Search

We explore finite size 3D effects in open axion haloscopes such as a dish antenna, a dielectric disk and a minimal dielectric haloscope consisting of a mirror and one dielectric disk. Particularly dielectric haloscopes are a promising new method for detecting dark matter axions in the mass range above $40\,\mu{\rm eV}$. By using two specialized independent approaches - based on finite element methods and Fourier optics - we compute the electromagnetic fields in these settings expected in the presence of an axion dark matter field. This allows us to study diffraction and near field effects for realistically sized experimental setups in contrast to earlier idealized 1D studies with infinitely extended mirrors and disks. We also study axion velocity effects and disk tiling. Diffraction effects are found to become less relevant towards larger axion masses and for the larger disk radii for example aimed at in full size dielectric haloscopes such as MADMAX. The insights of our study not only provide a foundation for a realistic modelling of open axion dark matter search experiments in general, they are in particular also the first results taking into account 3D effects for dielectric haloscopes., Comment: 41 pages, 20 figures; typos corrected, references updated

We develop a formalism to describe extensions of existing axion haloscope designs to those that possess directional sensitivity to incoming dark matter axion velocities. The effects are measurable if experiments are designed to have dimensions that approach the typical coherence length for the local axion field. With directional sensitivity, axion detection experiments would have a greatly enhanced potential to probe the local dark matter velocity distribution. We develop our formalism generally, but apply it to specific experimental designs, namely resonant cavities and dielectric disk haloscopes. We demonstrate that these experiments are capable of measuring the daily modulation of the dark matter signal and using it to reconstruct the three-dimensional velocity distribution. This allows one to measure the Solar peculiar velocity, probe the anisotropy of the dark matter velocity ellipsoid and identify cold substructures such as the recently discovered streams near to Earth. Directional experiments can also identify features over much shorter timescales, potentially facilitating the mapping of debris from axion miniclusters., Comment: 62 pages, 13 figures

We study the effect of the axion dark matter velocity in the recently proposed dielectric haloscopes, a promising avenue to search for well-motivated high mass ($40-400~\mu$eV) axions. We describe non-zero velocity effects for axion-photon mixing in a magnetic field and for the phenomenon of photon emission from interfaces between different dielectric media. As velocity effects are only important when the haloscope is larger than about 20% of the axion de Broglie wavelength, for the planned MADMAX experiment with 80 dielectric disks the velocity dependence can safely be neglected. However, an augmented MADMAX or a second generation experiment would be directionally sensitive to the axion velocity, and thus a sensitive measure of axion astrophysics., Comment: 36 pages, 16 figures; v3: typos and a small sign error corrected as in the erratum published as JCAP 05 (2018) E02

We study the underlying theory of dielectric haloscopes, a new way to detect dark matter axions. When an interface between different dielectric media is inside a magnetic field, the oscillating axion field acts as a source of electromagnetic waves, which emerge in both directions perpendicular to the surface. The emission rate can be boosted by multiple layers judiciously placed to achieve constructive interference and by a large transverse area. Starting from the axion-modified Maxwell equations, we calculate the efficiency of this new dielectric haloscope approach. This technique could potentially search the unexplored high-frequency range of 10--100 GHz (axion mass 40--400 $\mu$eV), where traditional cavity resonators have difficulties reaching the required volume., Comment: 71 pages, 34 figures; v2: minor changes, typo in (4.13) corrected, matches published version

We propose a new strategy to search for dark matter axions in the mass range of 40--400 $\mu$eV by introducing dielectric haloscopes, which consist of dielectric disks placed in a magnetic field. The changing dielectric media cause discontinuities in the axion-induced electric field, leading to the generation of propagating electromagnetic waves to satisfy the continuity requirements at the interfaces. Large-area disks with adjustable distances boost the microwave signal (10--100 GHz) to an observable level and allow one to scan over a broad axion mass range. A sensitivity to QCD axion models is conceivable with 80 disks of 1 m$^2$ area contained in a $10$ Tesla field., Comment: 6 pages, 3 figures. v2: minor changes to align with published version

