Analysis of the optical background sources in the ANTARES experiment and preliminary studies related to a larger scale detector

Il lavoro di tesi presentato si svolge nell'ambito dell'esperimento ANTARES, che prevede la costruzione nel Mediterraneo di un telescopio per la rivelazione di neutrini da sorgenti astrofisiche e mantiene una collaborazione con altri settori della scienza che studiano le profondità del mare. Il telescopio sfrutterà l'acqua del mare come mezzo Cherenkov per rivelare le particelle cariche prodotte dall'interazione dei neutrini con il mare stesso (o il fondale), e rivelerà la luce emessa tramite un array di fotomoltiplicatori distribuiti su dodici stringhe immerse a 2.4 km di profondità. L'energia di soglia per la ricostruzione di un evento è stimata intorno a 50 GeV e si calcola che la massima superficie efficace del telescopio sia dell'ordine di 0.1 km^2. Sorgenti di fondo per l'esperimento sono i neutrini e i muoni atmosferici, ma è presente anche un " fondo ottico" costituito dalla bioluminescenza e dalla luce Cherenkov risultante dai prodotti di decadimenti radioattivi dell'isotopo 40K presente nell'acqua. In questo lavoro di tesi è affrontato il problema del fondo ottico dell'esperimento, in particolar modo è stata effettuata un'analisi delle caratteristiche del fondo da bioluminescenza a partire dai dati disponibili grazie all'ultima stringa di test dell'esperimento, la Prototype Sector Line. Una migliore comprensione del fondo può aiutare ad ottimizzare gli algoritmi di trigger per ridurre il tempo morto del detector o aumentare l'efficacia della selezione. L'analisi presentata individua i parametri utili per simulare in maniera appropriata il fondo e valuta l'accuratezza delle simulazioni ora disponibili. La stessa analisi presenta inoltre interessanti risvolti nell'ambito della biologia, in quanto permette di estrarre informazioni sulla fauna bioluminescente del Mediterraneo, fino ad ora poco studiata. In vista dell'estensione dell'esperimento ad una fase ancora più promettente, con un telescopio dal volume sensibile dell'ordine del km^3, è stato inoltre tentato uno studio preliminare sulle potenzialità di un nuovo tipo di rivelatore di luce, l'Hybrid PhotoDiode (HPD), in sostituzione dei classici fotomoltiplicatori. Mediante un programma di simulazione dell'ottica elettronica è stato riprodotto un possibile modello di HPD in grado di soddisfare i requisiti dell'esperimento. I risultati di tale simulazione sono stati poi introdotti nel Monte Carlo dell'esperimento, per quantificarne (benché grossolanamente) gli effetti sulla ricostruzione degli eventi

gamma-ray DBSCAN: a clustering algorithm applied to Fermi-LAT gamma-ray data. I. Detection performances with real and simulated data

The Density Based Spatial Clustering of Applications with Noise (DBSCAN) is a topometric algorithm used to cluster spatial data that are affected by background noise. For the first time, we propose the use of this method for the detection of sources in gamma-ray astrophysical images obtained from the Fermi-LAT data, where each point corresponds to the arrival direction of a photon. We investigate the detection performance of the gamma-ray DBSCAN in terms of detection efficiency and rejection of spurious clusters, using a parametric approach, and exploring a large volume of the gamma-ray DBSCAN parameter space. By means of simulated data we statistically characterize the gamma-ray DBSCAN, finding signatures that differentiate purely random fields, from fields with sources. We define a significance level for the detected clusters, and we successfully test this significance with our simulated data. We apply the method to real data, and we find an excellent agreement with the results obtained with simulated data. We find that the gamma-ray DBSCAN can be successfully used in the detection of clusters in gamma-ray data. The significance returned by our algorithm is strongly correlated with that provided by the Maximum Likelihood analysis with standard Fermi-LAT software, and can be used to safely remove spurious clusters. The positional accuracy of the reconstructed cluster centroid compares to that returned by standard Maximum Likelihood analysis, allowing to look for astrophysical counterparts in narrow regions, minimizing the chance probability in the counterpart association. We find that gamma-ray DBSCAN is a powerful tool in the detection of clusters in gamma-ray data, this method can be used both to look for point-like sources, and extended sources, and can be potentially applied to any astrophysical field related with detection of clusters in data.Comment: Accepted for publication in A&

