Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 03/05

In der vorliegenden Dissertation werden Experimente zur Resonator-Quantenelektrodynamik auf einem Mikrofallenchip beschrieben. Dabei konnte u. a. erstmals einzelne, in einer Chipfalle gefangene Atome detektiert werden. Hierfür wurde im Rahmen dieser Arbeit ein neuartiger optischer Mikroresonator entwickelt, der sich dank seiner

Miniaturisierung mit der in unserer Arbeitsgruppe eingeführten Mikrofallentechnik zur Manipulation ultrakalter Atome kombinieren lässt. Für diesen Resonator werden Glasfaserenden als Spiegelsubstrate benutzt, zwischen denen sich eine stehende Lichtwelle ausbildet. Mit einem solchen Faser-Fabry{Perot-Resonator erzielen wir eine Finesse von bis zu F~37 000. Aufgrund der kleinen Modenvolumina kann trotz moderater Resonatorgüte die kohärente Wechselwirkung zwischen einem Atom und einem Photon so groß gemacht werden, dass das Regime der starken Atom-Resonator-Kopplung erreicht wird. Für die Ein-Atom-Ein-Photon-Kopplungsrate und die Ein-Atom-Ein-Photon-Kooperativität werden dabei Rekordwerte von g0 = 2Pi300MHz bzw. C0 = 210 erzielt.

Ebenso konnte erstmals das Regime starker Kopplung zwischen einem Bose-Einstein-Kondensat (BEC) und dem Feld eines Resonators hoher Güte erreicht werden. Das BEC wurde dabei mithilfe der magnetischen Mikrofallenpotentiale deterministisch an eine Position innerhalb des Resonators gebracht und zur Gänze in einen wohldefinierten

Bauch einer zusätzlichen optischen Dipol-Stehwellenfalle im Resonator umgeladen. Das Spektrum des gekoppelten Atom-Resonator-Systems wurde für unterschiedliche Atomzahlen

und Atom-Resonator-Verstimmungen vermessen, wobei ein kollektives Vakuum-Rabi-Splitting von mehr als 20GHz erreicht werden konnte.

Stars do not form in isolation, they form in groups known as star clusters.

Star clusters are seen in a wide range of galaxies and environments. Their presence reveals the history of the host galaxy and the processes of its star formation. During the last years star clusters have been deeply investigated in violent environments, while the properties of star clusters in more quiet environments have received less attention. Among all unperturbed environments we focus on 5 spiral galaxies with no signs of external perturbations: NGC 45, NGC 1313, NGC 4395, NGC 5236 and NGC 7793. Star clusters lying in these galaxies were observed through HST imaging and VLT spectroscopy. The analysis of star cluster masses, ages, sizes, and their positions on the galaxies, showed that star cluster formation is an ongoing process that depends on the local conditions. The observed star cluster

luminosity functions show values consistent with the expected alpha~-2.

We found an important number of globular clusters in NGC 45. Their properties are analyzed through photometry and spectroscopy. Photometry

suggests that these globular clusters belong to a single metal poor population. Spectroscopy confirmed this for the 8 brightest ones. Velocities indicate halo or bulge kinematics. Absorption spectrum features indicate ages of the order of Gyr and [alpha/Fe] values lower than the Milky Way globular clusters, but similar to dwarf galaxies in the local group.

This PhD thesis is a cumulative dissertation that consists of three papers.

The first paper describes the qualitative as well as quantitative nature of the electrostatic potential behind a macroparticle in a drifting collisional plasma and also the ion drag force taking into account plasma absorption on the grain surface. Plasma absorption on the grain surface is one of the fundamental properties of complex (dusty) plasmas which results in the "openness" of these systems. It is shown that in the considered regime plasma absorption determines completely the long-range potential. Physically, absorption of the drifting ions on the grain surface generates ion rarefaction behind the grain which opposes the ion focussing effect. In certain parameter regime ion rarefaction effect dominates over focussing effect which makes ion drag force to act in the opposite direction of ion flow i.e we obtain negative ion drag force.

In the second paper both the ion drag and electron drag forces acting on an absorbing grain have been calculated in the limit of highly collisional weakly ionized plasmas. The plasma is exposed to an external weak electric field so that both ions and electrons drift with subthermal velocities. The effect of plasma absorption reduces the absolute magnitude of the ion drag force and even can change its sign in certain parameter regimes whereas this effect increases the magnitude of the electron drag force. The total force which is the sum of electric, ion drag and electron drag forces is proportional to the electric field. The proportionality constant represents the effective charge. The calculated value of the effective charge turns out to be of the order of the actual charge. This fact implies that in this parameter regime the drag forces are of minor importance compared to the electric force.

The third paper deals with the isotropic potential distribution and the ion drag force acting on an isolated dust particle in highly collisional weakly anisotropic plasma. But unlike the previous two cases where plasma absorption and loss processes were neglected in the vicinity of the grain, in this work we consider these processes. The plasma production is assumed to be due to electron impact ionization whereas for the loss processes two different mechanisms are considered: in high pressure plasmas the loss is mainly due to electron-ion volume recombination and in low/moderate pressure gas discharges the plasma loss is due to ambipolar diffusion towards the chamber walls or electrodes. In the first case the potential consists of two exponential terms and the long range potential is of Yukawa type. In the second case the long range potential is Coulomb-like. It is found that the purely Debye-Hückel potential is obtained only in absence of ionization-recombination processes. The ion drag force is negative when the ionization rate is low, but becomes positive for sufficiently high ionization rate in both cases. The parameter regimes for the positive and negative ion drag forces have been identified.

