14,716 research outputs found
Production of multi-strange baryons in 7 TeV proton-proton collisions with ALICE
In the perspective of comparisons between proton-proton and heavy-ion
physics, understanding the production mechanisms (soft and hard) in pp that
lead to strange particles is of importance. Measurements of charged
multi-strange (anti-)baryons (Omega and Xi) are presented for pp collisions at
sqrt(s) = 7 TeV. This report is based on results obtained by ALICE (A Large Ion
Collider Experiment) from the 2010 data-taking.
Taking advantage of the characteristic cascade-decay topology, the
identification of Xi-, anti-Xi+, Omega- and anti-Omega+ can be performed, over
a wide range of momenta (e.g. from 0.6 to 8.5 GeV/c for Xi-, with the present
statistics analysed). The production at central rapidity (|y| < 0.5) as a
function of transverse momentum, dN/dptdy, is presented. These results are
compared to PYTHIA Perugia 2011 predictions.Comment: 6 pages, 3 figures, 1 table. Strangeness In Quark Matter (SQM 2011),
18-24 Sept. 2011, Krakow. To be published in Acta Physica Polonica B (APPB
Measurements of inclusive J/psi production in Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV with the ALICE experiment
Charmonium is a prominent probe of the Quark-Gluon Plasma (QGP), expected to
be formed in ultrarelativistic heavy-ion (A-A) collisions. It has been
predicted that the J/psi(c-cbar) particle is dissolved in the deconfined medium
created in A-A systems. However this suppression can be counterbalanced via
regeneration of the charm/anti-charm bound state in QGP or via statistical
production at the phase boundary. At LHC energies, the latter mechanisms are
expected to play a more important role, due to a charm production cross section
significantly larger than at lower energies.
Measurements obtained by the ALICE experiment for inclusive J/psi production
are shown, making use of Pb-Pb data at sqrt(s_NN) = 2.76 TeV, collected in 2010
and 2011. In particular, the focus is given on the nuclear modification factor,
R_AA, derived for forward (2.5 < y < 4) and mid rapidities (|y| < 0.9), both
down to zero transverse momentum (pT). The centrality, y and pT dependences of
R_AA are presented and discussed in the context of theoretical models, together
with PHENIX and CMS results.Comment: 8 pages, 7 figures. To be published in PoS. Proceedings of the Xth
QCHS conference (Quark Confinement and the Hadron Spectrum), 2012, 8-12
October 2012, Munich. See corresponding presentation under TUM indico :
http://intern.universe-cluster.de/indico/contributionDisplay.py?contribId=246&sessionId=36&confId=229
Gemini Planet Imager Observational Calibrations IV: Wavelength Calibration and Flexure Correction for the Integral Field Spectrograph
We present the wavelength calibration for the lenslet-based Integral Field
Spectrograph (IFS) that serves as the science instrument for the Gemini Planet
Imager (GPI). The GPI IFS features a 2.7" x 2.7" field of view and a 190 x 190
lenslet array (14.3 mas/lenslet) operating in , , , and bands with
spectral resolving power ranging from 35 to 78. Due to variations
across the field of view, a unique wavelength solution is determined for each
lenslet characterized by a two-dimensional position, the spectral dispersion,
and the rotation of the spectrum with respect to the detector axes. The four
free parameters are fit using a constrained Levenberg-Marquardt least-squares
minimization algorithm, which compares an individual lenslet's arc lamp
spectrum to a simulated arc lamp spectrum. This method enables measurement of
spectral positions to better than 1/10th of a pixel on the GPI IFS detector
using Gemini's facility calibration lamp unit GCAL, improving spectral
extraction accuracy compared to earlier approaches. Using such wavelength
calibrations we have measured how internal flexure of the spectrograph with
changing zenith angle shifts spectra on the detector. We describe the methods
used to compensate for these shifts when assembling datacubes from on-sky
observations using GPI.Comment: 12 pages, 8 figures. Proceedings of the SPIE, 9147-27
A Partially Reflecting Random Walk on Spheres Algorithm for Electrical Impedance Tomography
In this work, we develop a probabilistic estimator for the voltage-to-current
map arising in electrical impedance tomography. This novel so-called partially
reflecting random walk on spheres estimator enables Monte Carlo methods to
compute the voltage-to-current map in an embarrassingly parallel manner, which
is an important issue with regard to the corresponding inverse problem. Our
method uses the well-known random walk on spheres algorithm inside subdomains
where the diffusion coefficient is constant and employs replacement techniques
motivated by finite difference discretization to deal with both mixed boundary
conditions and interface transmission conditions. We analyze the global bias
and the variance of the new estimator both theoretically and experimentally. In
a second step, the variance is considerably reduced via a novel control variate
conditional sampling technique
Monte Carlo approximations of the Neumann problem
We introduce Monte Carlo methods to compute the solution of elliptic
equations with pure Neumann boundary conditions. We first prove that the
solution obtained by the stochastic representation has a zero mean value with
respect to the invariant measure of the stochastic process associated to the
equation. Pointwise approximations are computed by means of standard and new
simulation schemes especially devised for local time approximation on the
boundary of the domain. Global approximations are computed thanks to a
stochastic spectral formulation taking into account the property of zero mean
value of the solution. This stochastic formulation is asymptotically perfect in
terms of conditioning. Numerical examples are given on the Laplace operator on
a square domain with both pure Neumann and mixed Dirichlet-Neumann boundary
conditions. A more general convection-diffusion equation is also numerically
studied
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