arXiv subjects

Mauro Mariani

Publications and source records attributed to Mauro Mariani.

At least 19 recordsLinked to original sources

arXiv2026

Systematic study of the morphology and length of slow stable hybrid star branches

We introduce and systematically study the length of the slow stable hybrid star branch as a quantitative measure of the extended stability region that arises in hybrid neutron stars when the hadron-quark phase conversion is slow compared to the radial oscillation timescale. Combining generalized piecewise-polytropic hadronic equations of state of varying stiffness with a constant-speed-of-sound quark-matter model, we construct a large set of hybrid equations of state spanning a broad range of transition pressures, energy-density jumps, and quark-matter speeds of sound. We identify four morphological types for the slow stable branch in the mass-radius plane: waterfall branches that descend monotonically from the hadronic maximum mass, bridges that connect the hadronic branch to a second unconditionally stable hybrid branch, tails that extend briefly beyond the maximum mass of an unconditionally stable hybrid branch, and tail-bridges that combine features of the latter two. Their prevalence is governed primarily by the transition pressure and the energy-density jump, while the branch length is also significantly influenced by the stiffness of the hadronic sector and the quark-matter speed of sound. Imposing current astrophysical and microphysical constraints shows that viable long branches are predominantly of waterfall type, and that stiff hadronic equations of state -- strongly disfavored under the rapid-conversion assumption -- remain compatible with all current constraints within the slow-conversion framework. In the plane of transition baryon density versus density jump, slow stable configurations open a new region of viable parameter space inaccessible under rapid conversions.

astro-ph.HE↗

arXiv2026

How Neutron Star Observations Point Towards Exotic Matter: Existing Explanations and a Prospective Proposal

Multi-messenger astronomical observations of neutron stars, together with more precise calculations and constraints coming from dense matter microphysics, are generating tension with regard to equations of state models used to describe neutron star cores. Assuming an abrupt first-order phase transition with a slow conversion speed between phases, we propose different slow stable hybrid star configurations aiming to reconcile all current constraints simultaneously; within this framework, we also introduce a novel non-CSS parametrization to the quark matter equation of state and discuss its strengths and limitations. We analyze our model results in conjunction with a review of other relevant theoretical possibilities existing in the literature. We found that modern neutron star observations seem to favor the existence of some type of exotic matter in the neutron star cores; in particular, our slow stable hybrid star scenario remains a proposal capable of satisfying these constraints. However, due both to the existing skepticism regarding some of the adopted hypotheses in most extreme neutron star measurements and to the precise adjustment needed for the equation-of-state parameters, significant tension and open questions remain.

astro-ph.HE↗

arXiv2025

Towards a unified hadron-quark equation of state for neutron stars within the relativistic mean-field model

The equation of state of dense matter remains a central challenge in astrophysics and high-energy physics, particularly at supra-nuclear densities where exotic degrees of freedom like hyperons or deconfined quarks are expected to appear. Neutron stars provide a unique natural laboratory to probe this regime. In this work, we present EVA--01, a novel equation of state that provides a unified description of dense matter by incorporating both hadron and quark degrees of freedom within a single relativistic mean-field Lagrangian, from which the equation of state is derived at finite temperature. The model extends the density-dependent formalism by introducing a Polyakov-loop-inspired scalar field to dynamically govern the hadron-quark phase transition, following the approach of chiral mean-field models. The resulting model is consistent with a wide range of theoretical and observational constraints, including those from chiral effective field theory, massive pulsars, gravitational-wave events, and NICER data. We analyze its thermodynamic properties by constructing the QCD phase diagram, identifying the deconfinement, chiral, and nuclear liquid-gas transitions. As a first application, we model the evolution of proto-neutron stars using isentropic snapshots and explore the implications of the slow stable hybrid star hypothesis. Our findings establish EVA--01 as a robust and versatile framework for exploring dense matter, bridging the gap between microphysical models and multimessenger astrophysical observations.

astro-ph.HE↗

arXiv2025

The Gamma Expansion of the Level Two Large Deviation Rate Functional for Reversible Diffusion Processes

