HEP Group

Zagreb, Croatia

HEP Group

Zagreb, Croatia
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Sola J.,HEP Group
AIP Conference Proceedings | Year: 2014

An expanding universe is not expected to have a static vacuum energy density. The so-called cosmological constant Δ should be an approximation, certainly a good one for a fraction of a Hubble time, but it is most likely a temporary description of a true dynamical vacuum energy variable that is evolving from the inflationary epoch to the present day. We can compare the evolving vacuum energy with a Casimir device where the parallel plates slowly move apart ("expand"). The total vacuum energy density cannot be measured, only the effect associated to the presence of the plates, and then also their increasing separation with time. In the universe there is a nonvanishing spacetime curvature R as compared to Minkowskian spacetime that is changing with the expansion. The vacuum energy density must change accordingly, and we naturally expect δ Δ - R - H2. A class of dynamical vacuum models that trace such rate of change can be constructed. They are compatible with the current cosmological data, and conveniently extended can account for the complete cosmic evolution from the inflationary epoch till the present days. These models are very close to the ?CDM model for the late universe, but very different from it at the early times. Traces of the inherent vacuum dynamics could be detectable in our recent past. © 2014 AIP Publishing LLC.

Meloni D.,Third University of Rome | Morisi S.,HEP Group | Peinado E.,HEP Group
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2011

We study a model based on the dihedral group D4 in which the dark matter is stabilized by the interplay between a remnant Z2 symmetry, of the same spontaneously broken non-abelian group, and an auxiliary Z2 f introduced to eliminate unwanted couplings in the scalar potential. In the lepton sector the model is compatible with normal hierarchy only and predicts a vanishing reactor mixing angle, θ13=0. Since mν1=0, we also have a simple prediction for the effective mass in terms of the solar angle: |mββ|=|mν2|sin2θ⊙~10-3 eV. There also exists a large portion of the model parameter space where the upper bounds on lepton flavor violating processes are not violated. We incorporate quarks in the same scheme finding that a description of the CKM mixing matrix is possible and that semileptonic K and D decays mediated by flavor changing neutral currents are under control. © 2011 Elsevier B.V.

Cosic B.,University of Zagreb | Stanic Z.,HEP Group | Duic N.,University of Zagreb
Energy | Year: 2011

This paper provides methodology for regional analysis of biomass energy potential and for assessing the cost of the biomass at the power plant (PP) location considering transport distance, transport costs and size of the power plants. Also, methodology for determination of an upper-level price of the biomass which energy plant can pay to the external suppliers has been proposed. The methodology was applied on the case of Croatia and energy potential of biomass in the Croatian counties was calculated, using different methodologies, for wheat straw, corn stover and forestry residues, types of biomass considered economically viable at the moment. Results indicate that the average energy potential of wheat straw is 8.5. PJ, corn stover 7.2. PJ and forestry residues 5.9. PJ. © 2010 Elsevier Ltd.

Ceperic E.,HEP Group | Ceperic V.,University of Zagreb | Ceperic V.,Catholic University of Leuven | Baric A.,University of Zagreb
IEEE Transactions on Power Systems | Year: 2013

This paper presents a generic strategy for short-term load forecasting (STLF) based on the support vector regression machines (SVR). Two important improvements to the SVR based load forecasting method are introduced, i.e., procedure for generation of model inputs and subsequent model input selection using feature selection algorithms. One of the objectives of the proposed strategy is to reduce the operator interaction in themodel-building procedure. The proposed use of feature selection algorithms for automatic model input selection and the use of the particle swarm global optimization based technique for the optimization of SVR hyper-parameters reduces the operator interaction. To confirm the effectiveness of the proposed modeling strategy, the model has been trained and tested on two publicly available and well-known load forecasting data sets and compared to the state-of-the-art STLF algorithms yielding improved accuracy. © 2013 IEEE.

Meessen P.,HEP Group | Ortin T.,Institute Fisica Teorica UAM
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2012

We study the generalization of the Ansatz of Galli et al. (2011) [1] for non-extremal black holes of N=2, d=4 supergravities for a simple model of N=2, d=5 supergravity with a vector multiplet whose moduli space has two branches. We use the formalism of Ferrara, Gibbons and Kallosh (1997) [2], which we generalize to any dimension d. We find that the equations of motion of the model studied can be completely integrated without the use of our Anstaz (which is, nevertheless, recovered in the integration). The family of solutions found (common to both branches) is characterized by five independent parameters: the mass M, the electric charges q0, q1, the asymptotic value of the scalar at infinity Φ ∞ and the scalar charge Σ. The solutions have a singular horizon whenever Σ differs from a specific expression Σ 0(M,q 0,q 1,Φ ∞) (i.e. when there is primary scalar hair Σ-Σ 0≠0). The family of regular black holes interpolates between its two extremal limits. The supersymmetry properties of the extremal solutions depend on the choice of branch: one is always supersymmetric and the other non-supersymmetric in one branch and the reverse in the other one. © 2011 Elsevier B.V.

