Berryman and Gardner consider the possibility of a new force between quarks, which modifies the neutron star equation of state, particularly at supranuclear densities. Thereby making them heavy.
On 23 June 2020, the LIGO Scientific Collaboration and the Virgo Collaboration announced the discovery of a gravitational wave binary, GW190814. While one component of this binary is a 23 solar-mass black hole, the other component is a compact object of 2.6 solar mass, which may be too massive to be a neutron star, given our current knowledge of the nuclear matter equation of state. Now, J. Berryman and S. Gardner with the help of a new physics model (within a non-relativistic many body framework) proposed that, it is possible that a neutron star can be so heavy, if a new force exists between quarks and it modifies the neutron star equation of state, particularly at supranuclear densities. Their study recently appeared in Arxiv.
“This new force between quarks can make neutron stars for a fixed equation of state and many-body method both heavier and puffier.”— told Susan Gardner, professor at University of Kentucky and one of the author of the study
They considered minimal extensions of the Standard Model (SM) that give rise to new, short-range interactions between quarks. In particular, they considered U(1)X gauge symmetry extensions that couple to baryon number. If this symmetry is spontaneously broken it would give a gauge boson X. If this boson is heavier than the pion and not too strongly coupled, then its effects on nuclear matter (or on nucleon-nucleon force) are expected to be subdominant to those of the strong interactions, comprising at most some part of the empirical low energy constants (LECs). These effects can modify the neutron matter equation of state at supranuclear densities.
“This new interaction is largely shielded from constraints from low-energy experiments. In particular, its contribution to the nucleon-nucleon (NN) force can be hidden within the short-distance repulsion of the phenomenological NN force in the SM, yet it can modify the neutron matter equation of state (EoS) at supranuclear densities”— told S. Gardner
They also evaluated how this modification can impact a neutron star’s mass and radius to make the observed heavy compact object more probably a neutron star, rather than a black hole. By combining spin effects with new physics, it has been found that it could yield heavy neutron stars (NSs); thus more compact objects in excess of 2M may eventually be identified, promoting the possibility of new baryonic interactions.
“Although we have not resolved the nature of the ~ 2.6 M compact object in GW190814, this mechanism allows it to more naturally be a neutron star.”— told S. Gardner.
Finally, they concluded that their new physics scenario can be tested through studies of rare η and η’ decays and of X photoproduction at low-energy accelerators.
Reference: Jeffrey M. Berryman, Susan Gardner, “Neutron stars and the secret lives of quarks”, Arxiv, pp. 1-7, 2021. https://arxiv.org/abs/2105.05254
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