Fix a smooth Morse function $U\colon \mathbb{R}^{d}\to\mathbb{R}$ with finitely many critical points, and consider the solution of the stochastic differential equation \[ d\boldsymbol{x}_ε(t)=-\nabla U(\boldsymbol{x}_ε(t))\,dt \,+\,\sqrt{2ε}\, d\boldsymbol{w}_{t}\,, \] where $(\boldsymbol{w}_{t})_{t\ge0}$ represents a $d$-dimensional Brownian motion, and $ε>0$ a small parameter. Denote by $\mathcal{P}(\mathbb{R}^{d})$ the space of probability measures on $\mathbb{R}^d$, and by $\mathcal{I}_ε \colon \mathcal{P}(\mathbb{R}^{d})\to[0,\,\infty]$ the Donsker--Varadhan level two large deviations rate functional. We express $\mathcal{I}_ε$ as $\mathcal{I}_ε= ε^{-1} \mathcal{J}^{(-1)} + \mathcal{J}^{(0)} + \sum_{1\le p\le \mathfrak{q}} (1/θ^{(p)}_ε) \, \mathcal{J}^{(p)}$, where $\mathcal{J}^{(p)}\colon \mathcal{P}(\mathbb{R}^d) \to [0,+\infty]$ stand for rate functionals independent of $ε$ and $θ^{(p)}_ε$ for sequences such that $θ^{(1)}_ε\to\infty$, $θ^{(p)}_ε/ θ^{(p+1)}_ε\to 0$ for $1\le p< \mathfrak{q}$. The speeds $θ^{(p)}_ε$ correspond to the time-scales at which the diffusion $\boldsymbol{x}_ε(\cdot)$ exhibits a metastable behaviour, while the functional $\mathcal{J}^{(p)}$ represent the level two, large deviations rate functionals of the finite-state, continuous-time Markov chains which describe the evolution of the diffusion $\boldsymbol{x}_ε(\cdot)$ among the wells in the time-scale $θ^{(p)}_ε$.

math.PR↗

arXiv2025

Exploring the role of $d^*$ hexaquarks on quark deconfinement and hybrid stars

We investigate the impact of the $d^*$(2380) hexaquark on the equation of state (EoS) of dense matter within hybrid stars (HSs) using the Chiral Mean-Field model (CMF). The hexaquark is included as a new degree of freedom in the hadronic phase, and its influence on the deconfinement transition to quark matter is explored. We re-parametrize the CMF model to ensure compatibility with recent astrophysical constraints, including the observation of massive pulsars and gravitational wave events. Our results show that the presence of $d^*$ significantly modifies the EoS, leading to a softening at high densities and a consequent reduction in the predicted maximum stellar masses. Furthermore, we examine the possibility of a first-order deconfinement phase transition within the context of the extended stability branch of slow stable HSs (SSHSs). We find that the presence of hexaquarks can delay the deconfinement phase transition and reduce the associated energy density gap, affecting the structure and stability of HSs. Our results suggest that, as the hexaquark appearance tends to destabilize stellar configurations, fine tuning of model parameters is required to obtain both the presence of hexaquarks and quark deconfinement in these systems. In this scenario, the SSHS branch plays a crucial role in obtaining HSs with hexaquarks that satisfy current astrophysical constraints. Our work provides new insights into the role of exotic particles like $d^*$ in dense matter and the complex interplay between hadronic and quark degrees of freedom inside compact stellar objects.

nucl-th↗

arXiv2025

Consistent crust-core interpolation and its effect on non-radial neutron star oscillations