Recent studies of the cosmic microwave background, large scale structure, and big bang nucleosynthesis (BBN) show trends towards extra radiation. Within the framework of supersymmetric hadronic axion models, we explore two high-reheating-temperature scenarios that can explain consistently extra radiation and cold dark matter (CDM), with the latter residing either in gravitinos or in axions. In the gravitino CDM case, axions from decays of thermal saxions provide extra radiation already prior to BBN and decays of axinos with a cosmologically required TeV-scale mass can produce extra entropy. In the axion CDM case, cosmological constraints are respected with light eV-scale axinos and weak-scale gravitinos that decay into axions and axinos. These decays lead to late extra radiation which can coexist with the early contributions from saxion decays. Recent results of the Planck satellite probe extra radiation at late times and thereby both scenarios. Further tests are the searches for axions at ADMX and for supersymmetric particles at the LHC., Comment: 25 pages, 7 figures; v2: references added; v3: update based on Planck results, axino->axion+gravitino decays included in Sect.IV, references added

We calculate the rate for thermal production of axions and saxions via scattering of quarks, gluons, squarks, and gluinos in the primordial supersymmetric plasma. Systematic field theoretical methods such as hard thermal loop resummation are applied to obtain a finite result in a gauge-invariant way that is consistent to leading order in the strong gauge coupling. We calculate the thermally produced yield and the decoupling temperature for both axions and saxions. For the generic case in which saxion decays into axions are possible, the emitted axions can constitute extra radiation already prior to big bang nucleosynthesis and well thereafter. We update associated limits imposed by recent studies of the primordial helium-4 abundance and by precision cosmology of the cosmic microwave background and large scale structure. We show that the trend towards extra radiation seen in those studies can be explained by late decays of thermal saxions into axions and that upcoming Planck results will probe supersymmetric axion models with unprecedented sensitivity., Comment: 16 pages, 7 figures; v2: references added, minor clarifying additions, matches published version

We investigate the direct production of supersymmetric scalar taus at the LHC. We present the general calculation of the dominant cross section contributions for hadronic stau pair production within the MSSM, taking into account left-right mixing of the stau eigenstates. We find that b-quark annihilation and gluon fusion can enhance the cross sections by more than one order of magnitude with respect to the Drell-Yan predictions. For long-lived staus, we consider CMSSM parameter regions with such enhanced cross sections and possible consequences from recent searches. We find that regions of exceptionally small stau yields, favoured by cosmology, are in tension with a recent CMS limit on m_stau., Comment: 9 pages, 6 figures, Talk given at the workshop "School and Workshops on Elementary Particle Physics and Gravity" September 4-18, 2011 Corfu, Greece

We calculate dominant cross section contributions for stau pair production at hadron colliders within the MSSM, taking into account left-right mixing of the stau eigenstates. We find that b-quark annihilation and gluon fusion can enhance the cross sections by more than one order of magnitude with respect to the Drell-Yan predictions. These additional production channels are not yet included in the common Monte Carlo analysis programs and have been neglected in experimental analyses so far. For long-lived staus, we investigate differential distributions and prospects for their stopping in the collider detectors. New possible strategies are outlined to determine the mass and width of the heavy CP-even Higgs boson H0. Scans of the relevant regions in the CMSSM are performed and predictions are given for the current experiments at the LHC and the Tevatron. The obtained insights allow us to propose collider tests of cosmologically motivated scenarios with long-lived staus that have an exceptionally small thermal relic abundance., Comment: 49 pages, 13 figures; v2: references added, typos corrected, text streamlined, results unchanged

The axino, the fermionic superpartner of the axion, is a well-motivated candidate for cold dark matter if it is the lightest supersymmetric particle. Since the axino couples very weakly to the matter multiplets, the next-to-lightest supersymmetric particle (NLSP) has a long lifetime, which has important consequences for both cosmology and collider phenomenology. Assuming that a charged slepton is the NLSP, we calculate the complete leading one- and two-loop contributions to its decay. We analyze in detail constraints on the parameters space from cosmology and discuss how this scenario can be probed at colliders. Scenarios in which both the axino and the gravitino are lighter than the long-lived charged slepton are also explored with particular emphasis on cosmological constraints and collider phenomenology., Comment: 64 pages, 22 figures

We calculate the rate for thermal production of axions via scattering of quarks and gluons in the primordial quark-gluon plasma. To obtain a finite result in a gauge-invariant way that is consistent to leading order in the strong gauge coupling, we use systematic field theoretical methods such as hard thermal loop resummation and the Braaten-Yuan prescription. The thermally produced yield, the decoupling temperature, and the density parameter are computed for axions with a mass below 10 meV. In this regime, with a Peccei-Quinn scale above 6x10^8 GeV, the associated axion population can still be relativistic today and can coexist with the axion cold dark matter condensate., Comment: 5 pages, 4 figures; v2: typo in (15) corrected, one reference added; v3: minor changes, references added, matches published version

