Narrow Results

The Casimir effect is known as an attractive force induced by photon fields sandwiched by parallel conducting plates and appears even at zero temperature. It is well known that thermal fluctuations contribute to the Casimir energy, whereas the contribution from finite-density environments is not established. Here, we discuss the typical behaviors of the Casimir effect at finite density. We develop how to define the Casimir energy at finite density and its typical property. Our findings will be applied to fermion systems in quark matter and Dirac/Weyl semimetals.

We uncover novel Casimir effects emerging from quark fields in dense and thin quark matter under vanishing or nonzero magnetic field. Remarkably, in the dual chiral density wave (DCDW) phase, a candidate ground state for dense quark matter the Casimir energy oscillates as a function of the thickness. This discovery highlights a unique oscillatory Casimir phenomenon driven by QCD dynamics in extreme conditions.

Chiral symmetry breaking at finite baryon density is usually discussed in the context of quark matter, i.e. a system of deconfined quarks. Many systems like stable nuclei and neutron stars however have quarks confined within nucleons. In this paper we construct a Fermi sea of three-quark nucleon clusters and investigate the change of the quark condensate as a function of baryon density. We study the effect of quark clustering on the in-medium quark condensate and compare results with the traditional approach of modeling hadronic matter in terms of a Fermi sea of deconfined quarks.

The existence of deconfined quark matter in the superdense interior of neutron stars is a key question that has drawn considerable attention over the past few decades. Quark matter can comprise an arbitrary fraction of the star, from 0 for a pure neutron star to 1 for a pure quark star, depending on the equation of state of matter at high density. From an astrophysical viewpoint, these two extreme cases are generally expected to manifest different observational signatures. An intermediate fraction implies a hybrid star, where the interior consists of mixed or homogeneous phases of quark and nuclear matter, depending on surface and Coulomb energy costs, as well as other finite size and screening effects. In this review, we discuss what we can deduce about quark matter in neutron stars in light of recent exciting developments in neutron star observations. We state the theoretical ideas underlying the equation of state of dense quark matter, including color superconducting quark matter. We also highlight recent advances stemming from re-examination of an old paradigm for the surface structure of quark stars and discuss possible evolutionary scenarios from neutron stars to quark stars, with emphasis on astrophysical observations.

Quark matter both in terrestrial experiment and in astrophysics is briefly reviewed. Astrophysical quark matter could appear in the early Universe, in compact stars, and as cosmic rays. Emphasis is put on quark star as the nature of pulsars. Possible astrophysical implications of experiment-discovered sQGP are also concisely discussed.

We discuss the saturation mechanism for the nuclear matter equation of state in a chiral effective quark theory. The importance of the scalar polarizability of the nucleon is emphasized. The phase transition to color superconducting quark matter is also discussed.

We investigate the thermal conductivity (κ) of the quark matter at finite quark chemical potential (μ) and temperature (T), employing the Green–Kubo formula, for the SU(2) light-flavor sector with the finite current-quark mass m = 5 MeV. As a theoretical framework, we construct an effective thermodynamic potential from the (μ, T)-modified liquid-instanton model (mLIM). Note that all the relevant model parameters are designated as functions of T, using the trivial-holonomy caloron solution. By solving the self-consistent equation of mLIM, we acquire the constituent-quark mass M₀ as a function of T and μ, satisfying the universal-class patterns of the chiral phase transition. From the numerical results for κ, we observe that there emerges a peak at μ≈200 MeV for the low-T region, i.e. T≲100 MeV. As T increase over T≈100 MeV, the curve for κ is almost saturated as a function of T in the order of ~ 10^-1GeV², and grows with respect to μ smoothly. At the normal nuclear-matter density ρ₀ = 0.17 fm^-3, κ shows its maximum 6.22 GeV² at T≈10 MeV, then decreases exponentially down to κ≈0.2 GeV². We also compute the ratio of κ and the entropy density, i.e. κ/s as a function of (μ, T) which is a monotonically decreasing function for a wide range of T, then approaches a lower bound at very high T: κ/s_min≳0.3 GeV^-1 in the vicinity of μ = 0.

We present an analytical description of the phase transitions from a nucleon gas to nuclear matter and from nuclear matter to quark matter within the same model. The equation of state for quark and nuclear matter is encoded in the effective potential of a linear sigma model. We exploit an exact differential equation for its dependence upon the chemical potential μ associated to conserved baryon number. An approximate solution for vanishing temperature is used to discuss possible phase transitions as the baryon density increases. For a nucleon gas and nuclear matter we find a substantial density enhancement as compared to quark models which neglect the confinement to baryons. The results point out that the latter models are not suitable to discuss the phase diagram at low temperature.

We present a novel treatment of calculating the in-medium quark condensates. The advantage of this approach is that one does not need to make further assumptions on the derivatives of model parameters with respect to the quark current mass. The normally accepted model-independent result in nuclear matter is naturally reproduced. The change of the quark condensate induced by interactions depends on the incompressibility of nuclear matter. When it is greater than 265 MeV, the density at which the condensate vanishes is higher than that from the linear extrapolation. For the chiral condensate in quark matter, a similar model-independent linear behavior is found at lower densities, which means that the decreasing speed of the condensate in quark matter is merely half of that in nuclear matter if the pion-nucleon sigma commutator is six times the average current mass of u and d quarks. The modification due to QCD-like interactions is found to slow the decreasing speed of the condensate, compared with the linear extrapolation.

