Reaching percolation and conformal limits in neutron stars

Abstract

Generating an ensemble of equations of state that fulfill multimessenger constraints, we statistically determine the properties of dense matter found inside neutron stars (NSs). We calculate the speed of sound and trace anomaly and demonstrate that they are driven towards their conformal values at the center of maximally massive NSs. The local peak of the speed of sound is shown to be located at values of the energy and particle densities which are consistent with deconfinement and percolation conditions in QCD matter. We also analyze fluctuations of the net-baryon number density in the context of possible remnants of critical behavior. We find that the global maxima of the variance of these fluctuations emerge at densities beyond those found in the interiors of NSs.

Repulsive vector interaction as a trigger for the non-conformal peak in $V_s^2$

“…Basically, it interpolates between a regime where repulsion is high (the EoS is stiff) and a regime where repulsion low (the EoS is soft). Thanks to this property the model is able to predict a non-monotonic behavior for V 2 s which is in line with recent estimates [18,34,35]. Regarding the conformal measure the results indicate that a non-conformal peak in V 2 s is not in tension with the trace anomaly being positive for all densities, a result which agrees with a scenario proposed in Ref.…”

“…In this case, recent simulations [18] indicate that the most probable scenario is the one in which V 2 s is a non-monotonic function of n B , which in turn suggests the existence of at least one local maximum where V 2 s > 1/3. This possibility is supported by a large number of applications employing frameworks such as quarkyonic matter [19][20][21][22], models for dense QCD [23][24][25][26][27][28][29][30], as well as models based on the gauge/gravity duality [31][32][33], hadron percolation threshold [34], and Bayesian inference methods [35] among others. To emphasize the importance of V 2 s it is worth mentioning that changes in its slope can also provide important information related to the phase transitions and cross-overs that can take place within dense baryonic matter.…”

“…Also, remark that δ was chosen so as to give a smooth transition within a narrow 10 MeV width since taking δ → 0 could lead to discontinuities in V 2 s which do not seem to be observed in Refs. [18,34,35]. With this conservative choice one can anticipate that the transition from the repulsive/stiff phase to the non-repulsive/soft phase will be driven by a cross-over.…”

“…Finally, it should be emphasized that the shape of the curve generated with G V (µ) resembles some of those recently predicted in Refs. [18,34,35].…”

Reconciling Multi-messenger Constraints with Chiral Symmetry Restoration

“…As we have demonstrated in this work, the characteristics of the bulk EoS, such as the change of the logarithmic slope in the EoS, do not necessarily imply the existence of a hadron-quark phase transition as proposed in recent studies, e.g., [21]. We conclude that due to the anticipated near-future advances in multi-messenger astronomy, it will become inevitable to link the observed properties of NSs and their mergers to fundamental properties of strong interactions described by QCD, including chiral symmetry restoration, as well as emergence of conformal matter [23].…”