37,604 research outputs found
Ghost imaging with the human eye
Computational ghost imaging relies on the decomposition of an image into
patterns that are summed together with weights that measure the overlap of each
pattern with the scene being imaged. These tasks rely on a computer. Here we
demonstrate that the computational integration can be performed directly with
the human eye. We use this human ghost imaging technique to evaluate the
temporal response of the eye and establish the image persistence time to be
around 20 ms followed by a further 20 ms exponential decay. These persistence
times are in agreement with previous studies but can now potentially be
extended to include a more precise characterisation of visual stimuli and
provide a new experimental tool for the study of visual perception
Univariate Mean Change Point Detection: Penalization, CUSUM and Optimality
The problem of univariate mean change point detection and localization based
on a sequence of independent observations with piecewise constant means has
been intensively studied for more than half century, and serves as a blueprint
for change point problems in more complex settings. We provide a complete
characterization of this classical problem in a general framework in which the
upper bound on the noise variance, the minimal spacing
between two consecutive change points and the minimal magnitude of the
changes, are allowed to vary with . We first show that consistent
localization of the change points, when the signal-to-noise ratio , is impossible. In contrast, when
diverges with at the rate of at least
, we demonstrate that two computationally-efficient change
point estimators, one based on the solution to an -penalized least
squares problem and the other on the popular wild binary segmentation
algorithm, are both consistent and achieve a localization rate of the order
. We further show that such rate is minimax
optimal, up to a term
Optimal change point detection and localization in sparse dynamic networks
We study the problem of change point localization in dynamic networks models. We assume that we observe a sequence of independent adjacency matrices of the same size, each corresponding to a realization of an unknown inhomogeneous Bernoulli model. The underlying distribution of the adjacency matrices are piecewise constant, and may change over a subset of the time points, called change points. We are concerned with recovering the unknown number and positions of the change points. In our model setting, we allow for all the model parameters to change with the total number of time points, including the network size, the minimal spacing between consecutive change points, the magnitude of the smallest change and the degree of sparsity of the networks. We first identify a region of impossibility in the space of the model parameters such that no change point estimator is provably consistent if the data are generated according to parameters falling in that region. We propose a computationally-simple algorithm for network change point localization, called network binary segmentation, that relies on weighted averages of the adjacency matrices. We show that network binary segmentation is consistent over a range of the model parameters that nearly cover the complement of the impossibility region, thus demonstrating the existence of a phase transition for the problem at hand. Next, we devise a more sophisticated algorithm based on singular value thresholding, called local refinement, that delivers more accurate estimates of the change point locations. Under appropriate conditions, local refinement guarantees a minimax optimal rate for network change point localization while remaining computationally feasible
Competition and Economic Growth: an Empirical Analysis for a Panel of 20 OECD Countries
This paper aims at analyzing, from an empirical point of view, the relationship between product market competition and economic growth, using the data on multi-factor productivity for a panel of 20 OECD countries over a period 1995-2005, and considering the role of the distance from the technological frontier in the growth process. Section A examines the impact of economic freedom and of the distance to frontier on the level and on the growth rate of multi-factor productivity. The analysis distinguishes between the indicators of business freedom and trade freedom, as proxies for the competitive pressures coming from domestic market and from foreign market. Then, trade liberalizations are more beneficial for the countries far from the frontier, because they can exploit the opportunities given by international trade also in order to adopt the existing technologies developed by the advanced economies. On the other hand, business liberalizations are more advantageous for the countries close to the frontier, because the elimination of regulatory barriers increases the possibility of entry in the market and then rises the potential competition to the incumbent firms. Section B studies the effect of product market regulation, employment protection legislation and of the distance to frontier on the level and on the growth rate of multi-factor productivity. Product market liberalization as well as labour market deregulation determine an increase of total factor productivity: moreover, the interaction of market rigidities with the distance to the frontier mostly displays an innovationenhancing effect, since the positive effect of market liberalizations on TFP is higher for the countries close to the frontier, where the existing technology level would reinforce the incentive for innovation.multi-factor productivity; economic freedom; product market regulation; employment protection legislation; distance to frontier
