Resource
2015 EN
Michele Marrocco
Quantum optics is a field of research based on the quantum theory of light.Here, we show that the classical theory of light can be equally effective inexplaining a cornerstone of quantum optics: the quantization of the freeradiation field. The quantization lies at the heart of quantum optics and hasnever been obtained classically. Instead, we find it by taking into account thedegeneracy of the spherical harmonics that appear in multipole terms of theordinary Maxwell theory of the free electromagnetic field. In this context, thenumber of energy quanta is determined by a finite countable set of sphericalharmonics of higher order than the fundamental (monopole). This one plays,instead, the role of the electromagnetic vacuum that, contrary to the commonview, has its place in the classical theory of light.
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2015 EN
Giorgio Carta · Michele Brun · Antonio Baldi
This paper presents an auxetic medium, consisting of a two-dimensionalperforated sheet where the holes are arranged in a repetitive pattern. Thehexagonal disposition of the perforations makes the medium isotropic in theplane. It is shown that negative values of the Poisson's ratio can be achievedfor specific values of the dimensions and orientations of the holes. Theresults of the numerical simulations are confirmed by experimental tests, inwhich the Poisson's ratio of each specimen examined is evaluated from thedisplacement field obtained from the Digital Image Correlation (DIC) technique.The distribution of stresses in the medium is determined directly fromphotoelastic images. The auxetic structure proposed in this paper is easy tofabricate and can be very useful in several engineering applications.
Resource
2015 EN
Velleda Baldoni · Michele Vergne
These notes are an expanded version of a talk given by the second author. Ourmain interest is focused on the challenging problem of computing Kroneckercoefficients. We decided, at the beginning, to take a very general approach tothe problem of studying multiplicity functions, and we survey the variousaspects of the theory that comes into play, giving a detailed bibliography toorient the reader. Nonetheless the main general theorems involvingmultiplicities functions (convexity, quasi-polynomial behavior, Jeffrey-Kirwanresidues) are stated without proofs. Then, we present in detail our approach tothe computational problem, giving explicit formulae, and outlining an algorithmthat calculate many interesting examples, some of which appear in theliterature also in connection with Hilbert series.
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2015 EN
Benjamin Wolter · Michael G. Pullen · Matthias Baudisch
+7 more
Strong-field physics is currently experiencing a shift towards the use ofmid-IR driving wavelengths. This is because they permit conducting experimentsunambiguously in the quasi-static regime and enable exploiting the effectsrelated to ponderomotive scaling of electron recollisions. Initial measurementstaken in the mid-IR immediately led to a deeper understanding ofphoto-ionization and allowed a discrimination amongst different theoreticalmodels. Ponderomotive scaling of rescattering has enabled new avenues towardstime resolved probing of molecular structure. Essential for this paradigm shiftwas the convergence of two experimental tools: 1) intense mid-IR sources thatcan create high energy photons and electrons while operating within thequasi-static regime, and 2) detection systems that can detect the generatedhigh energy particles and image the entire momentum space of the interaction infull coincidence. Here we present a unique combination of these two essentialingredients, namely a 160\~kHz mid-IR source and a reaction microscopedetection system, to present an experimental methodology that provides anunprecedented three-dimensional view of strong-field interactions. The systemis capable of generating and detecting electron energies that span a six orderof magnitude dynamic range. We demonstrate the versatility of the system byinvestigating electron recollisions, the core process that drives strong-fieldphenomena, at both low (meV) and high (hundreds of eV) energies. The low energyregion is used to investigate recently discovered low-energy structures, whilethe high energy electrons are used to probe atomic structure via laser-inducedelectron diffraction. Moreover we present, for the first time, the correlatedmomentum distribution of electrons from non-sequential double-ionization drivenby mid-IR pulses.
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2015 EN
Dimitrios A. Gouliermis · David Thilker · Bruce G. Elmegreen
+25 more
We present a detailed clustering analysis of the young stellar populationacross the star-forming ring galaxy NGC 6503, based on the deep HST photometryobtained with the Legacy ExtraGalactic UV Survey (LEGUS). We apply acontour-based map analysis technique and identify in the stellar surfacedensity map 244 distinct star-forming structures at various levels ofsignificance. These stellar complexes are found to be organized in ahierarchical fashion with 95% being members of three dominant super-structureslocated along the star-forming ring. The size distribution of the identifiedstructures and the correlation between their radii and numbers of stellarmembers show power-law behaviors, as expected from scale-free processes. Theself-similar distribution of young stars is further quantified from theirautocorrelation function, with a fractal dimension of ~1.7 for length-scalesbetween ~20 pc and 2.5 kpc. The young stellar radial distribution sets theextent of the star-forming ring at radial distances between 1 and 2.5 kpc.About 60% of the young stars belong to the detected stellar structures, whilethe remaining stars are distributed among the complexes, still inside the ringof the galaxy. The analysis of the time-dependent clustering of youngpopulations shows a significant change from a more clustered to a moredistributed behavior in a time-scale of ~60 Myr. The observed hierarchy instellar clustering is consistent with star formation being regulated byturbulence across the ring. The rotational velocity difference between theedges of the ring suggests shear as the driving mechanism for this process. Ourfindings reveal the interesting case of an inner ring forming stars in ahierarchical fashion.
