No description available.

The work of this dissertation has been to prove that the coherence of multiphoton excitation can be studied by an appropriately phased and time delayed sequence of pulses. An application of this fundamental study of coherence has been made for the enhancement of third harmonic generation. The coherent recovery of the energy lost to the two photon absorption process enalled a larger propagation distance for the fundamental than in an interaction which is incoherent or coherent, but not using a 90 degree phase shifted pulse pair. Phase matching over this longer propagation distance gave an enhancement of third harmonic generation.

A pair of nonidentical two-level atoms, separated by a fixed distance R, interact through photon exchange. The system is described by a state vector which is assumed to be a superposition of four "essential states": (1) the first atom is excited, the second one is in the ground state, and no photon is present, (2) the first atom is in its ground state, the second one is excited, and no photon is present, (3) both atoms are in their ground states and a photon is present, and (4) both atoms are excited and a photon is also present. The system is initially in state (1). The probabilities of each atom being excited are calculated for both the minimally-coupled interaction and the multipolar interaction in the electric dipole approximation. For the minimally-coupled interaction Hamiltonian, the second atom has a probability of being instantaneously excited, so the interaction is not retarded. For the multipolar interaction Hamiltonian, the second atom is not excited before the retardation time, which agrees with special relativity. For the minimally-coupled interaction the nonphysical result occurs because the unperturbed Hamiltonian is not the energy operator in the Coulomb gauge. For the multipolar Hamiltonian in the electric dipole approximation the unperturbed Hamiltonian is the energy operator. An active view of unitary transformations in nonrelativistic quantum electrodynamics is used to derive transformation laws for the potentials of the electromagnetic field and the static Coulomb potential. For a specific choice of unitary transformation the transformation laws for the potentials are used in the minimally-coupled second-quantized Hamiltonian to obtain the multipolar Hamiltonian, which is expressed in terms of the quantized electric and magnetic fields.

No description available.

Universiẗat, Diss., 1999--Freiburg (Breisgau).

No description available.

Cette thèse présente la première mesure de la section efficace exclusive photon-photon-> di-leptons en collisions proton-proton à une énergie dans le centre de masse de 7 TeV dans l'expérience ATLAS au LHC. La mesure est réalisée avec toutes les données accumulées en 2011, pour une luminosité intégrée de 4.6/fb. La première partie de la thèse est dédiée à la description théorique de ce type d'interaction. Le calcul de l'amplitude de la réaction photon-photon-> di-leptons est bien connu dans le cadre du modèle standard de la physique des particules. Cependant, une complication apparaît au LHC avec la prise en compte des effets de taille finie du proton. Le calcul de cet effet est l'une de mes contributions personnelles. Il a donné lieu à une publication. La section efficace nominale est réduite d'environ 20% du fait de cet effet, ce qui n'est pas négligeable. La seconde partie de la thèse concerne la description des détecteurs de l'expérience ATLAS. J'y décris en particulier le travail que j'ai réalisé sur la simulation de certains de ces détecteurs, en particulier les détecteurs situés dans la ligne de vol des protons, qui permettent la mesure de la trajectoire de ces derniers. Enfin, la dernière partie de la thèse concerne la mesure expérimentale de la section efficace de la réaction photon-photon-> di-leptons en collisions proton-proton, comme mentionnée plus haut. Lorsque la paire de leptons est une paire de muons, on trouve que la section efficace exclusive est égale à 0.628 ± 0.032(stat) ± 0.021(syst) pb, pour le domaine cinématique suivant M(di-muons)>20 GeV, pT(muons)>10 GeV et |muons)<2.4. Lorsque la paire de leptons est une paire électron-positrons, nous avons obtenu une section efficace exclusive égale à 0.428 ± 0.035(stat) ± 0.018(syst) pb, pour M(ee)>24 GeV, pT(e)>12 GeV et |muons (e)|<2.4. De plus, ces mesures confirment que le facteur de réduction par rapport à la prédiction théorique nominale (sans prendre en compte les effets de taille finie du proton) est bien de 20% dans le domaine d'énergie de la mesure. Cette analyse a été publiée par la collaboration ATLAS. Une perspective pour cette mesure est de la réaliser à plus haute énergie avec les nouvelles données accumulées par l'expérience ATLAS à partir de 2015. De plus, une mesure directe de la trajectoire des protons pourrait aider à la sélection exclusive. / This thesis presents a first measurement of the exclusive cross section for the reaction photon-photon-> di-leptons in proton-proton collisions at center of mass energy of 7 TeV in the ATLAS experiment at the LHC. The measurement is realized with all data accumulated during the year 2011 with an integrated luminosity of 4.6/fb. The first part of the thesis is dedicated to the theoretical description of this kind of interactions. The calculus of the amplitude for the reaction photon-photon-> di-leptons is well known in the context of the standard model of particle physics. However, the situation at the LHC is more complex. It requires taking into account the finite size effects of the protons. The understanding of this effect and its evaluation is one of my personal contributions, which has been published. The nominal cross section is then reduced by about 20% following this effect, which is non negligible. The second part of the thesis concerns the description of the detectors of the ATLAS experiment. In particular, I describe the work I have realized on the simulation of some of these devices, mainly the detectors located in the direction of flight of the protons that allows a measurement of the trajectory of the scattered (intact) protons. Finally, the last part of the thesis is devoted to the experimental measurement of the cross section of the reaction photon-photon-> di-leptons in proton-proton collisions, as mentioned above. When the lepton pair is a pair of muons, we find an exclusive cross section of 0.628 ± 0.032(stat) ± 0.021(syst) pb, for the kinematical domain M(di-muons)>20 GeV, pT(muons)>10 GeV and |muons|<2.4. When the lepton pair is a pair of electrons (positrons), we have obtained an exclusive cross section of 0.428 ± 0.035(stat) ± 0.018(syst) pb, for M(ee)>24 GeV, pT(e)>12 GeV et |muons(e)|<2.4. Moreover, these measurements confirm that the reduction (suppression) factor with respect to the nominal theory prediction (without taking into account the finite size effect of the protons) is of the order of 20% in the energy domain of the measurement. A perspective for this work is to make the measurement at larger energy with the new data recorded by the ATLAS experiment since 2015. Moreover, a direct measurement of the trajectory of protons could help for the exclusive selection.

