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| Home > Publications database > Angular Analysis of $B \to K^\ast \ell\ell$ and Search for $B^+\to K^+ \tau \tau$ at the Bellle Experiment |
| Book/Dissertation / PhD Thesis | PUBDB-2016-03770 |
; ; ; ;
2016
Verlag Deutsches Elektronen-Synchrotron
Hamburg
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Please use a persistent id in citations: doi:10.3204/PUBDB-2016-03770
Report No.: DESY-THESIS-2016-025
Abstract: Rare decays of $B$ mesons are an ideal probe to search for phenomena beyond the Standard Model of particle physics, since contributions from new particles can affect the decays on the same level as Standard Model predictions.The rare decay of $B \to K^{(\ast)} \ell^+ \ell^-$offers the quark transition $b\to s \ell^+ \ell^-$, a flavor changing neutral current which is forbidden at tree level in the Standard Model.Higher order processes such as penguin or $W^+W^-$ box diagrams allow for these processes, leading to branching ratios of less than one in a million.Various extensions to the Standard Model predict influences of new physics, which can enhance or suppress branching ratios or lead to changes in angular distributions of the decay products. In order to produce a sufficient amount of $B$ mesons, the so-called $B$-factories were designed.One of them is the asymmetric-energy $e^+e^-$ collider KEKB with the Belle experiment, located in Tsukuba, Japan.During 1999 to 2010 the experiment recorded a total luminosity of $1~\mathrm{ab}^{-1}$ of electron positron collisions. In this thesis a comprehensive study of the flavor changing neutral current process $b\to s \ell\ell$ in $B$ meson decays with accompanying kaons is presented in two separate analyses.All three lepton modes, $e^+e^-$, $\mu^+\mu^-$ and $\tau^+\tau^-$ are investigated to search for evidence of physics beyond the Standard Model. The measurements are performed using the full Belle data sample of $772\times10^6$ $B\bar B$ pairs, recorded at the $\Upsilon(4S)$ resonance energy.The first analysis in this thesis covers the muon and electron modes in the decay of $B^0 \to K^\ast(892)^0 \ell^+ \ell^-$.To maximize signal efficiency and purity, neural networks are developed sequentially from the bottom to the top of the decay chain, transferring each time the output probability to the subsequent step such that most effective selection cuts are applied in the last stage based on all information combined.Reconstructed signal yields of both channels exceed previous $B$-factory results of Belle and BaBar measurements, enabling a full angular analysis in this decay with the Belle data for the first time.In total $117.6\pm12.4$ signal candidates for $B^0 \to K^\ast(892)^0 \mu^+ \mu^-$ and $69.4\pm 12.0$ signal events for $B^0 \to K^\ast(892)^0 e^+ e^-$ are observed in the data.The branching ratios of both modes are extracted and found to be in agreement with previous measurements. With the combined data of both channels, the differential decay rate is extracted in three angular dimensions in five bins of $q^2$, the di-lepton invariant mass squared.A series of spatial transformations is applied to reduce the number of free parameters of the differential decay rate from eight to three.With four different transformations the fit is independently sensitive to observables $P_4'$, $P_5'$, $P_6'$ and $P_8'$, which are optimized regarding uncertainties arising from form-factors.Altogether 20 independent three-dimensional maximum likelihood fits are performed extracting $P_{4,5,6}'$ or $P_8'$, the \kast longitudinal polarization $F_L$ and the transverse polarization asymmetry $A_T$. Results in the region of $q^2 <8~ \mathrm{\,Ge\kern -0.1em V}^{2}/c^4 $ are compared with Standard Model predictions and overall agreement is observed.One measurement is found to deviate by $\sim 2.1\sigma$ from the predicted value in the same direction and in the same $q^2$ region where the LHCb collaboration reported the so called $P_5'$ anomaly .The second analysis in this thesis is dedicated to the $\tau$ mode of $b\to s \ell\ell$ in the search for $B^+\to K^+\tau^+ \tau^-$. This mode is particularly interesting as new particles could couple to the high mass of $\tau$ leptons stronger than to $e$ and $\mu$. However, due to several neutrinos being present in the final state, it is difficult to find.The full reconstruction technique is used, which is unique for $e^+e^-$ colliders and makes it possible to find signatures of the decay.On the control channel, $B^+ \to K^+ \tau^+ \tau^- (K_S)$, it is shown that the reconstruction and background suppression methods work as expected and that they deliver consistent results in both data and Monte Carlo.This analysis demonstrates using simulated events and control channels on the Belle dataset, that the current upper limit can be improved by more than one order of magnitude compared to the current value to\begin{equation}{\cal B}^{Projected}(B^+\to K^+ \tau^+\tau^-) < 3.17\times 10^{-4},\end{equation}at $90\%$ confidence level including systematic uncertainties determined by an extensive study of all the sources.This limit will come close to the prediction of models developed in the context of Minimal Lepton Flavor Violation.
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