Nuclear Theory·nucl-th

Thursday·December 17, 2015

12 papers—4 primary·8 cross-listed

05
Oscillations of superfluid hyperon stars: decoupling scheme and g-modes
V.A. Dommes · M.E. Gusakov (Ioffe Institute)
We analyse the oscillations of general relativistic superfluid hyperon stars, following the approach suggested by Gusakov & Kantor and Gusakov et al. and generalizing it to the nucleon-hyperon matter. We show that the equations governing the oscillations can be split into two weakly coupled systems with the coupling parameters $s_{e}$ , $s_{μ}$ , and $s_{str}$ . The approximation $s_{e} = s_{μ} = s_{str} = 0$ (decoupling approximation) allows one to drastically simplify the calculations of stellar oscillation spectra. An efficiency of the presented scheme is illustrated by the calculation of sound speeds in the nucleon-hyperon matter composed of neutrons (n), protons (p), electrons (e), muons ( $μ$ ), as well as $Λ$ , $Ξ^{-}$ , and $Ξ^{0}$ -hyperons. However, the gravity oscillation modes (g-modes) cannot be treated within this approach, and we discuss them separately. For the first time we study the composition g-modes in superfluid hyperon stars with the $npe μ Λ$ core and show that there are two types of g-modes (`muonic' and ` $Λ$ --hyperonic') in such stars. We also calculate the g-mode spectrum and find out that the eigenfrequencies $ν$ of the superfluid g-modes can be exceptionally large (up to $ν \approx 742 Hz$ for a considered stellar model).
↳ astro-ph.SRastro-ph.HEgr-qcnucl-thMNRAS(2016)·26 citations
06
Pinning down QCD-matter shear viscosity in A+A collisions via EbyE fluctuations using pQCD + saturation + hydrodynamics
H. Niemi🇩🇪 · K. J. Eskola🇫🇮 · R. Paatelainen🇪🇸 · K. Tuominen🇫🇮
We compute the initial energy densities produced in ultrarelativistic heavy-ion collisions from NLO perturbative QCD using a saturation conjecture to control soft particle production, and describe the subsequent space-time evolution of the system with hydrodynamics, event by event. The resulting centrality dependence of the low- $p_{T}$ observables from this pQCD + saturation + hydro ("EKRT") framework are then compared simultaneously to the LHC and RHIC measurements. With such an analysis we can test the initial state calculation, and constrain the temperature dependence of the shear viscosity-to-entropy ratio $η / s$ of QCD matter. Using these constraints from the current RHIC and LHC measurements we then predict the charged hadron multiplicities and flow coefficients for the 5.023 TeV Pb+Pb collisions.
↳ hep-phnucl-thNPA(2016)·7 citations
07
Introductory lectures on lattice QCD at nonzero baryon number
Gert Aarts (Swansea University)🇬🇧
These lecture notes contain an elementary introduction to lattice QCD at nonzero chemical potential. Topics discussed include chemical potential in the continuum and on the lattice; the sign, overlap and Silver Blaze problems; the phase boundary at small chemical potential; imaginary chemical potential; and complex Langevin dynamics. An incomplete overview of other approaches is presented as well. These lectures are meant for postgraduate students and postdocs with an interest in extreme QCD. A basic knowledge of lattice QCD is assumed but not essential. Some exercises are included at the end.
↳ hep-lathep-phnucl-thJ.Phys.Conf.Ser.(2016)·233 citations
08
Heavy-flavour production in high-energy d-Au and p-Pb collisions
Andrea Beraudo🇮🇹 · Arturo De Pace🇮🇹 · Marco Monteno🇮🇹 · Marzia Nardi🇮🇹 · Francesco Prino🇮🇹
Soft-hadron measurements in high-energy collisions of small systems like p-Pb and d-Au show peculiar qualitative features (long-range rapidity correlations, flattening of the $p_{T}$ -spectra with increasing hadron mass and centrality, non-vanishing Fourier harmonics in the azimuthal particle distributions) suggestive of the formation of a strongly-interacting medium displaying a collective behaviour, with a hydrodynamic flow as a response to the pressure gradients in the initial conditions. Hard observables (high- $p_{T}$ jet and hadron spectra) on the other hand, within the current large systematic uncertainties, appear only midly modified with the respect to the benchmark case of minimum-bias p-p collisions. What should one expect for heavy-flavour particles, initially produced in hard processes but tending, in the nucleus-nucleus case, to approach kinetic equilibrium with the rest of the medium? This is the issue we address in the present study, showing how the current experimental findings are compatible with a picture in which the formation of a hot medium even in proton-nucleus collisions modifies the propagation and hadronization of heavy-flavour particles.
↳ hep-phnucl-exnucl-thJHEP(2016)·76 citations
09
Equation of state for cold and dense heavy QCD
Jonas Glesaaen🇩🇪 · Mathias Neuman🇩🇪 · Owe Philipsen🇩🇪
