Aufbau, Charakterisierung und Inbetriebnahme des 1.4 THz Kanals des Heterodyn-Empfängers GREAT

This thesis describes the characterisation and commissioning of the heterodyne receiver "German REceiver for Astronomy at Terahertz frequencies" (GREAT) aboard of the airborne "Stratospheric Observatory For Infrared Astronomy" (SOFIA). GREAT is a modular dual-channel-receiver designed for frequencies between 1.25 and 4.7 THz. Currently three single-pixel frequency channels are available. A fourth one is currently under construction and two multi-pixel systems are planned for 2014. In the frequency range observable with GREAT one can find a lot of transitions of molecules, such as rotational transitions of CO, and of atoms and ions, e.g the fine structure lines of C+ and O. Line emission of these transitions is often observed in photon-dominated regions (PDR) in space. These regions form at the edges of denser clouds of the interstellar medium which are illuminated by strong UV fields, e.g. of nearby OB-""stars. The UV-radiation heats up the gas and dust of the cloud and dissociates and ionizes the gas particles. These reactions depend on the intensity of the UV field, which drops down with increasing depth inside the clouds, and the density, which rises, leading to a characteristic stratification of the ions, atoms and molecules. After two short introductory chapters describing astronomy and receivers in the THz regime and especially the GREAT receiver and the SOFIA observatory, this thesis starts with a description of the build-up of the local oscillator (LO) for the L#1-channel (1.25-1.5 THz) of GREAT. This includes the design of a cross-channel LO housing and its certification by NASA and FAA. Then the used frequency multiplier chains, manufactured by Virginia Diodes Inc., are characterised. Afterwards the developed control electronics and security circuits for these chains are presented. The next chapter is dedicated to the commissioning of GREAT on SOFIA. GREAT's configuration during the first flights consisted of the L#1- and L#2-channel (1.82-1.92 THz). A short note on the adjustment of the cross-channel common optics is given. Then, the text presents a description of the developed alignment procedures and methods to derive the positions of the GREAT beam in SOFIA's focal plane and on its sub-reflector. Final values for pointing, telescope coupling and co-alignment between the two channels measured before the first and second flight series (April and July 2011) are given. The sensitivity and stability of the GREAT system are investigated. Noise temperatures of the L#1-channel lie in the range of 1200-1400 K, for the L#2-channel in the range of 1300-1800 K. The spectroscopic Allan times are above 100 seconds. The fifth chapter of this thesis presents results of the Cepheus B (CepB) project, one of the first observed with GREAT on SOFIA. CepB is part of a giant molecular cloud and shows sequential star formation. The UV radiation of a nearby OB star cluster illuminates the cloud surface, thereby creating several PDRs and possibly triggering new star formation processes. The existence of several embedded stars, including a luminous B-star, inside CepB is known. The aim of this project is to measure the dynamics of the triggering of star formation by UV radiation. [CII] and high rotational transitions of CO are used to trace the hot gas in CepB. A velocity resolved map containing the embedded star, two PDRs and two dense clumps was observed. Integrated-intensity maps are presented showing emission which is consistent with previous radio continuum and 13CO observations. A global velocity gradient across the map is visible in the velocity channel maps of [CII]. A value of (3.6 +/- 0.6) * 10^{-3} km/s*arcsec^{-1} was calculated which is consistent with former observations of Beuther et al. (2000). [CII] and CO(11-10) observations show a localized inversion of this gradient between the positions of two CO(11-10) emission maxima. Three possible scenarios are proposed for this discovery and further high resolution CO and [NII] observations to clarify this situation are suggested. Together with 13CO observations of Mookerjea et al. (2006), a clear stratification, consistent with the expected emission pattern of a simple 1-dimensional PDR, in a region of the map near the embedded star was observed. The embedded star is found to be the source of the UV radiation creating this PDR. The distances between the emission maxima were measured and compared with model calculations. The results are consistent with Mookerjea's and constrain the surface density of the PDR to 10^4 - 10^6 cm^{-3}

