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.
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.
15688477
Scaling and fluctuations in galaxy distribution: two tests to probe large scale structures
We present a brief introduction to the statistical properties of systems with large fluctuations. We point out that for such systems the relevant statistical quantities are scaling exponents and the nature of fluctuations is completely different from the one belonging to system with small fluctuations. We then propose two test to be performed on galaxy counts data as a function of apparent magnitude, which may clarify in a direct way the nature of the large scale galaxy clustering
14253637
Finite size effects on the galaxy number counts: Evidence for fractal behavior up to the deepest scale
We introduce and study two new concepts which are essential for the quantitative analysis of the statistical quality of the available galaxy samples. These are the dilution effect and the small scale fluctuations. We show that the various data that are considered as pointing to a homogenous distribution are all affected by these spurious effects and their interpretation should be completely changed. In particular, we show that finite size effects strongly affect the determination of the galaxy number counts, namely the number versus magnitude relation (N (< m)) as computed from the origin. When one computes N (< m) averaged over all the points of a redshift survey, one observes an exponent α = D/5 ≈ 0.4 compatible with the fractal dimension D ≈ 2 derived from the full correlation analysis. Instead the observation of an exponent α ≈ 0.6 at relatively small scales, where the distribution is certainly not homogeneous, is shown to be related to finite size effects. We conclude therefore that the observed counts correspond to a fractal distribution with dimension D ≈ 2 in the entire range 12 ⪅ m ⪅ 28, that is to say the largest scales ever probed for luminous matter. In addition our results permit to clarify various problems of the angular catalogs, and to show their compatibility with the fractal behaviour. We consider also the distribution of Radio-galaxies, Quasars and γ-ray bursts, and we show their compatibility with a fractal structure with D ≈ 1.6–1.8. Finally we have established a quantitative criterion that allows us to define and predict the statistical validity of a galaxy catalog (angular or three-dimensional).
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.
121134571
Procedures and results of the measurements on large area photomultipliers for the NEMO project
Abstract The selection of the photomultiplier plays a crucial role in the R&D activity related to a large-scale underwater neutrino telescope. This paper illustrates the main procedures and facilities used to characterize the performances of 72 large area photomultipliers, Hamamatsu model R7081 sel. The voltage to achieve a gain of 5×10 7 , dark count rate and single photoelectron time and charge properties of the overall response were measured with a properly attenuated 410 nm pulsed laser. A dedicated study of the spurious pulses was also performed. The results prove that the photomultipliers comply with the general requirements imposed by the project.
109494522
Design, realization and test of a rad-hard 2D-compressor and packing chip for high energy physics experiments
CARLOSv3 is a third version of a chip that plays a significant role in the data acquisition chain of the A Large Ion Collider Experiment Inner Tracking System experiment. It has been designed and realized with a 0.25 μm CMOS 3-metal rad-hard digital library. The chip elaborates and compresses, by means of a bi-dimensional compressor, data belonging to a so-called event. The compressor looks for cross-shaped clusters within the whole data set coming from the silicon detector. To test the chip a specific PCB has been designed; it contains the connectors for probing the ASIC with a pattern generator and a logic state analyzer. The chip is inserted on the PCB using a ZIF socket. This allows to test the 35 packaged samples out of the total amount of bare chips we have from the foundry. The test phase has shown that 32 out of 35 chips under test work well. It is planned to redesign a new version of the chip by adding extra features and to submit the final version of CARLOS upon the final DAQ chain will be totally tested both in Bologna and at CERN.
110590916
NEMO: Status of the Project
The activities towards the realisation of a km 3 Cherenkov neutrino detector, carried out by the NEMO Collaboration are described. Long term exploration of a 3500 m deep site close to the Sicilian coast has shown that it is optimal for the installation of the detector. A complete feasibility study, that has considered all the components of the detector as well as its deployment, has been carried out demonstrating that technological solutions exist for the realization of an underwater km 3 detector. The realization of a technological demonstrator (the NEMO Phase 1 project) is under way.
61680556
Realization and test of a 0.25$\mu$m rad-hard chip for ALICE ITS data acquisition chain
120517559
A PowerPC-based control system for the Read-Out-Driver module of the ATLAS IBL
The ATLAS experiment at LHC planned to upgrade the existing Pixel Detector with the insertion of an innermost silicon layer, called Insertable B-layer (IBL). A new front-end ASIC has been foreseen (named FE-I4) and it will be read out with improved off-detector electronics. In particular, the new Read-Out Driver card (ROD) is a VME-based board designed to process a four-fold data throughput. Moreover, the ROD hosts the electronics devoted to control operations whose main tasks are providing setup busses to access configuration registers on several FPGAs, receiving configuration data from external PCs, managing triggers and running calibration procedures. In parallel with a backward-compatible solution with a Digital Signal Processor (DSP), a new ROD control circuitry with a PowerPC embedded into an FPGA has been implemented. In this paper the status of the PowerPC-based control system will be outlined with major focus on firmware and software development strategies.