Color superconducting state has been known as a possible phase of quark matter with sufficiently large baryon number density so as for perturbative analysis to be valid. We point out that a color ferromagnetic state is another possible phase of such a sufficiently dense quark matter. Furthermore, we show under reasonable choices of parameters that the color ferromagnetic phase is energetically more favored than the color superconducting phase in the quark matter with smaller baryon number density. Supposing that increasing baryon density in neutron stars transforms nuclear matter into the quark matter of the color ferromagnetic phase, not color superconducting phase, we find that a critical mass of the neutron star with such an internal structure is about 1.6M_⊙.

The color-flavor locked (CFL) phase of strangelets is investigated in a quark mass density-dependent model. Parameters are determined by stability arguments. It is concluded that three solutions to the system equations can be found, corresponding, respectively, to positively charged, negatively charged, and nearly neutral CFL strangelets. The charge to baryon number of the positively charged strangelets is smaller than the previous result, while the charge of the negatively charged strangelets is nearly proportional in magnitude to the cubic-root of the baryon number. However, the positively charged strangelets are more stable compared to the other two solutions.

We describe work being done at Baylor University investigating the possibility of new states of mesonic matter containing two or more quark–antiquark pairs. To put things in context, we begin by describing the lattice approach to hadronic physics. We point out there is a need for a quark model which can give an overall view of the quark interaction landscape. A new application of the Thomas–Fermi (TF) statistical quark model is described, similar to a previous application to baryons. The main usefulness of this model will be to detect systematic energy trends in the composition of the various particles. It could be a key to identifying families of bound states, rather than individual cases. Numerical results based upon a set of parameters derived from a phenomenological model of tetraquarks are given.

We have studied phase transition from hadron matter to quark matter in the presence of high magnetic fields incorporating the trapped electron neutrinos at finite temperatures. We have used the density dependent quark mass (DDQM) model for the quark phase while the hadron phase is treated in the frame-work of relativistic mean field theory. It is seen that the energy density in the hadron phase at phase transition decreases with both magnetic field and temperature.

We present a brief review of the present status of the standard model of core collapse supernovae and neutron star formation outlining the basic concepts and paying attention to the possibility of a transition to quark matter. We evaluate the consequences of this transition on the whole explosion mechanism, analyze the possible generation of beamed gamma ray bursts, and discuss the nature of the compact star born as a result of the supernova explosion.

We analyze the possibility that bubbles of quark matter surviving the confinement phase transition might have become colour superconducting due to the enormous compression suffered by them.

Quark matter is expected to exist in the interior of compact stellar objects as neutron stars or even the more exotic strange stars, based on the Bodmer–Witten conjecture. Bare strange quark stars and (normal) strange quark-matter stars, those possessing a baryon (electron-supported) crust, are hypothesized as good candidates to explain the properties of a set of peculiar stellar sources such as the enigmatic X-ray source RX J1856.5-3754, some pulsars such as PSR B1828-11 and PSR B1642-03, and the anomalous X-ray pulsars and soft γ-ray repeaters. In the MIT bag model, quarks are treated as a degenerate Fermi gas confined to a region of space having a vacuum energy density B_bag (the Bag constant). In this note, we modify the MIT bag model by including the electromagnetic interaction. We also show that this version of the MIT model implies the anisotropy of the bag pressure due to the presence of the magnetic field. The equations of state of the degenerate quarks gases are studied in the presence of ultra strong magnetic fields. The behavior of a system made up of quarks having (or not) anomalous magnetic moment is reviewed. A structural instability is found, which is related to the anisotropic nature of the pressures in this highly magnetized matter. The conditions for the collapse of this system are obtained and compared to a previous model of neutron stars that is built on a neutron gas having anomalous magnetic moment.

In this article, we study higher-dimensional cosmological models with quark–gluon plasma in the context of general relativity. For this purpose, we consider quark–gluon plasma as a perfect fluid in the higher-dimensional universes. After solving Einstein's field equations, we have analyzed this matter for the different types of universes in the higher- and four-dimensional universes. Also, we have discussed the features of obtained solutions.

This paper provides a short overview of the multifaceted, possible role of quark matter for compact stars (neutron stars and strange quark matter stars). We began with a variational investigation of the maximum possible energy densities in the cores of neutron stars. This is followed by a brief discussion of the possible existence of quark matter in the cores of neutron stars and how such matter could manifest itself in neutron star observables. The possible presence of color superconducting strange quark matter nuggets in the crusts of neutron stars is reviewed next, and their impact on the pycnonuclear reaction rates in the crusts of neutron stars is discussed. The second part of the paper discusses the impact of ultra-strong electric fields on the bulk properties of strange quark matter stars and presents results of a preliminary study that models the thermal evolution of radio-quiet, X-ray bright, central compact objects (CCOs).

We investigate the hadron-quark phase transition inside neutron stars and obtain mass–radius relations for hybrid stars. The equation of state for the quark phase using the standard NJL model is too soft, leading to an unstable star and suggesting a modification of the NJL model by introducing a momentum cutoff dependent on the chemical potential. However, even in this approach, the instability remains. In order to remedy the instability we suggest the introduction of a vector coupling in the NJL model, which makes the EoS stiffer, reducing the instability. We conclude that the possible existence of quark matter inside the stars require high densities, leading to very compact stars.

An effective Lagrangian for quark pairing in nonlinear chromodielectric models (CDM) is derived in the leading order. The effective pairing coupling depends explicitly on the confinement field, χ, of the CDM manifesting a confinement effect on color superconductivity. The self-consistent gap equations are constructed and solved for the color-flavor locked phase. The pairing coupling, for the quartic χ potential used in the calculations, is inversely proportional to the second power of the χ field and to the χ mass squared. In the chiral solution the quarks do not pair, but in the chiral breaking solution, where χ is small and the effective pairing interaction is stronger, it does show pairing. Our results indicate that the vector channel of the gap is weaker than the scalar one.