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Quasi-normal oscillation modes of neutron stars provide a means to probe their interior composition using gravitational wave astronomy. We compute the frequencies and damping times of composition-dependent core g-modes of neutron stars containing quark matter employing linearized perturbative equations of general relativity. We find that ignoring background metric perturbations due to the oscillating fluid, as in the Cowling approximation, underestimates the g-mode frequency by up to 10% for higher mass stars, depending on the parameters of the nuclear equation of state and how the mixed phase is constructed. The g-mode frequencies are well-described by a linear scaling with the central lepton (or combined lepton and quark) fraction for nucleonic (hybrid) stars. Our findings suggest that neutron stars with and without quarks are manifestly different with regards to their quasi-normal g-mode spectrum, and may thus be distinguished from one another in future observations of gravitational waves from merging neutron stars., 14 pages, 10 figures

We combine equation of state of dense matter up to twice nuclear saturation density ($n_{\rm sat}=0.16\, \text{fm}^{-3}$) obtained using chiral effective field theory ($��$EFT), and recent observations of neutron stars to gain insights about the high-density matter encountered in their cores. A key element in our study is the recent Bayesian analysis of correlated EFT truncation errors based on order-by-order calculations up to next-to-next-to-next-to-leading order in the $��$EFT expansion. We refine the bounds on the maximum mass imposed by causality at high densities, and provide stringent limits on the maximum and minimum radii of $\sim1.4\,{\rm M}_{\odot}$ and $\sim2.0\,{\rm M}_{\odot}$ stars. Including $��$EFT predictions from $n_{\rm sat}$ to $2\,n_{\rm sat}$ reduces the permitted ranges of the radius of a $1.4\,{\rm M}_{\odot}$ star, $R_{1.4}$, by $\sim3.5\, \text{km}$. If observations indicate $R_{1.4}1/2$ for densities above $2\,n_{\rm sat}$, or that $��$EFT breaks down below $2\,n_{\rm sat}$. We also comment on the nature of the secondary compact object in GW190814 with mass $\simeq 2.6\,{\rm M}_{\odot}$, and discuss the implications of massive neutron stars $>2.1 \,{\rm M}_{\odot}\,(2.6\,{\rm M}_{\odot})$ in future radio and gravitational-wave searches. Some form of strongly interacting matter with $c^2_{s}>0.35\, (0.55)$ must be realized in the cores of such massive neutron stars. In the absence of phase transitions below $2\,n_{\rm sat}$, the small tidal deformability inferred from GW170817 lends support for the relatively small pressure predicted by $��$EFT for the baryon density $n_{\rm B}$ in the range $1-2\,n_{\rm sat}$. Together they imply that the rapid stiffening required to support a high maximum mass should occur only when $n_{\rm B} \gtrsim 1.5-1.8\,n_{\rm sat}$., 26 pages, 17 figures; accepted for publication in PRC, appendices and references updated

We present a thermodynamically consistent method by which equations of state based on nonrelativistic potential models can be modified so that they respect causality at high densities, both at zero and finite temperature (entropy). We illustrate the application of the method using the high density phase parametrization of the well known APR model in its pure neutron matter configuration as an example. We also show that, for models with only contact interactions, the adiabatic speed of sound is independent of the temperature in the limit of very large temperature. This feature is approximately valid for models with finite-range interactions as well, insofar as the temperature dependence they introduce to the Landau effective mass is weak. In addition, our study reveals that in first principle nonrelativistic models of hot and dense matter, contributions from higher than two-body interactions must be screened at high density to preserve causality., 12 pages, 12 figures

Baryon and quark superfluidity in the cooling of neutron stars are investigated. Observations could constrain combinations of the neutron or Lambda-hyperon pairing gaps and the star's mass. However, in a hybrid star with a mixed phase of hadrons and quarks, quark gaps larger than a few tenths of an MeV render quark matter virtually invisible for cooling. If the quark gap is smaller, quark superfluidity could be important, but its effects will be nearly impossible to distinguish from those of other baryonic constituents., Comment: 4 pages, 3 ps figures, uses RevTex(aps,prl). Submitted to Phys. Rev. Lett