Search for Squarks and Gluinos in τ-lepton Final States with the ATLAS Experiment

Suche nach Squarks und Gluinos in τ-Lepton Endzuständen mit dem ATLAS Experiment Das erfolgreiche und wohletablierte Standardmodell der Teilchenphysik ist in der Lage, die Mehrheit der beobachteten Phänomene auf der mikroskopischen Skala der Elementarteilchen zu beschreiben. Trotz seiner hohen Vorhersagekraft zeigt es offensichtliche Unzulänglichkeiten. Es ist zum Beispiel nicht in der Lage, die experimentell bestätigte Beobachtung von dunkler Materie zu erklären oder Gravitation auf der Quantenskala zu beschreiben. Supersymmetrie ist eine vielseitige und lange verfolgte Theorie, die Lösungen zu diesen Unzulänglichkeiten bietet und in die das Standardmodell als niederenergetische Näherung vollständig eingebettet werden kann. Das ATLAS Experiment am Large Hadron Collider stellt eine einzigartige Forschungsumgebung dar, um nach ersten Anzeichen von Supersymmetrie zu suchen. Die Präsenz neuer, stark wechselwirkender Teilchen—Squarks und Gluinos—wäre ein starker Indikator für die Existenz von Supersymmetrie. Vielversprechende und experimentell zugängliche Signaturen der Zerfälle kurzlebiger Squarks und Gluinos können die Präsenz von τ-Leptonen beinhalten. Diese Arbeit präsentiert die durchgeführte Analyse in der Suche nach in τ-Leptonen zerfallenden Squarks und Gluinos als Hinweise auf Supersymmetrie. Die Analysestrategie beinhaltet ein fortgeschrittenes Verfahren zur Abschätzung des Einflusses von Prozessen des Standardmodells auf die Suche. Verschiedene Ansätze in der eigentlichen Suche nach Signalen von Supersymmetrie werden implementiert und ihre Verhalten verglichen. Die Ergebnisse der Suche werden einer detaillierten statistischen Analyse und anschließender Interpretation unterzogen. Da diese Arbeit die Entdeckung der gesuchten neuen Teilchen nicht für sich beanspruchen kann, werden obere Ausschlussgrenzen auf die Modellparameter von Supersymmetrie sowie die generelle Anwesenheit beliebiger Modelle neuer Physik gesetzt.The successful and well-established Standard Model of Particle Physics is able to describe the majority of observed phenomena at the microscopic scale of elementary particles. Despite its high predictive power, it is also subject to certain manifest shortcomings. It is, for example, unable to explain the experimentally confirmed existence of dark matter or provide a description of gravity at the quantum scale. Supersymmetry is a versatile and long sought model that provides solutions to these shortcomings and into which the Standard Model can be fully embedded as a low-energy approximation. The ATLAS experiment at the Large Hadron Collider poses a unique experimental environment to search for first signs of Supersymmetry. The presence of new strongly interacting particles—squarks and gluinos—would be a strong indicator for the existence of Supersymmetry. Promising and experimentally accessible signatures of the decays of short-lived squarks and gluinos can comprise the presence of τ-leptons. This thesis presents the analysis carried out in the search for squarks and gluinos decaying to τ-leptons as signs for Supersymmetry. The analysis strategy comprises a sophisticated procedure to estimate the influence of Standard Model processes to the search. Different approaches in the actual search for signals of Supersymmetry are implemented and their performance is compared. The findings are made subject to a detailed statistical evaluation and subsequent interpretation. As no discovery of the sought new particles can be claimed, upper limits on the model parameters of Supersymmetry as well as on the general presence of any model of new physics are set

Velocity resolved [CII], [CI], and CO observations of the N159 star-forming region in the Large Magellanic Cloud: a complex velocity structure and variation of the column densities

