Uncertainties in radiative effects of the quarks in -background in the form of final state radiation (FSR) are significant when it comes to reducing all forms of systematics that can arise from measuring the jets energy. Analysis on FSR is in general conducted on different simulated samples where one has included the radiative effect using algorithms such as PYTHIA[29]. The hypothesis is that through the re-weighting of the -background nominal sample one could add a better representation of the FSR effect. Finding a simple way to include a better description of FSR would not only save time in the simulation process but it would also be a way to reduce the systematic errors originating from limited MC statistics. Due to statistical effects coming from the simulations one cannot use the basic approach to define the effect of FSR as simply the difference between nominal and FSR. Two methods are tested to estimate the FSR effects; the first method uses a set of efficiency factors to represent the signal regions, the second method is to add a weight to the events of the nominal sample. The first method show positive results, especially in SR2, compared to a basic analysis, with an uncertainty of the FSR effect of: SR1:±29% SR2: ±51% SR3: ±37%. While a basic analysis gave an uncertainty of ±42%, ±122% and 36%. The second method shows positive signs where the re-weighted sample moves closer to the behaviour of the FSR sample. However, both methods are based on insufficient amount of statistics to draw any absolute conclusions.

Cette thèse porte sur l'amélioration des performances du CAG, le gravimètre atomique développé au SYRTE. Cet instrument exploite des techniques d'interférométrie atomique pour mesurer l'accélération locale de la pesanteur g subit par un nuage d'atomes froids de 87Rb en chute libre. Les perfectionnements comme l'asservissement des puissances des faisceaux Raman et l'optimisation des paramètres tels que ceux pilotant la détection sont présentés dans ce manuscrit. La position initiale du nuage, sa vitesse moyenne, son expansion balistique dans les faisceaux Raman ainsi que leur évolution, sont autant de paramètres altérant les performances du CAG. Les inhomogénéités de couplage qui en résultent, modifient la symétrie de la fonction de sensibilité de l'interféromètre et le rendent sensible aux désaccords Raman constants. De plus, les désaccords Raman de type Doppler ne peuvent voir leur effet annulé par la technique de mesure mise en place pour rejeter les effets systématiques. L'asymétrie de l'interféromètre a été mesurée et une méthode pour la compenser est proposée. Enfin, plusieurs comparaisons avec différents gravimètres sont présentées. La comparaison internationale CCM.G-K2 a permis de confirmer l'exactitude du CAG, révélant notamment un écart type d'Allan de 5,7 10-9/Hz1/2. Finalement, une session de mesure d'un mois en vue commune avec un gravimètre supraconducteur iGrav est étudiée. Elle a permis la détermination du facteur d'échelle de l'iGrav à 0,1% dès un jour de mesure et 0,02% en moins d'un mois. L'écart type d'Allan sur le résidu du signal entre les gravimètres atteint alors 6 10-11g après 12h de mesure. / This thesis aims at pushing the performances of the atom gravimeter CAG developed at SYRTE. This instrument uses atom interferometry to measure the local gravity acceleration of a free falling 87Rb cold atomic cloud. The improvements of the Raman power control and parameters such as those driving the detection are presented in this thesis. The initial position of the cloud, its mean velocity, its ballistic expansion into the Raman beams and thus their evolution, are important parameters which affect the performances of the CAG. The resulting coupling inhomogeneities modify the symmetry of the sensitivity function of the interferometer and make it sensitive to constant Raman detuning. Moreover, the Raman detuning in a Doppler way cannot be canceled by our rejection technic of systematic effects. The asymmetry of the sensitivity function has been measured and a method is demonstrated to cancel it. Several comparisons between the CAG and different gravimeter types have been performed and their results are developed. The international comparison of absolute gravimeters CCM.G-K2 confirms the CAG accuracy budget. It reveals an Allan standard deviation of 5.710-9g/Hz1/2. In the end, a one month common view measurement with a superconducting gravimeter iGrav is studied. The determination of the iGrav scale factor at the level of 0.1% in a single day and 0.02% in less than a month is obtained with this long measurement. The Allan standard deviation of the gravity residual signal is 610-11g in 12h measurement time.

The new generation of astronomical instruments are not only in need of the highest sensitivity but require also well-controlled and known instrumental systematic effects. This is particularly relevant for projects dedicated to the study of the Cosmic Microwave background (CMB). Following the success of the Planck mission in providing the most detailed picture of the CMB temperature anisotropy to date, the next generation of CMB projects such as COrE (a potential future mission) and QUBIC (a ground based instrument)will be aiming to study the polarisation anisotropy of the CMB. However, the expectedB mode signal from the CMB is several orders of magnitude weaker than the temperature counterpart. Hence the calibration procedures will have to be more stringent than the ones that have been adopted for Planck, to get a proper detection of the primordialB mode signal. For instance, measurements for the receiver and optical systematic effects must be taken into consideration to get a proper reconstruction of the B modepower spectrum. This thesis is focused on the impact of real individual receiver and optical componentson the observation of the primordial B modes. To achieve this, several receiver and quasioptical components have been measured and modelled for their instrumental systematic effects. An analysis pipeline has also been developed, to assess the impact of such instrumental systematic effects on the observation of the primordial B modes. Using the results from the measurements and the analysis pipeline, the instrumental systematic effects that are of concern to the observation of primordial B modes have been identified. This is assuming that no effort has been made to mitigate such instrumental systematic effects.