Strange matter in compact stars (1711.11260v1)

Abstract: We discuss possible scenarios for the existence of strange matter in compact stars. The appearance of hyperons leads to a hyperon puzzle in ab-initio approaches based on effective baryon-baryon potentials but is not a severe problem in relativistic mean field models. In general, the puzzle can be resolved in a natural way if hadronic matter gets stiffened at supersaturation densities, an effect based on the quark Pauli quenching between hadrons. We explain the conflict between the necessity to implement dynamical chiral symmetry breaking into a model description and the conditions for the appearance of absolutely stable strange quark matter that require both, approximately masslessness of quarks and a mechanism of confinement. The role of strangeness in compact stars (hadronic or quark matter realizations) remains unsettled. It is not excluded that strangeness plays no role in compact stars at all. To answer the question whether the case of absolutely stable strange quark matter can be excluded on theoretical grounds requires an understanding of dense matter that we have not yet reached.

• The paper explores theoretical scenarios, observational constraints, and challenges regarding the potential existence and role of strange matter within compact stars like neutron stars.
• The "hyperon puzzle" highlights a challenge where expected hyperon presence could soften the equation of state below observed maximum neutron star masses in certain theoretical models.
• Current observational evidence for strange matter is inconclusive, but future gravitational wave detections and improved mass/radius measurements may provide critical insights.

Overview of "Strange Matter in Compact Stars"

The paper "Strange Matter in Compact Stars" by Thomas Klähn and David B. Blaschke explores the role and potential existence of strange matter within compact astrophysical objects, particularly neutron stars. The paper explores several scenarios by which strange quark matter might manifest within these stellar objects and discusses both theoretical and observational constraints that frame our understanding.

Neutron Stars: Masses, Radii, and Composition

Neutron stars, remnants of supernovae, are traditionally thought to consist primarily of neutrons, protons, and electrons. With masses often exceeding that of our sun and radii on the order of 10-15 kilometers, these dense stars offer a unique opportunity to paper matter under extreme conditions. Observations of pulsars like PSR J1614-2230 and PSR J0348 + 0432 provide crucial data on the upper limits of neutron star masses, which in turn guide theoretical models of dense matter.

Hyperon Puzzle

One notable challenge in this domain is the "hyperon puzzle". Hyperons, more massive particles than protons or neutrons, are expected to appear at baryon densities of two to three times nuclear saturation density. However, their presence typically softens the equation of state (EoS), which can reduce the maximum mass of neutron stars below observed limits. The hyperon puzzle is a significant concern for models utilizing Brueckner-Bethe-Goldstone calculations but less so for relativistic mean field models where additional repulsive interactions can stabilize the EoS.

Quark Matter and its Implications

Quark matter, specifically deconfined quark-gluon plasma, emerges as a candidate for the high-density core of neutron stars. The transition from hadronic to quark matter remains a contentious topic, with numerous models attempting to capture this transition's characteristics. Convincing integration of quark matter into the neutron star context necessitates addressing the interplay of chiral symmetry breaking and confinement mechanisms.

The diversity of potential quark matter phases, including color-flavor locked phases and other condensate structures, compounds the complexity. Effective relativistic models and Dyson-Schwinger equation approaches provide insights into these phases' properties, indicating that quark matter must incorporate significant repulsive interactions to support observational mass limits.

Strange Quark Matter Stability

The notion of absolutely stable strange quark matter, hypothesized to lower the energy per baryon and offer a stable end state for compact stars, remains speculative. While certain versions of the thermodynamic bag model suggest such stability, they rely on assumptions that conflict with aspects like chiral symmetry breaking. Models integrating chiral symmetry consider such scenarios less likely, pointing to a requirement for massless quarks in forming stable strange matter, which the symmetry-breaking processes typically prevent.

Prospective Observational Evidence

Presently, observational evidence for strange matter's role in compact stars is inconclusive. Future gravitational wave observations from events like neutron star mergers, along with more precise mass and radius measurements from initiatives like the NICER experiment, may provide further insights. However, whether strange quark matter manifests as a dominant phase within neutron stars, and if it is a naturally occurring state, remains unsolved.

Conclusions and Future Directions

The research concludes that while the existence of strange matter in compact stars—either as part of hadronic or quark phases—remains uncertain, its potential impact on neutron star structure cannot be ignored. Theoretical models are challenged to reconcile dynamical chiral symmetry breaking with the possibility of massless quark states leading to absolutely stable strange matter. Future developments in observations and the refinement of nuclear matter models could elucidate strange matter's role in the cosmos.