The [CII]158um line is one of the dominant cooling lines in star-forming active regions. The commonly assumed clumpy UV-penetrated cloud models predict a [CII] line profile similar to that of CO. However, recent spectral-resolved observations show that they are often very different, indicating a more complex origin of the line emission including the dynamics of the source region. The aim of our study is to investigate the physical properties of the star-forming ISM in the Large Magellanic Cloud (LMC) by separating the origin of the emission lines spatially and spectrally. In this paper, we focus on the spectral characteristics and the origin of the emission lines, and the phases of carbon-bearing species in the N159 star-forming region in the LMC. We mapped a 4'x(3-4)' region in N159 in [CII]158um and [NII]205um with the GREAT on board SOFIA, and in CO(3-2), (4-3), (6-5), 13CO(3-2), and [CI]3P1-3P0 and 3P2-3P1 with APEX. The emission of all transitions observed shows a large variation in the line profiles across the map and between the different species. At most positions the [CII] emission line profile is substantially wider than that of CO and [CI]. We estimated the fraction of the [CII] integrated line emission that cannot be fitted by the CO line profile to be 20%-50%. We derived the relative contribution from C+, C, and CO to the column density in each velocity bin. The contribution from C+ dominates the velocity range far from the velocities traced by the dense molecular gas, and the region located between the CO cores of N159 W and E. We estimate the contribution of the ionized gas to the [CII] emission using the ratio to the [NII] emission to be < 19% to the [CII] emission at its peak position, and <15% over the whole observed region. Using the integrated line intensities, we present the spatial distribution of I([CII])/I(FIR). (abridged for arXiv)Comment: 16 pages with 14 figures, accepted for publication in A&

L1599B: Cloud Envelope and C+ Emission in a Region of Moderately Enhanced Radiation Field

We study the effects of an asymmetric radiation field on the properties of a molecular cloud envelope. We employ observations of carbon monoxide (12CO and 13CO), atomic carbon, ionized carbon, and atomic hydrogen to analyze the chemical and physical properties of the core and envelope of L1599B, a molecular cloud forming a portion of the ring at approximately 27 pc from the star Lambda Ori. The O III star provides an asymmetric radiation field that produces a moderate enhancement of the external radiation field. Observations of the [CII] fine structure line with the GREAT instrument on SOFIA indicate a significant enhanced emission on the side of the cloud facing the star, while the [Ci], 12CO and 13CO J = 1-0 and 2-1, and 12CO J = 3-2 data from the PMO and APEX telescopes suggest a relatively typical cloud interior. The atomic, ionic, and molecular line centroid velocities track each other very closely, and indicate that the cloud may be undergoing differential radial motion. The HI data from the Arecibo GALFA survey and the SOFIA/GREAT [CII] data do not suggest any systematic motion of the halo gas, relative to the dense central portion of the cloud traced by 12CO and 13CO.Comment: 9 Figure

Spiral Growth and Step Edge Barriers

The growth of spiral mounds containing a screw dislocation is compared to the growth of wedding cakes by two-dimensional nucleation. Using phase field simulations and homoepitaxial growth experiments on the Pt(111) surface we show that both structures attain the same characteristic large scale shape when a significant step edge barrier suppresses interlayer transport. The higher vertical growth rate observed for the spiral mounds on Pt(111) reflects the different incorporation mechanisms for atoms in the top region and can be formally represented by an enhanced apparent step edge barrier.Comment: 11 pages, 4 figures, partly in colo

