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Giorgio Torrieri

Publications and source records attributed to Giorgio Torrieri.

At least 73 records · Page 4Linked to original sources
arXiv2015

The Unruh effect and oscillating neutrinos

We point out that neutrino oscillations imply an ambiguity in the definition of the vacuum and the coupling to gravity, with experimentally observable consequences due to the Unruh effect. In an accelerating frame, the detector should see a bath of mass Eigenstates neutrinos. In inertial processes, neutrinos are produced and absorbed as charge Eigenstates. The two cannot be reconciled by a spacetime coordinate transformation. This makes manifestations of the Unruh effect in neutrino physics a promising probe of both neutrinos and fundamental quantum field theory. In this respect, we suggest $p\rightarrow n +\ell^+ + {ν_\ell}$ ($\ell^+ = e^+, μ^+, τ^+$) transitions in strong electromagnetic fields as a promising avenue of investigation. In this essay we discuss this process both in the inertial and comoving frame, we briefly describe the experimental realization and its challenges, and close by speculating on possible results of such an experiment in different scenarios of fundamental neutrino physics

hep-ph↗
arXiv2015

Longitudinal hydrodynamics from event-by-event Landau initial conditions

We investigate three-dimensional ideal hydrodynamic evolution, with Landau initial conditions, incorporating event-by-event variation with many events and transverse density inhomogeneities. We show that the transition to boost-invariant flow occurs too late for realistic setups, with corrections of $\order{20-30\%}$ expected at freezeout for most scenarios. Moreover, the deviation from boost-invariance is correlated with both transverse flow and elliptic flow, with the more highly transversely flowing regions also showing the most violation of boost invariance. Therefore, if longitudinal flow is not fully developed at the early stages of heavy ion collisions, hydrodynamics where boost-invariance holds at mid-rapidity is inadequate to extract transport coefficients of the quark-gluon plasma. We conclude by arguing that developing experimental probes of boost invariance is necessary, and suggest some promising directions in this regard.

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arXiv2014

Analytical Solutions of Landau (1+1)-Dimensional Hydrodynamics

To help guide our intuition, summarize important features, and point out essential elements, we review the analytical solutions of Landau (1+1)-dimensional hydrodynamics and exhibit the full evolution of the dynamics from the very beginning to subsequent times. Special emphasis is placed on the matching and the interplay between the Khalatnikov solution and the Riemann simple wave solution at the earliest times and in the edge regions at later times. These analytical solutions collected and developed here serve well as a useful guide and cross-check in the development of complicated numerically-intensive relativistic hydrodynamical Monte Carlo simulations.

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arXiv2014

Flow harmonics within an analytically solvable viscous hydrodynamic model

Based on a viscous hydrodynamic model with anisotropically perturbed Gubser flow and isothermal Cooper-Frye freezeout at early times, we analytically compute the flow harmonics $v_n(p_T)$ and study how they scale with the harmonic number $n$ and transverse momentum, as well as the system size, shear and bulk viscosity coefficients, and collision energy. In particular, we find that the magnitude of shear viscous corrections grows linearly with $n$. The mixing between different harmonics is also discussed. While this model is rather simple as compared to realistic heavy-ion collisions, we argue that the scaling results presented here may be meaningfully compared to experimental data collected over many energies, system sizes, and geometries.

hep-ph↗
arXiv2014

On the origin of long-range azimuthal correlations in hadronic collisions

We review the models suggested, to date, as an explanation for the so called "ridge" phenomenon, an elongation in rapidity of 2-particle correlations seen at RHIC and LHC energies. We argue that these models can be divided into two phenomenologically distinct classes: "Hotspot+flow" driven correlations, where initial state correlations created by structures local in configuration space are collimated by transverse flow, and models where the azimuthal correlation is created through local partonic interactions in a high gluon density initial state. We further argue that the key to distinguish these two scenarios is to experimentally examine the role flow plays in the ridge. The measurement of a strong double ridge in $pA$ and $dA$ collisions allows a good opportunity to understand the ridge's origin. In this sense, particle-identified correlations are particularly promising way of testing the assumptions at the base of either of the two models above. The successful fit of the pA ridge by hydrodynamic models \cite{us}, therefore, suggests that the ridge is primarily a "final state effect" generated by radial expansion, although its scaling across system sizes is non-trivial to reconcile with hydrodynamic expectations. We close by discussing experimental observables capable of clarifying the situation.