No description available.

<p>Einstein's theory of general relativity is a cornerstone in the process of gaining increased understanding about problems of gravitational nature. It can be applied to problems on the huge length scales of cosmology and as far as we know it does not break down before the Planck scale is approached. Irrespective of scale, a perturbative approach is often a very useful way to reduce the Einstein system to manageable complexity and size.</p><p>The projects included in this thesis can be divided into three subcategories. In the first category the keyword is photon-photon scattering. General relativity predicts that scattering can take place on a flat background due to the curvature of space-time caused by the photons themselves. The coupling equations and cross-section are found and a comparison with the corresponding quantum field theoretical results is done to leading order. Moreover, photon-photon scattering due to exchange of virtual electron-positron pairs is considered as an effective field theory in terms of the Heisenberg-Euler Lagrangian resulting in a possible setup for experimental detection of this phenomenon using microwave cavities. The second category of projects is related to cosmology. Here linear perturbations around a flat FRW universe with a cosmological constant are considered and the corresponding temperature variations of the cosmic microwave background radiation are found. Furthermore, cosmological models of Bianchi type V are investigated using a method based on the invariant scheme for classification of metrics by Karlhede. The final category is slowly rotating stars. Here the problem of matching a perfect fluid interior of Petrov type D to an exterior axisymmetric vacuum solution is treated perturbatively up to second order in the rotational parameter.</p>

Einstein's theory of general relativity is a cornerstone in the process of gaining increased understanding about problems of gravitational nature. It can be applied to problems on the huge length scales of cosmology and as far as we know it does not break down before the Planck scale is approached. Irrespective of scale, a perturbative approach is often a very useful way to reduce the Einstein system to manageable complexity and size. The projects included in this thesis can be divided into three subcategories. In the first category the keyword is photon-photon scattering. General relativity predicts that scattering can take place on a flat background due to the curvature of space-time caused by the photons themselves. The coupling equations and cross-section are found and a comparison with the corresponding quantum field theoretical results is done to leading order. Moreover, photon-photon scattering due to exchange of virtual electron-positron pairs is considered as an effective field theory in terms of the Heisenberg-Euler Lagrangian resulting in a possible setup for experimental detection of this phenomenon using microwave cavities. The second category of projects is related to cosmology. Here linear perturbations around a flat FRW universe with a cosmological constant are considered and the corresponding temperature variations of the cosmic microwave background radiation are found. Furthermore, cosmological models of Bianchi type V are investigated using a method based on the invariant scheme for classification of metrics by Karlhede. The final category is slowly rotating stars. Here the problem of matching a perfect fluid interior of Petrov type D to an exterior axisymmetric vacuum solution is treated perturbatively up to second order in the rotational parameter.