22 search hits
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Study of QCD-like theories at nonzero temperatures and densities
(2012)
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Tian Zhang
- In this thesis I use effective models to investigate the properties of QCD-like
theories at nonzero temperature and baryon chemical potential.
First I construct a PNJL model using a lattice spin model with nearestneighbor
interactions for the gauge sector and four-fermion interactions for
the quarks in (pseudo)real representations of the gauge group. Calculating
the phase diagram in the plane of temperature and quark chemical potential
in QCD with adjoint quarks, it is qualitatively confirmed that the critical
temperature of the chiral phase transition is much higher than the deconfinement
transition temperature. At a chemical potential equal to half of the
diquark mass in the vacuum, a diquark Bose–Einstein condensation (BEC)
phase transition occurs. In the two-color case, a Ginzburg–Landau expansion
is used to study the tetracritical behavior around the intersection point
of the deconfinement and BEC transition lines which are both of second order.
A compact expression for the expectation value of the Polyakov loop in
an arbitrary representation of the gauge group is obtained for any number of
colors, which allows us to study Casimir scaling at both nonzero temperature
and chemical potential.
Subsequently I study the thermodynamics of two-color QCD (QC2D) at
high temperature and/or density using ZQCD, a dimensionally reduced superrenormalizable
effective theory, formulated in terms of a coarse grained
Wilson line. In the absence of quarks, the theory is required to respect the Z2
center symmetry, while the effects of quarks of arbitrary masses and chemical
potentials are introduced via soft Z2 breaking operators. Perturbative
matching of the effective theory parameters to the full theory is carried out
explicitly, and it is argued how the new theory can be used to explore the
phase diagram of two-color QCD.
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Interacting ultracold gases in optical lattices: non-equilibrium dynamics and effects of disorder
(2012)
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Julia Wernsdorfer
- This dissertation aims at giving a theoretical description of various applications of ultracold
gases. A particular focus is cast upon the dynamical evolution of bosonic condensates
in non-equilibrium by means of the time-dependent Gutzwiller method. Ground state
properties of strongly interacting fermionic atoms in box and speckle disordered lattices
are investigated via real-space dynamical mean-field theory. ...
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Commissioning of the ALICE High-Level Trigger
(2012)
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Jochen Thäder
- A new era in experimental nuclear physics has begun with the start-up of the
Large Hadron Collider at CERN and its dedicated heavy-ion detector system
ALICE. Measuring the highest energy density ever produced in nucleus-nucleus
collisions, the detector has been designed to study the properties of the created
hot and dense medium, assumed to be a Quark-Gluon Plasma.
Comprised of 18 high granularity sub-detectors, ALICE delivers data from
a few million electronic channels of proton-proton and heavy-ion collisions.
The produced data volume can reach up to 26 GByte/s for central Pb–Pb
collisions at design luminosity of L = 1027 cm−2 s−1 , challenging not only the
data storage, but also the physics analysis. A High-Level Trigger (HLT) has
been built and commissioned to reduce that amount of data to a storable value
prior to archiving with the means of data filtering and compression without the
loss of physics information. Implemented as a large high performance compute
cluster, the HLT is able to perform a full reconstruction of all events at the time
of data-taking, which allows to trigger, based on the information of a complete
event. Rare physics probes, with high transverse momentum, can be identified
and selected to enhance the overall physics reach of the experiment.
The commissioning of the HLT is at the center of this thesis. Being deeply
embedded in the ALICE data path and, therefore, interfacing all other ALICE
subsystems, this commissioning imposed not only a major challenge, but also a
massive coordination effort, which was completed with the first proton-proton
collisions reconstructed by the HLT. Furthermore, this thesis is completed with
the study and implementation of on-line high transverse momentum triggers.
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The production of _j63 [eta] and _w63 [omega] mesons in 3.5 GeV p+p interaction in HADES
(2011)
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Khaled Teilab
- The study of meson production in proton-proton collisions in the energy range
up to one GeV above the production threshold provides valuable information about
the nature of the nucleon-nucleon interaction. Theoretical models describe the interaction
between nucleons via the exchange of mesons. In such models, different
mechanisms contribute to the production of the mesons in nucleon-nucleon collisions.
The measurement of total and differential production cross sections provide information
which can help in determining the magnitude of the various mechanisms.
Moreover, such cross section information serves as an input to the transport calculations
which describe e.g. the production of e+e− pairs in proton- and pion-induced
reactions as well as in heavy ion collisions.
In this thesis, the production of ω and η mesons in proton-proton collisions at 3.5
GeV beam energy was studied using the High Acceptance DiElectron Spectrometer
(HADES) installed at the Schwerionensynchrotron (SIS 18) at the Helmholtzzenturm
f¨ur Schwerionenforschung in Darmstadt.
About 80 000 ω mesons and 35 000 η mesons were reconstructed. Total production
cross sections of both mesons were determined. Furthermore, the collected statistics
allowed for extracting angular distributions of both mesons as well as performing
Dalitz plot studies.