Taking into account effects of late energy injection, we examine big bang nucleosynthesis (BBN) constraints on axino dark matter scenarios with long-lived charged sleptons. We calculate 4-body slepton decays into the axino, a lepton, and a quark-antiquark pair since they govern late hadronic energy injection and associated BBN constraints. For supersymmetric hadronic axion models, we present the obtained hadronic BBN constraints and show that they can be more restrictive than the ones associated with catalyzed BBN via slepton-bound-state formation. From the BBN constraints on hadronic and electromagnetic energy release, we find new upper limits on the Peccei-Quinn scale., Comment: 8 pages, 5 figures; minor additions and changes, small changes to Figure 2, typos corrected, references added, matches published version

Considering axino cold dark matter scenarios with a long-lived charged slepton, we study constraints on the Peccei-Quinn scale f_a and on the reheating temperature T_R imposed by the dark matter density and by big bang nucleosynthesis (BBN). For an axino mass compatible with large-scale structure, m_axino \gtrsim 100 keV, temperatures above 10^9 GeV become viable for f_a > 3x10^12 GeV. We calculate the slepton lifetime in hadronic axion models. With the dominant decay mode being two-loop suppressed, this lifetime can be sufficiently large to allow for primordial bound states leading to catalyzed BBN of Lithium-6 and Beryllium-9. This implies new upper limits on f_a and on T_R that depend on quantities which will be probed at the Large Hadron Collider., Comment: 10 pages, 5 figures; small additions and minor changes, matches published version

The existence of dark matter provides strong evidence for physics beyond the Standard Model. Extending the Standard Model with the Peccei-Quinn symmetry and/or supersymmetry, compelling dark matter candidates appear. For the axion, the neutralino, the gravitino, and the axino, I review primordial production mechanisms, cosmological and astrophysical constraints, experimental searches, and prospects for experimental identification., Comment: 31 pages, 14 figures

In supersymmetric models with a long-lived stau being the lightest Standard Model superpartner, the stau abundance during primordial nucleosynthesis is tightly constrained. Considering the complete set of stau annihilation channels in the minimal supersymmetric Standard Model (MSSM) with real parameters for scenarios in which sparticle coannihilations are negligible, we calculate the decoupling of the lighter stau from the primordial plasma and identify processes which are capable to deplete the resulting stau abundance significantly. We find particularly efficient stau annihilation at the resonance of the heavy CP-even Higgs boson and for a lighter stau with a sizeable left-right mixing due to enhanced stau-Higgs couplings. Even within the constrained MSSM, we encounter both effects leading to exceptionally small values of the resulting stau abundance. Prospects for collider phenomenology are discussed and possible implications of our findings are addressed with emphasis on gravitino dark matter scenarios., Comment: 34 pages, 8 figures; one reference added; matches published version

The catalysis of nuclear reactions by negatively charged relics leads to increased outputs of primordial ^6Li and ^9Be. In combination with observational constraints on the primordial fractions of ^6Li and ^9Be, this imposes strong restrictions on the primordial abundance and the lifetime of charged relics. We analyze the constraints from the catalysis of ^9Be on supersymmetric models in which the gravitino is the lightest supersymmetric particle and a charged slepton--such as the lighter stau--the next-to-lightest supersymmetric particle (NLSP). Barring the special cases in which the primordial fraction of the slepton NLSP is significantly depleted, we find that the ^9Be data require a slepton NLSP lifetime of less than 6x10^3 seconds. We also address the issue of the catalytic destruction of ^6Li and ^9Be by late forming bound states of protons with negatively charged relics finding that it does not lead to any significant modification of the limit on the slepton lifetime., Comment: 31 pages, 8 figures; minor additions, matches published version

Considering gravitino dark matter scenarios with a long-lived charged slepton, we show that collider measurements of the slepton mass and its lifetime can probe not only the gravitino mass but also the post-inflationary reheating temperature TR. In a model independent way, we derive upper limits on TR and discuss them in light of the constraints from the primordial catalysis of lithium-6 through bound-state effects. In the collider-friendly region of slepton masses below 1 TeV, the obtained conservative estimate of the maximum reheating temperature is about TR=3\times 10^9 GeV for the limiting case of a small gluino-slepton mass splitting and about TR=10^8 GeV for the case that is typical for universal soft supersymmetry breaking parameters at the scale of grand unification. We find that a determination of the gluino-slepton mass ratio at the Large Hadron Collider will test the possibility of TR>10^9 GeV and thereby the viability of thermal leptogenesis with hierarchical heavy right-handed Majorana neutrinos., Comment: 10 pages, 3 figures; revised version (references added, minor corrections)