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arXiv2014

Dynamics and Stability of Chiral Fluid

Starting from the linear sigma model with constituent quarks we derive the chiral fluid dynamics where hydrodynamic equations for the quark fluid are coupled to the equation of motion for the order-parameter field. In a static system at thermal equilibrium this model leads to a chiral phase transition which, depending on the choice of the quark-meson coupling constant, could be a crossover or a first order one. We investigate the stability of the chiral fluid in the static and expanding backgrounds by considering the evolution of perturbations with respect to the mean-field solution. In the static background the spectrum of plane-wave perturbations consists of two branches, one corresponding to the sound waves and another to the sigma-meson excitations. For large couplings these two branches "cross" and the excitation spectrum acquires exponentially growing modes. The stability analysis is also done for the Bjorken-like background solution by explicitly solving the time-dependent differential equation for perturbations in the eta-space. In this case the growth rate of unstable modes is significantly reduced.

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arXiv2013

Quantum ideal hydrodynamics on the lattice

After discussing the problem of defining the hydrodynamic limit from microscopic scales, we give an introduction to ideal hydrodynamics in the Lagrange picture, and show that it can be viewed as a field theory, which can be quantized using the usual Feynman sum-over-paths prescription. We then argue that this picture can be connected to the usually neglected thermal microscopic scale in the hydrodynamic expansion. After showing that this expansion is generally non-perturbative, we show how the lattice can be used to understand the impact quantum and thermal fluctuations can have on the fluid behavior.

hep-lat↗
arXiv2013

Mass hierarchy in identified particle distributions in proton-lead collisions

We study the mass dependence for identified-particle average transverse momentum and harmonic flow coefficients in proton-lead (p-Pb) collisions, recently measured at the LHC. The collective mechanism in the p-Pb system predicts a specific mass ordering in these observables: the growth of the average transverse momentum with the particle mass and a mass splitting of the elliptic flow coefficient, i.e., smaller differential elliptic flow of protons than pions for p_T<2 GeV. This provides an opportunity to distinguish between the collective scenario and the mechanism based on the initial gluon dynamics in the evolution of the p-Pb system.

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arXiv2013

Hydrodynamic models of particle production

Viscous hydrodynamics gives a satisfactory description of the transverse momentum spectra, of the elliptic and triangular flow, and of the femtoscopic correlations for particles produced in relativistic heavy-ion collisions. On general grounds, a similar collective behavior has been predicted for proton-lead (p-Pb) collisions at the LHC. We present results of the hydrodynamics calculation of the elliptic and triangular flow in p-Pb. We discuss the mass dependence of flow coefficients and of the average transverse momentum for identified particles.

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arXiv2013

Multi-particle correlations, many particle systems, and entropy in effective field theories

We discuss the treatment, in an effective field theory, of multi-particle correlations within a "large" system. We show that the act of coarse-graining necessarily introduces violations of unitarity in the evolution of states where the particle number is not defined. For an interacting system, such unitarity violations can cascade from the ultraviolet scale to the infrared in a "short" time. Hence, an effective field theory will be grossly inadequate for describing multi-particle correlations and related observables, even far away from the cut-off scale $Λ$. We furthermore argue that if the system is strongly coupled at $Λ$, than its final state {\em in the Effective Field Theory} (EFT) will appear as the highest entropy state if only low cumulants and correlations of the EFT degrees of freedom are measured. Heuristically, this can serve as an explanation of how "entropy" is created in a microscopically unitary evolution of a Quantum Field Theory (QFT). We conclude by discussing how these considerations might provide a clue to the apparent thermalization in a hadronic collision even in comparatively small systems, as well as the so-called black hole information paradox; We argue the "paradoxes" are likely to be artifacts of using an effective theory beyond its domain of validity.

hep-th↗
arXiv2013

Quarkyonic Percolation and deconfinement at finite density and number of colors

We examine the interplay between the percolation and the deconfinement phase transitions of Yang-Mills matter at finite temperature, quark chemical potential $μ_Q$ and number of colors $N_c$. We find that, whereas the critical $N_c$ for percolation goes down with density, the critical $N_c$ for confinement generally goes up. Because of this, Yang-Mills matter falls into two qualitatively different regimes: the "low-$N_c$ limit", where percolation does not occur because matter deconfines before it percolates, and the high-$N_c$ limit, where there are three distinct phases characterizing Yang-Mills matter at finite temperature and density: confined, deconfined and confined but percolating matter. The latter can be thought of as the recently conjectured "quarkyonic phase". We attempt to estimate the critical $N_c$, to see if the percolating phase can occur in our world. We find that, while percolation will not occur at normal nuclear density as in the large-$N_c$ limit, a sliver of the phase diagram in $N_c$, energy density and baryonic density where percolation occurs while confinement persists is possible. We conclude by speculating on the phenomenological properties of such percolating "quarkyonic" matter, suggest avenues to study it quantitatively, and look for it in experiment.

