July 14th 2014

July 15th 2014

• Paolo Dondero: "Searches for direct stop production within the ATLAS experiment"

  INFN Pavia, Università di Pavia

  09:50-10:15

  The ATLAS experiment at the LHC, in parallel with the discovery of the Higgs boson is looking for sign of physics which goes beyond the Standard Model of Electroweak interactions. Among possible theories for physics beyond Standard Model, Supersymmetry seems to be the one of the most most promising. This theory infact addresses the Standard Model naturalness problem and offers a perfect candidate for the dark matter. Within this scenario the search supersymmetric partner of the stop quark, called stop, plays a key role. The ATLAS Experiment has developed a dedicated strategy for the discovery of this particle, focusing on achieving a complete coverage of the available parameter space for this particle, based on the combined search for all of its possible decay modes. The results obtained using the complete ATLAS 2012 statistics will be presented, targeting different decay modes and explaining the procedure to obtain the exclusion limits on the existance of a supersymmetric partner of the top quark at the electroweak scale.

• Luca Agostino: "The LAGUNA/LBNO physics potential for Long Baseline neutrino physics"

  APC

  10:15-10:40

  The LAGUNA/LBNO collaboration proposes a new generation neutrino experiment to address fundamental questions in particle and astroparticle physics. The experiment consists of a far detector, LAr double phase TPC, the fiducial mass of the detector is set to 20 kt in its first stage. The detector will be situated at 2300 km from CERN: this long baseline provides a unique opportunity to study the neutrino flavour oscillations over the first and second oscillation maxima and to explore the L/E behaviour. The near detector is based on a high-pressure argon gas TPC situated at CERN. During my talk I will detail the physics potential of this experiment for determining without ambiguity the mass hierarchy (MH) in its first stage and discovering CP‐violation (CPV) using the CERN SPS beam with a power of 750 kw. The impact of the assumptions on the knowledge of the oscillation parameters and the systematic errors are very important and will be shown in detail to prove the force of the experiment assuming realistic and conservative parameter values.

• Giancarlo Panizzo: "New Physics signals from measurable polarization asymmetries at LHC "

  Università degli Studi di Trieste and INFN - Udine Group

  10:40-11:05

  A new type of Z polarization asymmetry in bottom-Z production at LHC, called A_FB^(b,LHC), can be realistically measurable and would provide the determination of the so-called Ab parameter, whose available measured value still appears to be in disagreement with the Standard Model prediction. This polarization can be measured independently of a possible existence of Supersymmetry. If Supersymmetry is found, a second polarization, i.e. the top longitudinal polarization in top-charged Higgs production, would neatly identify the tanβ parameter. In this case, the value of Ab should be in agreement with the Standard Model. If Supersymmetry does not exist, a residual disagreement of Ab from the Standard Model prediction would be a clean signal of New Physics of "non-Supersymmetric" origin.

• Martina Gerbino: "Neutrino Physics in the Era of Precision Cosmology"

  University of Rome 'Sapienza'

  11:30-11:55

  I will talk about how a combination of different datasets from both cosmology and particle physics can constrain neutrino properties. Neutrino physics is one of the most promising area for scientific research. We know from oscillation experiments that they have a mass; however, oscillations are insensitive to the absolute mass scale. On the other hand, cosmological experiments like Planck are sensitive enough to put the tightest constraints on the total mass of neutrino species. Encouraging results on this topic are also coming from the combination of different cosmological datasets, such as cosmic microwave background (CMB) and Large Scale Structure (LSS) observations. A recent analysis of galaxy survey dataset from BOSS collaboration shows a preference for nonzero neutrino mass at the 3.3 sigma. Constraining precisely the neutrino mass will shed light on neutrino physics, helping investigate properties such as the mass hierarchy, the absolute mass scale, the origin of neutrino masses and the Dirac or Majorana nature of neutrinos. Moreover, future experiments are expected to put more stringent constraints on other parameters, like the number of neutrino families, or the possible existence of non-standard interactions. Finally, combining cosmological data directly with particle physics experiments could further boost our knowledge in neutrino sector. As an example, if neutrinos were Majorana particles, the joint combination of cosmology and double-beta decay experiments will be able to constrain the Majorana effective mass, the CP violating phase and one of the two Majorana phases in the very near future.

