14013791
Initial Conditions, Discreteness and Non-Linear Structure Formation in Cosmology
In this lecture we address three different but related aspects of the initial continuous fluctuation field in standard cosmological models. Firstly we discuss the properties of the so-called Harrison-Zeldovich like spectra. This power spectrum is a fundamental feature of all current standard cosmological models. In a simple classification of all stationary stochastic processes into three categories, we highlight with the name “super- homogeneous” the properties of the class to which models like this, with P(0) = 0, belong. In statistical physics language they are well described as glass-like. Secondly, the initial continuous density field with such small amplitude correlated Gaussian fluctuations must be discretised in order to set up the initial particle distribution used in gravitational N-body simulations. We discuss the main issues related to the effects of discretisation, particularly concerning the effect of particle induced fluctuations on the statistical properties of the initial conditions and on the dynamical evolution of gravitational clustering.
14466810
Basic properties of galaxy clustering in the light of recent results from the Sloan Digital Sky Survey
We discuss some of the basic implications of recent results on galaxy correlations published by the SDSS collaboration. In particular we focus on the evidence which has been recently presented for the scale and nature of the transition to homogeneity in the galaxy distribution, and results which describe the dependence of clustering on luminosity. The two questions are in fact strictly entangled, as the stability of the measure of the amplitude of the correlation function depends on the scale at which the mean density becomes well defined. We note that the recent results which indicate the convergence to well defined homogeneity in a volume equivalent to that of a sphere of radius 70 Mpc/h, place in doubt previous detections of luminosity bias from measures of the amplitude of the correlation function. We emphasize that the way to resolve these issues is to first use, in volume limited samples corresponding to different ranges of luminosity, the unnormalized two point statistics to establish the scale (and value) at which the mean density becomes well defined. We note also that the recent SDSS results for these statistics are in good agreement with those obtained by us through analyses of many previous samples, confirming in particular that the galaxy distribution is well described by a fractal dimension D 2 up to a scale of at least 20 Mpc/h. We discuss critically the agreement of this new data with current theoretical models.
31106687
Gravitational dynamics of an infinite shuffled lattice of particles.
We study, using numerical simulations, the dynamical evolution of self-gravitating point particles in static Euclidean space, starting from a simple class of infinite "shuffled lattice" initial conditions. These are obtained by applying independently to each particle on an infinite perfect lattice a small random displacement, and are characterized by a power spectrum (structure factor) of density fluctuations which is quadratic in the wave number k, at small k. For a specified form of the probability distribution function of the "shuffling" applied to each particle, and zero initial velocities, these initial configurations are characterized by a single relevant parameter: the variance delta(2) of the "shuffling" normalized in units of the lattice spacing l. The clustering, which develops in time starting from scales around l, is qualitatively very similar to that seen in cosmological simulations, which begin from lattices with applied correlated displacements and incorporate an expanding spatial background. From very soon after the formation of the first nonlinear structures, a spatiotemporal scaling relation describes well the evolution of the two-point correlations. At larger times the dynamics of these correlations converges to what is termed "self-similar" evolution in cosmology, in which the time dependence in the scaling relation is specified entirely by that of the linearized fluid theory. Comparing simulations with different delta, different resolution, but identical large scale fluctuations, we are able to identify and study features of the dynamics of the system in the transient phase leading to this behavior. In this phase, the discrete nature of the system explicitly plays an essential role.
15984757
Complexity in cosmic structures
We discuss correlation properties of a general mass density field introducing a classification of structures based on their complexity. Standard cosmological models for primordial mass fluctuations are characterized by a sort of large-scale stochastic order, that we call super-homogeneity to highlight the fact that mass fluctuations increase as a function of scale in the slowest-possible way for any stochastic mass field. On the other hand, the galaxy spatial distribution show complex structures with a high degree of inhomogeneity and fractal-like spatial correlations up to some relevant cosmological scale. The theoretical problem of cosmological structure formation should then explain the growth of strongly correlated and non-linear structures from the very uniform field of density fluctuations given as standard initial condition.
9116544
Gravitational dynamics of an infinite shuffled lattice: Particle coarse-graining, nonlinear clustering, and the continuum limit.
We study the evolution under their self-gravity of particles evolving from infinite "shuffled lattice" initial conditions. We focus here specifically on the comparison between the evolution of such a system and that of "daughter" coarse-grained particle distributions. These are sparser (i.e., lower density) particle distributions, defined by a simple coarse-graining procedure, which share the same large-scale mass fluctuations. We consider both the case that such coarse-grainings are performed (i) on the initial conditions, and (ii) at a finite time with a specific additional prescription. In numerical simulations we observe that, to a first approximation, these coarse-grainings represent well the evolution of the two-point correlation properties over a significant range of scales. We note, in particular, that the form of the two-point correlation function in the original system, when it is evolving in the asymptotic "self-similar" regime, may be reproduced well in a daughter coarse-grained system in which the dynamics are still dominated by two-body (nearest neighbor) interactions. This provides a simple physical description of the origin of the form of part of the asymptotic nonlinear correlation function. Using analytical results on the early time evolution of these systems, however, we show that small observed differences between the evolved system and its coarse-grainings at the initial time will in fact diverge as the ratio of the coarse-graining scale to the original interparticle distance increases. The second coarse-graining studied, performed at a finite time in a specified manner, circumvents this problem. It also makes it more physically transparent why gravitational dynamics from these initial conditions tends toward a self-similar evolution. We finally discuss the precise definition of a limit in which a continuum (specifically Vlasov-type) description of the observed linear and nonlinear evolution should be applicable. This requires the introduction of an additional intrinsic length scale (e.g., a physical smoothing in the force at small scales), which is kept fixed as the particle density diverges. In this limit the different coarse-grainings are equivalent and leave the evolution of the "mother" system invariant.