Shear viscosity \eta and entropy density s of a hadronic resonance gas are calculated using the Chapman-Enskog and virial expansion methods using the K-matrix parametrization of hadronic cross sections which preserves the unitarity of the T -matrix. In the \pi-K-N-\eta- mixture considered, a total of 57 resonances up to 2 GeV were included. Comparisons are also made to results with other hadronic cross sections such as the Breit-Wigner (BW) and, where available, experimental phase shift parametrizations. Hadronic interactions forming resonances are shown to decrease the shear viscosity and increase the entropy density leading to a substantial reduction of \eta/s as the QCD phase transition temperature is approached., Comment: 16 pages and 14 figures

We study various non-equilibrium properties of a mixture comprised of pions, kaons and nucleons. The concentration of the constituents is chosen to mimic those expected in the mid-rapidity interval of CERN and RHIC experiments. The heavier kaons and nucleons are found to equilibrate more slowly than pions. These results shed new light on the influence of collective flow effects on the transverse momentum distributions of kaons and nucleons versus those of pions in ultrarelativistic nuclear collisions.

Nuclear matter at high density requires a relativistic description. Recent two-loop calculations in local quantum field theory models at finite density yield disastrously large contributions. We argue that the composite structure of nucleons ought to soften these contributions. This argument is supported by a model calculation in which we find very strong reductions in the Lamb shift and vacuum fluctuation contributions. At nuclear matter densities and beyond, the total-loop result is perturbatively small in comparison to the one-loop calculation.

We study a simple quantum-mechanical model that illustrates various conceptual questions associated with color transparency.

The QCD sum-rule approach for a nuclear medium is developed. The medium dependence of the neutron-proton mass difference calculated from this approach gives effects in nuclei which have direct relevance for the resolution of the Okamoto-Nolen-Schiffer anomaly.

We show that the direct URCA process can occur in neutron stars if the proton concentration exceeds some critical value in the range 11--15%. The proton concentration, which is determined by the poorly known symmetry energy of matter above nuclear density, exceeds the critical value in many current calculations. If it occurs, the direct URCA process enhances neutrino emission and neutron star cooling rates by a large factor compared to any process considered previously.

Models that are successful in hadronic spectroscopy, viz., the nonrelativistic quark model, bag models, and QCD sum rules, are used to shed light on the Okamoto-Nolen-Schiffer anomaly in nuclear physics.

We demonstrate that the largest measured mass of a neutron star establishes an upper bound to the energy density of observable cold baryonic matter. An equation of state-independent expression satisfied by both normal neutron stars and self-bound quark matter stars is derived for the largest energy density of matter inside stars as a function of their masses. The largest observed mass sets the lowest upper limit to the density. Implications from existing and future neutron star mass measurements are discussed.

The photon emissivity from the bremsstrahlung process ee-> ee��occuring in the electrosphere at the bare surface of a strange quark star is calculated. For surface temperatures T, 17 pages, 6 figures

The differential rates and emissivities of neutrino pairs from an equilibrium plasma are calculated for the wide range of density and temperature encountered in astrophysical systems. New analytical expressions are derived for the differential emissivities which yield total emissivities in full agreement with those previously calculated. The photon and plasmon pair production and absorption kernels in the source term of the Boltzmann equation for neutrino transport are provided. The appropriate Legendre coefficients of these kernels, in forms suitable for multi-group flux-limited diffusion schemes are also computed., 27 pages and 10 figures. Submitted to Phys. Rev. C

Invariant mass distributions of the hadronic decay products from resonances formed in relativistic heavy-ion collision (RHIC) experiments are investigated with a view to disentangle the effects of thermal motion and the phase space of decay products from those of intrinsic changes in the structure of resonances at the freeze-out conditions. Analytic results of peak mass shifts for the cases of both equal and unequal mass decay products are derived. The shift is expressed in terms of the peak mass and width of the vacuum or medium-modified spectral functions and temperature. Examples of expected shifts in meson (e.g., $\ensuremath{\rho},$ $\ensuremath{\omega},$ and $\ensuremath{\sigma})$ and baryon (e.g., $\ensuremath{\Delta})$ resonances that are helpful to interpret recent RHIC measurements at BNL are provided. Although significant downward mass shifts are caused by widened widths of the $\ensuremath{\rho}$ meson in medium, a downward shift of at least 50 MeV in its intrinsic mass is required to account for the reported downward shift of 60--70 MeV in the peak of the $\ensuremath{\rho}$ invariant mass distribution. An observed downward shift from the vacuum peak value of the $\ensuremath{\Delta}$ distinctively signals a significant downward shift in its intrinsic peak mass, since unlike for the $\ensuremath{\rho}$ meson, phase space functions produce an upward shift for the $\ensuremath{\Delta}$ isobar.