• Ivan Delvecchio: "Tracing the symbiotic growth between black holes and galaxies"

  University of Bologna

  11:55-12:20

  The tight and surprising interplay between super massive black holes and their host galaxies has been investigated by a number of studies. I will summarize the main observational evidences for such co-evolution scenario and also describe the most recent findings obtained from the Herschel satellite in this context.

• Ekaterina Karukes: "Modeling the Mass Distribution in the Spiral Galaxy NGC 3198"

  International School for Advanced Studies

  12:20-12:45

  We present the mass modeling of the spiral galaxy NGC 3198. We model the Rotation Curve (RC) in the framework of two models for the Dark Matter distribution: the NFW halo proposed by Navarro, Frenk and White and the Burkert halo proposed by Burkert. The HI observations of the spiral galaxy NGC 3198 were taken from the Westerbork Hydrogen Accretion in LOcal GALaxies (HALOGAS) survey. We find that the Universal Rotation Curve mass model (stellar disk + Burkert halo + gas disk) fits the observational data accurately. Instead, the NFW (halo + stellar disk + gas disk) mass model gives non-physical values of NFW halo parameters. We also derive the value of the local Dark Matter density without globally mass modeling the galaxy and show that starting from some galactocentric radius ( 20 kpc) it is possible to fit the density just including the Dark Matter component. Finally, we show that the dark halo density distribution predicted by the Cold Dark Matter (CDM) theory fails to fit the data.

• Stefano Camera: "Cosmology on the Largest Scales with Intensity Mapping"

  CENTRA, Instituto Superior Técnico, Universidade de Lisboa

  12:45-13:10

  The large-scale structure of the Universe supplies crucial information about the physical processes at play at early times. Unresolved maps of the intensity of 21 cm emission from neutral hydrogen, HI, at redshifts 1

• Claudia Mancuso: "Deep radio counts of star forming galaxies"

  SISSA

  13:10-13:35

  We have extended to the radio band the hybrid evolutionary model for star-forming galaxies worked out by Cai et al. (2013), that accurately reproduces the available data on their epoch-dependent luminosity functions, source counts and redshift distributions over a broad wavelength range, extending from mid-IR to mm wavelengths. The model combines a physical model for proto-spheroidal galaxies, dominating the cosmic star-formation rate at z = 1.5, with a phenomenological model for normal and starburst galaxies at lower z. Our extension to the radio band takes into account both the synchrotron and the free-free emission. The latter is found to dominate the 95 GHz counts of star-forming galaxies by Mocanu et al. (2013). On the contrary, radio counts at < 10 GHz are dominated by synchrotron emission, with the free-free gradually taking over at higher and higher frequencies. After having checked that the model accurately accounts for the observed sub-mJy counts at all radio frequencies we have worked out predictions for number counts and redshift distributions at the frequencies covered by SKA.

• Stefania Pandolfi: "Dark Energy coupling with electromagnetism as seen from future low redshift probes"

  Dark Cosmology Centre

  14:30-14:55

  Beyond the standard cosmological model the late-time accelerated expansion of the universe can be reproduced by the introduction of an additional dynamical scalar field. In this case, the field is expected to be naturally coupled to the rest of the theory's fields, unless a (still unknown) symmetry suppresses this coupling. Therefore, this would possibly lead to some observational consequences, such as space-time variations of nature's fundamental constants. In this talk I'll investigate the coupling between a dynamical Dark Energy model and the electromagnetic field, and the corresponding evolution of the fine structure constant (α) with respect to the standard local value α0. In particular, I will show derived joint constraints on two dynamical Dark Energy model parametrizations (the Chevallier-Polarski-Linder and Early Dark Energy model) and on the coupling with electromagnetism ζ, forecasting future low-medium redshift observations, such as supernovae and weak lensing measurements from the Euclid experiment, high-resolution spectroscopy measurements of fundamental couplings and the redshift drift from the European Extremely Large Telescope. I'll also consider the case where the field driving the α evolution is not the one responsible for cosmic acceleration and investigate how future observations can constrain this scenario.

