125444963
Causality and the superhorizon power spectrum
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
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.
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.
119390803
Statistical Physics for Cosmic Structures
The recent observations of galaxy and dark matter clumpy distributions have provided new elements to the understanding of the problem of cosmological structure formation. The strong clumping characterizing galaxy structures seems to be present in the overall mass distribution and its relation to the highly isotropic Cosmic Microwave Background Radiation represents a fundamental problem. The extension of structures, the formation of power-law correlations characterizing the strongly clustered regime and the relation between dark and visible matter are the key problems both from an observational and a theoretical point of view. We discuss recent progresses in the studies of structure formation by using concepts and methods of statistical physics.
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.
109891384
Production and test of a readout chip for the ALICE SDD experiment
The paper summarizes the design, the fabrication and test of a chip for the silicon drift detector experiment, part of the A Large Ion Collider Experiment (ALICE) at CERN. The chip performs data reduction via bidimensional compression and packing for the readout chain of the experiment. The chip interfaces with front-end electronics and with the counting room. It is synchronized with a 40 MHz system master clock and configured via a serial signal. The work presents the tests that were performed to characterize the chip and it exploits the final yield of 89% over 700 fabricated chips. The whole tests were performed in laboratory and the chip was also tested in a test beam at CERN in November 2004.
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.
122993753
Recent results and perspectives of the NEMO project
Abstract The latest results and the activities towards the realization of a km 3 Cherenkov neutrino detector carried out by the NEMO Collaboration are described. The realization of a Phase-1 project has validated all relevant technologies proposed for the realization of the km 3 detector on a test site at 2000 m depth. The realization of a new infrastructure on the candidate Capo Passero site (for Phase-2 project) will provide the possibility to test detector components at 3500 m depth.
56367755
Design of an integrated particle detector-cell based on latchup effect
The paper describes an innovative idea for a silicon pixel detector. The principle is based on latchup effect that is common in to-date CMOS technologies working in a radiation environment. A prototype was constructed in the past with commercial components and here is described in detail an integrated version of the design. In principle the detector has low triggering and dead time, can operate at room temperature, does not require a high voltage power supply and is intrinsically tolerant to radiation. As a latchup-based detector can be constructed using state-of-the-art CMOS technologies, here it is presented a design of a prototype that will be implemented on a commercial 0.35 μm Bi-CMOS technology and tested at Electronics Department of Torino Politecnico.
110341132
Development of an electronic board for a neutrino telescope project
The NEMO (NEutrino Mediterranean Observatory) collaboration is involved in research and development for the construction of an underwater km3 scale Cherenkov neutrino detector. The detector will consist of about four thousands of optical modules that interface with coaxial cables to electronics cards. The detector is connected to the shore by an electro-optical cable for data transmission and power supply. The board also provides signal synchronization, filtering, data compression and packing. We describe the details of this electronic control part, which has been developed using commercial components and the very high-speed, Hardware Description Language (VHDL). The design was implemented on a programmable device. A test-bench system was also designed using a PC-based acquisition board running on the National Instrument LabVIEW environment.