To model the structure of neutron stars (NSs) theoretically,it is common to consider layers with different density regimes. Matching the equation of state (EoS) for the crust and core and obtaining a suitable description of these extreme conditions are crucial for understanding the properties of these compact objects. In this work, we construct ten different NS EoSs incorporating three distinct crust models, which are connected to the core using a thermodynamically and causally consistent formalism. For cold NSs, we propose a linear relationship between pressure and energy density in a narrow region between the crust and core, effectively establishing an interpolation function in the pressure-baryonic chemical potential plane. We then compare this EoS matching method with the classical approach, which neglects causal and thermodynamic consistency. We solve the Tolman-Oppenheimer-Volkoff equation to obtain the mass-radius relationship and compare our results with observational constraints on NSs. Furthermore, we investigate the influence of the new matching formalism on non-radial oscillation frequencies and damping times. Our findings suggest that the method used to glue the crust and core EoS impacts NS observables, such as the radius, oscillation frequencies, and damping times of non-radial modes, which may be crucial for interpreting future gravitational wave observations from neutron star mergers or isolated pulsars. The effects are particularly noticeable for low-mass NSs, regardless of the specific EoS model chosen. In particular, we find that the $p_1$ oscillation mode exhibits significant differences in frequencies among alternative matching methods, whereas the fundamental $f$-mode remains unaffected by changes in crust models or interpolation schemes.

astro-ph.HE↗

arXiv2024

Could a slow stable hybrid star explain the central compact object in HESS~J1731-347?

We explore an alternative explanation for the low-mass ultra-compact star in the supernova remnant HESS~J1731-347 using a model-agnostic approach to construct hybrid equations of state. The hadronic part of the hybrid equation of state is constructed using a generalized piecewise polytropic scheme, while the quark phase is described by the generic constant speed of sound model. We assume an abrupt first-order hadron-quark phase transition with a slow conversion speed between phases. Our equations of state align with modern Chiral Effective Field Theory calculations near nuclear saturation density and are consistent with perturbative Quantum Chromodynamics calculations at high densities. Using this theoretical framework, we derive a wide range of hybrid equations of state capable of explaining the light compact object in HESS~J1731-347 in a model-independent manner, without fine-tuning. These equations of state are also consistent with modern astronomical constraints from high-mass pulsar timing, NICER observations, and multimessenger astronomy involving gravitational waves. Our results support the hypothesis that the compact object in HESS~J1731-347 could plausibly be a slow stable hybrid star.

astro-ph.HE↗

arXiv2024

Asymptotic Properties of Random Homology Induced by Diffusion Processes

We investigate the asymptotic behavior, in the long time limit, of the random homology associated to realizations of stochastic diffusion processes on a compact Riemannian manifold. In particular a rigidity result is established: if the rate is quadratic, then the manifold is a locally trivial fiber bundle over a flat torus, with fibers being minimal in a weighted sense (that is, regarding the manifold as a metric measured space, with the invariant probability being the weight measure). Surprisingly, this entails that at least for some classes of manifolds, the homology of non-reversible processes relaxes to equilibrium slower than its reversible counterpart (as opposed to the respective empirical measure, which relaxes faster).

math.PR↗

arXiv2024

Metastability between the clicks of Muller's ratchet

We prove the existence and uniqueness of a quasi-stationary distribution for three stochastic processes derived from the model of Muller's ratchet. This model was invented with the aim of evaluating the limitations of an asexual reproduction mode in preventing the accumulation of deleterious mutations through natural selection alone. The main considered model is non-classical, as it is a stochastic diffusion evolving on an irregular set of infinite dimension with hard killing on an hyperplane. We are nonetheless able to prove exponential convergence in total variation to the quasi-stationary distribution even in this case. The parameters in this last convergence result are directly related to the core parameters of Muller's ratchet. The speed of convergence to the quasi-stationary distribution is deduced both for the infinite dimensional model and for approximations with a large yet finite number of potential mutations. Likewise, we give uniform moment estimates of the empirical distribution of mutations in the population under quasi-stationarity.

math.PR↗

arXiv2024

Quark-hadron pasta phase in neutron stars: the role of medium-dependent surface and curvature tensions