In supersymmetric extensions of the Standard Model, the lightest neutralino, the gravitino, and the axino can appear as the lightest supersymmetric particle and as such provide a compelling explanation of the non-baryonic dark matter in our Universe. For each of these dark matter candidates, I review the present status of primordial production mechanisms, cosmological constraints, and prospects of experimental identification., Comment: 13 pages, 8 figures; Submitted for the SUSY07 proceedings; revised version (comments and references added, typos corrected)

Catalyzed big bang nucleosynthesis (CBBN) can lead to an overproduction of ^6Li in gravitino dark matter scenarios in which the lighter stau is the lightest Standard Model superpartner. Based on a treatment using the state-of-the-art result for the catalyzed ^6Li production cross section, we update the resulting constraint within the framework of the constrained minimal supersymmetric Standard Model (CMSSM). We confront our numerical findings with recently derived conservative limits on the gaugino mass parameter and the reheating temperature., Comment: 4 pages, 4 figures; Submitted for the SUSY07 proceedings

We investigate gravitino dark matter scenarios in which the primordial ^6Li production is catalyzed by bound-state formation of long-lived negatively charged particles X^- with ^4He. In the constrained minimal supersymmetric Standard Model (CMSSM) with the stau^- as the X^-, the observationally inferred bound on the primordial ^6Li abundance allows us to derive a rigid lower limit on the gaugino mass parameter for a standard cosmological history. This limit can have severe implications for supersymmetry searches at the Large Hadron Collider and for the reheating temperature after inflation., Comment: 4 pages, 2 figures; note and paragraph added; references updated

The identity of dark matter is one of the greatest puzzles of our Universe. Its solution may be associated with supersymmetry which is a fundamental space-time symmetry that has not been verified experimentally so far. In many supersymmetric extensions of the Standard Model of particle physics, the lightest supersymmetric particle cannot decay and is hence a promising dark matter candidate. The lightest neutralino, which appears already in the minimal supersymmetric model, can be identified as such a candidate in indirect and direct dark matter searches and at future colliders. As the superpartner of the graviton, the gravitino is another candidate for the lightest superparticle that provides a compelling explanation of dark matter. While it will neither be detected in indirect or direct searches nor be produced directly at accelerators, the analysis of late-decaying charged particles can allow for an experimental identification of the gravitino at future accelerators. In this way, the upcoming experiments at the CERN Large Hadron Collider may become a key to the understanding of our Universe., Comment: 14 pages, 4 figures, to appear in the Jahrbuch 2007 (German only) of the Max Planck Society

Considering gravitino dark matter scenarios, we study constraints on the reheating temperature of inflation. We present the gauge-invariant result for the thermally produced gravitino yield to leading order in the Standard Model gauge couplings. Within the framework of the constrained minimal supersymmetric Standard Model (CMSSM), we find a maximum reheating temperature of about 10^7 GeV taking into account bound-state effects on the primordial $^6$Li abundance. We show that late-time entropy production can relax this constraint significantly. Only with a substantial entropy release after the decoupling of the lightest Standard Model superpartner, thermal leptogenesis remains a viable explanation of the cosmic baryon asymmetry within the CMSSM., Comment: 13 pages, 15 figures, revised version accepted for publication (reheating phase considered, improved treatment of entropy production, revised last section, references added)

Considering scenarios in which the gravitino is the lightest supersymmetric particle and a charged slepton the next-to-lightest supersymmetric particle (NLSP), we discuss cosmological constraints on the masses of the gravitino and the NLSP slepton. The presented mass bounds are crucial for gravitino dark matter studies and potential gravitino signatures at future colliders., Comment: 4 pages, 1 figure; to appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 2006

Considering gravitino dark matter scenarios, we obtain the full gauge-invariant result for the relic density of thermally produced gravitinos to leading order in the Standard Model gauge couplings. For the temperatures required by thermal leptogenesis, we find gaugino mass bounds which will be probed at future colliders. We show that a conceivable determination of the gravitino mass will allow for a unique test of the viability of thermal leptogenesis in the laboratory., Comment: 5 pages, 3 figures, revised version matches published version

