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Electron identification with a likelihood method and measurements of di-electrons for the CBM-TRD
(2017)
In this work a likelihood method has been implemented and investigated as particle identification algorithm for the CBM-TRD.
The creation of the probability distributions for the likelihood method via V0-topologies seems to be feasible and the purity of the obtained samples is sufficient for the usage in the likelihood method.
The comparison between the ANN and the likelihood method shows no differences in the identification performance. The pion suppression factor reaches the same values for the same electron identification efficiencies and the yields of the resulting di-lepton signals are comparable. The signal-to-background ratios for both methods have the same values and show a value of about 10−2 in the invariant mass range of minv = 1.5 - 2.5 GeV/c2, which is expected to be sufficient to provide access to the thermal in-medium and QGP radiation.
The investigation of a detector system without a TRD shows no pion suppression for a momentum above p = 6 GeV/c. Therefore, the background contributions increase drastically and the signal-to-background ratio decreases at all invariant masses, but especially in the invariant mass range of minv = 1.5 - 2.5 GeV/c2.
The background contributions in the invariant mass range of minv = 1.5 - 2.5 GeV/c 2 are also influenced by the selected electron identification efficiency of the TRD, which significantly shifts the fraction of the eπ contributions relative to the total number of pairs.
Anisotropic collective flow of protons resulting from non-central heavy ion collisions is a unique hadronic observable providing information about the early stage of the nuclear collision. The analysis of collective flow in the energy regime between 1-2 AGeV enables the study of the phase diagram of hadronic matter at a high baryochemical potential µb, as well as the analysis of the equation of state at densities up to the threefold of the ground state density ρ0.
The algorithms of the standard event plane method and the scalar product method are used to analyse directed and elliptic flow of protons in a centrality range of 0-40 % most central events.
Prior to the analysis of experimental data, the respective influence of the reconstruction procedure on the algorithms is examined using Monte Carlo simulations based on the Ultra relativistic Quantum Molecular Dynamics (UrQMD) model.
Subsequently, experimental data measured in April 2012 with the High Acceptance DiElectron Spectrometer (HADES) is analysed using both methods. About 7.3 · 109 Au+Au events at a kinetic beam energy of 1.23 AGeV, equivalent to a centre of mass energy of √sNN = 2.42 GeV were recorded. A multi-differential analysis is feasible as the HADES detector provides a good transverse momentum and rapidity coverage.
Both algorithms result in identical values for directed and elliptic flow across all centrality classes within the observable phase space of protons. The calculated integrated value of v2 at mid rapidity is in good agreement with world data.
In April and May 2012 data on Au+Au collisions at beam energies of Ekin = 1.23A GeV were collected with the High Acceptance Di-Electron Spectrometer (HADES) at the GSI Helmholtzzentrum für Schwerionenforschung facility in Darmstadt, Germany. In this thesis, the production of deuterons in this collision system is investigated.
A total number of 2.1 × 109 Au+Au events is selected, containing the most central 0-40% of events. After particle identification, based on a mass determination via time-of-flight and momentum and on a measurement of the energy loss, the transverse mass spectra of the deuteron candidates are extracted for various rapidities and subsequently corrected for acceptance and efficiency.
The inverse slope parameter of a Boltzmann fit applied to the transverse mass spectra at midrapidity, which is referred to as the effective temperature, is extracted. For a static thermal source, this parameter corresponds to the kinetic freeze-out temperature Tkin and is therefore expected to be smaller or equal to the chemical freeze-out temperature Tchem. The extracted effective temperature of Tef f = (190 ± 10) MeV however exceeds the chemical freeze-out temperature that was obtained by a statistical model fit to different particle yields. The effective temperatures of various particle species, obtained in previous analyses, suggest a systematic rise with increasing particle mass, which is confirmed by the deuteron results.
