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Diego Casadei

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Published work

12 published item(s)

preprint2026arXiv

A Unified Framework for Structured Flow Modeling: From Continuous Fields to Data-Driven Representations

Many dynamical systems can be described in terms of structured flows combining source/sink behavior, cyclic dynamics, and topology-constrained transport. These features arise across a wide range of domains, including physical, engineered, and data-driven systems. This work provides a unified perspective on such systems by connecting continuous formulations based on the Helmholtz-Hodge decomposition with discrete and data-driven representations. We review the recently proposed Graph Vector Field (GVF) framework, which enables a decomposition of complex dynamics into gradient, curl, and harmonic components on simplicial complexes, offering both expressivity and interpretability. We then introduce a hierarchy of alternative modeling approaches, including parametric conditional models, linear graph dynamical systems, and reduced Hodge representations, which trade expressive power for computational tractability and reduced data requirements. A key contribution of this work is a cross-domain validation strategy that leverages datasets from well-understood physical systems to verify model correctness and assess robustness independently of the target application domain. This approach enables a systematic evaluation of the trade-offs between model complexity, interpretability, and predictive performance. The resulting framework supports an iterative modeling methodology in which highly expressive models are used as diagnostic tools to identify dominant mechanisms, guiding the construction of simplified models tailored to practical constraints. This work highlights the broad applicability of structured flow modeling and provides a foundation for scalable and interpretable analysis of complex dynamical systems.

preprint2015arXiv

Correlation of hard X-ray and white light emission in solar flares

A statistical study of the correlation between hard X-ray and white light emission in solar flares is performed in order to search for a link between flare-accelerated electrons and white light formation. We analyze 43 flares spanning GOES classes M and X using observations from RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) and HMI (Helioseismic and Magnetic Imager). We calculate X-ray fluxes at 30 keV and white light fluxes at 6173 Å summed over the hard X-ray flare ribbons with an integration time of 45 seconds around the peak hard-X ray time. We find a good correlation between hard X-ray fluxes and excess white light fluxes, with a highest correlation coefficient of 0.68 for photons with energy of 30 keV. Assuming the thick target model, a similar correlation is found between the deposited power by flare-accelerated electrons and the white light fluxes. The correlation coefficient is found to be largest for energy deposition by electrons above ~50 keV. At higher electron energies the correlation decreases gradually while a rapid decrease is seen if the energy provided by low-energy electrons is added. This suggests that flare-accelerated electrons of energy ~50 keV are the main source for white light production.

preprint2015arXiv

Reference analysis of the signal + background model in counting experiments II. Approximate reference prior

The objective Bayesian treatment of a model representing two independent Poisson processes, labelled as "signal" and "background" and both contributing additively to the total number of counted events, is considered. It is shown that the reference prior for the parameter of interest (the signal intensity) can be well approximated by the widely (ab)used flat prior only when the expected background is very high. On the other hand, a very simple approximation (the limiting form of the reference prior for perfect prior background knowledge) can be safely used over a large portion of the background parameters space. The resulting approximate reference posterior is a Gamma density whose parameters are related to the observed counts. This limiting form is simpler than the result obtained with a flat prior, with the additional advantage of representing a much closer approximation to the reference posterior in all cases. Hence such limiting prior should be considered a better default or conventional prior than the uniform prior. On the computing side, it is shown that a 2-parameter fitting function is able to reproduce extremely well the reference prior for any background prior. Thus, it can be useful in applications requiring the evaluation of the reference prior for a very large number of times. [The published version JINST 9 (2014) T10006 has a typo in the normalization $N$ of eq.(2.6) that is fixed here.]

preprint2015arXiv

ROOT - A C++ Framework for Petabyte Data Storage, Statistical Analysis and Visualization