Impacts of pure shocks in the BHR71 bipolar outflow

During the formation of a star, material is ejected along powerful jets that impact the ambient material. This outflow regulates star formation by e.g. inducing turbulence and heating the surrounding gas. Understanding the associated shocks is therefore essential to the study of star formation. We present comparisons of shock models with CO, H2, and SiO observations in a 'pure' shock position in the BHR71 bipolar outflow. These comparisons provide an insight into the shock and pre-shock characteristics, and allow us to understand the energetic and chemical feedback of star formation on Galactic scales. New CO (Jup = 16, 11, 7, 6, 4, 3) observations from the shocked regions with the SOFIA and APEX telescopes are presented and combined with earlier H2 and SiO data (from the Spitzer and APEX telescopes). The integrated intensities are compared to a grid of models that were obtained from a magneto-hydrodynamical shock code which calculates the dynamical and chemical structure of these regions combined with a radiative transfer module based on the 'large velocity gradient' approximation. The CO emission leads us to update the conclusions of our previous shock analysis: pre-shock densities of 1e4 cm-3 and shock velocities around 20-25 km s-1 are still constrained, but older ages are inferred ( 4000 years). We evaluate the contribution of shocks to the excitation of CO around forming stars. The SiO observations are compatible with a scenario where less than 4% of the pre-shock SiO belongs to the grain mantles. We infer outflow parameters: a mass of 1.8x1e-2 Msun was measured in our beam, in which a momentum of 0.4 Msun km s-1 is dissipated, for an energy of 4.2x1e43erg. We analyse the energetics of the outflow species by species. Comparing our results with previous studies highlights their dependence on the method: H2 observations only are not sufficient to evaluate the mass of outflows.Comment: 14 pages, 10 figures, 4 Tables, accepted in A&

The Inception of Star Cluster Formation Revealed by [CII] Emission Around an Infrared Dark Cloud

We present SOFIA-upGREAT observations of [CII] emission of Infrared Dark Cloud (IRDC) G035.39-00.33, designed to trace its atomic gas envelope and thus test models of the origins of such clouds. Several velocity components of [CII] emission are detected, tracing structures that are at a wide range of distances in the Galactic plane. We find a main component that is likely associated with the IRDC and its immediate surroundings. This strongest emission component has a velocity similar to that of the $^{13}$CO(2-1) emission of the IRDC, but offset by $\sim3\:{\rm km\:s}^{-1}$ and with a larger velocity width of $\sim9\:{\rm km\:s}^{-1}$. The spatial distribution of the [CII] emission of this component is also offset predominantly to one side of the dense filamentary structure of the IRDC. The CII column density is estimated to be of the order of $\sim10^{17}-10^{18}\,{\rm cm}^{-2}$. We compare these results to the [CII] emission from numerical simulations of magnetized, dense gas filaments formed from giant molecular cloud (GMC) collisions, finding similar spatial and kinematic offsets. These observations and modeling of [CII] add further to the evidence that IRDC G035.39-00.33 has been formed by a process of GMC-GMC collision, which may thus be an important mechanism for initiating star cluster formation.Comment: 5 pages, 4 figures, submitted to MNRAS Letter

Measurement of the cross-section and charge asymmetry of WW bosons produced in proton-proton collisions at s=8\sqrt{s}=8 TeV with the ATLAS detector

This paper presents measurements of the $W^+ \rightarrow \mu^+\nu$ and $W^- \rightarrow \mu^-\nu$ cross-sections and the associated charge asymmetry as a function of the absolute pseudorapidity of the decay muon. The data were collected in proton--proton collisions at a centre-of-mass energy of 8 TeV with the ATLAS experiment at the LHC and correspond to a total integrated luminosity of 20.2~\mbox{fb^{-1}}. The precision of the cross-section measurements varies between 0.8% to 1.5% as a function of the pseudorapidity, excluding the 1.9% uncertainty on the integrated luminosity. The charge asymmetry is measured with an uncertainty between 0.002 and 0.003. The results are compared with predictions based on next-to-next-to-leading-order calculations with various parton distribution functions and have the sensitivity to discriminate between them.Comment: 38 pages in total, author list starting page 22, 5 figures, 4 tables, submitted to EPJC. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2017-13

Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at √ s = 8 TeV with the ATLAS detector

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb−1 of √ s = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between Emiss T > 150 GeV and Emiss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presente