Meessen P.,HEP Group | Ortin T.,Institute Fisica Teorica UAM CSIC
Nuclear Physics B | Year: 2012

We discuss the structure of supersymmetric solutions in the timelike case to general gauged . N=2 . d=4 supergravity theories coupled to non-Abelian vector multiplets and hypermultiplets. © 2012 Elsevier B.V.

Meessen P.,HEP Group | Ortin T.,Institute Fisica Teorica UAM CSIC | Perz J.,Institute Fisica Teorica UAM CSIC | Shahbazi C.S.,Institute Fisica Teorica UAM CSIC
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2012

We rewrite the Ferrara-Gibbons-Kallosh (FGK) black-hole effective action of N= 2, d= 4, 5 supergravities coupled to vector multiplets, replacing the metric warp factor and the physical scalars with real variables that transform in the same way as the charges under duality transformations, which simplifies the equations of motion. For a given model, the form of the solution in these variables is the same for all spherically symmetric black holes, regardless of supersymmetry or extremality. © 2012 Elsevier B.V.

Bauer F.,HEP Group | Sola J.,HEP Group | Stefancic H.,Ruder Boskovic Institute
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2010

We present an unconventional approach for addressing the old cosmological constant (CC) problem in a class of F (R, G) models of modified gravity. For a CC of arbitrary size and sign the corresponding cosmological evolution follows an expansion history which strikingly resembles that of our real universe. The effects of the large CC are relaxed dynamically and there is no fine-tuning at any stage. In addition, the relaxation mechanism alleviates the coincidence problem. The upshot is that a large cosmological constant and the observed cosmic expansion history coexist peacefully in the Relaxed Universe. This model universe can be thought of as an interesting preliminary solution of the cosmological constant problem, in the sense that it provides a successful dynamical mechanism able to completely avoid the fine-tuning problem (the toughest aspect of the CC problem). However, since the Relaxed Universe is formulated within the context of modified gravity, it may still suffer of some of the known issues associated with these theories, and therefore it can be viewed only as a toy-model proposal towards a final solution of the CC problem. © 2010 Elsevier B.V. All rights reserved.

Soldo B.,HEP Group
Applied Energy | Year: 2012

Publishing papers in the area of forecasting natural gas consumption has begun in the middle of last century and led to a tremendous surge in research activities in the past decade. This paper presents a state-of-the-art survey of forecasting natural gas consumption. Purpose of this paper is to provide analysis and synthesis of published research in this area from beginning to the end of 2010, insights on applied area, used data, models and tools to achieve usable results, in order to be helpful base for future researchers. © 2011 Elsevier Ltd.

Sola J.,HEP Group
Journal of Physics: Conference Series | Year: 2013

The cosmological constant (CC) term in Einstein's equations, Λ, was first associated to the idea of vacuum energy density. Notwithstanding, it is well-known that there is a huge, in fact appalling, discrepancy between the theoretical prediction and the observed value picked from the modern cosmological data. This is the famous, and extremely difficult, "CC problem". Paradoxically, the recent observation at the CERN Large Hadron Collider of a Higgs-like particle, should actually be considered ambivalent: on the one hand it appears as a likely great triumph of particle physics, but on the other hand it wide opens Pandora's box of the cosmological uproar, for it may provide (alas!) the experimental certification of the existence of the electroweak (EW) vacuum energy, and thus of the intriguing reality of the CC problem. Even if only counting on this contribution to the inventory of vacuum energies in the universe, the discrepancy with the cosmologically observed value is already of 55 orders of magnitude. This is the (hitherto) "real" magnitude of the CC problem, rather than the (too often) brandished 123 ones from the upper (but fully unexplored!) ultrahigh energy scales. Such is the baffling situation after 96 years of introducing the Λ-term by Einstein. In the following I will briefly (and hopefully pedagogically) fly over some of the old and new ideas on the CC problem. Since, however, the Higgs boson just knocked our door and recalled us that the vacuum energy may be a fully tangible concept in real phenomenology, I will exclusively address the CC problem from the original notion of vacuum energy, and its possible "running" with the expansion of the universe, rather than venturing into the numberless attempts to replace the CC by the multifarious concept of dark energy.

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