• Jessica Alfonsi: "Quantum lattice models for the excited-state properties of graphene nanoribbons"

  University of Padova

  09:00-09:25

  The theoretical investigation of the interplay between geometrical and electronic properties in graphene-based systems such as graphene nanoribbons (GNRs) is currently a hot topic in condensed matter physics because of its crucial importance both for the understanding of physics in low-dimensional systems and the design of new technological applications. In this talk I will first survey the basics concepts needed to understand the electronic properties of GNRs in the framework of the tight-binding model; electronic band structures and optical selection rules in GNRs for both longitudinally and transversely polarized photons will be discussed and compared to those known for similar graphene-based systems such as single-walled nanotubes (SWNTs). The exact diagonalization (ED) method for the half-filled Hubbard model will be presented as a full many-body approach going beyond perturbative and mean-field techniques for elucidating the excited-state spectra of these systems. I will show how the use of open-ended 1D and 2D quantum lattice models allows to capture the main features of the excited state properties of zigzag and armchair GNRs in the low-intermediate correlation regime, respectively. Simulation results based on density-matrix renormalization group (DMRG) algorithm will also be shown in order to address the size dependence of optical excitations for medium-to-large width graphene nanostripes.

• Francesco Floris: "Plasmonic Structures for Emitting and Sensing Devices"

  Department of Physics - University of Pavia

  09:25-09:50

  Plasmonics is a way to concentrate optical energy into spatial regions lower than the diffraction limit together with enhancement and squeezing of an electromagnetic (E.M.) field at a metal-dielectric interface. My research is based on the development of a platform consisting in a plasmonic nano-structured surface based on a gold film embedding a two-dimensional array of polymeric pillars covered with a ultra-thin sol-gel film of a few nanometres in thickness. The main activity has been focused on defining and improving the sol-gel process to conformal deposit such thin films. Several devices have been prepared and characterized by atomic force microscopy (AFM), Fourier transform micro reflectance (FT-R) and spectroscopic ellipsometry (SE) techniques. Besides structural and optical characterization of the studied structure and thin film, a modelling via finite-difference time-domain (FDTD) simulations have been developed in order to give a feedback on the device quality and interpret its optical response. Experimental results have been found to very well agree with FDTD simulations (e.g. reflectance spectra and E.M. field expansions) and remarkable performances as surface plasmon resonance (SPR) based optical sensor, for both biological and medical applications, have been founded. Moreover, plasmonic-photonic coupled devices, funded on the interaction between surface plasmons based structures and active photonic media, represent a very promising field for nano-scale applications. In this context, by doping the sol-gel precursors with dye characterized by absorption and emission in the visible (VIS) and near-infrared (NIR) spectral regions, preliminary results confirm that photoluminescence and Raman features of such emitters establish a coupling with respect to the plasmonic resonances and shape effects, involving both the dye and plasmonic resonances, have been observed.

• Maria Barbara Maccioni: "Band offsets at Ga2O3/(GaIn)2O3 interfaces"

  University of Cagliari, department of Physics

  09:50-10:15

  We'll report the results of a study on the electronic (gap and band offset) and local structural properties of Ga2O3 in monoclinic phase, using the density functional theory (DFT). The large bandgap of Ga2O3 can be tuned, for example considering the substitution of In for Ga in the (GaxIn1−x)2O3 alloy. In this way should be possible to realize nanostructured optoelectronic devices, based on Ga2O3/(GaIn)2O3 junctions, such as power field effect transistors (Power FETs), ligth emitter devices (LEDs) or solid state lasers, wavelength-tunable optical filters and photodetectors. Gallium- and Indium-based semiconductors (Ga2O3, In2O3 and their solid solutions) have recently started attracting interest as a novel materials system for high-power transport devices as well as for ultraviolet optical absorbers and emitters, owing to their wider band gap compared to GaN and SiC. Ga2O3, In2O3 and their solid solutions, over most of their mixing range, are insulating but it is thought that they could be easily dopable (especially n-type). Moreover, they can be made ferromagnetic through magnetic doping and undergo metal-insulator transitions in special doping conditions. In addition, resorting to an appropriated optimization of physical properties and nanostructuration of Gallium- and Indium-based semiconductors layers of chosen composition, it is possible to tune their key properties (such as band gaps, interface band offsets, vibrational absorptions, as well as, potentially, the magnetic behavior) leading overall to novel multi-functional nanomaterials, nanostructures and devices.