A previously derived three-dimensional effective lattice theory describing the thermodynamics of QCD with heavy quarks in the cold and dense region is extended through order $\sim u^{5} κ^{8}$ in the combined character and hopping expansion of the original four-dimensional Wilson action. The systematics of the effective theory is investigated to determine its range of validity in parameter space. We demonstrate the severe cut-off effects due to lattice saturation, which afflict any lattice results at finite baryon density independent of the sign problem or the quality of effective theories, and which have to be removed by continuum extrapolation. We then show how the effective theory can be solved analytically by means of a linked cluster expansion, which is completely unaffected by the sign problem, in quantitative agreement with numerical simulations. As an application, we compute the cold nuclear equation of state of heavy QCD. Our continuum extrapolated result is consistent with a polytropic equation of state for non-relativistic fermions.
↳ hep-lathep-phnucl-thJHEP(2016)·32 citations
10
Lifetime of the eta' meson at low temperature
E. Perotti (Uppsala U.)🇸🇪 · C. Niblaeus (Stockholm U.)🇸🇪 · S. Leupold (Uppsala U.)🇸🇪
This work constitutes one part of an investigation of the low-temperature changes of the properties of the eta' meson. In turn these properties are strongly tied to the U(1)_A anomaly of Quantum Chromodynamics. The final aim is to explore the interplay of the chiral anomaly and in-medium effects. We determine the lifetime of an eta' meson being at rest in a strongly interacting medium as a function of the temperature. To have a formally well-defined low-energy limit we use in a first step Chiral Perturbation Theory for a large number of colors. We determine the pertinent scattering amplitudes in leading and next-to-leading order. In a second step we include resonances that appear in the same mass range as the eta' meson. The resonances are introduced such that the low-energy limit remains unchanged and that they saturate the corresponding low-energy constants. This requirement fixes all coupling constants. We find that the width of the eta' meson is significantly increased from about 200 keV in vacuum to about 10 MeV at a temperature of 120 MeV.
↳ hep-phnucl-thNPA(2016)·0 citations
11
DBI scalar field theory for QGP hydrodynamics
Horatiu Nastase🇧🇷
A way to describe the hydrodynamics of the quark-gluon plasma using a DBI action is proposed, based on the model found by Heisenberg for high energy scattering of nucleons. The expanding plasma is described as a shockwave in a DBI model for a real scalar standing in for the pion, and I show that one obtains a fluid description in terms of a relativistic fluid that near the shock is approximately ideal ( $η ≃ 0$ ) and conformal. One can introduce an extra term inside the square root of the DBI action that generates a shear viscosity term in the energy-momentum tensor near the shock, as well as a bulk viscosity, and regulates the behaviour of the energy density at the shock, making it finite. The resulting fluid satisfies the relativistic Navier-Stokes equation with $u^{μ}, ρ, P, η$ defined in terms of $ϕ$ and its derivatives. One finds a relation between the parameters of the theory and the QGP thermodynamics, $\a / \b^{2} = η / (s T)$ , and by fixing $\a$ and $\b$ from usual (low multiplicity) particle scattering, one finds $T \propto m_{π}$ .
↳ hep-thhep-phnucl-thPRD(2016)·7 citations
12
Low-Energy QCD in the Delta Regime
Matthew E. Matzelle🇺🇸 · Brian C. Tiburzi🇺🇸
We investigate properties of low-energy QCD in a finite spatial volume, but with arbitrary temperature. In the limit of small temperature and small cube size compared to the pion Compton wavelength, Leutwyler has shown that the effective theory describing low-energy QCD reduces to that of quantum mechanics on the coset manifold, which is the so-called delta regime of chiral perturbation theory. We solve this quantum mechanics analytically for the case of a $U (1)_{L} \times U (1)_{R}$ subgroup of chiral symmetry, and numerically for the case of $S U (2)_{L} \times S U (2)_{R}$ . We utilize the quantum mechanical spectrum to compute the mass gap and chiral condensate, and investigate symmetry restoration in a finite spatial volume as a function of temperature. Because we obtain the spectrum for non-zero values of the quark mass, we are able to interpolate between the rigid rotor limit, which emerges at vanishing quark mass, and the harmonic approximation, which is referred to as the p-regime. We find that the applicability of perturbation theory about the rotor limit largely requires lighter-than-physical quarks. As a stringent check of our results, we raise the temperature to that of the inverse cube size. When this condition is met, the quantum mechanics reduces to a matrix model. The condensate we obtain in this limit agrees with that determined analytically in the epsilon regime.
↳ hep-lathep-phnucl-thPRD(2016)·10 citations

Affiliations

first authorsco-authorsvia INSPIRE