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H. Stefancic

Publications and source records attributed to H. Stefancic.

18 recordsLinked to original sources
arXiv2011

Solving for Micro- and Macro- Scale Electrostatic Configurations Using the Robin Hood Algorithm

We present a novel technique by which highly-segmented electrostatic configurations can be solved. The Robin Hood method is a matrix-inversion algorithm optimized for solving high density boundary element method (BEM) problems. We illustrate the capabilities of this solver by studying two distinct geometry scales: (a) the electrostatic potential of a large volume beta-detector and (b) the field enhancement present at surface of electrode nano-structures. Geometries with elements numbering in the O(10^5) are easily modeled and solved without loss of accuracy. The technique has recently been expanded so as to include dielectrics and magnetic materials.

physics.comp-ph↗
arXiv2011

New approach to 3D electrostatic calculations for micro-pattern detectors

We demonstrate practically approximation-free electrostatic calculations of micromesh detectors that can be extended to any other type of micropattern detectors. Using newly developed Boundary Element Method called Robin Hood Method we can easily handle objects with huge number of boundary elements (hundreds of thousands) without any compromise in numerical accuracy. In this paper we show how such calculations can be applied to Micromegas detectors by comparing electron transparencies and gains for four different types of meshes. We demonstrate inclusion of dielectric material by calculating the electric field around different types of dielectric spacers.

physics.ins-det↗
arXiv2010

Probability distribution of the vacuum energy density

As the vacuum state of a quantum field is not an eigenstate of the Hamiltonian density, the vacuum energy density can be represented as a random variable. We present an analytical calculation of the probability distribution of the vacuum energy density for real and complex massless scalar fields in Minkowski space. The obtained probability distributions are broad and the vacuum expectation value of the Hamiltonian density is not fully representative of the vacuum energy density.

hep-th↗
arXiv2006

"Expansion" around the vacuum: how far can we go from Lambda?

The cosmological constant ($Λ$), i.e., the energy density stored in the true vacuum state of all existing fields in the Universe, is the simplest and the most natural possibility to describe the current cosmic acceleration. However, despite its observational successes, such a possibility exacerbates the well known $Λ$ problem, requiring a natural explanation for its small, but nonzero, value. In this paper we discuss how different our Universe may be from the $Λ$CDM model by studying observational aspects of a kind of "expansion" around the vacuum given by the equation of (EOS) $p_{d}=-ρ_{d} - A ρ_{d}^α$. In different parameter regimes such a parametrization is capable of describing both quintessence-like and phantom-like dark energy, transient acceleration, and various (non)singular possibilities for the final destiny of the Universe, including singularities at finite values of the scale factor, the so-called "Big Rip", as well as sudden future singularities. By using some of the most recent cosmological observations we show that if the functional form of the dark energy EOS has additional parameters very little can be said about their values from the current observational results, which postpones, until the arrival of more precise observational data, a definitive answer to the question posed above.

astro-ph↗
arXiv2006

Wikipedias: Collaborative web-based encyclopedias as complex networks

Wikipedia is a popular web-based encyclopedia edited freely and collaboratively by its users. In this paper we present an analysis of Wikipedias in several languages as complex networks. The hyperlinks pointing from one Wikipedia article to another are treated as directed links while the articles represent the nodes of the network. We show that many network characteristics are common to different language versions of Wikipedia, such as their degree distributions, growth, topology, reciprocity, clustering, assortativity, path lengths and triad significance profiles. These regularities, found in the ensemble of Wikipedias in different languages and of different sizes, point to the existence of a unique growth process. We also compare Wikipedias to other previously studied networks.

physics.soc-ph↗
arXiv2005

Renormalization-group running cosmologies - a scale-setting procedure

For cosmologies including scale dependence of both the cosmological and the gravitational constant, an additional consistency condition dictated by the Bianchi identities emerges, even if the energy-momentum tensor of ordinary matter stays individually conserved. For renormalization-group (RG) approaches it is shown that such a consistency relation ineluctably fixes the RG scale (which may have an explicit as well as an implicit time dependence), provided that the solutions of the RG equation for both quantities are known. Hence, contrary to the procedures employed in the recent literature, we argue that there is no more freedom in identification of the RG scale in terms of the cosmic time in such cosmologies. We carefully set the RG scale for the RG evolution phrased in a quantum gravity framework based on the hypothetical existence of an infrared (IR) fixed point, for the perturbative regime within the same framework, as well as for an evolution within quantum field theory (QFT) in a curved background. In the latter case, the implications of the scale setting for the particle spectrum are also briefly discussed.

