arXiv subjects

A. Capella

Publications and source records attributed to A. Capella.

At least 19 recordsLinked to original sources

arXiv2024

Nonlinear Stability of Static Néel Walls in Ferromagnetic Thin Films

In this paper, the nonlinear (orbital) stability of static 180^\circ Néel walls in ferromagnetic films, under the reduced wave-type dynamics for the in-plane magnetization proposed by Capella, Melcher and Otto [CMO07], is established. It is proved that the spectrum of the linearized operator around the static Néel wall lies in the stable complex half plane with non-positive real part. This information is used to show that small perturbations of the static Néel wall converge to a translated orbit belonging to the manifold generated by the static wall.

math.AP↗

arXiv2023

A Physics Based Surrogate Model in Bayesian Uncertainty Quantification involving Elliptic PDEs

The paper addresses Bayesian inferences in inverse problems with uncertainty quantification involving a computationally expensive forward map associated with solving a partial differential equations. To mitigate the computational cost, the paper proposes a new surrogate model informed by the physics of the problem, specifically when the forward map involves solving a linear elliptic partial differential equation. The study establishes the consistency of the posterior distribution for this surrogate model and demonstrates its effectiveness through numerical examples with synthetic data. The results indicate a substantial improvement in computational speed, reducing the processing time from several months with the exact forward map to a few minutes, while maintaining negligible loss of accuracy in the posterior distribution.

stat.ME↗

arXiv2016

Enhanced resolution in Argon and Neon spectra using a Super-Resolution algorithm

This paper presents the principles and application of a super-resolution (SR) technique, based on a L1-Norm minimization procedure. In particular, the technique is applied to improve low-resolution resolution spectra as obtained from the optogalvanic effect in Neon and Argon discharges over the 413-423 nm wavelength range. By applying the super-resolution algorithm to the experimental data, a surprising 70-fold reduction of the linewidth is achieved allowing to resolve prior indistinguishable peaks. It is found that further improvements on the resolution are limited by the signal to noise ratio of the original spectra. The importance of a suitable mathematical representation of the experiment and the discussion on other applications of this super-resolution technique in spectroscopy are also presented.

physics.plasm-ph↗

arXiv2013

A Hydrodynamical Mechanism for Generating Astrophysical Jets

Whenever in a classical accretion disk the thin disk approximation fails interior to a certain radius, a transition from Keplerian to radial infalling trajectories should occur. We show that this transition is actually expected to occur interior to a certain critical radius, provided surface density profiles are steeper than $Σ(R) \propto R^{-1/2}$, and further, that it probably corresponds to the observationally inferred phenomena of thick hot walls internally limiting the extent of many stellar accretion disks. Infalling trajectories will lead to the convergent focusing and concentration of matter towards the very central regions, most of which will simply be swallowed by the central object. We show through a perturbative hydrodynamical analysis, that this will naturally develop a well collimated pair of polar jets. A first analytic treatment of the problem described is given, proving the feasibility of purely hydrodynamical mechanisms of astrophysical jet generation. The purely hydrodynamic jet generation mechanism explored here complements existing ideas focusing on the role of magnetic fields, which probably account for the large-scale collimation and stability of jets.

astro-ph.HE↗

arXiv2013

Proton-proton multiplicity distributions at LHC and the Pomeron intercept

We compute the proton-proton multiplicity distributions at LHC energies in the framework of a multiple scattering model assuming a Poisson distribution for each inelastic collision. Multiple scattering is essential to broaden the multiplicity distribution. We obtain approximate KNO scaling for small pseudo-rapidity intervals ($|η| < 0.5$) and sizable KNO scaling violations for larger ones, in agreement with experiment.

hep-ph↗

arXiv2012

Charged multiplicities in pp and AA collisions at LHC

The mid-rapidity charged particle multiplicities in pp and AA collisions at LHC energies are described in the framework of a generalized eikonal model with shadowing corrections incorporated in AA. We show that the pp data require a Pomeron intercept close to 1.2, higher than the conventional one close to 1.1. An $s^{0.11}$ energy dependence is obtained in the LHC range and beyond. The size and centrality dependence of the AA multiplicity at $\sqrt{s} = 2.76$ TeV is reproduced and its energy dependence is predicted.