An explanation can be the influence of a collective expansion with a radial expansion velocity βr. By fitting a Siemens-Rasmussen function to the transverse mass spectra, the global temperature of T = (100 ± 8) MeV and radial expansion velocity βr = 0.37 ± 0.01 are obtained. This temperature is still very high and only takes into account the production of deuteron nuclei.
The simultaneous fit of a blast-wave function to the transverse mass spectra of deuterons and other particles, as obtained by previous analyses, considers a velocity profile for the radial expansion velocity and takes into account the production of various particle species. The resulting global temperature Tkin = (68 ± 1) MeV and average transverse expansion velocity hβri = 0.341 ± 0.003 are within the expected range for the collision energy.
The Siemens-Rasmussen fits are also used to extrapolate the transverse mass spectra into unmeasured regions, to integrate them and obtain a rapidity-dependent count rate. This count rate exhibits a thermal shape for central events and shows increasing spectator contributions for more peripheral events.
The invariant yield spectra of the deuterons are compared to those of protons, as obtained by a previous analysis, in the context of a nucleon coalescence model. The hereby extracted nucleon coalescence factor B2 = (4.6 ± 0.1) × 10−3 agrees with the expected result for the beam energy that was studied.
Ziel der Simulationsstudien in dieser Arbeit war es, die Leistungsfähigkeit des Transition Radiation Detectors zur Identifikation von leichten Kernen und Hyperkernen im CBM-Experiment zu untersuchen. Die Trennung von Helium und Deuterium
mithilfe ihres spezifischen Energieverlustes im TRD ist zentral, um eine Rekonstruktion des seltenen Hyperkerns 6 ΛΛHe mit einem hohen Signal-zu-Untergrund-Verhältnisse zu leisten. Zur Erfüllung der Anforderungen, die sich aus dem CBM-Forschungsprogramm ergeben, wird eine Auflösung des Energieverlustes dEdx von Helium von höchstens 30 % verlangt...
Das CBM-Experiment konzentriert sich auf die Untersuchung der Eigenschaften des Quark-Gluon-Plasmas bei hohen Netto-Baryonendichten und moderaten Temperaturen. An der zukünftigen Beschleunigeranlage FAIR an der GSI findet das Experiment, neben vielen anderen Experimenten, ihren Platz. Der TRD ist, neben dem RICH, STS und TOF, einer der zentralen Detektoren im CBM-Experiment. Der TRD nutzt dabei den physikalischen Effekt der Übergangsstrahlung, die durch ein geladenes Teilchen beim Durchqueren einer Grenze zweier Medien mit unterschiedlichen Dielektrizitätskonstanten mit einer gewissen Wahrscheinlichkeit entsteht, um Elektronen von Pionen trennen zu können. Im Jahr 2017 wurde an der DESY 4 TRD-Prototypen in einer Teststrahlzeit getestet. Dabei handelt es sich um große TRD-Module mit den Maßen 95 · 95 cm2 , was dem finalen Design sehr nahe kommt. Die Untersuchung der DESY-Daten in Kapitel 5 brachte große Problematiken in den Daten zum Vorschein. Die Hauptprobleme der DESY-Daten sind: 1) Bug des SPADIC-Chips 2.0, bei der FN-Trigger zeitlich verschoben wurden; 2) schwache und suboptimale Trigger-Bedingung, wodurch sehr viel Rauschen aufgenommen wurde. Die Daten müssen für weitere Auswertung aufbereitet werden, wobei sehr viel Information und Statistik verloren geht, da einige Daten durch diverse Probleme nicht mehr rekonstruierbar sind. Kapitel 6 beschäftigt sich mit der Simulation der Detektorantwort und geht genauer auf die einzelnen Schritte, die zur Simulation des vom SPADIC erzeugten Pulses benötigt werden, ein. Am Ende werden Ergebnisse aus beiden Datensätzen miteinander verglichen. Um einen optimalen Vergleich zu gewähren, wird die Simulation bestmöglich an die Einstellungen in der Teststrahlzeit angepasst. Hauptsächlich geht es um die Erhöhung des Gasgains und der Verschiebung der Peaking-Zeit des Pulses. Im Allgemeinen können wir in der Simulation einige Effekte, die auch in den DESY-Daten vorkommen, nachsimulieren. Wir erhalten zum Teil sehr unterschiedliche Ergebnisse in der Simulation, deren Richtigkeit nicht verifiziert werden kann, da die Daten aufgrund der Probleme unzuverlässig werden. Durch die Analyse der DESY-Daten konnten wir die Problematik in den Daten besser verstehen. Eine sinnvolle Anpassung der Simulation wird durch die Unzuverlässigkeit der DESY-Daten unmöglich. Für die Optimierung der Simulation müsste man einen Vergleich mit neueren, zuverlässigeren Daten aus zukünftigen Teststrahlzeiten nehmen.
