arXiv subjects

Rico Zöllner

Publications and source records attributed to Rico Zöllner.

15 recordsLinked to original sources

arXiv2026

Two-Fluid Schwarzschild Solution

We consider the interior two-fluid Schwarzschild solution. That is a model of compact (neutron) stars with admixed dark matter. The two constant energy densities generalize the interior Schwarschild solution to the case of two fluids (representing standard model matter and dark matter, for instance), interacting mutually solely by the common gravitational field. In general, the two-fluid core is surrounded by a one-fluid corona (envelope). Only for special parameters (the model depends on energy densities $e_1,e_2$ and central pressures $p_{\mathrm{c}1},p_{\mathrm{c}2}$ of the fluids), both fluids occupy the same region. Despite the sum of two energy densities in that region, the compactness bound of 8/9 is not exceeded. A conceivable object is one with a stark leakage of one fluid (e.g. dark matter) beyond the two-fluid core.

gr-qc↗

arXiv2025

A New Perspective on Double-S Curve Motions of Higher Order and Optimal Motion Planning

This paper presents and proves an equation for the time horizon of symmetric trajectories with zero boundary conditions and bounded derivatives of arbitrary order. This equation holds regardless of the number of phases comprising the associated motion. This avoids case distinctions in calculations. Application examples of motions with minimum time, minimum velocity, and minimum acceleration are discussed. Furthermore, an algorithm is derived that reduces the time minimization problem to solving a system of equations. This algorithm avoids nested case distinctions and complex optimizations.

math.OC↗

arXiv2025

On the Number of Posets

This paper presents combinatorial facts dealing with the number of unlabeled partially ordered sets (posets) refined by the number of arcs in the Hasse diagram (sequence A342447 in OEIS). The main result is that the differences with respect to the number of points in this sequence become stationary if the number of points is sufficiently high. These differences are proposed as the new sequence A376894. In addition, the underlying combinatorial and graph theoretical arguments were used to extend some further OEIS sequences.

math.CO↗

arXiv2025

Core-Corona Decomposition of Very Compact (Neutron) Stars: Accounting for Current Data of XTE J1814-338

A core-corona decomposition of compact (neutron) star models was compared with the current mass-radius data of the outlier XTE~J1814-338. The corona (which may also be dubbed the envelope, halo or outer crust) is assumed to be of Standard Model matter, with an equation of state that is supposed to be faithfully known and accommodates nearly all other neutron star data. The core, solely parameterized by its mass, radius and transition pressure, presents a challenge regarding its composition. We derived a range of core parameters needed to describe the current data of XTE J1814-338.

astro-ph.HE↗

arXiv2024

Core-corona decomposition of compact (neutron) stars compared to NICER data including XTE J1814-338

A core-corona decomposition of compact (neutron) star models is compared to recent NICER data of masses and radii. It is in particular interesting to capture the outlier XTE~J1814-338. Instead of integrating the TOV equations from the center to surface, we follow here another pathway by accommodating all uncertainties of the equation(s) of state (EoS) at supra-nuclear density or/and an unknown dark matter admixture in a parameterization of the core by its radius $r_x$, the included mass $m_x$ and the pressure $p_x$ at $r_x$. The corona, which may be dubbed also envelope or halo or outer crust, is assumed to be of standard-model matter where the EoS is supposed to be faithfully known.

astro-ph.HE↗

arXiv2024

Towards a warm holographic equation of state by an Einstein-Maxwell-dilaton model

The holographic Einstein-Maxwell-dilaton model is employed to map state-of-the-art lattice QCD thermodynamics data from the temperature ($T$) axis towards the baryon-chemical potential ($μ_B$) axis aimed at gaining a warm equation of state (EoS) of deconfined QCD matter which can be supplemented with a cool and confined part suitable for subsequent compact (neutron) star (merger) investigations. The model exhibits a critical end point (CEP) at $T_\mathrm{CEP} = \mathcal{O}(100)$ MeV and $μ_{B \, \mathrm{CEP}} = 500 \ldots 700$ MeV with emerging first-order phase transition (FOPT) curve which extends to large values of $μ_B$ without approaching the $μ_B$ axis. We consider the impact and peculiarities of the related phase structure on the EoS for the employed dilaton potential and dynamical coupling parameterizations. These seem to prevent to design an overall trustable EoS without recourse to hybrid constructions.

hep-th↗

arXiv2022

Exotic Cores with and without Dark-Matter Admixtures in Compact Stars

We parameterize the core of compact spherical star configurations by a mass ($m_x$) and a radius ($r_x$) and study the resulting admissible areas in the total-mass vs. total-radius plane. The employed fiducial equation-of-state models of the corona at radii $r \ge r_x$ and pressures $p \le p_x = p(r = r_x)$ are that (i) of constant sound velocity and (ii) a proxy of DY$Δ$ DD-ME2 provided by Buchdahl's exactly solvable ansatz. The core ($r < r_x$) may contain any type of material, e.g. Standard-Model matter with unspecified equation of state or/and an unspecified Dark-Matter admixture. Employing a toy model for the cool equation of state with first-order phase transition we discuss also the mass-radius relation of compact stars with an admixture of Dark Matter in a Mirror-World scenario.