• Sofia Pazzagli: "Energy transfer from a single organic molecule to a graphene monolayer"

  University of Florence

  10:15-10:40

  The problem of a single atom or molecule close to a planar layered medium represents a paradigmatic system in near field physics. A quantitative analysis of the dipole radiation and energy distribution for different geometries is essential e.g. in the quest for efficient light-matter interfaces, on which optical sensing and photon-based communication protocols strongly rely. Among other materials, grapheme - a one-atom-thick layer of carbon atoms - is now very well known for its unique optical, electronic and mechanical properties, resulting from its gapless band structure and locally linear dispersion relation. The effects occurring when a quantum emitter is in close proximity to such a purely two-dimensional material are still largely unexplored, although they potentially unveil a new scenario for the physics of strong light-matter interaction. Moreover, based on the near-field interaction between biomolecules and graphene, a wealth of sensing applications have been proposed. In this talk a full statistical study of the coupling between a graphene monolayer sheet and single emitters made of individual organic DiBenzoTerrylene (DBT) molecules embedded in thin Anthracene crystals is presented and discussed. When the DBT:Anth system is interfaced with a graphene monolayer sheet, the main energy relaxation mechanism for the excited emitter is the non-radiative energy transfer to graphene, which results in a reduction of the DBT excited-state lifetime. Time-correlated single-photon counting (TCSPC) is used, employing a home-built epifluorescence scanning confocal microscope, to collect the lifetime statistical distribution from 150 DBT molecules spincoated on graphene. The comparison with the fluorescence lifetime statistical distribution collected from 75 molecules spincoated on a simple quartz substrate demonstrates that graphene induces a substantial change in the molecule photophysics. Moreover, the experimental result confirms the analytical model that describes the rate of transferred energy to graphene with a universal scaling law, which depends on the inverse fourth power of the molecule-interface distance. Because of its strong-distance dependence, this study represents a first proof of principle of the working mechanism for a graphene-based ruler in the nm-range, in which the emitters are used as markers.

• Roberto Piras: "Synthesis and characterization of Bi2S3 nanocrystals for photovoltaic applications."

  University of Cagliari

  10:40-11:05

  Bismuth sulfide is a promising n-type semiconductor for solar energy conversion. We have explored the colloidal synthesis of Bi2S3 nanocrystals, with the aim of employing them in the fabrication of solution-processable solar cells and to replace toxic heavy metals chalcogenides like PbS or CdS, that are currently employed in such devices. We compare different methods to obtain Bi2S3 colloidal quantum dots, including the use of environmentally benign reactants, through a organometallic synthesis. Different sizes and shapes were obtained according to the synthesis parameters and the growth process has been rationalized with the help of theoretical calculations, comparing the predicted morphology with systematic physical-chemistry characterization of nanocrystals by X-ray diffraction, FT-IR absorbance, Transmission Electron Microscopy. Future plans for the project are the study of transport properties in films of Bi2S3 nanocrystals, including the treatment with surfactants for ligand exchange procedures, and the fabrication of prototype solar cells with Bi2S3 nanocrystals.

• Iustyna Vasilchenko: "Influence of the phosphorous precursors on the optical properties of SiO2-HfO2-P2O5 planar waveguides"

  University of Trento, department of physics

  11:30-11:55

  The needs for compact and efficient photonic devices obtained by rare earth-activated glasses, such as fibers and lightwave planar circuits, still drive the research of novel glass composition and optimized fabrication protocols. It’s demonstrated by a rich literature and by several devices present on the market that sol-gel processes can be effectively used for integrated optics. Miniature size and performances of integrated lasers and amplifiers are mainly linked to rare earth doping level, which must be maximized, without quenching the fluorescence of the doped material. Cross relaxation between neighboring erbium ions leads to the quenching of the fluorescence and toо fast de-excitation within erbium clusters. The clusters have the strongest influence on the decrease of the lifetime. Hence, it is important to eliminate them by dissolving the rare-earth ions into the glass matrix. A way to do this is to improve the silica glass matrix by adding co-doping agents, such as P2O5 or A12O3. We have already demonstrated that the silica-hafnia system is a viable route for the preparation of active waveguides by sol-gel approach. On the basis of that, we have investigated planar waveguides with compositions (70-x)SiO2-30HfO2 -хP2O5 (х= 5, 10 mol %), activated by 0.5 mol% Er3+ ions. Planar waveguides were fabricated by sol-gel method and dip-coating technique. The main attention was paid on the influence of different phosphorous oxides precursors, on the fabrication process and on the optical parameters. Prepared planar waveguides were characterized by modal spectroscopy, optical transmittance, time resolved photoluminescence measurements. This research is performed in the framework of MAE (2013–2015) and CNR-PAS (2014-2016) projects.

