arXiv subjects

Dake Zhou

Publications and source records attributed to Dake Zhou.

9 recordsLinked to original sources

arXiv2024

Bayesian evidence for two peaks in the sound speed in cold dense QCD

I show that in addition to the well-known peak inside massive neutron stars, the sound speed in cold dense QCD matter likely exhibits another peak above neutron star densities before it asymptotes to $c_s=\sqrt{C_s}=\sqrt{1/3}$. Based on the framework reported in arxiv:2408.16738, this approach does not rely on any assumption about the ultra-dense matter not realized in nature. Current multimessenger observation of neutron stars favors the two-peak scenario with a Bayes factor $5.1_{-0.7}^{+0.9}$, where the uncertainties are systematics due to models of neutron star inner cores. This evidence grows to $27_{-8}^{+11}$ if the $2.6M_\odot$ component of GW190814 is a neutron star. The trough in $C_s$ separating the two peaks is inferred to be below $0.05^{+0.04}_{-0.02}$ (at the $50\%$ level) if $2.6M_\odot$ neutron stars exist. The second peak above $1/3$ beyond baryon chemical potential $\mu_B=1.6-1.9$ GeV most likely signals non-perturbative effects in cold quark matter, for instance color superconductivity.

nucl-th↗

arXiv2024

Bounds on the minimum sound speed above neutron star densities

We show that the existence of massive neutron stars and asymptotic freedom of QCD place robust upper bounds on the lowest sound speed of the ultra-dense matter unattainable in neutron stars. Centered on worst-case scenarios, our limits are the most conservative among physical equations of state in the density range $\sim 2-40 n_0$. Discovery of $\gtrsim 2.6 M_\odot$ neutron stars, in combination with current multimessenger astrophysical constraints on the equation of state, would strongly support first-order phase transitions at high baryon densities.

nucl-th↗

arXiv2023

Dwarf (Twin) Neutron Stars I: Did GW170817 Involve One?

Dwarf neutron stars are stable twins of neutron stars but with a maximum mass less than that of neutron stars. Their existence brings into concordance the seemingly conflicting information on the size of neutron stars inferred from gravitational waves from GW170817, from the NICER mission, and from the PREX-II experiment. Their distinctive characteristics lead to rich and falsifiable predictions that are expected to be tested in the near future. If corroborated, the existence of dwarf neutron stars would substantially improve our understanding of the QCD phase diagram and offer valuable insights into the dark sector.

astro-ph.HE↗

arXiv2023

Reexamining constraints on neutron star properties from perturbative QCD

The implications of perturbative QCD (pQCD) calculations on neutron stars are carefully examined. While pQCD calculations above baryon chemical potentials $\mu_B\simeq2.4$ GeV demonstrate the potential of ruling out a wide range of neutron star equations of state (EOSs), such constraints only affect the most massive neutron stars in the vicinity of the Tolman-Oppenheimer-Volkoff (TOV) limit, resulting in constraints that are orthogonal to current or expected astrophysical bounds. In the most constraining scenario, pQCD considerations favor low values of the squared speed sound $C_s$ at high $\mu_B$ relevant for the most massive neutron stars, but leave predictions of the radii and tidal deformabilities almost unchanged. Such considerations become irrelevant if the maximum speed of sound squared inside neutron stars does not exceed about $C_{s,\mathrm{max}}\simeq0.5$, or if pQCD breaks down below $\mu_B\simeq2.9$ GeV. Furthermore, the large pQCD uncertainties preclude any meaningful bounds on the neutron star EOS at the moment. Interestingly, if pQCD predictions for the pressure at around $\mu_B\simeq2.5$ GeV are refined and found to be low ($\lesssim 1.5$ GeV/fm$^3$), evidence for a soft neutron star inner core EOS in combination with the existence of two-solar-mass pulsars would indicate the presence of color superconductivity beyond neutron star densities. We point out that two-solar-mass pulsars place robust upper bounds on this non-perturbative effect and require the pairing gap to be less than $\Delta_\mathrm{CFL}\lesssim500$ MeV at $\mu_B\simeq2.5$ GeV.

