141,937 research outputs found
Velocity dispersion in N-body simulations of CDM models
This work reports on a study of the spatially coarse-grained velocity
dispersion in cosmological N-body simulations (OCDM and Lambda-CDM models) as a
function of time (redshifts z=0-4) and of the coarsening length (0.6-20 Mpc/h).
The main result is the discovery of a polytropic relationship I_1 ~ rho^{2-eta}
between the velocity-dispersion kinetic energy density of the coarsening cells,
I_1, and their mass density, rho. The exponent eta, dependent on time and
coarsening scale, is a compact measure of the deviations from the naive virial
prediction eta_virial=0. This relationship supports the ``polytropic
assumption'' which has been employed in theoretical models for the growth of
cosmological structure by gravitational instability.Comment: Minor, unimportant changes. Matches published versio
A SpiNNaker Application: Design, Implementation and Validation of SCPGs
In this paper, we present the numerical results of the implementation
of a Spiking Central Pattern Generator (SCPG) on a SpiNNaker
board. The SCPG is a network of current-based leaky integrateand-
fire (LIF) neurons, which generates periodic spike trains that correspond
to different locomotion gaits (i.e. walk, trot, run). To generate
such patterns, the SCPG has been configured with different topologies,
and its parameters have been experimentally estimated. To validate our
designs, we have implemented them on the SpiNNaker board using PyNN
and we have embedded it on a hexapod robot. The system includes a
Dynamic Vision Sensor system able to command a pattern to the robot
depending on the frequency of the events fired. The more activity the
DVS produces, the faster that the pattern that is commanded will be.Ministerio de Economía y Competitividad TEC2016-77785-
The exact renormalization group in Astrophysics
The coarse-graining operation in hydrodynamics is equivalent to a change of
scale which can be formalized as a renormalization group transformation. In
particular, its application to the probability distribution of a
self-gravitating fluid yields an "exact renormalization group equation" of
Fokker-Planck type. Since the time evolution of that distribution can also be
described by a Fokker-Planck equation, we propose a connection between both
equations, that is, a connection between scale and time evolution. We finally
remark on the essentially non-perturbative nature of astrophysical problems,
which suggests that the exact renormalization group is the adequate tool for
them.Comment: World Scientific style, 6 pages, presented at the 2nd Conference on
the Exact RG, Rome 200
Measurement of the inclusive production cross sections for forward jets and for dijet events with one forward and one central jet in pp collisions at sqrt(s) = 7 TeV
The inclusive production cross sections for forward jets, as well for jets in
dijet events with at least one jet emitted at central and the other at forward
pseudorapidities, are measured in the range of transverse momenta pt = 35-150
GeV/c in proton-proton collisions at sqrt(s) = 7 TeV by the CMS experiment at
the LHC. Forward jets are measured within pseudorapidities 3.2<|eta|<4.7, and
central jets within the |eta|<2.8 range. The double differential cross sections
with respect to pt and eta are compared to predictions from three approaches in
perturbative quantum chromodynamics: (i) next-to-leading-order calculations
obtained with and without matching to parton-shower Monte Carlo simulations,
(ii) PYTHIA and HERWIG parton-shower event generators with different tunes of
parameters, and (iii) CASCADE and HEJ models, including different non-collinear
corrections to standard single-parton radiation. The single-jet inclusive
forward jet spectrum is well described by all models, but not all predictions
are consistent with the spectra observed for the forward-central dijet events.Comment: Submitted to the Journal of High Energy Physic
Study of W boson production in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV
A measurement is presented of W-boson production in PbPb collisions carried
out at a nucleon-nucleon (NN) centre-of-mass energy sqrt(s[NN]) of 2.76 TeV at
the LHC using the CMS detector. In data corresponding to an integrated
luminosity of 7.3 inverse microbarns, the number of W to mu mu-neutrino decays
is extracted in the region of muon pseudorapidity abs(eta[mu])<2.1 and
transverse momentum pt[mu]>25 GeV. Yields of muons found per unit of
pseudorapidity correspond to (159 +/- 10 (stat.) +/- 12 (syst.)) 10E-8 W(plus)
and (154 +/- 10 (stat.) +/- 12 (syst.)) 10E-8 W(minus) bosons per minimum-bias
PbPb collision. The dependence of W production on the centrality of PbPb
collisions is consistent with a scaling of the yield by the number of
incoherent NN collisions. The yield of W bosons is also studied in a sample of
pp interactions at sqrt(s)= 2.76 TeV corresponding to an integrated luminosity
of 231 inverse nanobarns. The individual W(plus) and W(minus) yields in PbPb
and pp collisions are found to agree, once the neutron and proton content in Pb
nuclei is taken into account. Likewise, the difference observed in the
dependence of the positive and negative muon production on pseudorapidity is
consistent with next-to-leading order perturbative QCD calculations.Comment: Submitted to Physics Letters
Quark masses in QCD: a progress report
Recent progress on QCD sum rule determinations of the light and heavy quark
masses is reported. In the light quark sector a major breakthrough has been
made recently in connection with the historical systematic uncertainties due to
a lack of experimental information on the pseudoscalar resonance spectral
functions. It is now possible to suppress this contribution to the 1% level by
using suitable integration kernels in Finite Energy QCD sum rules. This allows
to determine the up-, down-, and strange-quark masses with an unprecedented
precision of some 8-10%. Further reduction of this uncertainty will be possible
with improved accuracy in the strong coupling, now the main source of error. In
the heavy quark sector, the availability of experimental data in the vector
channel, and the use of suitable multipurpose integration kernels allows to
increase the accuracy of the charm- and bottom-quarks masses to the 1% level.Comment: Invited review paper to be published in Modern Physics Letters
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