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arXiv2013

Photon Signals from Quarkyonic Matter

We calculate the Bremsstrahlung photon spectrum emitted from dynamically evolving quarkyonic matter, and compare this spectrum with that of a high chemical potential quark-gluon plasma as well as to a hadron gas. We find that the transverse momentum distribution and the harmonic coefficient is markedly different in the three cases. The transverse momentum distribution of quarkyonic matter can be fit with an exponential, but is markedly steeper than the distribution expected for the quark-gluon plasma or a hadron gas, even at the lower temperatures expected in the critical point region. The quarkyonic elliptic flow coefficient fluctuates randomly from event to event, and within the same event at different transverse momenta. The latter effect, which can be explained by the shape of quark wavefunctions within quarkyonic matter, might be considered as a quarkyonic matter signature, provided initial temperature is low enough that the quarkyonic regime dominates over deconfinement effects, and the reaction-plane flow can be separated from the fluctuating component.

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arXiv2012

Phenomenology of quarkyonic percolation at FAIR

We will give an introduction to the concept of quarkyonic matter, presenting an overview of what is meant by this term in the literature. We will then argue that the quarkyonic phase, as defined in the original paper, is a percolation-type phase transition whose phase transition line is strongly curved in $ρ_B-N_c$ space, where $N_c$ is the number of colors and $ρ_B$ the baryon density. With a toy model estimate, we show that it might be possible to obtain a percolating but confined phase at $N_c=3,N_f=2$ at densities larger than one baryon per one baryon size. We conclude by discussing how this phase can be observed at FAIR.

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arXiv2012

Elliptic flow at varying energy heavy ion collisions: Partonic vs hadronic dynamics

We examine whether the breakdown in elliptic flow quark number scaling observed at the Relativistic Heavy Ion Collider (RHIC) energy scan is related to the turning off of deconfinement by testing the hypothesis that hydrodynamics and parton coalescence always apply, but are obscured, at lower energies, by variations in the widths of quark and anti-quark rapidity distribution. We find that this effect is enough to spoil quark number scaling in elliptic flow. A lack of scaling in data therefore does not signal the absence of partonic degrees of freedom and hadronization by coalescence. In a coalescing partonic fluid, however, elliptic flow of anti-baryons should be the greater than that of baryons, since antibaryons contain a greater admixture of partons from the highly flowing midrapidity region. Intriguingly, purely hadronic dynamics has a similar dependence of baryon-anti-baryons $v_2$ as purely partonic dynamics, again because antibaryons tend to come from regions where the deviation of the system from hydrodynamic behavior is at its smallest. The opposite trend observed in experiment is therefore an indication that we might be misunderstanding the origin of $v_2$. We finish by discussing possible explanations of this, and suggest experimental measurements capable of clarifying the situation.

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arXiv2012

Scaling of elliptic flow in heavy ion collisions

The common interpretation of $v_2$ in heavy ion collisions is that it is produced by hydrodynamic flow at low transverse momentum and by parton energy loss at high transverse momentum. In this talk we discuss this interpretation in view of the dependence of $v_2$ with energy, rapidity and system size, and show that it might not be trivial to reconcile these models with the relatively simple scaling found in experiment

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arXiv2012

Quarkyonic percolation in dense nuclear matter

We examine the phase diagram of hadronic matter when the number of colours $N_c$, as well as temperature and density, are varied. We show that in this regime a new percolation phase transition is possible, and examine the implications of this transition for extrapolations to physical QCD of the large-N_c limit.

hep-ph↗
arXiv2012

Statistical hadronization phenomenology in $K/π$ fluctuations at ultra-relativistic energies

We discuss the information that can be obtained from an analysis of fluctuations in heavy ion collisions within the context of the statistical model of particle production. We then examine the recently published experimental data on ratio fluctuations, and use it to obtain constraints on the statistical properties (physically relevant ensemble, degree of chemical equilibration, scaling across energies and system sizes) and freeze-out dynamics (amount of reinteraction between chemical and thermal freeze-out) of the system.

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