Computational workflow optimization for magnetic fluctuation measurements of 3D nano-tetrapods
(2021)
The detailed understanding of micro–and nanoscale structures, in particular their magnetization dynamics, dominates contemporary solid–state physics studies. Most investigations already identified an abundance of phenomena in one–and two–dimensional nanostructures. The following thesis focuses on the magnetic fingerprint of three–dimensional CoFe nano–magnets, specifically the temporal development of their hysteresis loop. These nano–magnets were grown in a tetrahedral pattern on top of a highly susceptible home–build GaAs/AlGaAs micro–Hall sensor using focused electron beam induced deposition (FEBID).
During the measurements, utmost efforts were employed to exemplify current best research practices. The data life cycle of the present thesis is based upon open–source data science tools and packages. Data acquisition and analysis required self–written automated algorithms to handle the extensive quantity of data. Existing instrumental-controlling software was improved, and new Python packages were devised to analyze and visualize the gathered data. The open–source Python data analysis framework (ana) was developed to facilitate computational reproducibility. This framework transparently analyses and visualizes the gathered data automatically using Continuous Analysis tools based on GitLab and Continuous Integration. This automatization uses bespoke scripts combined with virtualization tools like Docker to facilitate reproducible and device–independent results.
The hysteresis loops reveal distinct differences in subsequently measured loops with identical initial experimental parameters, originating from the nano–magnet’s magnetic noise. This noise amplifies in regions where switching processes occur. In such noise–prone regions, the time–dependent scrutinization reveals presumably thermally induced metastable magnetization states. The frequency–dependent power spectral density uncovers a characteristic 1/f² behavior at noise–prone regions with metastable magnetization states.
During RUN3 (2021-2023) of the Large Hadron Collider, the Time Projection Chamber (TPC) of ALICE will be operated with quadruple stacks of Gas Electron Multipliers (GEMs). This technology will allow to overcome the rate limitation due to the gated operation of the Multi-Wire Proportional Chambers (MWPCs) used in RUN1 (2009-2013) and RUN2 (2015-2018).
As part of the Upgrade project, long-term irradiation tests, so called "ageing tests", have been carried out. A test setup with a detector using a quadruple stack of 10x10cm2 GEMs was built and operated in Ar-CO2 and Ne-CO2-N2 gas mixtures. The detector performance such as gas gain and energy resolution were monitored continuously. In addition, outgassing tests of materials used for the assembly process of the upgraded TPC were performed. To reach the expected dose of the GEM-based TPC, the detector was operated at much higher gains than the TPC. It was found, that the GEMs could keep their performance within the projected lifetime of the TPC. Most of the tested materials showed no negative impact on the detector. For the tested epoxy adhesive no certain conclusion could be drawn.
At much higher doses than expected for the upgraded TPC, a new phenomenon was observed, which changed the hole geometry of the GEMs and led to a degradation of the energy resolution. Even though its occurrence is not expected during the lifetime of the GEM-based TPC, simulations were carried out to study this effect more systematically. The simulations confirmed, that a change of the hole geometries of the GEMs, lead to an increase of the local gain variation, which results in a decrease of the energy resolution.