Two-pion Bose-Einstein correlations in central Pb-Pb collisions at = 2.76 TeV
The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb
collisions at TeV at the Large Hadron Collider is
presented. We observe a growing trend with energy now not only for the
longitudinal and the outward but also for the sideward pion source radius. The
pion homogeneity volume and the decoupling time are significantly larger than
those measured at RHIC.Comment: 17 pages, 5 captioned figures, 1 table, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/388
Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV
We report the first measurement of charged particle elliptic flow in Pb-Pb
collisions at 2.76 TeV with the ALICE detector at the CERN Large Hadron
Collider. The measurement is performed in the central pseudorapidity region
(||<0.8) and transverse momentum range 0.2< < 5.0 GeV/. The
elliptic flow signal v, measured using the 4-particle correlation method,
averaged over transverse momentum and pseudorapidity is 0.087 0.002
(stat) 0.004 (syst) in the 40-50% centrality class. The differential
elliptic flow v reaches a maximum of 0.2 near = 3
GeV/. Compared to RHIC Au-Au collisions at 200 GeV, the elliptic flow
increases by about 30%. Some hydrodynamic model predictions which include
viscous corrections are in agreement with the observed increase.Comment: 10 pages, 4 captioned figures, published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/389
Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at TeV
Inclusive transverse momentum spectra of primary charged particles in Pb-Pb
collisions at = 2.76 TeV have been measured by the ALICE
Collaboration at the LHC. The data are presented for central and peripheral
collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross
section. The measured charged particle spectra in and GeV/ are compared to the expectation in pp collisions at the same
, scaled by the number of underlying nucleon-nucleon
collisions. The comparison is expressed in terms of the nuclear modification
factor . The result indicates only weak medium effects ( 0.7) in peripheral collisions. In central collisions,
reaches a minimum of about 0.14 at -7GeV/ and increases
significantly at larger . The measured suppression of high- particles is stronger than that observed at lower collision energies,
indicating that a very dense medium is formed in central Pb-Pb collisions at
the LHC.Comment: 15 pages, 5 captioned figures, 3 tables, authors from page 10,
published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/98
GLAST, the Gamma-ray Large Area Space Telescope
GLAST, a detector for cosmic gamma rays in the range from 20 MeV to 300 GeV,
will be launched in space in 2005. Breakthroughs are expected in particular in
the study of particle acceleration mechanisms in space and of gamma ray bursts,
and maybe on the search for cold dark matter; but of course the most exciting
discoveries could come from the unexpected.Comment: 6 pages, 3 figures. Invited talk at the hird International Workshop
"New Worlds in Astroparticle Physics", September 2000, University of the
Algarve. Faro, Portugal. To be published in the Proceeding
Solutions of a two-particle interacting quantum walk
We study the solutions of the interacting Fermionic cellular automaton
introduced in Ref. [Phys Rev A 97, 032132 (2018)]. The automaton is the
analogue of the Thirring model with both space and time discrete. We present a
derivation of the two-particles solutions of the automaton, which exploits the
symmetries of the evolution operator. In the two-particles sector, the
evolution operator is given by the sequence of two steps, the first one
corresponding to a unitary interaction activated by two-particle excitation at
the same site, and the second one to two independent one-dimensional Dirac
quantum walks. The interaction step can be regarded as the discrete-time
version of the interacting term of some Hamiltonian integrable system, such as
the Hubbard or the Thirring model. The present automaton exhibits scattering
solutions with nontrivial momentum transfer, jumping between different regions
of the Brillouin zone that can be interpreted as Fermion-doubled particles, in
stark contrast with the customary momentum-exchange of the one dimensional
Hamiltonian systems. A further difference compared to the Hamiltonian model is
that there exist bound states for every value of the total momentum, and even
for vanishing coupling constant. As a complement to the analytical derivations
we show numerical simulations of the interacting evolution.Comment: 16 pages, 6 figure
D meson nuclear modification factors in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV with the ALICE detector
The measurement of D meson production provides key tests for parton
energy-loss models, which predict that charm quarks should experience less
in-medium energy loss than light quarks and gluons. The ALICE experiment has
measured the production of prompt D^(0), D^(+) and D^(*+) mesons in pp and
Pb-Pb collisions at the LHC at sqrt(s) = 7 and 2.76 TeV and at sqrt(s_NN) =
2.76 TeV, respectively, via the exclusive reconstruction of their hadronic
decay. The pT-differential production yields in the range 2 < pT < 16 GeV/c at
central rapidity, |y| < 0.5, were used to calculate the nuclear modification
factor. A suppression of a factor 3 to 4 for transverse momenta larger than 5
GeV/c in the 20% most central collisions was observed. Preliminary results in
an extended pT-range, using the data sample collected during the 2011 Pb-Pb
run, together with the first measurement of D^(+)_(s) nuclear modification
factor will be shown
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