24 research outputs found
Exact numerical simulation of power-law noises
Many simulations of stochastic processes require colored noises: I describe
here an exact numerical method to simulate power-law noises: the method can be
extended to more general colored noises, and is exact for all time steps, even
when they are unevenly spaced (as may often happen for astronomical data, see
e.g. N. R. Lomb, Astrophys. Space Sci. {\bf 39}, 447 (1976)). The algorithm has
a well-behaved computational complexity, it produces a nearly perfect Gaussian
noise, and its computational efficiency depends on the required degree of noise
Gaussianity.Comment: 14 postscript figures, accepted for publication on Phys. Rev.
Artifacts with uneven sampling of red noise
The vast majority of sampling systems operate in a standard way: at each tick
of a fixed-frequency master clock a digitizer reads out a voltage that
corresponds to the value of some physical quantity and translates it into a bit
pattern that is either transmitted, stored, or processed right away. Thus
signal sampling at evenly spaced time intervals is the rule: however this is
not always the case, and uneven sampling is sometimes unavoidable.
While periodic or quasi-periodic uneven sampling of a deterministic signal
can reasonably be expected to produce artifacts, it is much less obvious that
the same happens with noise: here I show that this is indeed the case only for
long-memory noise processes, i.e., power-law noises with . The resulting artifacts are usually a nuisance although they can be
eliminated with a proper processing of the signal samples, but they could also
be turned to advantage and used to encode information.Comment: 5 figure
Quantum Fluctuations of the Gravitational Field and Propagation of Light: a Heuristic Approach
Quantum gravity is quite elusive at the experimental level; thus a lot of
interest has been raised by recent searches for quantum gravity effects in the
propagation of light from distant sources, like gamma ray bursters and active
galactic nuclei, and also in earth-based interferometers, like those used for
gravitational wave detection. Here we describe a simple heuristic picture of
the quantum fluctuations of the gravitational field that we have proposed
recently, and show how to use it to estimate quantum gravity effects in
interferometers.Comment: LaTeX2e, 8 pages, 2 eps figures: Talk presented at QED2000, 2nd
Workshop on Frontier Tests of Quantum Electrodynamics and Physics of the
Vacuum; included in conference proceeding
Compton sources for the observation of elastic photon-photon scattering events
We present the design of a photon-photon collider based on conventional Compton gamma sources for the observation of elastic \u3b3\u3b3 scattering. Two symmetric electron beams, generated by photocathodes and accelerated in linacs, produce two primary gamma rays through Compton backscattering with two high energy lasers. The elastic photon-photon scattering is analyzed by start-to-end simulations from the photocathodes to the detector. A new Monte Carlo code has been developed ad hoc for the counting of the QED events. Realistic numbers of the secondary gamma yield, obtained by using the parameters of existing or approved Compton devices, a discussion of the feasibility of the experiment and of the nature of the background are presented
Emergent Properties of Tumor Microenvironment in a Real-life Model of Multicell Tumor Spheroids
Multicellular tumor spheroids are an important {\it in vitro} model of the
pre-vascular phase of solid tumors, for sizes well below the diagnostic limit:
therefore a biophysical model of spheroids has the ability to shed light on the
internal workings and organization of tumors at a critical phase of their
development. To this end, we have developed a computer program that integrates
the behavior of individual cells and their interactions with other cells and
the surrounding environment. It is based on a quantitative description of
metabolism, growth, proliferation and death of single tumor cells, and on
equations that model biochemical and mechanical cell-cell and cell-environment
interactions. The program reproduces existing experimental data on spheroids,
and yields unique views of their microenvironment. Simulations show complex
internal flows and motions of nutrients, metabolites and cells, that are
otherwise unobservable with current experimental techniques, and give novel
clues on tumor development and strong hints for future therapies.Comment: 20 pages, 10 figures. Accepted for publication in PLOS One. The
published version contains links to a supplementary text and three video
file
Search for gravitational-wave bursts in the third Advanced LIGO-Virgo run with coherent WaveBurst enhanced by Machine Learning
This paper presents a search for generic short-duration gravitational-wave
(GW) transients (or GW bursts) in the data from the third observing run of
Advanced LIGO and Advanced Virgo. We use coherent WaveBurst (cWB) pipeline
enhanced with a decision-tree classification algorithm for more efficient
separation of GW signals from noise transients. The machine-learning (ML)
algorithm is trained on a representative set of noise events and a set of
simulated stochastic signals that are not correlated with any known signal
model. This training procedure preserves the model-independent nature of the
search. We demonstrate that the ML-enhanced cWB pipeline can detect GW signals
at a larger distance than previous model-independent searches. The sensitivity
improvements are achieved across the broad spectrum of simulated signals, with
the goal of testing the robustness of this model-agnostic search. At a
false-alarm rate of one event per century, the detectable signal amplitudes are
reduced up to almost an order of magnitude, most notably for the single-cycle
signal morphologies. This ML-enhanced pipeline also improves the detection
efficiency of compact binary mergers in a wide range of masses, from stellar
mass to intermediate-mass black holes, both with circular and elliptical
orbits. After excluding previously detected compact binaries, no new
gravitational-wave signals are observed for the two-fold Hanford-Livingston and
the three-fold Hanford-Livingston-Virgo detector networks. With the improved
sensitivity of the all-sky search, we obtain the most stringent constraints on
the isotropic emission of gravitational-wave energy from short-duration burst
sources.Comment: 15 pages, 7 figure
Gleam: the GLAST Large Area Telescope Simulation Framework
This paper presents the simulation of the GLAST high energy gamma-ray
telescope. The simulation package, written in C++, is based on the Geant4
toolkit, and it is integrated into a general framework used to process events.