ROOT is an object-oriented C++ framework conceived in the high-energy physics (HEP) community, designed for storing and analyzing petabytes of data in an efficient way. Any instance of a C++ class can be stored into a ROOT file in a machine-independent compressed binary format. In ROOT the TTree object container is optimized for statistical data analysis over very large data sets by using vertical data storage techniques. These containers can span a large number of files on local disks, the web, or a number of different shared file systems. In order to analyze this data, the user can chose out of a wide set of mathematical and statistical functions, including linear algebra classes, numerical algorithms such as integration and minimization, and various methods for performing regression analysis (fitting). In particular, ROOT offers packages for complex data modeling and fitting, as well as multivariate classification based on machine learning techniques. A central piece in these analysis tools are the histogram classes which provide binning of one- and multi-dimensional data. Results can be saved in high-quality graphical formats like Postscript and PDF or in bitmap formats like JPG or GIF. The result can also be stored into ROOT macros that allow a full recreation and rework of the graphics. Users typically create their analysis macros step by step, making use of the interactive C++ interpreter CINT, while running over small data samples. Once the development is finished, they can run these macros at full compiled speed over large data sets, using on-the-fly compilation, or by creating a stand-alone batch program. Finally, if processing farms are available, the user can reduce the execution time of intrinsically parallel tasks - e.g. data mining in HEP - by using PROOF, which will take care of optimally distributing the work over the available resources in a transparent way.

preprint2014arXiv

Objective Bayesian analysis of "on/off" measurements

In high-energy astrophysics, it is common practice to account for the background overlaid with the counts from the source of interest with the help of auxiliary measurements carried on by pointing off-source. In this "on/off" measurement, one knows the number of photons detected while pointing to the source, the number of photons collected while pointing away of the source, and how to estimate the background counts in the source region from the flux observed in the auxiliary measurements. For very faint sources, the number of detected photons is so low that the approximations which hold asymptotically are not valid. On the other hand, the analytical solution exists for the Bayesian statistical inference, which is valid at low and high counts. Here we illustrate the objective Bayesian solution based on the reference posterior and compare the result with the approach very recently proposed by \citet{knoetig2014}, discussing its most delicate points. In addition, we propose to compute the significance of the excess with respect to the background-only expectation with a method which is able to account for any uncertainty on the background and is valid for any photon count. This method is compared to the widely used significance formula by \citet{LiMa83}, which is based on asymptotic properties.

preprint2014arXiv

Solar Flare Measurements with STIX and MiSolFA

Solar flares are the most powerful events in the solar system and the brightest sources of X-rays, often associated with emission of particles reaching the Earth and causing geomagnetic storms, giving problems to communication, airplanes and even black-outs. X-rays emitted by accelerated electrons are the most direct probe of solar flare phenomena. The Micro Solar-Flare Apparatus (MiSolFA) is a proposed compact X-ray detector which will address the two biggest issues in solar flare modeling. Dynamic range limitations prevent simultaneous spectroscopy with a single instrument of all X-ray emitting regions of a flare. In addition, most X-ray observations so far are inconsistent with the high anisotropy predicted by the models usually adopted for solar flares. Operated at the same time as the STIX instrument of the ESA Solar Orbiter mission, at the next solar maximum (2020), they will have the unique opportunity to look at the same flare from two different directions: Solar Orbiter gets very close to the Sun with significant orbital inclination; MiSolFA is in a near-Earth orbit. To solve the cross-calibration problems affecting all previous attempts to combine data from different satellites, MiSolFA will adopt the same photon detectors as STIX, precisely quantifying the anisotropy of the X-ray emission for the first time. By selecting flares whose footpoints (the brightest X-ray sources, at the chromosphere) are occulted by the solar limb for one of the two detectors, the other will be able to study the much fainter coronal emission, obtaining for the first time simultaneous observations of all interesting regions. MiSolFA shall operate on board of a very small satellite, with several launch opportunities, and will rely on moiré imaging techniques.

preprint2012arXiv

Estimating the selection efficiency

The measurement of the efficiency of an event selection is always an important part of the analysis of experimental data. The statistical techniques which are needed to determine the efficiency and its uncertainty are reviewed. Frequentist and Bayesian approaches are illustrated, and the problem of choosing a meaningful prior is explicitly addressed. Several practical use cases are considered, from the problem of combining different samples to complex situations in which non-unit weights or non-independent selections have been used. The Bayesian approach allows to find analytical expressions which solve even the most complicate problems, which make use of the family of Beta distributions, the conjugate priors for the binomial sampling.

