Small radii of neutron stars as an indication of novel in-medium effects

TL;DR

This paper proposes a new in-medium effect involving a U-boson that could explain very small neutron star radii below 10 km, challenging existing models and awaiting future observational tests.

Contribution

Introduction of a novel in-medium term in the equation of state, potentially due to U-boson exchange, to account for smaller neutron star radii without disrupting nuclear model success.

Findings

New term can produce smaller radii predictions below 10 km.

U-boson exchange may be responsible for in-medium effects.

Model remains consistent with current nuclear physics constraints.

Abstract

At present, neutron star radii from both observations and model predictions remain very uncertain. Whereas different models can predict a wide range of neutron star radii, it is not possible for most models to predict radii that are smaller than about 10 km, thus if such small radii are established in the future they will be very difficult to reconcile with model estimates. By invoking a new term in the equation of state that enhances the energy density, but leaves the pressure unchanged we simulate the current uncertainty in the neutron star radii. This new term can be possibly due to the exchange of the weakly interacting light U-boson with appropriate in-medium parameters, which does not compromise the success of the conventional nuclear models. The validity of this new scheme will be tested eventually by more precise measurements of neutron star radii.