Furthermore the effect of methane as quench gas on GEMs was studied, even though this gas is not foreseen to be used in the TPC. From ageing tests with single-wire proportional counters it is well known that hydrocarbons are produced in the plasma of the avalanches, which cover the electrodes and lead to a degradation of the detector performance. Even though GEMs have a quite different geometry, the ageing tests showed, that also this technology tends to methane-induced ageing. A loss of gas gain as well as a degradation of the energy resolution due to deposits on the electrodes was monitored. A qualitative and quantitative comparison between ageing in GEMs and proportional counters was performed.
In thesis I investigate the possibility that at the smallest length scale (Planck scale) the very notion of "dimension" needs to be revisited. Due to "quantum effects" spacetime might become very turbulent at these scales and properties like those of "fractals" emerge, including a "scale dependent dimension". It seems that this "spontaneous dimensional reduction" and the appearance of a minimal physical length are very general effects that most approaches to quantum gravity share. Main emphasis is given to the"spectral dimension" and its calculation for strings and p-branes.
In this thesis, Planck size black holes are discussed. Specifically, new families of black holes are presented. Such black holes exhibit an improved short scale behaviour and can be used to implement gravity self-complete paradigm. Such geometries are also studied within the ADD large extra dimensional scenario. This allows black hole remnant masses to reach the TeV scale. It is shown that the evaporation endpoint for this class of black holes is a cold stable remnant. One family of black holes considered in this thesis features a regular de Sitter core that counters gravitational collapse with a quantum outward pressure. The other family of black holes turns out to nicely fit into the holographic information bound on black holes, and lead to black hole area quantization and applications in the gravitational entropic force. As a result, gravity can be derived as emergent phenomenon from thermodynamics.
The thesis contains an overview about recent quantum gravity black hole approaches and concludes with the derivation of nonlocal operators that modify the Einstein equations to ultraviolet complete field equations.
Die Druckmessung in Tieftemperatur-Vakuumsystemen stellt ein großes messtechnisches Problem dar. Für die in solchen Systemen auftretenden Drücke im UHV und XHV-Bereich werden meist Ionisationsmanometer vom Glühkathodentyp zur Druckmessung verwendet. Diese haben jedoch den entscheidenden Nachteil, dass durch die Verwendung einer Glühkathode zur Erzeugung freier Elektronen eine große Wärmelast in das System eingekoppelt wird. Dies führt zu einer Störung des thermischen Gleichgewichts und damit zu einer Verfälschung der Druckmessung. Weiterhin muss diese zusätzliche Wärmelast abgeführt werden, was vor allem bei kryogenen Vakuumsystemen einen erheblichen Mehraufwand darstellt.
Um dieses Problem zu umgehen, wurde ein Ionisationsmanometer entwickelt, dessen Glühkathode durch eine kalte Elektronenquelle ersetzt wurde. Der verwendete Feldemitter, eine kommerziell erhältliche CNT-Kathode, wurde gegenüber dem Anodengitter einer Extraktormessröhre positioniert. Mit diesem Aufbau wurden die Charakteristika von Kathode und Messröhre sowohl bei Raumtemperatur als auch unter kryogenen Vakuumbedingungen untersucht.
Dabei konnte gezeigt werden, dass die modifizierte Messröhre auch bei einer Umgebungstemperatur von 6 K ohne funktionale Einbußen betrieben werden kann und der gemessene Ionenstrom über mehrere Dekaden linear mit dem von einer Extraktormessröhre mit Glühkathode gemessenen Referenzdruck ansteigt. Des Weiteren konnte gezeigt werden, dass der Extraktor mit CNT-Kathode unter diesen kryogenen Bedingungen deutlich sensitiver auf geringe Druckschwankungen reagiert als sein Äquivalent mit Glühkathode.