Strange Matter

TL;DR

This work tackles the uncertain nature of ultra-dense matter inside pulsar-like objects by comparing neutron-star and strange-star hypotheses under Quantum Chromodynamics. It surveys six EOS frameworks for strange matter, including the MIT bag model, polytropic, Lennard-Jones (strangeon) and related approaches, and analyzes their implications for mass–radius relations, surface properties, glitches, and binary mergers. Through TOV modeling and multi-messenger constraints (notably GW170817), it argues that stiff EOS can support $M_{\rm TOV}$ well above $2M_\odot$ and that strange-star signatures may appear in tidal deformabilities, post-merger dynamics, and electromagnetic counterparts. The paper also explores strangeon nuggets as dark-matter candidates and outlines observational tests across surface phenomena, mergers, and potential dark-matter signals, highlighting the role of future multi-messenger astronomy in confirming or refuting the strange-matter paradigm.

Abstract

Pulsar-like objects are extremely compact, with an average density that exceeds nuclear saturation density, where the fundamental strong interaction plays an essential role, particularly in the low-energy regime. The internal structures and properties of those objects are profoundly connected to phenomena such as supernova explosions, gamma-ray bursts, fast radio bursts, high/low-mass compact stars, and even to issues like dark matter and cosmic rays. However, due to the non-perturbative nature of quantum chromodynamics, significant uncertainties remain in our current understanding of the composition and equation of state (EOS) for the dense matter inside them. Drawing on three-flavour symmetry and the strong coupling between light quarks, this paper presents a novel perspective on the nature of pulsars: they are actually composed of strange matter, in the form of either strange quark matter or strangeon (analogous to nucleons and representing multibaryon states with three-flavour symmetry) matter. As both strange quark matter and strangeon matter contain non-zero strangeness, we refer to them collectively as ``strange matter'', and to the corresponding compact stars as ``strange stars''. We then briefly introduce several physical models describing strange matter and present the resulting structures and properties of strange stars. This includes discussions on the EOSs, surface properties, mass-radius relations, glitches, binary compact star mergers, and dark matter. Furthermore, we will explore how observational properties of pulsar-like objects support the strange star model.

Figures (10)

• Figure 1: The triangle of light-quark flavors. The points inside this triangle define the states with certain quark number densities of u, d and s quarks, indicated by the heights of one point to one of the triangle edges. The gray level denotes the charge-mass-ratio of quarks ($R$). Normal nuclei are around point A ($R=1/2$). Neutron stars are around point n and strange stars are around point s, both of which have nearly charge-neutrality ($R=0$). Another possible scenario is strangeon stars located also at point s, which are made of strangeons (see Section \ref{['sec:strange2strangeon']}).
• Figure 2: A schematic illustration of a lattice cell in strong matter. A spherical bag (centered at point "O") resides in the cell, which is linked to the neighbouring bags through the six windows (the red circles) on the bag's surface. The size of each window is characterized by the angle $\theta$ with $\cos\theta = a/(2r_{\rm bag})$, where $a$ is the lattice constant and $r_{\rm bag}$ the bag radius. Taken from Ref. Miao2022_IJMPE0-2250037.
• Figure 3: The $M$-$R$ curves of strange stars predicted by various models. The black dot-dashed line shows the results of the MIT bag model ($B^{1/4}=145\ \rm MeV$, $\Delta=0$, $a_4=1$), the red dashed line represents the polytropic model ($n=0.5$), the blue solid line represents the Lennard-Jones model ($n_{\rm s}=0.36\ \rm /fm^3$, $u_0=30$ MeV, $N_{\rm q}=18$), the green solid line represents the H dibaryon model ($\alpha_{\rm BR}=0.15$, $n_{\rm s}=2n_0$), the cyan dash-dotted line represents the corresponding state model ($u_0=40$ MeV, $r_0=2.5$ fm, $N_{\rm q}=18$), and the black dotted line represents the linked-bag model ($B_2=162.3\ \rm MeV/fm^3$, $B_3=100\ \rm MeV/fm^3$, $z_0=2.843$). For comparison, the pink dot-dashed line shows the results of neutron star model AP4.
• Figure 4: The recovery coefficient $Q$ as functions of glitch size $\Delta \Omega/\Omega$, where the parameter $a$ represents different degrees of exponential recovery. The observational values for the Crab pulsar, the Vela pulsar and several other pulsars are indicated by the red circles, blue triangles and black crosses, respectively. Taken from Ref. Lai2018_MNRAS476-3303.
• Figure 5: Frequency residual in the recovery for a glitch of PSR J1852-0635 in the framework of strangeon star model, where $h$ is the depth of the cracking site below the surface, $R$ the stellar radius, and $\tau$ the viscous time-scale. The red solid circles are the data points extracted from observation, while the first circle at $t=0$ is derived from the glitch size and before the glitch. Taken from Ref. Lai2023_MNRAS523-3967.