We address the role of fluctuations in strongly interacting matter during the dense stages of a heavy-ion collision through its electromagnetic emission. Fluctuations of isospin charge are considered in a thermal system at rest as well as in a moving hadronic fluid at fixed proper time within a finite bin of pseudo-rapidity. In the former case, we use general thermodynamic relations to establish a connection between fluctuations and the space-like screening limit of the retarded photon self-energy, which directly relates to the emissivities of dileptons and photons. Effects of hadronic interactions are highlighted through two illustrative calculations. In the latter case, we show that a finite time scale $\tau$ inherent in the evolution of a heavy-ion collision implies that equilibrium fluctuations involve both space-like and time-like components of the photon self-energy in the system. Our study of non-thermal effects, explored here through a stochastic treatment, shows that an early and large fluctuation in isospin survives only if it is accompanied by a large temperature fluctuation at freeze-out, an unlikely scenario in hadronic phases with large heat capacity. We point out prospects for the future which include: (1) A determination of the Debye mass of the system at the dilute freeze-out stage of a heavy-ion collision, and (2) A delineation of the role of charge fluctuations during the dense stages of the collision through a study of electromagnetic emissivities., Comment: 12 pages ReVTeX incl. 4 ps-figs

Photon production cross sections in intermediate energy heavy-ion reactions are calculated using detailed balance. The model gives substantial photon yields and reproduces qualitatively the experimental observations. A possible influence of the ..delta..-isobar degree of freedom on the high energy photon yield is pointed out.

Prompt emission of nucleons is observed in time-dependent Hartree-Fock calculations of $^{16}\mathrm{O}$+$^{93}\mathrm{Nb}$ collisions. These nucleons are emitted in the forward direction in about 2\ifmmode\times\else\texttimes\fi{}10$^{\ensuremath{-}22}\mathrm{s}$ after the contact of the ions, with a mean velocity somewhat less than the beam velocity. At $E(\mathrm{lab})=204$ MeV, the estimated total nucleon cross section is \ensuremath{\sim}230 mb. For central collisions a lower energy threshold of 130 MeV is established. Force dependence of the results is studied in detail.

The nucleon exchange process in deep-inelastic collisions is investigated by studying the one-way current constructed using the Wigner transform of the time-dependent Hartree-Fock one-body density matrix. Detailed calculations for head-on $^{16}\mathrm{O}$+$^{16}\mathrm{O}$ collisions at three different bombarding energies have been performed. A combination of two simple models for the contributions from relative motion and tunneling is shown to predict one-way currents close to the time-dependent Hartree-Fock results.

The relationship between the maximum neutron-star mass and observable parameters of the equation of state is explored. In particular, the roles of the nuclear incompressibility and the symmetry energy are considered. It is concluded that, for realistic symmetry energies, the compression modulus cannot, by itself, be severely limited by observed neutron-star masses. Several directions for further study are suggested.

The asymmetry dependence of the incompressibility at fixed density, the density at fixed pressure, and the isobaric incompressibility are studied for neutron-rich nuclear matter at arbitrary densities. Simple expressions valid to all orders in density and to second order in asymmetry are derived involving the density derivatives of the Landau parameters of the symmetric system. Illustrative calculations are performed with different equations of state.

Nuclear matter overlap integrals between two nuclei are calculated for density distributions generated by folding a short range function over arbitrarily shaped sharp surfaces of the two nuclei. Explicit formulas are derived for two choices of the folding function, viz., a Gaussian and a Yukawa function. An approximation scheme is given to facilitate a quick evaluation of the overlap integrals and is found to give results which are in good agreement with the exact results over a wide range of masses and spheroidal deformations of the two nuclei.

A unified approach is used to describe quasielastic and fusion processes in heavy-ion reactions at near barrier energies. The spin distributions for fusion are used in conjunction with transition-state theory to calculate fission fragment angular distributions. Results for the reaction /sup 16/O+/sup 208/Pb show that when transfer contributions are small, satisfactory agreement with data is obtained. The role of fragment anisotropy as a useful probe of entrance channel effects at energies close to the barrier is pointed out.

It is shown that no definite conclusions can be drawn regarding the magnitude of energy dissipation in fission during the descent from the saddle point, by a comparison of the calculated deformation and interaction energies of fission fragments near scission, with the experimental fragment excitation and kinetic energies. This is contrary to the conclusions drawn by Schultheis and Schultheis on the basis of a similar analysis. The reasons for this discrepancy are also discussed.