The gravitino is a promising candidate for cold dark matter. We study cosmological constraints on scenarios in which the gravitino is the lightest supersymmetric particle and a charged slepton the next-to-lightest supersymmetric particle (NLSP). We obtain new results for the hadronic nucleosynthesis bounds by computing the 4-body decay of the NLSP slepton into the gravitino, the associated lepton, and a quark-antiquark pair. The bounds from the observed dark matter density are refined by taking into account gravitinos from both late NLSP decays and thermal scattering in the early Universe. We examine the present free-streaming velocity of gravitino dark matter and the limits from observations and simulations of cosmic structures. Assuming that the NLSP sleptons freeze out with a thermal abundance before their decay, we derive new bounds on the slepton and gravitino masses. The implications of the constraints for cosmology and collider phenomenology are discussed and the potential insights from future experiments are outlined. We propose a set of benchmark scenarios with gravitino dark matter and long-lived charged NLSP sleptons and describe prospects for the Large Hadron Collider and the International Linear Collider., Comment: 51 pages, 20 figures, revised version matches published version (results unchanged, JHEP style used, figures replaced with new high-quality figures, typos corrected, references added)

The axino and the gravitino are extremely weakly interacting candidates for the lightest supersymmetric particle (LSP). We demonstrate that either of them could provide the right amount of cold dark matter. Assuming that a charged slepton is the next-to-lightest supersymmetric particle (NLSP), we discuss how NLSP decays into the axino/gravitino LSP can provide evidence for axino/gravitino dark matter at future colliders. We show that these NLSP decays will allow us to estimate the value of the Peccei-Quinn scale and the axino mass if the axino is the LSP. In the case of the gravitino LSP, we illustrate that the gravitino mass can be determined. This is crucial for insights into the mechanism of supersymmetry breaking and can lead to a microscopic measurement of the Planck scale., Comment: 6 pages, 7 figures, to appear in the proceedings of the "2005 International Linear Collider Workshop" (LCWS05), Stanford, 2005

We compute the thermal axino production rate in supersymmetric QCD to leading order in the gauge coupling. Using hard thermal loop resummation and the Braaten-Yuan prescription, we obtain a finite result in a gauge-invariant way, which takes into account Debye screening in the hot quark-gluon-squark-gluino plasma. The relic axino density from thermal reactions in the early Universe is evaluated assuming the axino is the lightest supersymmetric particle and stable due to R-parity conservation. From the comparison with the WMAP results, we find that axinos could provide the dominant part of cold dark matter, for example, for an axino mass of 100 keV and a reheating temperature of 10^6 GeV., Comment: 5 pages, 2 figures. Talk given at Strong and Electroweak Matter 2004, Helsinki, Finland, June 16-19, 2004

The axino is the fermionic superpartner of the axion. Assuming the axino is the lightest supersymmetric particle and stable due to R-parity conservation, we compute the relic axino density from thermal reactions in the early Universe. From the comparison with the WMAP results, we find that thermally produced axinos could provide the dominant part of cold dark matter, for example, for an axino mass of 100 keV and a reheating temperature of 10^6 GeV., Comment: 4 pages, 2 figures. Talk presented at 39th Rencontres de Moriond, "Exploring the Universe," La Thuile, Italy, March 28 - April 4, 2004

The axino is a promising candidate for dark matter in the Universe. It is electrically and color neutral, very weakly interacting, and could be - as assumed in this study - the lightest supersymmetric particle, which is stable for unbroken R-parity. In supersymmetric extensions of the standard model, in which the strong CP problem is solved via the Peccei-Quinn mechanism, the axino arises naturally as the fermionic superpartner of the axion. We compute the thermal production rate of axinos in supersymmetric QCD. Using hard thermal loop resummation, we obtain a finite result in a gauge-invariant way, which takes into account Debye screening in the hot quark-gluon-squark-gluino plasma. The relic axino abundance from thermal scatterings after inflation is evaluated. We find that thermally produced axinos could provide the dominant part of cold dark matter, for example, for an axino mass of 100 keV and a reheating temperature of 10^6 GeV., Comment: 33 pages, 7 figures, 1 table, erratum added

We present the loop-loop correlation model that allows us to compute confining flux tubes and to examine their effects and manifestations in high-energy reactions., Comment: 1 page, no figure. Contribution to Confinement 2003, Tokyo, July 21-24