1069360
Fluctuations in galaxy counts: A new test for homogeneity vs. fractality
Fractal properties are usually characterized by means of various statistical tools which deal with spatial average quantities. Here we focus on the determination of fluctuations around the average counts and we develop a test for the study of galaxy distribution both in redshift and magnitude space. Fluctuations in the counts of galaxies, in a fractal distribution, are of the same order of the average number at all scales as a function of redshift and magnitude. We point out that the study of these kinds of fluctuations can be a powerful test to understand the nature of galaxy clustering at very large scales.
72023452
Preliminary timing measurements on a data acquisition chain for a SiPM-based detector for prostate imaging
Prostate cancer (PC) is one of the most common diseases in western countries and a leading cause of cancer death. A variety of treatment options are available but a precise disease characterization is needed: evaluation of cancer location, size, and extent and an indication of tumor aggressiveness. The current standard for diagnosing PC is transrectal ultrasound guided sextant biopsy. A novel multidisciplinary approach is required. Imaging may play a key role provided that dedicated prostate imagers and procedures are available: considerable improvements have been achieved in diagnosis with the Magnetic Resonance Imaging (MRI) and nuclear medicine (Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT)) techniques. Nevertheless, due to sub-optimal prostate imaging geometries with these generic large instruments preventing separation of the signal from surrounding organs, the sensitivity, spatial resolution and lesion contrast detected are lower compared to what can be potentially achievable with optimized dedicated prostate imagers and procedures. Fully exploiting the Time Of Flight (TOF) capability would allow not only to increase the Signal-to-Noise ratio (SNR) / Noise Equivalent Count Rate (NECR) but also to get rid of the huge background coming from neighboring organs i.e. the bladder. Recently a new research project was initiated by a large INFN collaboration, and a “TOF-PET and MRI for prostate cancer diagnosis and follow up experiment” (TOPEM) was financed as a 3-year experiment by the Italian “INFN Commissione Scientifica Nazionale V” with the goal of designing, building and testing in phantom tests an endorectal PET-TOF probe compatible with MRI.
109308730
Heavy-Ions induced SEE effects measurements for the STRURED ASIC
With the aim of developing a radiation-tolerant circuit, a digital test microelectronic device has been designed and fabricated by using a standard-cell library of a 130-nm CMOS technology, including three different architectures to correct circuit malfunctions induced by the occurrence of Single-Event Effects (SEEʼs). SEEʼs are one of the main reasons of failures affecting electronic circuits operating in harsh radiation environments, such as in experiments performed at High Energy Physics (HEP) colliders or in apparatus to be operated in Space. On the same digital circuit specifically designed, three redundant architectures added to a basic scheme have been implemented in order to evaluate their effectiveness to prevent SEE. This may give an indication on their usage in future digital circuits specifically designed for the above mentioned applications. We present the results of SEE cross section measurements performed on a test digital device exposed to a high energy heavy ion beam at the SIRAD irradiation facility of the INFN National Laboratories of Legnaro (Padova Italy).
119249031
Measurement of the atmospheric muon flux with the NEMO Phase-1 detector
Abstract The NEMO Collaboration installed and operated an underwater detector including prototypes of the critical elements of a possible underwater km 3 neutrino telescope: a four-floor tower (called Mini-Tower) and a Junction Box. The detector was developed to test some of the main systems of the km 3 detector, including the data transmission, the power distribution, the timing calibration and the acoustic positioning systems as well as to verify the capabilities of a single tridimensional detection structure to reconstruct muon tracks. We present results of the analysis of the data collected with the NEMO Mini-Tower. The position of photomultiplier tubes (PMTs) is determined through the acoustic position system. Signals detected with PMTs are used to reconstruct the tracks of atmospheric muons. The angular distribution of atmospheric muons was measured and results compared to Monte Carlo simulations.
109271274
Proposal of a sparsification circuit for mixed-mode MAPS detectors
Abstract This paper shows the design of a new mixed-mode chip built up of a bidimensional matrix of Monolithic Active Pixel Sensor (MAPS), and of an off-pixel digital readout sparsification circuit. The mixed-mode approach extends the performance of the chip as both the matrix and the readout logic have been developed separately. In particular, the matrix of MAPS has been described with a VHDL model and used as a macro-cell block within a bigger digital design that includes the readout logic. The global place-and-route has been also digitally carried out. The design was submitted via the STM 0.13 μm CMOS technology and readout logic is based on standard cells by implementing an optimized token-like technique. The proposed fast readout architecture should extend the flexibility of the MAPS devices to be also used in first level triggers on tracks in vertex detectors. The design, which has been developed within the SLIM5 Collaboration, is aimed at overcoming the readout speed limit of future large-matrix pixel detectors for particle tracking, by matching the requirements of future high-energy physics experiments.
46500747
Implementation of a bidimensional compressor for a high-energy physics experiment
CARLOSv3 is the third version of a chip that plays a significant role in the data acquisition chain of the A Large Ion Collider Experiment (ALICE) silicon drift detector (SDD). ALICE is one of the foremost high-energy physics experiments (HEPE) conducted within the Large Hadron Collider at CERN, the European Organization for Nuclear Research in Geneva. CARLOSv3 was principally designed and built for the on-line compression of the input dataset originating from a physical bidimensional silicon sensor. To compress a bidimensional dataset, a bidimensional data compressor was required. The compressor was designed for the ALICE SDD Experiment but could be applied to all experiments in which an incoming stream dataset originates from a bidimensional sensor.