The ω and η mesons were reconstructed via their decay into three pions (π+π−π0)
in the exclusive reaction pp −→ ppπ+π−π0. The charged particles were identified
via their characteristic energy loss, via the measurement of their time of flight and
momentum, or using kinematics.
The neutral pion was reconstructed using the missing mass method. A kinematic
fit was applied to improve the resolution and to select events in which a π0 was
produced.
The correction of measured yields for the effects of spectrometer acceptance was done
as a function of four variables (two invariant masses and two angles). Systematic
studies of the acceptance for different input distributions were performed.
The measured yields were normalized to the number of measured events of elastic
scattering. Systematic errors due to the methods of the data analysis and the
background subtraction were investigated.
Production angular distributions of ω and η mesons were measured. Both mesons
exhibit a slightly anisotropic angular distribution.
The Dalitz plot of ω meson production shows indications of resonant production.
However, the deviation of the distribution from the one expected by phase space
simulations is not large.
The Dalitz plot of η meson production shows a signal of the production via the
N(1535) resonance, The contribution of N(1535) to the production was quantified
to be about 47%. The angular distribution of η mesons does not show significant
differences between resonant and non resonant production.
The total production cross section of ω mesons in the reaction pp −→ ppω was
determined to be 106.5 ± 0.9 (stat) ± 7.9 (sys) [μb] where stat indicates statistical
error and sys indicates systematic error, while that of η mesons was determined to
be 136.9 ± 0.9 (stat) ± 10.1 (sys) [μb] in the reaction pp −→ ppη
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Strongly correlated ultracold gases in disordered optical lattices
(2012)
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Denis Semmler
- Seit Anbeginn der Festkörperphysik ist die Frage, warum manche Materialien
metallisch sind, andere dagegen isolierend, von zentraler Bedeutung. Eine erste
Erklärung wurde durch die Bändertheorie [23, 44] gegeben. Die Elektronen
sind dem periodischen Potential der Rumpfatome ausgesetzt, wodurch ein
Energiespektrum bestehend aus Bändern erzeugt wird und die Füllung dieser
Bänder bestimmt die Leitungseigenschaften des Festkörpers. ...
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Resonant pickups for non-destructive single-particle detection in heavy-ion storage rings and first experimental results
(2013)
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Mohammad Shahab Sanjari
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A numerical renormalization group approach to dissipative quantum impurity systems
(2011)
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David Roosen
- The miniaturization of electronics is reaching its limits.
Structures necessary to build integrated circuits from semiconductors are
shrinking and could reach the size of only a few atoms within the next few
years. It will be at the latest at this point in time that the physics of
nanostructures gains importance in our every day life.
This thesis deals with the physics of quantum impurity models.
All models of this class exhibit an identical structure: the simple and
small impurity only has few degrees of freedom. It can be built out of a
small number of atoms or a single molecule, for example. In the simplest
case it can be described by a single spin degree of freedom, in many
quantum impurity models, it can be treated exactly. The complexity of the
description arises from its coupling to a large number of fermionic or
bosonic degrees of freedom (large meaning that we have to deal with particle
numbers of the order of 10^{23}). An exact treatment thus remains
impossible. At the same time, physical effects which arise in quantum
impurity systems often cannot be described within a perturbative theory,
since multiple energy scales may play an important role. One example for
such an effect is the Kondo effect, where the free magnetic moment of the
impurity is screened by a "cloud" of fermionic particles of the quantum
bath.
The Kondo effect is only one example for the rich physics stemming from
correlation effects in many body systems. Quantum impurity models, and the
oftentimes related Kondo effect, have regained the attention of experimental
and theoretical physicists since the advent of quantum dots, which are
sometimes also referred to as as artificial atoms. Quantum dots offer a
unprecedented control and tunability of many system parameters. Hence, they
constitute a nice "playground" for fundamental research, while being
promising candidates for building blocks of future technological devices as
well.
Recently Loss' and DiVincenzo's proposal of a quantum computing scheme
based on spins in quantum dots, increased the efforts of
experimentalists to coherently manipulate and read out the spins of quantum
dots one by one.
In this context two topics are of paramount importance for future
quantum information processing:
since decoherence times have to be large enough to allow for good error
correction schemes, understanding the loss of phase coherence in quantum
impurity systems is a prerequisite for quantum computation in these
systems.
Nonequilibrium phenomena in quantum impurity systems also have to be
understood, before one may gain control of manipulating quantum bits.
As a first step towards more complicated nonequilibrium situations,
the reaction of a system to a quantum quench, i.e. a sudden change of
external fields or other parameters of the system can be investigated.
We give an introduction to a powerful numerical method used in this field
of research, the numerical renormalization group method, and apply this
method and its recent enhancements to various quantum impurity systems.
The main part of this thesis may be structured in the following way:
- Ferromagnetic Kondo Model,
- Spin-Dynamics in the Anisotropic Kondo and the Spin-Boson Model,
- Two Ising-coupled Spins in a Bosonic Bath,
- Decoherence in an Aharanov-Bohm Interferometer.