A detailed simulation of the electronic signals inside Silicon detectors has
been provided and it is used for the particle tracking, which is handled by a
dedicated software. A unique repository for the geometrical description of the
detector has been realized using the XML language and a C++ library to access
this information has been designed and implemented.Comment: 10 pages, Late
VIP-2 at LNGS: An experiment on the validity of the Pauli Exclusion Principle for electrons
Abstract. We are experimentally investigating possible violations of standard quantum mechanics predictions in the Gran Sasso underground laboratory in Italy. We test with high precision the Pauli Exclusion Principle and the collapse of the wave function (collapse models). We present our method of searching for possible small violations of the Pauli Exclusion Principle (PEP) for electrons, through the search for \u201canomalous\u201d X-ray transitions in copper atoms. These transitions are produced by \u201cnew\u201d electrons (brought inside the copper bar by circulating current) which can have the possibility to undergo Pauli-forbidden transition to the 1s level already occupied by two electrons. We describe the VIP2 (VIolation of the Pauli Exclusion Principle) experimental data taking at the Gran Sasso underground laboratories. The goal of VIP2 is to test the PEP for electrons in agreement with the Messiah-Greenberg superselection rule with unprecedented accuracy, down to a limit in the probability that PEP is violated at the level of 10^ 1231. We show preliminary experimental results and discuss implications of a possible violation
Experimental tests of Quantum Mechanics: from Pauli Exclusion Principle Violation to spontaneous collapse models
The Pauli exclusion principle (PEP) and, more generally, the spin-statistics connection, is at the very basis of our understanding of matter. The PEP spurs, presently, a lively debate on its possible limits, deeply rooted in the very foundations of Quantum Field Theory. Therefore, it is extremely important to test the limits of its validity. Quon theory provides a suitable mathematical framework of possible violation of PEP, where the q violation parameter translates into a probability of violating PEP. Experimentally, setting a bound on PEP violation means confining the q-parameter to a value very close to either 1 (for bosons) or -1 (for fermions). The VIP (Violation of the Pauli exclusion principle) experiment established a limit on the probability that PEP is violated by electrons, using the method of searching for PEP forbidden atomic transitions in copper. We describe the experimental method, the obtained results, both in terms of the q-parameter and as probability of PEP violation, we briefly discuss the results and present future plans to go beyond the actual limit by upgrading the experimental technique using vetoed new spectroscopic fast Silicon Drift Detectors. We mention as well the possibility of using a similar experimental technique to search for eventual X-rays generated as a signature of the spontaneous collapse of the wave function, predicted by continuous spontaneous localization type theories
GLAST Large Area Telescope simulation tools
This paper presents the simulation of the GLAST high energy gamma-ray telescope. The simulation package, written in C++, is based on the Geant4 toolkit, and it is integrated into a general framework used to process events. A detailed simulation of the electronic signals inside silicon detectors has been provided and it is used for the particle tracking, which is handled by a dedicated software. A unique repository for the geometrical description of the detector has been realized using the XML language and a C++ library to access this information has been designed and implemented. A new event display based on the HepRep protocol is being implemented. The GLAST satellite parameters derived from the simulation are used in a fast simulator to obtain a "snapshot" of the gamma-ray sky. This paper outlines the contribution developed by the Italian GLAST software group