• Ina Bisha: "Sodium-Galactose Transporter (SGLT): Na+ and Galactose are released independently from the inward-facing conformation. "

  SISSA

  11:55-12:20

  Sodium-Galactose Transporter (SGLT) is a secondary active symporter able to accumulate sugars like glucose/galactose into cells using the electrochemical gradient of Na+ across the membrane. This transport is believed to occur via an alternating-access mechanism in which the protein, switching from an outward to an inward-conformation, guarantees a correct uptake of sugar molecules important in intestinal absorption and renal reabsorption. Previous investigations of the releasing mechanism of the two ligands into the cytoplasm have led some contradictory conclusions about the gating role of Y263 and the conformational state of the transporter. In this study, using classical molecular dynamics (MD) simulations and bias-exchange metadynamics, we clarify the the role important residues and characterize the free energy barriers of the releasing process toward the intracellular side as being of the order of 11-12 kcal/mol for both the ion and the substrate. Most importantly and surprisingly, we find that the exit of Na+ and galactose is non-concerted.

• Sabrina Stella: "Use of GPUs to boost the performance of a lattice-free tumor growth model "

  Dept. of Physics - University of Trieste

  12:20-12:45

  We recently developed a computational model of tumor growth. It is based on a "bottom-up" approach, where a mathematical description of the single cell metabolism, biomechanics and cell cycle, is used to describe the overall tumor growth dynamics. The model was implemented from scratch in C++ language. At the moment the software is able to simulate the growth of multicellular tumor spheroids up to more than one million cells; the results were experimentally validated. The simulation program is very demanding and simulation time severely limits the integration of additional biological details, indeed, at the moment, a full simulation requires tens of days to be completed. Therefore, we are working on a new version of the code that exploits Graphical Processing Units (GPU) to boost performance. Here we describe the new code that, while it executes in parallel on the GPU, can lead to a considerable speed up. Some preliminary data of the expected speed up for a CPU-GPU simulation will be given.

• Andrea Russomando: "An innovative radio-guided surgery technique for complete resection of tumors"

  Università di Roma la Sapienza

  12:45-13:10

  Some small residuals of tumor could remain after the operation. The surgeon can't detect them, and this could cause a lower chance of survival of the patient. The radioguided surgery (RGS) is a technique that enables the surgeon to remove them. Today RGS is based on gamma probe, but, due the high penetration power of the photons, could be used only on selected cases (there are limits on the spatial resolution of the probe). In order to increase the field of applications, we are developing an intraoperative beta minus probe, to detect the residuals of tumor marked with electrons emitters. We create a prototype and tested it on phantoms. Extrapolating the results (also with simulations based on FLUKA) we are able to detect small volumes (less than 0.1 ml) in one second, delivering a dose to the patients 10 time lower than the dose normally used.

• Laura Giacopetti: "Degradation of Cd-yellow Paints: ab initio study of the adsorption of oxygen and water on (10-10) CdS surface"

  CNR-IOM Cagliari - University of Cagliari

  13:10-13:35

  The cadmium yellow paints (CdS) used in impressionist and modernist paintings in early 1900s are undergoing several deterioration processes including whitening and discoloration. A relevant effect produced at the surface of the paintings is the growth of discolored crusts formed mainly by white globular hydrated cadmium sulfate (CdSO4*nH2O) and cadmium carbonate (CdCO3). Recent studies ascribe to an initial photo-oxidation process of CdS the input for the formation of such whitish compounds. The following reactions leading to the observed aforementioned crusts on the yellow paint layer still need further understanding. In facts, despite the large number of experimental studies on altered yellow paints the origins and roles of these compounds remain unclear. In order to understand the oxidation and hydration mechanisms, at atomic level, of these whitish globules, we present the early stages of a theoretical study on the interaction between the hexagonal (10-10) surface of CdS and O2 and H2O molecules to simulate the combined effects of exposure to air and humidity. To this end the clean surface of CdS was simulated according to the slab geometry and was next covered by oxygen, water molecules and a combination of O2+H2O in different concentrations. More specifically, we determined the favorite adsorption sites and calculated the adsorption energies of the different molecules on top of the surface. The details of the electronic structure of the interactions are given via the bonding charge analysis along with a thorough description of the geometry. For this purpose, we adopted a first principles method within the framework of the Density Functional Theory (DFT) in the Generalized Gradient Approximation (GGA-PBE) with the use of ultrasoft pseudopotentials as implemented in the quantum-ESPRESSO package.