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Gas system, gas quality monitor and detector control of the ALICE Transition Radiation Detector and studies for a pre-trigger data read-out system
(2012)
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Nora Pitz
- The main purpose of the Transition Radiation Detector (TRD) located in the central
barrel of ALICE (A Large Ion Collider Experiment) is electron identification
for separation from pions at momenta pt > 1 GeV/c, since in this momentum range
the measurements of the specific energy loss (dE/dx) of the Time Projection Chamber
(TPC) is no longer sufficient. Furthermore, it provides a fast trigger for high
transverse momentum charged particles (pt > 3 GeV/c) and makes a significant
contribution to the optimization of the tracking of reaction products in heavy-ion
collisions. Its whole setup comprises 18 supermodules out of which 13 are presently
operational and mounted cylindrically around the beam axis of the Large Hadron
Collider (LHC). A supermodule contains either 30 or 24 chambers, each consisting of
a radiator for transition radiation creation, a drift and an amplifying region followed
by the read-out electronics. In total, the TRD is an array of 522 chambers operated
with about 28 m3 of a Xe-CO2 [85-15%] gas mixture.
During the work of this thesis, the testing, commissioning, operation and maintenance
of detector parts, the gas system and its online quality monitor, improvements
on the detector control user-interface and studies about a new pre-trigger module
for data read-out have been accomplished.
The TRD gas system mixes, distributes and circulates the operational gas mixture
through the detector. Its overall optimization has been achieved by minimizing gas
leakage, surveying, controlling, maintaining and continuously improving it as well
as designing and carrying out upgrades.
Gas quality monitors of the type \GOOFIE" (Gas prOportional cOunter For drIfting
Electrons) can be used in gaseous detectors as on-line monitors of the electron
drift velocity, gain and gas properties. One of these devices has been implemented
within the TRD gas system, while another one surveys the gas of the TPC. Both
devices had to be adapted to the specific needs of the detectors, were under constant
surveillance and control, and needed to be further developed on both hardware and
software side.
To improve the operation of the TRD, modifications on its DCS software (Detector
Control System) used for monitoring, controlling, operating, regulating and configuring of hardware and computing devices have been carried out. The DCS is
designed to enable an operator to interact with equipment through user interfaces
that display the information from the system. The main focus of this work was laid
on the optimization of the usability and design of the user interface.
The front-end electronics of the TRD require an early start signal (\pre-trigger")
from the fast forward detectors or the Time-Of-Flight detector during the running
periods. The realization of a new hardware concept for the read-out of the TRD
pre-trigger system has been studied and first tests were performed. This new module
called PIMDDL (Pre-trigger Interface Module Detector Data Link) is meant to
acquire all data necessary to simulate and predict the full pre-trigger functionality,
and to verify its proper operation. Furthermore, it shall provide all functionalities of
the so-called Control Box Bottom as well as keep the functionalities of the already
existing PIM (Pre-trigger Interface Module) in order to combine and replace these
two modules in the future.
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Ultrasonic and magnetic investigations in frustrated low-dimensional spin systems
(2012)
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Thanh Cong Pham
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Electroweak quantum chemistry: Parity violation in spectra of chiral molecules containing heavy atoms
(2012)
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Sophie Nahrwold
- The intriguing effects of electroweak induced parity violation (PV) in molecules have yet to be observed, but experiments on molecular PV promise to provide fascinating insights. They potentially offer a novel testing ground for the low energy sector of the standard model and, in addition, a successful measurement of PV differences between the two enantiomers of a chiral molecule could promote a deeper understanding of molecular chirality, by essentially establishing a new link between particle physics and biochemistry. A key challenge in the design of such experiments is the identification of suitable molecules, which in turn requires widely applicable computational schemes for the prediction of PV experimental signals. To this end, a quasirelativistic density functional theory approach to the calculation of PV effects in nuclear magnetic resonance (NMR) spectra of chiral molecules has been developed and implemented during the course of this thesis. It includes relativistic as well as electron--correlation effects and has been used extensively in the screening of molecules possibly suited for a first observation of molecular PV. Some relevant compound classes have been identified, but none of their selected representatives are predicted to exhibit PV NMR frequency shifts that can be detected under current experimental restrictions. In order to advance the design of molecules which exhibit particularly large PV signals in experiments, systematic effects on PV NMR frequency splittings such as scaling with nuclear charge, conformational dependence and the impact of atomic substitution around the NMR active nucleus have been studied. Previously predicted scaling laws were confirmed and it was determined that the environment of the NMR active nucleus, both in terms of conformation and atomic composition, can be tuned to increase PV frequency shifts by several orders of magnitude. In addition to molecules suited for NMR experiments, a fascinating chiral actinide compound was studied with regard to PV frequency shifts in vibrational spectra. This compound displays the largest such shift ever predicted for an existing molecule, which lies well within the attainable experimental resolution. The challenge now lies in making it compatible with current experimental setups.