Resource
2015 EN
Yongxin Chen · Tryphon T. Georgiou · Michele Pavon
Monge-Kantorovich optimal mass transport (OMT) provides a blueprint forgeometries in the space of positive densities -- it quantifies the cost oftransporting a mass distribution into another. In particular, it providesnatural options for interpolation of distributions (displacement interpolation)and for modeling flows. As such it has been the cornerstone of recentdevelopments in physics, probability theory, image processing, time-seriesanalysis, and several other fields. In spite of extensive work and theoreticaldevelopments, the computation of OMT for large scale problems has remained achallenging task. An alternative framework for interpolating distributions,rooted in statistical mechanics and large deviations, is that of Schroedingerbridges (entropic interpolation). This may be seen as a stochasticregularization of OMT and can be cast as the stochastic control problem ofsteering the probability density of the state-vector of a dynamical systembetween two marginals. In this approach, however, the actual computation offlows had hardly received any attention. In recent work on Schroedinger bridgesfor Markov chains and quantum evolutions, we noted that the solution can beefficiently obtained from the fixed-point of a map which is contractive in theHilbert metric. Thus, the purpose of this paper is to show that a similarapproach can be taken in the context of diffusion processes which i) leads to anew proof of a classical result on Schroedinger bridges and ii) provides anefficient computational scheme for both, Schroedinger bridges and OMT. Weillustrate this new computational approach by obtaining interpolation ofdensities in representative examples such as interpolation of images.
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2015 EN
Michele Maggiore
Recent work has shown that non-local modifications of gravity involving termssuch as $m^2R\Box^{-2}R$ (and no cosmological constant) provide aphenomenologically viable alternative to $\Lambda$CDM. We first discuss thepossibility that such non-local terms emerge in the far infrared from therunning of a coupling constant associated to the $R^2$ term inhigher-derivative gravity, which, depending on the UV completion of the theory,can be asymptotically free in the ultraviolet and strongly coupled in theinfrared. In this scenario the mass scale $m$ of the non-local model emergesfrom dimensional transmutation, similarly to $\Lambda_{\rm QCD}$ for stronginteractions, leading to a technically natural value and to a novelunderstanding of the scale associated to dark energy. Motivated by thesefindings, we then explore the possibility of generating strong infrared effectsin Einstein gravity, with no $R^2$ terms, as a consequence of thehigher-derivative term generated by the conformal anomaly.
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2015 EN
Michele Borassi · Pierluigi Crescenzi · Andrea Marino
Closeness is an important centrality measure widely used in the analysis ofreal-world complex networks. In particular, the problem of selecting the k mostcentral nodes with respect to this measure has been deeply analyzed in the lastdecade. However, even for not very large networks, this problem iscomputationally intractable in practice: indeed, Abboud et al have recentlyshown that its complexity is strictly related to the complexity of theAll-Pairs Shortest Path (in short, APSP) problem, for which no subcubic"combinatorial" algorithm is known. In this paper, we propose a new algorithmfor selecting the k most closeness central nodes in a graph. In practice, thisalgorithm significantly improves over the APSP approach, even though itsworst-case time complexity is the same. For example, the algorithm is able tocompute the top k nodes in few dozens of seconds even when applied toreal-world networks with millions of nodes and edges. We will alsoexperimentally prove that our algorithm drastically outperforms the mostrecently designed algorithm, proposed by Olsen et al. Finally, we apply the newalgorithm to the computation of the most central actors in the IMDBcollaboration network, where two actors are linked if they played together in amovie.
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2015 EN
Sergio Cecotti · Michele Del Zotto
Seiberg and Witten have shown that in N=2 SQCD with $N_f=2N_c=4$ theS-duality group PSL(2,Z) acts on the flavor charges, which are weights ofSpin(8), by triality. There are other N=2 SCFTs in which SU(2) SYM is coupledto strongly-interacting non-Lagrangian matter: their matter charges are weightsof $E_6$, $E_7$ and $E_8$ instead of Spin(8). The S-duality group PSL(2,Z) actson these weights: what replaces Spin(8) triality for the $E_6,E_7,E_8$ rootlattices? In this paper we answer the question. The action on the matter charges of (afinite central extension of) PSL(2,Z) factorizes trough the action of theexceptional Shephard--Todd groups $G_4$ and $G_8$ which should be seen ascomplex analogs of the usual triality group $\mathfrak{S}_3\simeq\mathrm{Weyl}(A_2)$. Our analysis is based on the identification of S-duality for SU(2) gaugeSCFTs with the group of automorphisms of the cluster category of weightedprojective lines of tubular type.
Resource
2015 EN
Michele Della Morte
We review recent lattice results for quark masses and low-energy hadronicparameters relevant for flavor physics. We do that by describing the FLAGinitiative, with emphasis on its scope and rating criteria. The emergingpicture is that while for light quantities a large number of computations usingdifferent approaches exist, and this increases the overall confidence on thefinal averages/estimates, in the heavy-light case the field is less advancedand, with the exception of decay constants, only a few computations areavailable. The precision reached for the light quantities is such thatelectromagnetic (EM) corrections, beyond the point-like approximation, arebecoming relevant. We discuss recent computations of the spectrum based ondirect simulations of QED+QCD. We also present theoretical developments forincluding EM effects in leptonic decays. We conclude describing recent resultsfor the $K \to \pi \pi$ transition amplitudes and prospects for tacklinghadronic decays on the lattice.