The top quark plays a special role in the Standard Model of Particle Physics. With its enormous mass of about 170 GeV it is as heavy as a gold atom and it is the only quark with a mass near the electroweak scale. Together with the W boson mass, the top quark mass allows indirect constraints on the mass of the hypothetical Higgs boson, which might hold the clue to the origin of mass.

Top pair production with a semileptonic decay is the "golden channel" for mass measurements, due to a large branching fraction and a relatively low background contamination compared to other decay channels. Top mass measurements based on this decay, performed with the matrix element method, have always been among the single best measurements in the world. In 2007, the top mass world average broke the 1% level of precision. Its measurement is no longer dominated by statistical but instead by systematic uncertainties. The reduction of systematic uncertainties has therefore become a key issue for further progress.

This thesis introduces two new developments in the treatment of b jets. The first improvement is an optimization in the way b identification information is used. It leads to an enhanced separation between signal and background processes and reduces the statistical uncertainty by about 16%. The second improvement determines differences in the detector response and thus the energy scales of light jets and b jets. Thereby, it addresses the major source of systematic uncertainty in the latest top mass measurements.

The Large Hadron Collider will collide protons with protons at a center-of-mass energy of up to 14 TeV. New physics phenomena and new particles are predicted to be detectable with the ATLAS detector at the Large Hadron Collider. One of these predicted new particles beyond the Standard Model are leptoquarks.

This thesis deals with the search for scalar second generation leptoquarks produced in pairs. Second generation leptoquarks decay into a muon-type lepton and a quark. In this thesis the decay of both second generation leptoquarks into a muon and a quark is considered. Since pair production is studied the final state consists of two high-energetic muons and two high-energetic jets.

This thesis studies second generation leptoquarks with masses of mLQ = 300 GeV, mLQ = 400 GeV, mLQ = 600 GeV and mLQ = 800 GeV. The best cut variables for the discrimination between the signal and the main Standard Model backgrounds ttbar and Z/gamma* found in this analysis are the pT of the muons, ST (the scalar sum of the transverse momenta of the two selected muons and the transverse energies of the two selected jets), the mass of the selected dimuon system and the reconstructed leptoquark mass.

The latter three cut variables have been optimized for a discovery with a 5 significance including the systematic uncertainties and trigger efficiencies.

Second generation leptoquarks have been excluded up to the mass of 300 GeV with a 95% confidence level at present experiments. The expected integrated luminosities needed for a 5 discovery of the tested second generation leptoquark masses with the ATLAS detector have been calculated. This thesis shows that for a disocvery with 5 significance of a second generation leptoquark with mLQ = 300 GeV and mLQ = 400 GeV an expected integrated luminosity of 1.51 pb−1 and 7.42 pb−1 is needed respectively; this corresponds to a very early phase, i.e. the first few months, of the Large Hadron Collider run. For the discovery with a 5 significance of second generation leptoquarks with masses of mLQ = 600 GeV and mLQ = 800 GeV an expected integrated luminosity of 103.3 pb−1 and 663 pb−1 is needed respectively; this corresponds to several months and about half a year to a year of the Large Hadron Collider run respectively.

This thesis presents an observational study of one of the nearest star-forming regions, the ρ Ophiuchi cluster, with the goal of characterising stellar activity in young stellar

objects. Variability is a common characteristic of pre-main-sequence (PMS) stars and originates from mechanisms related to magnetic fields, accretion discs, and circumstellar extinction. Variability surveys in the infrared (IR) can probe stellar and circumstellar environments and provide information about the dynamics of the on going magnetic and accretion processes. To this end, a large-field (∼0.8 deg2), near-IR photometric survey of ρ Ophiuchi has been conducted using the Wide Field Camera

(WFCAM) at the United Kingdom Infrared Telescope (UKIRT), which monitored the cluster over 14 nights in two consecutive years. Statistical tools, such as the multi-band

cross correlation index and the reduced chi-square, were used to disentangle signals of variability from noise. Variability in the near-IR is found to be present in half of the known population of ρ Ophiuchi. The behaviours shown are several and can be associated with the existence of spots on the stellar surface, variations in circumstellar extinction, or changes in the geometry of an accretion disc.

Furthermore, this thesis explores the feasibility of using photometric variability as a criterion for selection of new low-luminosity candidates of galactic star forming regions

down to very low masses. The use of this new selection method could help remove the biases from other techniques. For example, it has the advantage of identifying young

sources that do not show IR-excess or have small Hα emission. The use of this relatively new technique should contribute significantly to obtain a full census of the young objects in star formation regions, which remains one of the most important goals of galactic star formation studies. Using variability, a new population of objects has been uncovered, that is believed to be part of the ρ Ophiuchi cluster. Using low-resolution near-IR spectroscopy, 4 candidate members have been confirmed as PMS stars.

Finally, the survey presented in this thesis is the first of its kind in the IR, both in large area surveyed and sensitivity, taking advantage of recent developments in IR astronomical instrumentation. Of future relevance for IR variability studies is the new IR-imager at the Very Large Telesope (VLT), HAWK-I (High-Acuity Wide-field K band Imager), the most advanced IR camera on an 8-meter telescope at the present moment. The instrument was assembled and commissioned during the progress of this thesis, and is presented here, giving emphasis to the contribution of the author to the project.