arXiv subjects

Antonio Dobado

Publications and source records attributed to Antonio Dobado.

At least 55 records · Page 3Linked to original sources

arXiv2012

Bulk viscosity and the phase transition of the linear sigma model

In this work we deal with the critical behavior of the bulk viscosity in the linear sigma model (LSM) as an example of a system which can be treated by using different techniques. Starting from the Boltzmann-Uehling-Uhlenbeck equation we compute the bulk viscosity over entropy density of the LSM in the large-N limit. We search for a possible maximum of the bulk viscosity over entropy density at the critical temperature of the chiral phase transition. The information about this critical temperature, as well as the effective masses, is obtained from the effective potential. We find that the expected maximum (as a measure of the conformality loss) is absent in the large N in agreement with other models in the same limit. However, this maximum appears when, instead of the large-N limit, the Hartree approximation within the Cornwall-Jackiw-Tomboulis (CJT) formalism is used. Nevertheless, this last approach to the LSM does not give rise to the Goldstone theorem and also predicts a first order phase transition instead of the expected second order one. Therefore both, the large-N limit and the CJT-Hartree approximations, should be considered as complementary for the study of the critical behavior of the bulk viscosity in the LSM.

hep-ph↗

arXiv2011

The existence of a two-solar mass neutron star constrains the gravitational constant G_N at strong field

In General Relativity there is a maximum mass allowed for neutron stars that, if exceeded, entails their collapse into a black hole. Its precise value depends on details of the nuclear matter equation of state about which we are much more certain thanks to recent progress in low-energy effective theories. The discovery of a two-solar mass neutron star, near that maximum mass, when analyzed with modern equations of state, implies that Newton's gravitational constant in the star cannot exceed its value on Earth by more than 8% at 95% confidence level. This is a remarkable leap of ten orders of magnitude in the gravitational field intensity at which the constant has been constrained.

gr-qc↗

arXiv2011

Bulk viscosity and energy-momentum correlations in high energy hadron collisions

We show how the measurement of appropriately constructed particle-energy/momentum correlations allows access to the bulk viscosity of strongly interacting hadron matter in heavy ion collisions. This measurement can be performed by the LHC and RHIC experiments in events with high-particle multiplicity, following up on existing estimates of the shear viscosity based on elliptic flow.

hep-ph↗

arXiv2011

Bulk viscosity of low-temperature strongly interacting matter

We study the bulk viscosity of a pion gas in unitarized Chiral Perturbation Theory at low and moderate temperatures, below any phase transition to a quark-gluon plasma phase. We argue that inelastic processes are irrelevant and exponentially suppressed at low temperatures. Since the system falls out of chemical equilibrium upon expansion,a pion chemical potential must be introduced, so we extend the existing theory to include it. We control the zero modes of the collision operator and Landau's conditions of fit when solving the Boltzmann equation with the elastic collision kernel. The dependence of the bulk viscosity with temperature is reminiscent of the findings of Fernandez-Fraile and Gomez Nicola, while the numerical value is closer to that of Davesne. In the zero-temperature limit we correctly recover the vanishing viscosity associated to a non-relativistic monoatomic gas.

hep-ph↗

arXiv2010

Brief introduction to viscosity in hadron physics

We introduce the concept of viscosity (both shear and bulk) in the context of hadron physics and in particular the meson gas, highlighting the current theoretical efforts to connect possible measurements of the viscosities to underlying physics such as a phase transition or the trace anomaly.

hep-ph↗

arXiv2010

Viscosity near phase transitions

Probably the most enticing observation in theoretical physics during the last decade was the discovery of the great amount of consequences obtained from the AdS/CFT conjecture put forward by Maldacena. In this work we review how this correspondence can be used to address hydrodynamic properties such as the viscosity of some strongly interacting systems. We also employ the Boltzmann equation for those systems closer to low-energy QCD, and argue that this kind of transport coefficients can be related to phase transitions, in particular the QGP/hadronic phase transition studied in heavy ion collisions.

hep-th↗

arXiv2010

Black holes in modified gravity theories

In the context of $f(R)$ gravity theories, the issue of finding static and spherically symmetric black hole solutions is addressed. Two approaches to study the existence of such solutions are considered: first, constant curvature solutions, and second, the general case (without imposing constant curvature) is also studied. Performing a perturbative expansion around the Einstein-Hilbert action, it is found that only solutions of the Schwarzschild-(Anti-) de Sitter type are present (up to second order in perturbations) and the explicit expressions for these solutions are provided in terms of the $f(R)$ function. Finally we consider the thermodynamics of black holes in Anti-de Sitter space-time and study their local and global stability.

gr-qc↗

arXiv2009

Minimum of $η/s$ and the phase transition of the Linear Sigma Model in the large-N limit

We reexamine the possibility of employing the viscosity over entropy density ratio as a diagnostic tool to identify a phase transition in hadron physics to the strongly coupled quark-gluon plasma and other circumstances where direct measurement of the order parameter or the free energy may be difficult. It has been conjectured that the minimum of eta/s does indeed occur at the phase transition. We now make a careful assessment in a controled theoretical framework, the Linear Sigma Model at large-N, and indeed find that the minimum of eta/s occurs near the second order phase transition of the model due to the rapid variation of the order parameter (here the sigma vacuum expectation value) at a temperature slightly smaller than the critical one.

hep-ph↗

arXiv2009

Radiative corrections to the Higgs potential in the LH model

In this work we compute the radiative corrections to the Higgs mass and the Higgs quartic couplings coming from the Higgs sector itself and the scalar fields $ϕ$ in the Littlest Higgs (LH) model. The restrictions that the new contributions set on the parameter space of the models are also discussed. Finally this work, together with our three previous papers, complete our program addressed to compute the relevant contributions to the Higgs low-energy effective potential in the LH model and the analysis of their phenomenological consequences.

