arXiv subjects

Giorgio Torrieri

Publications and source records attributed to Giorgio Torrieri.

At least 19 recordsLinked to original sources

arXiv2026

Gaussian non relativistic spontaneously stochastic hydrodynamics

We study the non-relativistic limit of Gaussian covariant hydrodynamics [1]. We argue that the condition of incompressibility provides additional symmetries matching relativistic hydrodynamics but incompressibility must break down at a ``microscopic`` scale. We then develop the renormalization group equations for average and fluctuations w.r.t. that scale, to understand its effect on flows at intermolecular distances where hydrodynamics gives way to statistical mechanics. The resulting dynamics naturally incorporates spontaneous stochasticity as a macroscopic back reaction of statistical mechanics fluctuations, as well as features reminiscent of anomalous dissipation and ``wild solutions`` as renormalization group counterterms. We frame these considerations into both a phenomenological discussion of the limits of applicability of fluid dynamics, and a discussion of where physics might shed some light on the mathematical issues associated with turbulence.

physics.flu-dyn↗

arXiv2026

When Weak Fields Arent Weak: Post-Newtonian effective theory and the Dark Matter Puzzle

Post-Newtonian theory is considered a reliable effective expansion of General Relativity in the weak-field and slow-motion limit. We argue that such a belief is misplaced. In generic many-body relativistic dynamics, the absence of globally conserved charges in the region of interest and non-integrability can drive strong sensitivity to angular-momentum exchange across inhomogeneous curvature, invalidating naive power counting in an effective theory expansion. Building on general lessons from effective field theory, we derive an explicit breakdown criterion that delineates when post-Newtonian truncations become unreliable despite small local potentials and velocities. This supplies a controlled systematic for weak-field mass inference, relevant to the dark matter puzzle in astrophysics and cosmology.

gr-qc↗

arXiv2026

Hydrodynamic Initial Conditions in Small Systems from Proton Phase-Space Entropy

The experimental observation of collective behaviour in proton-proton and proton-nucleus collisions poses a fundamental theoretical question regarding the proper characterization of the initial state underlying hydrodynamic evolution. While relativistic hydrodynamics requires an initial condition (IC) characterized by an entropy current, corresponding to a maximally mixed state, the microscopic description of the proton is based on inherently quantum objects, that are projections of pure states. We show that the appropriate matching between proton wave function and classical hydrodynamics emerges from the coarse-graining of its phase-space distribution quantified by the Wehrl-like entropy. This entropy provides a semi-classical, positive-definite measure of the density of accessible microstates at a given resolution scale, and therefore constitutes the appropriate quantity to characterize entropy deposition in small collision systems.

hep-ph↗

arXiv2026

Gaussian pseudogauge invariant hydrodynamics with spin

Extending the Gaussian covariant hydrodynamics approach [1] using torsion as an auxiliary field we formulate a fluctuating hydrodynamics with spin which is covariant with respect to pseudo-gauge transformations as well as generally covariant with respect to foliations. This is done via the second order gravitational Ward identities, derived here in the torsionful case. This ensures that, while angular momentum observables depend covariantly on the pseudo-gauge, the dynamics is pseudo-gauge independent, thus clarifying the role of the pseudo-gauge in hydrodynamics with spin

hep-th↗

arXiv2026

Hall Viscosity in the Quark-Gluon Plasma

We study the Hall viscosity of the quark gluon plasma (QGP) created in non-central heavy-ion collisions. In the presence of a strong magnetic field or vorticity, rotational symmetry is broken from O(3) to O(2), allowing for two independent Hall viscosities associated with shear deformations transverse and parallel to the symmetry-breaking direction. We find the corresponding constitutive relations by extending the kinetic-theory mechanism to three spatial dimensions and provide parametric estimates of the Hall viscosities under realistic QGP conditions. Both kinetic-theory and holographic estimates indicate that Hall viscosities are comparable in magnitude to the shear viscosity at zero magnetic field. We further show that Hall viscous stresses at hydrodynamic initialization can be as large as standard viscous corrections and identify observable consequences in flow and event-plane correlations.