hep-ph↗

arXiv2009

Resolving the J/ψRHIC puzzles at LHC

Experiments with gold-gold collisions at RHIC have revealed (i) stronger suppression of charmonium production at forward rapidity than at midrapidity and (ii) the similarity between the suppression degrees at RHIC and SPS energies. To describe these findings we employ the model that includes nuclear shadowing effects, calculated within the Glauber-Gribov theory, rapidity-dependent absorptive mechanism, caused by energy-momentum conservation, and dissociation and recombination of the charmonium due to interaction with co-moving matter. The free parameters of the model are tuned and fixed by comparison with experimental data at lower energies. A good agreement with the RHIC results concerning the rapidity and centrality distributions is obtained for both heavy Au+Au and light Cu+Cu colliding system. For pA and A+A collisions at LHC the model predicts stronger suppression of the charmonium and bottomonium yields in stark contrast to thermal model predictions.

hep-ph↗

arXiv2008

Can the RHIC J/psi puzzle(s) be settled at LHC?

One observes strong suppression effects for hard probes, e.g. the production of J/psi or high-pT particles, in nucleus-nucleus (AA) collisions at RHIC. Surprisingly, the magnitude of the suppression is quite similar to that at SPS. In order to establish whether these features arise due to the presence of a thermalized system of quarks and gluons formed in the course of the collision, one should investigate the impact of suppression mechanisms which do not explicitly involve such a state. We calculate shadowing for gluons in the Glauber-Gribov theory and propose a model invoking a rapidity-dependent absorptive mechanism motivated by energy-momentum conservation effects. Furthermore, final state suppression due to interaction with co-moving matter (hadronic or pre-hadronic) has been shown to describe data at SPS. We extend this model by including the backward reaction channel, i.e. recombination of open charm, which is estimated directly from pp data at RHIC. Strong suppression of charmonium both in pA and AA collisions at LHC is predicted. This is in stark contrast with the predictions of models assuming QGP formation and thermalization of heavy quarks.

hep-ph↗

arXiv2008

Charmonium dynamics in dA and AA at RHIC and LHC

We discuss features of charmonium suppression at $\sqtr{s} = 200$ GeV within the framework of the Glauber-Gribov theory and the comovers interaction model. The latter approach has been extended by allowing for secondary charmonium production due to recombination of $c\bar{c}$ pairs in the medium, estimated from pp data at the same bombarding energy. Centrality and rapidity dependence of the nuclear modification factor for J/psi in d+Au, Cu+Cu and Au+Au collisions at RHIC are reproduced without fitting a single model parameter. A strong suppression of J/psi is predicted for LHC energies.

hep-ph↗

arXiv2008

J/psi suppression and the nuclear absorption decrease with increasing energy

In the field theoretical Glauber-Gribov framework in the eikonal approximation, we show that, in hadron-nucleus collisions in the high-energy limit and under especific conditions, the rescattering contributions to the J/psi production cross-section vanish, leading to an A^1 dependence on the atomic number A -- except for nuclear effects in the nucleus structure function, the so-called shadowing. We show that the RHIC data on $J/ψ$ production in d-Au collisions can be described with nuclear shadowing alone (sigma_{abs} = 0). The corresponding results for Au-Au collisions are also presented and compared with available data.

hep-ph↗

arXiv2007

Charmonium dissociation and recombination at RHIC and LHC

Charmonium production at heavy-ion colliders is considered within the comovers interaction model. The formalism is extended by including possible secondary J/psi production through recombination and an estimate of recombination effects is made with no free parameters involved. The comovers interaction model also includes a comprehensive treatment of initial-state nuclear effects, which are discussed in the context of such high energies. With these tools, the model properly describes the centrality and the rapidity dependence of experimental data at RHIC energy, $\sqrt{s}$ = 200 GeV, for both Au+Au and Cu+Cu collisions. Predictions for LHC, $\sqrt{s}$ = 5.5 TeV, are presented and the assumptions and extrapolations involved are discussed.

