Constraining Physics Beyond Standard Model with the Decay Bs0 -> tau+ tau-

The Standard Model of Particle Physics is very successful in describing fundamental interactions from the energy scale of atomic physics to near the TeV regime. However, it is also incomplete as it fails to explain the characteristic pattern of masses and mixing parameters of quarks and leptons, and as it does not provide a candidate for Dark Matter, which is five times more abundant than ordinary matter in the Universe. The search for New Physics beyond the Standard Model is thus the highest priority of Particle Physics today. The recent discovery of a Higgs-like particle at CERNs Large Hadron Collider suggests that New Physics emerge in the TeV energy range. However, direct searches have failed to obtain positive evidence so far. In this situation, indirect probes such as precision measurements in flavour physics play a crucial role in elucidating the experimental situation, as they can test energy scales far above the TeV. The aim of my proposal is to obtain the first, direct measurement of the decay B0s + - using the Y(5S) dataset of the Belle experiment. This decay is sensitive to numerous extensions of the Standard Model, such as the type II two Higgs doublet model (which includes supersymmetric models), the leptoquark model, or models with a flavour changing Z` boson. This decay is also the prime candidate for explaining the anomalously large CP- violating like-sign dimuon charge asymmetry in B meson decays observed by D0. Despite the efforts of CDF and LHCb the experimental situation is still not settled and my project will finally clarify the issue. My research will be based on the 121fb-1 data sample collected by the Belle experiment at the center-of-mass energy of the Y(5S) resonance (10.87 GeV), which I have analyzed extensively for my Ph.D. research. My experience at Belle places me in a unique position for tackling this challenging study. In fact, as the tau- leptons decay before reaching any active surface of the detector, the final state contains up to four neutrinos, making this decay inaccessible for hadron collider experiments such as CDF, D0 or LHCb. The decay B0 + - , the analogous mode in the Y(4S) dataset, has already been successfully studied at lepton colliders: Based on the BaBar study, I expect an experimental sensitivity in the B0s + - branching fraction of 1.95%. This is amply sufficient to clarify the D0 claim and even lies within the signal region of some New Physics models. The total amount of funding requested for my project is EUR 229,223.26. This includes my own funding as Principal Investigator for a duration of 36 months, working contracts for diploma students and travel expenses for attending Belle collaboration meetings.

The Standard Model of Particle Physics is a very successful theory which describes the fundamental interactions between elementary particles. However, it is incomplete as all known elementary particles only account for about 5% of the total energy in the universe and several questions remain unanswered, e.g. concerning the elementary particles masses. Thus, searches for New Physics beyond the Standard Model have the highest priority in particle physics today, but direct searches did not show any clear signs of new physics processes.For this reasons indirect searches, such as precision measurements in flavour physics, are significant methods to observe smallest deviations from the Standard Model predictions. This project was initiated to perform the first direct measurement of the decay B0s??+?- at the Belle experiment in Tsukuba, Japan. Several theories predict that the branching ratio of this decay is significantly influenced by new physics processes.The basis for this study is the so-called ?(5S) data sample collected by the Belle experiment, which contains about 7.11.3 million B0s meson pairs. As the ? leptons decay before reaching any active surface of the detector, the final state of this decay contains up to four neutrinos which do not leave any energy signal or visible track in the detector. In order to reduce the additional background arising from this lost information, not only the signal decay (B0s??+?-) is reconstructed, but also the second B0s meson. Thus, the daughter particles from the second meson cannot produce any background on the signal side.Within this project this method is realized by developing and implementing a software which reconstructs more than 5000 exclusive B0s decays and saves the results. In order to reduce the number of incorrectly reconstructed events more than 100 neural networks have been trained to distinguish signal events from background.