nucl-th↗

arXiv2026

The shape of transverse momentum spectra in hybrid hydrodynamic models

We study the scaled transverse momentum spectra over a wide parameter space of state-of-the-art hydrodynamic simulation models in order to learn what information can be obtained from the shape of identified-particle spectra -- previously observed to be surprisingly universal across centrality and collision systems in both experimental data and hydrodynamic simulations. We study its sensitivity to each of 17 model parameters in the context of 4 different models for particlization when switching from the hydro description to the kinetic theory afterburner. We find that the strongest sensitivity is to parameters relating to bulk viscosity, free-streaming time, and the $\texttt{T$_\mathrm{R}$ENTo}$ nucleon width parameter $w$. However, we find that the model generally has surprisingly little flexibility in describing the scaled spectrum observable, despite the large number of parameters. Within this small range of parameter dependence, we further find significant tension in a simultaneous description of momentum-integrated observables. In particular, while the mean transverse momentum prefers a large value of the nucleon width parameter $w$, a small value is required to obtain scaled spectra that are consistent with experimental measurements. We speculate on the origin of these model tensions and possible missing physics in the commonly-used $\texttt{T$_\mathrm{R}$ENTo}$+free streaming+hydro+afterburner simulation model.

nucl-th↗

arXiv2026

$f_2(1270)\to\pi+\pi$ as a probe of spin and vorticity in heavy-ion collisions

The correlation between vorticity and spin alignment in heavy-ion collisions can be probed through polarization measurements of hadrons, whose total spin originates from both constituent-quark spins and orbital angular momentum in the quark-model framework. To motivate such experimental studies, we calculate the general angular distribution of produced pion in $f_2(1270)\to\pi+\pi$ using interaction Lagrangian and helicity formalism and check that both methods yield the same result. The distribution is given as a function of angle between pion and initial quantization axis of $f_2$ and the spin density matrix element of $f_2$. Its diagonal entries and $\rho_{20}$ component were computed assuming local thermal equilibrium and blast wave model for different centrality classes, hence given as a function of azimuthal angle with respect to the impact parameter.

nucl-th↗

arXiv2026

Gaussian fluctuating Generally covariant diffusion

We extend the previously developed [1] generally covariant formalism to include diffusion of conserved charges, and comment on the seming difference between the chemica potential term and the diffusion term

hep-th↗

arXiv2025

QCD Wehrl and entanglement entropies in a gluon spectator model at small-$x$

Recent studies have shown that hadronic multiplicity in deep inelastic scattering can be associated with entanglement entropy. However, such definitions are intrinsically longitudinal and do not capture the full phase-space structure of the proton. In this work, we investigate the proton Wehrl entropy constructed from the gluon Husimi distribution, which provides a positive phase-space description within the present definitions and model calculations. Within this framework, we employ a gluon light-front spectator model based on soft-wall AdS/QCD-inspired wave functions, with free parameters constrained by global NNPDF fits, allowing us to compute both parton distribution functions and Wigner distributions. The Husimi distribution is obtained via Gaussian smearing of the Wigner distribution with width given by the saturation scale in the GBW model. We show that from a normalized Husimi distribution one can decompose the Wehrl entropy into an entanglement entropy term and a residual term associated with transverse degrees of freedom. Numerical results for the proton entanglement entropy are shown and compared with CMS data, while the Wehrl entropy is presented for different values of the virtuality.

hep-ph↗

arXiv2025

Hydrodynamical Initial State in Small Systems From the Phase-Space Entropy

The experimental observation of collective behavior in proton-proton and proton-nucleus collisions poses a fundamental theoretical question regarding the proper characterization of the initial state underlying hydrodynamic evolution. While relativistic hydrodynamics requires an initial condition characterized by an entropy current, corresponding to a maximally mixed state, the microscopic description of the proton is based on inherently quantum objects that probes projections of pure states. We show that the appropriate matching between hadronic structure and classical hydrodynamics emerges from the coarse-graining of the partonic Wigner distribution, leading naturally to a Husimi distribution and its associated Wehrl entropy. This entropy provides a semiclassical, positive-definite measure of the density of accessible microstates at a given resolution scale, and therefore constitutes the appropriate quantity to characterize entropy deposition in small collision systems.

hep-ph↗

arXiv2025

Event-by-event vortex rings in fixed-target p+Ar collisions

We present event-by-event simulations for central asymmetric p+Ar collisions at $\sqrt{s_\mathrm{NN}} = 68$ GeV to investigate the formation and evolution of vortex-ring structures from the early-stage longitudinal flow velocity profile. Our predictions for their imprints on Lambda hyperon's polarization observables are complementary to those presented in Ref. [Phys.Rev.C 110 (2024) 5, 054908] and can be explored in the future fixed-target collisions at the Large Hadron Collider beauty (LHCb) experiment.

nucl-th↗

arXiv2025

Probing the onset of collectivity via scaled particle spectra in ultrarelativistic nuclear collisions