gr-qc↗

arXiv2021

Holographic bottomonium formation in a cooling strong-interaction medium at finite baryon density

The shrinking of the bottomonium spectral function towards narrow quasi-particle states in a cooling strong-interaction medium at finite baryon density is followed within a holographic bottom-up model. The 5-dimensional Einstein-dilaton-Maxwell background is adjusted to lattice-QCD results of sound velocity and susceptibilities. The zero-temperature bottomonium spectral function is adjusted to experimental $Υ$ ground-state mass and first radial excitations. At baryo-chemical potential $μ_B = 0$, these two pillars let emerge the narrow quasi-particle state of the $Υ$ ground state at a temperature of about 150 MeV. Excited states are consecutively formed at lower temperatures by about 10 (20) MeV for the $2S$ ($3S$) vector states. The baryon density, i.e. $μ_B > 0$, pulls that formation pattern to lower temperatures. At $μ_B =$ 200 MeV, we find a shift by about 15 MeV.

hep-th↗

arXiv2020

Holographic vector meson melting in a thermal gravity-dilaton background related to QCD

A holographic model of probe vector mesons (quarkonia) is presented, where the dynamical gravity-dilaton background is adjusted to the thermodynamics of 2 +1 flavor QCD with physical quark masses. The vector meson action is modified to account for various quark masses. We focus on the $Φ$, $J/ψ$ and $Υ$ meson melting in agreement with hadron phenomenology in heavy-ion collisions at LHC, that is the formation of hadrons at the observed freeze-out temperature of 155 MeV.

hep-ph↗

arXiv2020

Phase structures emerging from holography with Einstein gravity -- dilaton models at finite temperature

Asymptotic AdS Riemann space-times in five dimensions with a black brane (horizon) sourced by a fully back-reacted scalar field (dilaton) offer -- via the holographic dictionary -- various options for the thermodynamics of the flat four-dimensional boundary theory, uncovering Hawking-Page, first-order and second-order phase transitions up to a cross-over or featureless behavior. The relation of these phase structures to the dilaton potential is clarified and illustrating examples are presented. Having in mind applications to QCD we study probe vector mesons with the goal to figure out conditions for forming Regge type series of radial excitations and address the issue of meson melting.

hep-th↗

arXiv2017

Holographic vector mesons in a dilaton background

Within a holographic framework, we consider vector mesons riding on a gravity-dilaton background. The latter one is determined directly from a Schrödinger equivalent potential which delivers a proper $ρ$ meson Regge trajectory. The mapping on the dilaton potential yields a thermodynamic phase structure with a first-order transition.

hep-th↗

arXiv2017

Holography at QCD-$T_{\rm c}$

Within an extended soft wall model, we study the temperature dependence of the lowest vector meson states. Scales are adjusted by using as input the $ρ$ meson mass in vacuum and the velocity of sound from lattice QCD which displays a minimum at $T_{\rm c}$. The melting of the $ρ$ meson occurs at temperatures $\mathcal{O}(T_{\rm c})$.

hep-ph↗

arXiv2017

Extended soft wall model with background related to features of QCD thermodynamics

The soft wall model is extended to accommodate at the same time (i) approximately linear $ρ$ meson Regge trajectories at zero temperature $T$, (ii) various options for the thermodynamics with reference to QCD (cross over or second-order transition or first-order transition at $T_c$), and (iii) the appearance of vector meson states at $T \leq T_c$. While the vector meson masses display some modest model dependence very near to $T_c$, they stay below $T_c$ to good accuracy independent of the temperature, that is nearly as at $T=0$, thus being very consistent with the thermo-statistical models widely employed in analyses of the hadron yields in relativistic heavy-ion collisions in a region where baryon densitiy effects can be neglected and the vacuum hadron masses are used.

hep-ph↗

arXiv2016

Extended soft-wall model for the QCD phase diagram

The soft-wall model, emerging as bottom-up holographic scenario anchored in the AdS/CFT correspondence, displays the disappearance of normalisable modes referring to vector mesons at a temperature $T_{\dis}$ depending on the chemical potential $μ$, $T_{\dis}(μ)$. We explore options for making $T_{\dis}(μ)$ consistent with the freeze-out curve $T_{\rm f.o.}(μ)$ from relativistic heavy-ion collisions and the cross-over curve $T_{\rm c}(μ)$ from QCD at small values of $μ$.

hep-th↗

arXiv2016

Holographically emulating sequential versus instantaneous disappearance of vector mesons in a hot environment

Descent extensions of the soft-wall model are used to accommodate two variants of Regge trajectories of vector meson excitations. At non-zero temperatures, various options for either sequential or instantaneous disappearance of vector mesons as normalisable modes are found, thus emulating deconfinement at a certain temperature in the order of the (pseudo-) critical temperature of QCD. The crucial role of the blackness function, which steers the thermodynamic properties of the considered system, is highlighted.

hep-ph↗