preprint2011arXiv

Reference analysis of the signal + background model in counting experiments

The model representing two independent Poisson processes, labelled as "signal" and "background" and both contributing at the same time to the total number of counted events, is considered from a Bayesian point of view. This is a widely used model for the searches of rare or exotic events in presence of some background source, as for example in the searches performed by the high-energy physics experiments. In the assumption of some prior knowledge about the background yield, a reference prior is obtained for the signal alone and its properties are studied. Finally, the properties of the full solution, the marginal reference posterior, are illustrated with few examples.

preprint2011arXiv

Statistical methods used in ATLAS for exclusion and discovery

The statistical methods used by the ATLAS Collaboration for setting upper limits or establishing a discovery are reviewed, as they are fundamental ingredients in the search for new phenomena. The analyses published so far adopted different approaches, choosing a frequentist or a Bayesian or a hybrid frequentist-Bayesian method to perform a search for new physics and set upper limits. In this note, after the introduction of the necessary basic concepts of statistical hypothesis testing, a few recommendations are made about the preferred approaches to be followed in future analyses.

preprint2010arXiv

Reconstruction of stop quark mass at the LHC

The cascade mass reconstruction approach was applied to simulated production of the lightest stop quark at the LHC in the cascade decay $\tilde{g} \to \sTop \, \Top \to \tildeχ_{2}^{0} \, \Top \, \Top \to \tilde{\ell}_{R} \, \ell \, \Top \, \Top \to \tildeχ_{1}^{0} \, \ell \, \ell \, \Top \, \Top$ with top quarks decaying into hadrons. The stop quark mass was reconstructed assuming that the masses of gluino, slepton and of the two lightest neutralinos were reconstructed in advance. A data sample set for the SU3 model point containing 400k SUSY events was generated which corresponded to an integrated luminosity of about 20 $\rm fb^{-1}$ at 14 TeV. These events were passed through the AcerDET detector simulator, which parametrized the response of a generic LHC detector. The mass of the $\tilde{t}_{1}$ was reconstructed with a precision of about $10\%$.

preprint2006arXiv

Searches for Cosmic Antimatter

We know from experimental high energy physics that whenever matter is created, an equal amount of antimatter is also created. However, we live in a large region of the universe where the antimatter can not constitute more than a very small fraction of the total mass. The cosmic antimatter problem has been addressed since the beginning of modern cosmology, but no definite answer has been formulated despite of the several approaches that can be found in the literature. In this paper we will make a historical review and we will focus on the experimental techniques that has been proposed to reveal directly and indirectly the presence of cosmic antimatter in the universe. Indirect searches can be carried on with the measurements of the electromagnetic radiation in the microwave and gamma-ray range, and of the neutrino flavour, whereas direct searches lay on the measurement of the cosmic rays and probe shorter distances. Finally, the current limits on the cosmic antimatter to matter ratio are compared to the sensitivity of future experiments.

preprint2004arXiv

The Local Interstellar Spectrum of Cosmic Ray Electrons

The direct measurements of electrons and positrons over the last 30 years, corrected for the solar effect in the force-field approximation, are considered. The resulting overall electron spectrum may be fitted with a single power law above few GeV with spectral index ($γ_{-} = 3.41 \pm 0.02$), consistent with the spectral index of the positron spectrum ($γ_{+} = 3.40 \pm 0.06$), therefore suggesting a common acceleration process for both species. We propose that the engine was a shock wave originating from the last supernova explosion among those that formed the local bubble. In addition, at low energy, the electron spectrum measured during the last $A+$ solar phase is damped, whereas the positron spectrum is well represented by a single power law down to the lowest inferred local interstellar energy (0.8 GeV). We suggest that this difference arises from a time- and charge-dependent effect of the solar modulation that is not taken into account by the force-field approximation.