We investigate the properties of the hadron-quark mixed phase, often termed the \textit{pasta} phase, expected to exist in the cores of massive neutron stars. To construct the equations of state (EoS), we combine an analytical representation based on the APR EoS for hadronic matter with the MIT bag model featuring vector interactions for quark matter. For modeling the mixed phase, we utilize the compressible liquid drop model that consistently accounts for finite-size and Coulomb effects. Unlike most previous analyses that treated surface tension as a constant free parameter and neglected curvature tension, we employ microphysical calculations using the multiple reflection expansion formalism to determine these parameters, while also ensuring their self-consistency with the EoS. We construct an extensive set of mixed hybrid EoSs by varying model parameters, solve the stellar structure equations to obtain neutron star mass-radius relationships, and select the models that satisfy current astrophysical constraints. Our findings closely align with calculations using a constant surface tension in terms of EoS stiffness and resulting stellar structure. However, they reveal significant differences in the types of geometric structures and their prevalence ranges within the mixed phase. Specifically, curvature effects enhance the emergence of tubes and bubbles at high densities despite the large value of surface tension, while suppressing the existence of drops and rods at low densities.

nucl-th↗

arXiv2024

Wasserstein Asymptotics for Brownian Motion on the Flat Torus and Brownian Interlacements

We study the large time behavior of the optimal transportation cost towards the uniform distribution, for the occupation measure of a stationary Brownian motion on the flat torus in $d$ dimensions, where the cost of transporting a unit of mass is given by a power of the flat distance. We establish a global upper bound, in terms of the limit for the analogue problem concerning the occupation measure of the Brownian interlacement on $\R^d$. We conjecture that our bound is sharp and that our techniques may allow for similar studies on a larger variety of problems, e.g. general diffusion processes on weighted Riemannian manifolds.

math.PR↗

arXiv2023

On the Destabilization of High-Mass Neutron Stars by the Emergence of $d^*$-Hexaquarks

We study the effects of the first nontrivial hexaquark, $d^*$(2380), on the equation of state of dense neutron star matter and investigate the consequences of its existence for neutron stars. The matter in the core regions of neutron stars is described using density-dependent relativistic mean-field theory. Our results show that within the parameter spaces examined in our paper, (i) the critical density at which the $d^*$ condensate emerges lies between 4 and 5 times the nuclear saturation density, (ii) $d^*$ hexaquarks are found to exist only in rather massive neutron stars, (iii) only relatively small fractions of the matter in the core of a massive neutron star may contain hexaquarks.

nucl-th↗

arXiv2023

Constraining self-interacting fermionic dark matter in admixed neutron stars using multimessenger astronomy

We investigate the structure of admixed neutron stars with a regular hadronic component and a fraction of fermionic self-interacting dark matter. Using two limiting equations of state for the dense baryonic interior, constructed from piecewise generalised polytropes, and an asymmetric self-interacting fermionic dark component, we analyse different scenarios of admixed neutron stars depending on the mass of dark fermions $m_χ$, interaction mediators $m_ϕ$, and self-interacting strengths $g$. We find that the contribution of dark matter to the masses and radii of neutron stars leads to tension with mass estimates of the pulsar J0453+1559, the least massive neutron star, and with the constraints coming from the GW170817 event. We discuss the possibilities of constraining dark matter model parameters $g$ and $y \equiv m_χ/m_ϕ$, using current existing knowledge on neutron star estimations of mass, radius, and tidal deformability, along with the accepted cosmological dark matter freeze-out values and self-interaction cross-section to mass ratio, $σ_\mathrm{SI}/m_χ$, fitted to explain Bullet, Abell, and dwarf galaxy cluster dynamics. By assuming the most restrictive upper limit, $σ_\mathrm{SI}/m_χ< 0.1$ cm$^2$/g, along with dark matter freeze-out range values, the allowed $g$-$y$ region is $0.01 \lesssim g \lesssim 0.1$, with $0.5 \lesssim y \lesssim 200$. For the first time, the combination of updated complementary restrictions is used to set constraints on self-interacting dark matter.

astro-ph.HE↗

arXiv2023

Asteroseismology using quadrupolar f-modes revisited: breaking of universal relationships in the slow hadron-quark conversion scenario

In this work, we consider polar perturbations and we calculate the frequency and damping time of the quadrupolar fundamental f -mode of compact objects, constructed using a wide range of model-independent hybrid equations of state that include quark matter. We give special attention to the impact of the hadron-quark conversion speed that, in the slow case, gives rise to a branch of slow stable hybrid stars. Moreover, we study the validity of universal relationships proposed in the literature and find out that none of them remains valid when slow stable hybrid stars are taken into account. This fact could constrain the applicability of asteroseismology methods with fundamental modes designed to estimate the properties of pulsating compact objects. We hope that this result could be tested with the start up of the third-generation gravitational wave observatories, which might shed some light on the f -mode emission from compact objects.