astro-ph.HE↗

arXiv2023

Framework for Multi-messenger Inference from Neutron Stars: Combining Nuclear Theory Priors

We construct an efficient parameterization of the pure neutron-matter equation of state (EoS) that incorporates the uncertainties from both chiral effective field theory ($\chi$EFT) and phenomenological potential calculations. This parameterization yields a family of EoSs including and extending the forms based purely on these two calculations. In combination with an agnostic inner core EoS, this parameterization is used in a Bayesian inference pipeline to obtain constraints on the e os parameters using multi-messenger observations of neutron stars. We specifically considered observations of the massive pulsar J0740+6620, the binary neutron star coalescence GW170817, and the NICER pulsar J0030+0451. Constraints on neutron star mass-radius relations are obtained and compared. The Bayes factors for the different EoS models are also computed. While current constraints do not reveal any significant preference among these models, the framework developed here may enable future observations with more sensitive detectors to discriminate them.

astro-ph.HE↗

arXiv2021

Dark Lepton Superfluid in Proto-Neutron Stars

We find that sub-GeV neutrino portal bosons that carry lepton number can condense inside a proto-neutron star (newly born neutron star). These bosons are produced copiously and form a Bose-Einstein condensate for a range of as yet unconstrained coupling strengths to neutrinos. The condensate is a lepton number superfluid with transport properties that differ dramatically from those encountered in ordinary dense baryonic matter. We discuss how this phase could alter the evolution of proto-neutron stars and comment on the implications for neutrino signals and nucleosynthesis.

hep-ph↗

arXiv2019

Constraining the neutron-matter equation of state with gravitational waves

We show how observations of gravitational waves from binary neutron star (BNS) mergers over the next few years can be combined with insights from nuclear physics to obtain useful constraints on the equation of state (EoS) of dense matter, in particular, constraining the neutron-matter EoS to within 20% between one and two times the nuclear saturation density $n_0\approx 0.16\ {\text{fm}^{-3}}$. Using Fisher information methods, we combine observational constraints from simulated BNS merger events drawn from various population models with independent measurements of the neutron star radii expected from x-ray astronomy (the Neutron Star Interior Composition Explorer (NICER) observations in particular) to directly constrain nuclear physics parameters. To parameterize the nuclear EoS, we use a different approach, expanding from pure nuclear matter rather than from symmetric nuclear matter to make use of recent quantum Monte Carlo (QMC) calculations. This method eschews the need to invoke the so-called parabolic approximation to extrapolate from symmetric nuclear matter, allowing us to directly constrain the neutron-matter EoS. Using a principal component analysis, we identify the combination of parameters most tightly constrained by observational data. We discuss sensitivity to various effects such as different component masses through population-model sensitivity, phase transitions in the core EoS, and large deviations from the central parameter values.

astro-ph.HE↗

arXiv2019

Dark halos around neutron stars and gravitational waves

We find that a class of models of MeV-GeV dark matter in which dark matter interacts strongly can be constrained by the observation of gravitational waves from neutron star mergers. Trace amounts of dark matter, either produced during the supernova or accreted later, can alter the structure of neutron stars and influence their tidal polarizability. We focus on models of dark matter interacting by the exchange of light vector gauge bosons that couple to a conserved dark charge. In these models, dark matter accumulated in neutron stars can extend to large radii and enhance their tidal polarizability. Gravitational waves detected from the first binary neutron star merger GW170817 places useful constraints on such not-so compact objects. Dark halos, if present, also predict a greater variability of neutron star tidal polarizabilities than expected for ordinary neutron stars.

hep-ph↗

arXiv2018

Neutron stars exclude light dark baryons

Exotic new particles carrying baryon number and with mass of order the nucleon mass have been proposed for various reasons including baryogenesis, dark matter, mirror worlds, and the neutron lifetime puzzle. We show that the existence of neutron stars with mass greater than 0.7 $M_\odot$ places severe constraints on such particles, requiring them to be heavier than 1.2 GeV or to have strongly repulsive self-interactions.

hep-ph↗