European and United States farmers’ markets: similarities, differences and potential developments

Farmers’ markets have always been the usual way of buying and selling rural products in the Western world. With the advent of supermarkets, farmers’ markets rapidly disappeared in many nations. However, in countries such as France and Italy, which place a high priority on food origin and regional specialisation, some farmers’ markets continued to exist, partly due to their mechanisms to identify and promote locally grown foods. The consumer’s desire to re-establish a bond with local food products, local growers and producers, together with the growing concern for food freshness and healthiness have been key drivers for the renaissance of farmers’ markets occurred in the latest years in many European countries and in the United States. Several studies, conducted across Europe and in the U.S., have shown rising consumers’ consideration of farmers’ markets as important sources of household food shopping and increasing attention of farmers to this sale channel, in a period in which their share of the “food dollar” is continuing to decrease. The objectives of this paper are to contribute to extend understanding of the main features of the European and United States farmers’ markets and raise critical questions about their potential development in the modern food system.Farmers’ markets, Local foods, Europe, U.S.A., Agricultural and Food Policy,

Searching for continuous gravitational wave sources in binary systems

We consider the problem of searching for continuous gravitational wave sources orbiting a companion object. This issue is of particular interest because the LMXB's, and among them Sco X-1, might be marginally detectable with 2 years coherent observation time by the Earth-based laser interferometers expected to come on line by 2002, and clearly observable by the second generation of detectors. Moreover, several radio pulsars, which could be deemed to be CW sources, are found to orbit a companion star or planet, and the LIGO/VIRGO/GEO network plans to continuously monitor such systems. We estimate the computational costs for a search launched over the additional five parameters describing generic elliptical orbits using match filtering techniques. These techniques provide the optimal signal-to-noise ratio and also a very clear and transparent theoretical framework. We provide ready-to-use analytical expressions for the number of templates required to carry out the searches in the astrophysically relevant regions of the parameter space, and how the computational cost scales with the ranges of the parameters. We also determine the critical accuracy to which a particular parameter must be known, so that no search is needed for it. In order to disentangle the computational burden involved in the orbital motion of the CW source, from the other source parameters (position in the sky and spin-down), and reduce the complexity of the analysis, we assume that the source is monochromatic and its location in the sky is exactly known. The orbital elements, on the other hand, are either assumed to be completely unknown or only partly known. We apply our theoretical analysis to Sco X-1 and the neutron stars with binary companions which are listed in the radio pulsar catalogue.Comment: 31 pages, LaTeX, 6 eps figures, submitted to PR

Robust Moment Closure Method for the Chemical Master Equation

The Chemical Master Equation (CME) is used to stochastically model biochemical reaction networks, under the Markovian assumption. The low-order statistical moments induced by the CME are often the key quantities that one is interested in. However, in most cases, the moments equation is not closed; in the sense that the first $n$ moments depend on the higher order moments, for any positive integer $n$. In this paper, we develop a moment closure technique in which the higher order moments are approximated by an affine function of the lower order moments. We refer to such functions as the affine Moment Closure Functions (MCF) and prove that they are optimal in the worst-case context, in which no a priori information on the probability distribution is available. Furthermore, we cast the problem of finding the optimal affine MCF as a linear program, which is tractable. We utilize the affine MCFs to derive a finite dimensional linear system that approximates the low-order moments. We quantify the approximation error in terms of the $$ induced norm of some linear system. Our results can be effectively used to approximate the low-order moments and characterize the noise properties of the biochemical network under study

The astrophysical science case for a decihertz gravitational-wave detector

We discuss the astrophysical science case for a decihertz gravitational-wave mission. We focus on unique opportunities for scientific discovery in this frequency range, including probes of type IA supernova progenitors, mergers in the presence of third bodies, intermediate mass black holes, seeds of massive black holes, improved sky localization, and tracking the population of merging compact binaries