astro-ph↗
arXiv2005

Hint for Quintessence-like Scalars from Holographic Dark Energy

We use the generalized holographic dark energy model, in which both the cosmological constant (CC) and Newton's constant G_N are scale-dependent, to set constraints on the renormalization-group (RG) evolution of both quantities phrased within quantum field theory (QFT) in a curved background. Considering the case in which the energy-momentum tensor of ordinary matter stays individually conserved, we show from the holographic dark energy requirement that the RG laws for the CC and G_N are completely determined in terms of the lowest part of the particle spectrum of an underlying QFT. From simple arguments one can then infer that the lowest-mass fields should have a Compton wavelength comparable with the size of the current Hubble horizon. Hence, although the models with the variable CC (or with both the CC and the G_N varying) are known tolead to successful cosmologies without introducing a new light degree of freedom, we nonetheless find that holography actually brings us back to the quintessence proposal. An advantage of having two different components of the vacuum energy in the cosmological setting is also briefly mentioned.

astro-ph↗
arXiv2004

"Winner takes it all": strongest node rule for evolution of scale free networks

We study a novel model for evolution of complex networks. We introduce information filtering for reduction of the number of available nodes to a randomly chosen sample, as stochastic component of evolution. New nodes are attached to the nodes that have maximal degree in the sample, which is a deterministic component of network evolution process. This fact is a novel for evolution of scale free networks and depicts a possible new route for modeling network growth. We present both simulational and theoretical results for network evolution. The obtained degree distributions exhibit an obvious power-law behavior in the middle with the exponential cut off in the end. This highlights the essential characteristics of information filtering in the network growth mechanisms.

cond-mat.dis-nn↗
arXiv2004

Cabibbo-suppressed decays of the Omega_c^0 - feedback to the Xi_c^+ lifetime

We investigate a possible background of the type Omega_c^0 --> Xi_c^+ pi^- to the CLEO Xi_c^+ lifetime measurement. This decay mode may lead to an overestimate of the Xi_c^+ decay length and, therefore, increase the measured Xi_c^+ lifetime. The branching ratio Gamma(Omega_c^0 --> Xi_c^+ pi^-)/Gamma(Omega_c^0 --> Omega^- pi^+) is analyzed in the framework of the pole model and the modified current algebra. We find that the Omega_c^0 --> Xi_c^+ pi^- decay mode could not generate a substantial systematic error in the Xi_c^+ lifetime measurement. Also, it cannot significantly reduce the disagreement between theoretical and experimental values of the Xi_c^+ lifetime.

hep-ph↗
arXiv2003

Renormalization-group running of the cosmological constant and the fate of the universe

For a generic quantum field theory we study the role played by the renormalization-group (RG) running of the cosmological constant (CC) in determining the ultimate fate of the universe. We consider the running of the CC of generic origin (the vacuum energy of quantum fields and the potential energy of classical fields), with the RG scale proportional to the (total energy density$\rm{)^{1/4}}$ as the most obvious identification. Starting from the present-era values for cosmological parameters we demonstrate how the running can easily provide a negative cosmological constant, thereby changing the fate of the universe, at the same time rendering compatibility with critical string theory. We also briefly discuss the recent past in our scenario.

hep-ph↗
arXiv2002

Cabibbo suppressed decays and the $Ξ_{c}^{+}$ lifetime

The problem of the $Ξ_{c}^{+}$ lifetime is considered in the framework of {\em Heavy-Quark Expansion} and $SU(3)_{flavor}$ symmetry. The lifetime of $Ξ_{c}^{+}$ is expressed in terms of measurable inclusive quantities of the other two charmed baryons belonging to the same $SU(3)_{flavor}$ multiplet in a model-independent way. In such a treatment, inclusive decay rates of singly Cabibbo suppressed decay modes have a prominent role. An analogous approach is applied to the multiplet of charmed mesons yielding interesting predictions on $D_{s}^{+}$ properties. The results obtained indicate that a more precise measurement of inclusive decay quantities of some charmed hadrons (such as $Λ_{c}^{+}$) that are more amenable to experiment can contribute significantly to our understanding of decay properties of other charmed hadrons (such as $Ξ_{c}^{+}$) where discrepancies or ambiguities exist.