We develop a loop-loop correlation model for a unified description of static color dipole potentials, confining QCD strings, and hadronic high-energy reactions with special emphasis on saturation effects manifesting S-matrix unitarity at ultra-high energies. The model combines perturbative gluon exchange with the non-perturbative stochastic vacuum model which describes color confinement via flux-tube formation of color fields. We compute the chromo-field distributions of static color dipoles in various SU(N_c) representations and find Casimir scaling in agreement with recent lattice QCD results. We investigate the energy stored in the confining string and use low-energy theorems to show consistency with the static quark-antiquark potential. We generalize Meggiolaro's analytic continuation from parton-parton to dipole-dipole scattering and obtain a Euclidean approach to high-energy scattering that allows us in principle to calculate S-matrix elements in lattice QCD. In this approach we compute high-energy dipole-dipole scattering with the Euclidean loop-loop correlation model. Together with a universal energy dependence and reaction-specific wave functions, the result forms the basis for a unified description of proton-proton, pion-proton, kaon-proton, photon-proton, and photon-photon reactions in good agreement with experimental data for cross sections, slope parameters, and structure functions. The obtained impact parameter profiles for proton-proton and longitudinal photon-proton reactions and the impact parameter dependent gluon distribution of the proton xG(x,Q^2,b) show saturation at ultra-high energies in accordance with unitarity constraints., Comment: 116 pages, 27 figures, doctoral thesis (Dissertation), University of Heidelberg

We compute total and differential elastic cross sections of high-energy hadronic collisions in the loop-loop correlation model that provides a unified description of hadron-hadron, photon-hadron, and photon-photon reactions. The impact parameter profiles of pp and gamma*p collisions are calculated. For ultra-high energies the hadron opacity saturates at the black disc limit which tames the growth of the hadronic cross sections in agreement with the Froissart bound. We compute the impact parameter dependent gluon distribution of the proton xG(x,Q^2,b) and find gluon saturation at small Bjorken x. These saturation effects manifest S-matrix unitarity in hadronic collisions and should be observable in future cosmic ray and accelerator experiments at ultra-high energies. The c.m. energies and Bjorken x at which saturation sets in are determined and LHC and THERA predictions are given., Comment: 11 pages, 6 figures. Talk at the 14th Topical Conference on Hadron Collider Physics (HCP2002), Karlsruhe, Germany, Sept. 29 - Oct. 5, 2002

The recent status of hard thermal photon production in relativistic heavy ion collisions is reviewed and the current rates are presented with emphasis on corrected bremsstrahlung processes in the quark-gluon plasma (QGP) and quark-hadron duality. Employing Bjorken hydrodynamics with an EOS supporting the phase transition from QGP to hot hadron gas (HHG), thermal photon spectra are computed. For SPS 158 GeV Pb+Pb collisions, comparison with other theoretical results and the WA98 direct photon data indicates significant contributions due to prompt photons. Extrapolating the presented approach to RHIC and LHC experiments, predictions of the thermal photon spectrum show a QGP outshining the HHG in the high-pT-region., Comment: 20 pages with 8 figures. v3: Erratum to [Phys. Lett. B 510 (2001) 98] with correctly labeled Figs. 2, 4, and 5 added

A systematic investigation of hard thermal photon spectra from central ultra-relativistic heavy ion collisions is presented with emphasis on the effects of bremsstrahlung processes in the quark-gluon plasma (QGP). Bremsstrahlung photon production in the quark-gluon plasma has recently been considered within the Braaten-Pisarski method in thermal QCD, where rates have been found that exhibit the same order in the coupling constants as those describing the lowest order processes, Compton scattering and q\bar{q}-annihilation. The impact of these bremsstrahlung photon production rates on the thermal photon spectra is studied systematically within a simple, well understood one-fluid hydrodynamical model that describes an only longitudinally expanding fireball (1+1 Bjorken scaling hydrodynamics). It is found that the bremsstrahlung processes enhance the thermal photon yield from the QGP by about one order of magnitude over the complete considered p_{\perp}-range independent of the choice of the model parameters. Experimental upper limits on direct photon production in fixed target 200 A GeV S + Au collisions at the CERN SPS are also considered and used to extract upper limits for the initial temperature of the QGP, where the QGP bremsstrahlung processes are found to make a difference of about 15 to 20 MeV depending on the temperature at which the phase transition is assumed. In comparison with other theoretical studies, the importance of reaction features not described in the simple model are estimated and interesting elements for a future extension of this systematic investigation are identified, which will be of great interest in prospect of the upcoming experiments at the BNL RHIC and the CERN LHC., Comment: 110 pages, 40 figures, diploma thesis (Diplomarbeit), University of Giessen