astro-ph.HE↗

arXiv2023

A model-agnostic analysis of hybrid stars with reactive interfaces

We study hybrid stars considering the effects on stellar stability of the hadron-quark conversion speed at the sharp interface. The equation of state is constructed by combining a model-agnostic hadronic description with a constant speed of sound model for quark matter. We show that current LIGO/Virgo, NICER, low-density nuclear and high-density perturbative QCD constraints can be satisfied in two scenarios, with low and high transition pressures. If the conversion speed at the interface is slow, a new class of dynamically stable hybrid objects is possible and very stiff hadronic equations of state cannot be discarded. Densities tens of times larger than the nuclear saturation density are possible at the center of these objects. We discuss possible formation mechanisms for the new class of hybrid stars and smoking guns for their observational identification.

nucl-th↗

arXiv2022

Constraining mass, radius and tidal deformability of compact stars with axial wI modes: new universal relations including slow stable hybrid stars

We revisit asteroseismology with quadrupolar wI modes and present universal relationships for its fundamental and first overtone. In contrast to relationships proposed in the literature, our universal relationships are capable of including slow stable hybrid stars that appear when considering slow sharp hadron-quark phase transitions. We show that, if the frequency and damping time of the fundamental mode of a given pulsating object are measured, its mass, radius, and dimensionless tidal deformability can be inferred. Moreover, we show that the errors of such estimates are smaller than a few percent for the mass and radius. For the dimensionless tidal deformability, the errors are -- for compact objects with $M\gtrsim 1.4\,M_\odot$ -- in general smaller than ~100%. Comparison with previous universal relationships shows that the ones proposed in this work produce better estimates of the mass and radius of totally stable compact objects.

astro-ph.HE↗

arXiv2022

Breaking of universal relationships of axial wI-modes in hybrid stars: rapid and slow hadron-quark conversion scenarios

Multi-messenger astronomy with gravitational waves is a blooming area whose limits are not clear. After the first detection of binary black hole merger and the famous event GW170817 and its electromagnetic counterpart, the compact-object astrophysical community is starting to grasp the physical implications of such event and trying to improve numerical models to compare with future observations. Moreover, recent detections made by the NICER collaboration increased the tension between several theoretical models used to describe matter in the inner core of compact objects. In this paper, we focus on quadrupolar purely spacetime wI-modes of oscillating compact objects described using a wide range of hybrid equations of state able to include several theoretical possibilities of exotic matter in the inner core of such stars. We study the case in which a sharp first order hadron-quark phase transition occurs and explore the scenarios of rapid and slow phase conversions at the interface. We put special attention on the validity of universal relationships for the oscillation frequency and damping time that might help unravel the mysteries hidden at the inner cores of compact objects. We show that, within the slow conversion regime where extended branches of hybrid stars appear, universal relationships for wI-modes proposed in the literature do not hold.

astro-ph.HE↗

arXiv2022

Oscillating Magnetized Color Superconducting Quark Stars

The main objective of this work is to study the structure, composition, and oscillation modes of color superconducting quark stars with intense magnetic fields. We adopted the MIT bag model within the color superconductivity CFL framework, and we included the effects of strong magnetic fields to construct the equation of state of stable quark matter. We calculated observable quantities, such as the mass, radius, frequency, and damping time of the oscillation fundamental $f$ mode of quark stars, taking into account current astrophysical constraints. The results obtained show that color superconducting magnetized quark stars satisfy the constraints imposed by the observations of massive pulsars and gravitational wave events. Furthermore, the quantities associated with the oscillation $f$ mode of these objects fit the universal relationships for compact objects. In the context of the new multi-messenger gravitational wave astronomy era and the future asteroseismology of neutron stars, we hope that our results contribute to the understanding of the behavior of dense matter and compact objects.

astro-ph.HE↗