hep-ph↗

arXiv2008

eta/s is critical (at phase transitions)

The viscosity over entropy density ratio, or KSS number, can help isolate the critical point in the hadron phase-diagram in Relativistic Heavy Ion Collisions. We argue that this quantity does have a minimum at a phase transition or crossover. Although indications from conventional non-relativistic gases point out to even a divergence in eta/s when the phase-transition is first-order, since the critical exponent is rather low, this will be more difficult to ascertain in RHIC or FAIR. The experimental data are more likely to reveal a discontinuity for a first order phase transition or a smooth minimum at a crossover.

hep-ph↗

arXiv2008

$η/s$ and Phase Transitions

We present a calculation of eta/s for the meson gas (zero baryon number), with the viscosity computed within unitarized NLO chiral perturbation theory, and confirm the observation that eta/s decreases towards the possible phase transition to a quark-gluon plasma/liquid. The value is somewhat higher than previously estimated in LO chi PT. We also examine the case of atomic Argon gas to check the discontinuity of eta/s across a first order phase transition. Our results suggest employing this dimensionless number, sometimes called KSS number (in analogy with other ratios in fluid mechanics such as Reynolds number or Prandtl number) to pin down the phase transition and critical end point to a cross-over in strongly interacting nuclear matter between the hadron gas and quark and gluon plasma/liquid.

hep-ph↗

arXiv2008

Towards the effective potential of the Littlest Higgs model

We compute the relevant parameters of the combined Higgs and ϕscalar effective potential in the Littlest Higgs (LH) model. These parameters are obtained as the sum of two kind of contributions. The first one is the one-loop radiative corrections coming from fermions and gauge bosons. The second one is obtained at the tree level from the higher order effective operators needed for the ultraviolet completion of the model. Finally we analyze the restrictions that the requirement of reproducing the standard electroweak symmetry breaking of the SM set on the LH model parameters.

hep-ph↗

arXiv2008

The status of the KSS bound and its possible violations (How perfect can a fluid be?)

In this work we briefly review the Kovtun-Son-Starinet (KSS) computation of the ratio eta/s for quantum field theories with gravitational dual and the related conjecture that it is bound from below by 1/(4 pi). We discuss the validity of the bound and the nature of its possible violations, its relevance for RHIC, its connection with phase transitions and other related issues.

hep-ph↗

arXiv2008

Could dark matter or neutrinos discriminate between the enantiomers of a chiral molecule?

We examine the effect of cold dark matter on the discrimination between the two enantiomers of a chiral molecule. We estimate the energy difference between the two enantiomers due to the interaction between fermionic WIMPs (weak interacting massive particles) and molecular electrons on the basis that electrons have opposite helicities in opposite enantiomers. It is found that this energy difference is completely negligible. Dark matter could then be discarded as an inductor of chiroselection between enantiomers and then of biological homochirality. However, the effect of cosmological neutrinos, revisited with the currently accepted neutrino density, would reach, in the most favorable case, an upper bound of the same order of magnitude as the energy difference obtained from the well known electroweak electron-nucleus interaction in some molecules.

astro-ph↗

arXiv2008

An introduction to the dark energy problem

In this work we review briefly the origin and history of the cosmological constant and its recent reincarnation in the form of the dark energy component of the universe. We also comment on the fundamental problems associated to its existence and magnitude which require and urgent solution for the sake of the internal consistency of theoretical physics.

astro-ph↗

arXiv2007

Indirect signals from braneworlds

It has been suggested that our universe could be a 3-dimensional brane where the SM fields live embedded in a D-dimensional space-time. In flexible raneworlds, in addition to the SM fields, new degrees of freedom appear on the brane associated to brane fluctuations. These new fields, known as branons, are standard WIMPs (Weakly Interacting Massive Particles) and therefore natural dark matter candidates, whose spontaneous annihilations can provide first evidences for this scenario.

astro-ph↗

arXiv2007

The Higgs effective potential in the Littlest Higgs model at the one-loop level

In this work we compute the contributions to the Higgs effective potential coming from the fermion and gauge boson sectors at the one-loop level in the context of the SU(5)/SO(5) Littlest Higgs (LH) model using a cutoff Lambda and including all finite parts. We consider both, the (SU(2)xU(1))_{1} x (SU(2)xU(1))_{2} and the (SU(2)xU(1))_{1} x (SU(2)xU(1)) gauge group versions of the LH model. We also show that the Goldstone bosons present in the model do not contribute to the effective potential at the one-loop level. Finally, by neglecting the contribution of higher dimensional operators, we discuss the restrictions that the new one-loop contributions set on the parameter space of the LH model and the need to include higher loop corrections to the Higgs potential.

hep-ph↗

arXiv2007

On the electroweak symmetry breaking in the Littlest Higgs model

In the SU(5)/SO(5) little Higgs models radiative corrections give rise to the SU(2)_L x U(1)_Y symmetry breaking. In this work we start a program for a detailed determination of the relevant terms of the Higgs effective potential by computing the contribution of the b, t and T quarks at the one-loop level, as an starting point for higher-loops computation. In spite of the fact that some two-loop level contributions are well known to be important, we use our preliminary one-loop result to illustrate that, by demanding the effective potential to reproduce exactly the Standard Model Higgs potential, and in particular the relation m_H^2= 2 lambda v^2= 2 mu^2, it will be possible to set new constraints on the parameter space of the Littlest Higgs model when the computation of all the relevant contributions to the Higgs effective potential is completed.

hep-ph↗