Dark Energy as an off-shell Tachyon Background in four-dimensional Strings

We consider a time-dependent bosonic string in graviton, dilaton and tachyon backgrounds, for which it was already shown that conformal invariance is respected to all orders in alpha' and in any space-time dimension. Assuming that the tachyon is off-shell, we show in this note that the specific time-homogeneity of the corresponding space time effective action leads to a power-law expanding Universe, that can be accelerating or decelerating, depending on the range of parameter space considered. We interpret this result as dark energy. We arrive at this result without requiring the knowledge of the structure of the string effective action, which is not known to all orders in alpha'. Moreover, in our approach, the background configurations are consistent in a four-dimensional space time, without any need for extra dimensions.Comment: 10 pages, typos correcte

On higher-order corrections in a four-fermion Lifshitz model

We study a flavour-violating four-fermion interaction in the Lifshitz context, in 3+1 dimensions and with a critical exponent z=3. This model is renormalizable, and features dynamical mass generation, as well as asymptotic freedom. At one-loop, it is only logarithmically divergent, but the superficial degree of divergence of the two-point functions is 3. We calculate the two-loop corrections to the propagators, and show that, at this order, the Lorentz-violating corrections to the IR dispersion relation are quadratic in the cut off. Furthermore, these corrections are too important to represent a physical effect. As a consequence, the predictive power of the model in terms of Lorentz-violating effects in the propagation of particles is limited.Comment: 20 pages, 3 figures, comments adde

Harmonic Maps into Homogeneous Spaces According to a Darboux Homogeneous Derivative

Our purpose is to use a Darboux homogenous derivative to understand the harmonic maps with values in homogeneous space. We present a characterization of these harmonic maps from the geometry of homogeneous space. Furthermore, our work covers all type of invariant geometry in homogeneous space

Emergent relativistic-like Kinematics and Dynamical Mass Generation for a Lifshitz-type Yukawa model

We study the Infra Red (IR) limit of dispersion relations for scalar and fermion fields in a Lifshitz-type Yukawa model, after dressing by quantum fluctuations. Relativistic-like dispersion relations emerge dynamically in the IR regime of the model, after quantum corrections are taken into account. In this regime, dynamical mass generation also takes place, but in such a way that the particle excitations remain massive, even if the bare masses vanish. The group velocities of the corresponding massive particles of course are smaller than the speed of light, in a way consistent with the IR regime where the analysis is performed. We also comment on possible extensions of the model where the fermions are coupled to an Abelian gauge field

mfEGRA: Multifidelity Efficient Global Reliability Analysis through Active Learning for Failure Boundary Location

This paper develops mfEGRA, a multifidelity active learning method using data-driven adaptively refined surrogates for failure boundary location in reliability analysis. This work addresses the issue of prohibitive cost of reliability analysis using Monte Carlo sampling for expensive-to-evaluate high-fidelity models by using cheaper-to-evaluate approximations of the high-fidelity model. The method builds on the Efficient Global Reliability Analysis (EGRA) method, which is a surrogate-based method that uses adaptive sampling for refining Gaussian process surrogates for failure boundary location using a single-fidelity model. Our method introduces a two-stage adaptive sampling criterion that uses a multifidelity Gaussian process surrogate to leverage multiple information sources with different fidelities. The method combines expected feasibility criterion from EGRA with one-step lookahead information gain to refine the surrogate around the failure boundary. The computational savings from mfEGRA depends on the discrepancy between the different models, and the relative cost of evaluating the different models as compared to the high-fidelity model. We show that accurate estimation of reliability using mfEGRA leads to computational savings of $\sim$46% for an analytic multimodal test problem and 24% for a three-dimensional acoustic horn problem, when compared to single-fidelity EGRA. We also show the effect of using a priori drawn Monte Carlo samples in the implementation for the acoustic horn problem, where mfEGRA leads to computational savings of 45% for the three-dimensional case and 48% for a rarer event four-dimensional case as compared to single-fidelity EGRA

Orientation in the diagnosis of intraocular neoplasia, through intraocular fine needle aspiration (IFNA)

IFNA is a safe diagnostic technique that allows diagnostic of neoplastic and inflammatory processes of the eye1. In cats melanoma is common in the eye, skin and oral cavity2. In the eyep, local infiltration to systemic metastases may occur2. Being, the IFNA a method that assists in the diagnosis and therapeutics1,3,4,5. An european female cat, seven years old, was presented to consultation with brown patches of progressive growth in the right iris for four months. The pupillary light reflex, dazzle and menace response were present, and the ocular echography showed an area with iris thickening and with hyperechogenicity1. Due to clinical suspicion of iris melanoma, it was requested radiographic examination of the thorax, abdominal ultrasonography, hematological and biochemical blood analysis1,2, revealing no alterations. The IFNA was performed under general anesthesia introducing the a 26 gauge needle near the limbus and directed to the iris lesion with the bevel facing the surface of the mass3,4. The technique was performed without complications. The cytology using hematoxylin and eosin staining revealed moderate cellularity composed of rounded cells with moderate nuclear pleomorphism, bulky, pale cytoplasm1. The atypical round cells were suggestive of round cell amelanotic melanoma. Due to the cytological orientation, the enucleation of right eye was advised to the owner, which was declined.The diagnosis of ocular melanoma can be directed on clinical grounds and confirmed by anatomopathological exams2. IFNA is a practical, method in aiding the differentiation of neoplastic from non-neoplastic processes. It also helps guiding the surgical procedure and establishing prognosis1,3,4,5

Neutrino oscillations in a Robertson-Walker Universe with space time foam

In Phys. Rev. D77 (2008) 105001, we have studied decoherence models for flavour oscillations in four-dimensional stochastically fluctuating space times and discussed briefly the sensitivity of current terrestrial and astrophysical neutrino experiments to such models. In this addendum we extend these results to incorporate the effects due to the expansion of the Universe, so that our analysis can be useful in studies of extragalactic high-energy neutrinos, such as those coming from Gamma Ray Bursts at cosmological distances. Unfortunately for some microscopic models of foam, constructed in the string theory framework, we arrive at pessimistic conclusions about the detectability of the decoherence effects via flavour oscillation meaurements.Comment: Addendum to Phys. Rev. D77 (2008) 10500

Comments on branon dressing and the Standard Model

This technical note shows how Electrodynamics and a Yukawa model are dressed after integrating out perturbative brane fluctuations, and it is found that first order corrections in the inverse of the brane tension occur for the fermion and scalar wave functions, the couplings and the masses. Nevertheless, field redefinitions actually lead to effective actions where only masses are dressed to this first order. We compare our results with the literature and find discrepancies at the next order, which, however, might not be measurable in the valid regime of low-energy brane fluctuations.Comment: 12 page

Liouville-Lifshitz theory in 3+1 dimensions

We consider a four-dimensional theory in the z=3 Lifshitz context, with an exponential (Liouville) potential. We determine the exact renormalized potential of the theory and derive the non-perturbative relation between the renormalized and bare couplings. In addition, we show that Lorentz symmetry is naturally generated by quantum fluctuations in the infrared regime, and conclude that the model can be relevant to High Energy Physics