}$, as indicated by the labels. The observational constraints for the HESS J1731-347 HESS-2023 and for the XTE J1814-338 Kini-2024Baglio-2013 are also indicated.
• Figure 5: a) The M-R dependence and b) the dark matter mass fraction $f_{ \chi}$ considering a cutoff on dark neutron decay for the interaction parameters $z_{\chi}=220$ MeV and $z_{\chi n}=200$ MeV (red line) and $z_{\chi}=350$ MeV and $z_{\chi n}=300$ MeV (green line). The cutoff of the dark neutron decay has been specified at twice the nuclear saturation density $n_s$. The observational constraints for the HESS J1731-347 HESS-2023 and for the XTE J1814-338 Kini-2024Baglio-2013 are also indicated.