hep-ph↗

arXiv2007

Thermal Dileptons at LHC

We predict dilepton invariant-mass spectra for central 5.5 ATeV Pb-Pb collisions at LHC. Hadronic emission in the low-mass region is calculated using in-medium spectral functions of light vector mesons within hadronic many-body theory. In the intermediate-mass region thermal radiation from the Quark-Gluon Plasma, evaluated perturbatively with hard-thermal loop corrections, takes over. An important source over the entire mass range are decays of correlated open-charm hadrons, rendering the nuclear modification of charm and bottom spectra a critical ingredient.

hep-ph↗

arXiv2007

pi0 Fixed pT suppression and elliptic flow at LHC

Using a final state interaction model which describes the data on these two observables, at RHIC, we make predictions at the LHC -- using the same cross-section and pT-shift. The increase in the medium density between these two energies (by a factor close to three) produces an increase of the fixed pT pi0 suppression by a factor 2 at large pT and of v2 by a factor 1.5.

hep-ph↗

arXiv2007

Multiplicities and $J/ψ$ suppression at LHC energies

We present our predictions on multiplicities and $J/ψ$ suppression at LHC energies. Our results take into account shadowing effects in the initial state and final state interactions with the hot medium. We obtain 1800 charged particles at LHC and the $J/ψ$ suppression increases by a factor 5 to 6 compared to RHIC.

hep-ph↗

arXiv2006

Radial Flow in a Final State Interaction Model

In the framework of a final state interaction model, we show that the so-called radial flow, i.e. the almost linear increase of the inverse slope $T$ with the mass of the produced particle, is already contained in the initial condition -- with a slope $ $ (the so-called strength of the average radial transverse flow) which is larger than the measured one. While the precise value of the slope depends on the details of the model, the above result has a very general basis -- namely the increase with increasing $p_T$ of the fixed $p_T$ suppression, in the low $p_T$ region.

hep-ph↗

arXiv2006

Elliptic Flow and Fixed p_T Suppression in a Final State Interaction Model

It has been shown that a final state interaction model, used to describe J/psi suppression, can also describe the fixed p_T suppression of the pi^0 (and charged pions) yield at all values of p_T, with a final state interaction cross-section sigma close to one milibarn. We propose an extension of the model to the pion motion in the transverse plane - which introduces a dependence of the suppression on the azimuthal angle theta_R. Using the same value of sigma, we obtain values of the elliptic flow v_2 close to the experimental ones, for all values of p_T, including the soft p_T region.

hep-ph↗

arXiv2006

Elliptic Flow in a Final State Interaction Model

We propose a final state interaction model to describe the fixed $p_T$ suppression of the yield of particles at all values of $p_T$. We make an extension of the model to the motion in the transverse plane which introduces a dependence of the suppression on the azimuthal angle $θ_R$. We obtain values of the elliptic flow $v_{2}$ close to the experimental ones for all values of $p_T$.

hep-ph↗

arXiv2005

J/$ψ$ suppression at $\sqrt{\bf s} =$ 200 GeV in the comovers interaction model

The yield of $J/ψ$ per binary nucleon-nucleon collision in $AuAu$ and $CuCu$ collisions at $\sqrt{s} = 200$ GeV is computed in the framework of the dual parton model, supplemented with final state interaction (comovers interaction). For the latter we use the same value of the cross-section, $σ_{co} = 0.65$ mb, which describes the anomalous $J/ψ$ suppression observed at CERN-SPS energies. Several possibilities for the value of the absorptive cross-section are considered. Shadowing is introduced in both the comovers and the $J/ψ$ yields. A comparison with the results at CERN-SPS, including a prediction for $InIn$ collisions, is also presented.

hep-ph↗