Rare B Meson Decays in the ATLAS Experiment

The ATLAS Experiment is a general-purpose particle detector at the Large Hadron Collider (LHC) at CERN in Geneva. The LHC is a proton-proton storage ring, in which dense proton clouds interact at predefined collision -2 -1 points, at a collision energy of 14 TeV and an expected design luminosity of 1034 s . While one of the main goals of the ATLAS Experiment is the discovery of the Higgs boson, the universal design of the detector allows a broad variety of measurements, like supersymmetry, the search for dark matter candidates and the precise measurement of CP violation. Due to the high production rate of beauty quark-antiquark pairs at these energies a whole set of measurements in this field is planned. One main topic of B-physics at ATLAS is the detailed study of the properties and decays of the entire family of B mesons (Bd , B+ , Bs and Bc and their anti-particles). The electroweak transition of a b quark into an s quark is of special interest. Within the Standard Model of particle physics this transitions is only allowed through highly suppressed one-loop box diagrams or penguin loop diagrams, but is forbidden as flavor changing neutral current (FCNC). Extensions of the Standard Model like supersymmetry or technicolor directly influence the yet unknown branching ratio of the exclusive decay of a Bs meson into two muons. The additionally allowed transitions could lead to a branching ratio significantly higher than the expected Standard Model value. Therefore, the measurement of higher branching ratios compared to the Standard Model calculations is a direct evidence for physics beyond the Standard Model. Until the end of this project in December 2013 the expected integrated luminosity of 10 fb-1 should not only allow the measurement of an upper limit of the branching ratio, but a direct measurement of the Standard Model predictions is feasible. An additional electroweak transition of a b into an s quark can be found in the decay of the neutral Bd meson into a K* meson and two muons. Recent measurements of this branching ratio at the B factories Belle and BaBar and the CDF detector at Tevatron did not show any evidence for physics beyond the Standard Model. Nevertheless, calculations of this decay within supersymmetric extensions of the Standard Model deliver distributions like the forward-backward A FB asymmetry or the K* meson polarization, which strongly depend on the choice of the model. An integrated luminosity of 10 fb-1 allows a detailed analysis of these distributions and therefore could give direct evidence for physics beyond the Standard Model.

In this project data taken with the ATLAS detector in the year 2011 at the LHC collider at CERN have been analyzed. Physics observables have been measured that are sensitive to signs of physics beyond the Standard Model of particle physics. With the available data important new results have been obtained, but no deviations from the predictions of the Standard Model have been observed. The two processes that have been looked at were (1) the rare decay of the neutral B_s meson into a pair of oppositely charged muons and (2) the semi-rare decay of the B_0 meson also into a pair of oppositely charged muons, but with neutral K* meson in addition. According to the theory both processes proceed via the electroweak transition of a b quark into an s quark, which is highly suppressed because it is only allowed at higher orders. It is these higher orders where physics effects beyond the Standard Model would contribute to. (1) For the rare decay B_s -> +- a so-called blind analysis was performed, meaning that the analysis had been prepared and tuned without using data from the signal mass region. Using ATLAS data from the year 2011 no signal events were observed. This allowed to calculate an upper limit on the branching ratio of 1.5*10^(-8) at 95% confidence level, where the value predicted by the Standard Model is 3.54*10^(-9). The analysis within the ATLAS collaboration continued by analyzing the four times bigger data sample of the year 2012, but according to the project proposal the work of the project team had to shift to the second topic. (2) In the case of the semi-rare B_0 -> K* +- decay an elaborate statistical analysis using a multidimensional Maximum Likelihood fit method has been developed. Applying it to the ATLAS data sample of the year 2011 about 466 signal events were found. Thus it was possible to measure specific properties of the decay particles, namely the forward backward asymmetry of the muon pair as well as the longitudinal polarization of the neutral K*. Within statistical and systematic errors these results are in agreement with the expectations of the Standard Model and also with measurements from other experiments. These new results have been presented at an international conference and are also the main scientific content of a PhD thesis that has been written within this project.