2,989 research outputs found
Low-mass dielectron measurements in pp, p-Pb, and Pb-Pb collisions with ALICE at the LHC
Dielectrons are an excellent probe for the QCD matter created in created in
ultra-relativistic heavy-ion collisions, since they are emitted during the
whole evolution of the collision and do not interact strongly with the medium.
To isolate the QGP signals, measurement of the dielectron production in vacuum
and its modifications due to the presence of cold nuclear matter is necessary.
We present and discuss results from a low magnetic field detector setup in
proton-proton collisions at TeV, as well as the measurement of
dielectron production in pp, p-Pb, and Pb-Pb collisions at TeV.Comment: Contribution to QM2019, submitted to Nuclear Physics, Section
Future ALICE upgrades for Run 4 and Beyond
For Run 4, ALICE is pioneering the construction of truly cylindrical tracking
layers, which will improve the measurements of heavy-flavour hadrons and
dielectrons. In addition, a Forward Calorimeter (FoCal) for the measurement of
direct photons is being developed to access the gluon distributions of nucleons
and nuclei at low . For Run 5 and beyond, ALICE 3 is proposed. It combines a
unique high-resolution vertex detector with a silicon pixel tracker and modern
particle identification solutions over a large acceptance. This will permit
heavy-flavour and dielectron measurements with unprecedented precision to
access fundamental properties and dynamics of the quark-gluon plasma (QGP). We
will discuss the upgrade plans, report on R\&D results for ITS3 and FoCal, and
present the requirements and concepts for ALICE 3.Comment: Contribution to the proceedings for INTERNATIONAL CONFERENCE ON
ULTRA-RELATIVISTIC NUCLEUS-NUCLEUS COLLISIONS (Quark Matter 2022
Multiple Imputation von fehlenden Werten mit Daten über Unterernährung und Kindersterblichkeit
In dieser Arbeit werden die Auswirkungen einer Ersetzung von fehlenden Werten auf das Ergebnis einer Regressionsanalyse untersucht. Grundlage ist eine Untersuchung von Klasen (2000) über die Unterschiede im Zusammenhang zwischen Unterernährung und Kindersterblichkeit in Afrika und Südasien. In dem Makro-Datensatz, welcher 101 Entwicklungsländer umfasst, fällt etwa ein Drittel der 273 Beobachtungen weg, da für verschiedene verwendete Variablen die Werte fehlen. Die so verloren gegangenen Informationen sollen in dieser Untersuchung genutzt werden um die Schätzergebnisse zu verbessern. Hierzu wird ein Verfahren zur multiplen Imputation verwandt, in welchem mit einem Data-Augmentation-Verfahren mehrere vervollständigte Datensätze generiert werden, mit welchen dann getrennt Schätzungen durchgeführt werden. Die Ergebnisse der Schätzungen werden dann miteinander kombiniert. Durch die Auswertung mehrerer vervollständigter Datensätze wird eine höhere Effizienz der Schätzer erreicht. Ein Vergleich von Regressionsanalysen, die mit dem vervollständigten Daten durchgeführt wurden, mit einer Complete-case-Analyse hat gezeigt, dass sich bestimmte Koeffizienten in ihrer Größenordnung geändert haben. Bei manchen Koeffizienten sind unplausible Vorzeichen aus der Complete-case Analyse verschwunden. Es ist also vorteilhaft, bei Problemen mit fehlenden Werten moderne Imputationsverfahren zu verwenden. Die wesentlichen Ergebnisse aus der Untersuchung von Klasen (2000) konnten dennoch bestätigt werden. Durch die Ersetzung der fehlenden Werte konnten noch eine Reihe von Variablen zugänglich gemacht werden, die in den bisherigen Untersuchungen nicht verwendet wurden, da dadurch auf noch mehr Beobachtungen hätte verzichtet werden müssen
Threshold energy for sub-barrier fusion hindrance phenomenon
The relationship between the threshold energy for a deep sub-barrier fusion
hindrance phenomenon and the energy at which the regime of interaction changes
(the turning-off of the nuclear forces and friction) in the sub-barrier capture
process, is studied within the quantum diffusion approach. The quasielastic
barrier distribution is shown to be a useful tool to clarify whether the slope
of capture cross section changes at sub-barrier energies.Comment: 4 pages, 4 figures (accepted in Eur. Phys. J. A
Sub-barrier capture reactions with O and Ca beams
Various sub-barrier capture reactions with beams O and Ca
are treated within the quantum diffusion approach. The role of neutron transfer
in these capture reactions is discussed. The quasielastic and capture barrier
distributions are analyzed and compared with the recent experimental data.Comment: 8 pages, 9 figures, will be published in EPJA. arXiv admin note:
substantial text overlap with arXiv:1211.433
Sub-barrier capture reactions with O beams
Various sub-barrier capture reactions with beams O are treated
within the quantum diffusion approach. The role of neutron transfer in these
capture reactions is discussed.Comment: 3 pages, 4 figure
Directed and elliptic flow in heavy ion collisions from MeV/nucleon to GeV/nucleon
Recent data from the NA49 experiment on directed and elliptic flow for Pb+Pb
reactions at CERN-SPS are compared to calculations with a hadron-string
transport model, the Ultra-relativistic Quantum Molecular Dynamics (UrQMD)
model.
The rapidity and transverse momentum dependence of the directed and elliptic
flow, i.e. and , are investigated. The flow results are compared to
data at three different centrality bins. Generally, a reasonable agreement
between the data and the calculations is found. Furthermore, the energy
excitation functions of and from MeV to GeV are explored within the UrQMD framework and discussed in the
context of the available data. It is found that, in the energy regime below
GeV, the inclusion of nuclear potentials is necessary to
describe the data. Above GeV beam energy, the UrQMD model starts to
underestimate the elliptic flow. Around the same energy the slope of the
rapidity spectra of the proton directed flow develops negative values. This
effect is known as the third flow component ("antiflow") and cannot be
reproduced by the transport model. These differences can possibly be explained
by assuming a phase transition from hadron gas to quark gluon plasma at about
GeV.Comment: 19 pages, minor changes and modified title as published in PR
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