• Eva Peccenini: "Advanced imaging systems for diagnostic investigations applied to Cultural Heritage."

  Institution

  14:30-14:55

  The diagnostic investigations are an important resource in the studies on Cultural Heritage to enhance the knowledge on execution techniques, materials and conservation status of a work of art. In this field, due to the great historical and artistic value of the objects, preservation is the main concern; for this reason, new technological equipment have been designed and developed in the Physics Departments of the Universities of Ferrara and Bologna to enhance the non-invasive approach to the study of pictorial artworks and other objects of cultural interest. IR reflectography, X-ray radiography and computed tomography (CT), applied to works of art, are joined by the same goal to get hidden information on execution techniques and inner structure pursuing the non-invasiveness of the methods, although using different setup and physical principles. In this work transportable imaging systems to investigate large objects directly in the museums and galleries are presented. In particular, 2D scanning devices for IR reflectography and X-ray radiography, CT systems and their applications on Cultural Heritage are described.

• Eduardo Henrique Silva Bittencourt: "Dynamical Bridges"

  Sapienza Università di Roma

  14:55-15:20

  I will present that the dynamics of a given field (scalar, vector or spinor) defined in the Minkowski space can be mapped into another dynamical equation defined in a curved space which has no gravitational character. This means that the solutions of the former are also solutions of the latter. Then, we introduce interactions and analyzed the results of the physical point of view.

• Giacomo Fragione: "Gravity and Thermodynamics: fundamental principles and gravothermal instability"

  University of Rome 'La Sapienza'

  15:20-15:45

  In the dynamical evolution of globular clusters, stellar encounters strongly contribute in phase space mixing of stellar orbits. In this scenario, thermodynamics plays a central role in the gravitational equilibrium and stability of the clusters. On the other hand, the observations of luminosity profiles suggest a unique distribution function allowing the study of the evolution as a sequence of thermodynamic transformations, keeping constant the distribution of the star velocities, like in the framework of Boltzmann statistical mechanics. Then, we can construct equilibrium models with a different approach by applying thermodynamic principles to a Boltzmann distribution function, with an Hamiltonian which contains an effective potential depending on the kinetic energy of the stars. We also obtain new relations for the thermodynamical equilibrium in presence of gravity and introduce the concept of thermodynamic and kinetic temperature and pressure. The models present regions with positive and negative specific heat, producing thermodynamic instabilities which drive the clusters towards the so called gravothermal catastrophe.

• Enrico Ubaldi: "Modeling Time Varying Networks with Memory"

  University of Parma

  15:45-16:10

  The large majority of research in network science has been focused on static representations of systems and on the characterization of dynamical processes unfolding on their fabrics. However, the time scales describing the evolution of the networks' topology are in general comparable to the time scales characterizing the dynamical processes taking place on top of them. Examples are the diffusion of information on online networks or the spreading of sexually transmitted diseases in the population. These observations, together with the recent availability of large scale and time resolved datasets, has ignited the development of a new paradigm: time varying network. The introduction of time poses new fundamental questions and challenges that just started to be addressed. In particular, the evolution of nodes connectivity (ego-net), non-Markovian properties, different type of correlations and high order structures still challenge our understanding and modeling efforts. In this work we aim to fill this gap. We define a generative model able to describe the realistic microscopic as well as the macroscopic evolution of time-varying networks. The key ingredient is a simple local non-Markovian reinforcement process that allows a full analytical description of the model evolution. Interestingly, this mechanism captures the ego-net dynamics of a very diverse set of real time-varying networks ranging from phone and different types of online interactions to co-authorships in scientific journals. Our results suggest that all the potentially different aspects of human behavior driving these networks can be described by a single universal process.

• Fulvio Sbisà: "The critical tension in the Cascading DGP model"

  ICG Portsmouth, UK

  16:10-16:35

  Despite the LaCDM cosmological models provide a good fit to the observational data, the late time acceleration problem and the cosmological constant problem are not yet understood. One way to approach these problems is by modifying the law of the gravitational interaction at large distances (modified gravity models). An interesting class of these models is provided by the so-called Brane Induced Gravity models. I will discuss the presence of ghost instabilities in one of these models, the Cascading DGP model, and will focus on the geometrical interpretation of the critical tension, which divides stable configurations from unstable configurations.