hep-ph↗
arXiv2001

Renormalization-group running of the cosmological constant and its implication for the Higgs boson mass in the Standard Model

The renormalization-group equation for the zero-point energies associated with vacuum fluctuations of massive fields from the Standard Model is examined. Our main observation is that at any scale the running is necessarily dominated by the heaviest degrees of freedom, in clear contradistinction with the Appelquist & Carazzone decoupling theorem. Such an enhanced running would represent a disaster for cosmology, unless a fine-tuned relation among the masses of heavy particles is imposed. In this way, we obtain $m_H \simeq 550 GeV$ for the Higgs mass, a value safely within the unitarity bound, but far above the more stringent triviality bound for the case when the validity of the Standard Model is pushed up to the grand unification (or Planck) scale.

hep-ph↗
arXiv2000

Lifetime-difference pattern of heavy hadrons

The preasymptotic effects originating from the four-quark operators are disentangled from the other contributions by considering appropriate combinations of inclusive decay rates. Under the assumption of the isospin and heavy-quark symmetry, a set of relations connecting charmed and beauty decays is obtained without invoking specific models. The results are compared with other approaches and confronted with experiment.

hep-ph↗
arXiv2000

Preasymptotic effects in beauty decays

Large preasymptotic effects in beauty decays have been found using heavy-quark and SU(3) symmetry, as well as experimental data on charmed hyperons. Contrary to rather uniform beauty-meson lifetimes, a much larger spread of beauty-baryon lifetimes is predicted. However, it is highly unlikely that, theoretically, the $ τ(Λ_b)/τ(B_d^0) $ ratio, which at present deviates more than $1σ$ from the experimental result, can be lowered below 0.9.

hep-ph↗
arXiv1999

Inclusive decays and lifetimes of doubly charmed baryons

We extend the analysis of weak decays of heavy hadrons to the case of doubly charmed baryons, Ξ_{cc}^{++}, Ξ_{cc}^{+} and Ω_{cc}^{+}. Doubly charmed baryons are modeled as a heavy-light system containing a heavy cc-diquark and a light quark. Such a model leads to preasymptotic effects in semileptonic and nonleptonic decays which are essentially proportional to the meson wave function. Very clear predictions for semileptonic branching ratios and lifetimes of doubly charmed baryons are obtained.

hep-ph↗
arXiv1999

Enhancement of preasymptotic effects in inclusive beauty decays

We extend Voloshin's recent analysis of charmed and beauty hyperon decays based on SU(3) symmetry and heavy quark effective theory, by introducing a rather moderate model-dependence, in order to obtain more predictive power, e.g. the values of lifetimes of the (Λ_{b},Ξ_{b}) hyperon triplet and the lifetime of Ω_{b}. In this way we obtain an improvement of the ratio τ(Λ_{b})/τ(B_{d}^{0}) \sim 0.9 and the hierarchy of lifetimes τ(Λ_{b}) \simeq τ(Ξ_{b}^{0}) < τ(Ξ_{b}^{-}) < τ(Ω_{b}) with lifetimes of Ξ_{b}^{-} and Ω_{b} exceeding the lifetime of Λ_{b} by 22% and 35%, respectively.

hep-ph↗
arXiv1999

Inclusive decays and lifetimes of doubly charmed baryons

The analysis of singly charmed hadrons has been extended to the case of doubly charmed baryons, $Ξ_{cc}^{++}$, $Ξ_{cc}^{+}$ and $Ω_{cc}^{+}$. Doubly charmed baryons are described as a system containing a heavy $cc$-diquark and a light quark, similarly as in a heavy-light meson. This leads to preasymptotic effects in semileptonic and nonleptonic decays which are essentially proportional to the meson wave function. Interplay between preasymptotic effects in semileptonic and/or nonleptonic decay rates leads to very clear predictions for semileptonic branching ratios and lifetimes of doubly charmed baryons.

hep-ph↗