We identify a novel scaling in the transverse momentum spectra of produced particles, obtained by removing the global scales of multiplicity and mean transverse momentum. Hydrodynamic simulations and experimental data reveal an almost universal scaled spectrum across centralities, systems, and even small systems, pointing to its origin in the collective, fluid-like dynamics of the QGP. Comparing this observable with Bayesian a priori distributions shows its independent constraining power on QCD transport properties, while also exposing limitations of current models. A detailed posterior analysis will be pursued in future work, opening a new avenue to refine our understanding of collectivity in heavy-ion collisions.

nucl-th↗

arXiv2025

Causality of polarizeable dissipative fluids from Lagrangian hydrodynamics

We perform a causality analysis on the dispersion relation of hydrodynamics with spin well as shear and bulk viscosity, including the relaxation times for all these quantities. We find that the interplay of the three relaxational scales, for shear and bulk viscosity as well as polarization, leads to non-trivial effects on the dispersion relation. Unexpectedly, the presence of polarization leads to lower effective viscosity and a longer relaxation time, and the presence of viscosity leads to lower limits as well as upper ones on the group velocity and constraints relating polarization to viscosity relaxation times. We conclude with a qualitative discussion on how these results impact phenomenology, specifically the low effective viscosity in strongly interacting matter as well as shear-vorticity coupling.

nucl-th↗

arXiv2025

The quantum Newton's bucket: Active and passive rotations in quantum theory

Motivated both by classical physics problems associated with ``Newton's bucket'' and recent developments related to QCD in rotating frames of reference relevant to heavy ion collisions, we discuss the difference between ``active'' and ``passive'' rotations in quantum systems. We examine some relevant potentials and give general symmetry arguments to give criteria where such rotations give the same results. We close with a discussion of how this can be translated to quantum field theory.

quant-ph↗

arXiv2025

Chiral oscillations in finite time quantum field theory

We demonstrate how chiral oscillations of a massive Dirac field can be described within quantum field theory using a finite-time interaction picture approach, where the mass term in the Lagrangian is treated as a perturbative coupling between massless fields of definite chirality. We derive the formula for chiral oscillations at the fourth order in the perturbative expansion, obtaining a result consistent with the formula derived by means of other methods. Furthermore, we illustrate how the perturbative framework of chiral oscillations can effectively describe production processes where an electron must exhibit both left chirality and positive helicity, as in decay $\pi^- \to e^- + {\bar \nu}_e$. Finally, we argue that, in this perturbative view, chiral oscillations are also essential for detecting the decay products in such processes.

hep-ph↗

arXiv2024

The need for a nonlocal expansion in general relativity

Motivated by known facts about effective field theory and non-Abelian gauge theory, we argue that the post-Newtonian approximation might fail even in the limit of weak fields and small velocities for wide-extended rotating bodies, where angular momentum spans significant spacetime curvature. We construct a novel dimensionless quantity that samples this breakdown, and we evaluate it by means of existing analytical solutions of rotating extended bodies and observational data. We give estimates for galaxies and binary systems, as well as our home in the Cosmos, Laniakea. We thus propose that a novel effective field theory of general relativity might be needed to account for the onset of nonlocal angular momentum effects.

gr-qc↗

arXiv2024

Meson spin alignment and baryon polarization from coalescence with spin-vorticity non-equilibrium

We estimate both the polarization of spin $1/2$ baryons and the spin density matrix coefficients of spin $1$ mesons using a thermal model that incorporates vorticity and polarization to describe quarks, along with a coalescence formalism where spin and vorticity are not in equilibrium. We find that while our model is not predictive due to its considerable number of free parameters, it has the potential to explain several seemingly puzzling features of experimental spin measurements, such as the absence of baryon polarization alongside significant vector meson spin alignment. We suggest the measuring meson spin off-diagonal matrix elements, and examining the dependence of polarization observables on azimuthal angles, as methods to falsify this model and gain insights into the freezeout details of baryon and meson spin structure.

hep-ph↗

arXiv2024

The Hanbury-Brown and Twiss effect in inflationary cosmological perturbations

The simplest model of inflation is based around an inflaton field that starts in a coherent false vacuum state with a positive cosmological constant, rolls slowly to the true vacuum and relaxes to it via reheating. We examine whether the scale of the transition from coherence to chaoticity can be examined via the Hanbury-Brown and Twiss (HBT) effect, in parallel with analogous problems of heavy ion physics (the ``pion laser'' and the thermalizing glasma). We develop an ansatz which contains a definition of ''chaoticity'' which parallels that of the usual setups where HBT is used. However, we also discuss the differences between the inflationary setup and more mainstream uses of HBT and conclude that these are more significant than the similarities, making the use of the developed methodology uncertain.

gr-qc↗