Decay at the Belle II Experiment

Universität Hamburg Fachbereich Physik Bachelorarbeit in der Belle II Gruppe des Deutschen Elektronen Synchrotron, DESY A sensitivity study of the B0 ! K∗(892)0 m+ m− decay at the Belle II experiment Merle Schreiber 1. Gutachterin: Prof. Dr. Caren Hagner 2. Gutachter: Dr. habil. Alexander Glazov Betreuer: Dr. Simon Wehle 21. November 2017 Abstract Although the Standard Model of particle physics is well established by many experiments some phenomena are observed that cannot be explained by the Standard Model alone. Further- more recent evaluations of experimental data yield results deviating from the Standard Model's predictions with a significance of up to 4s −5s. The B0 ! K∗0 l+ l− decay is one of those where new physics might arise. The Belle detector is being upgraded in order to achieve more precise measurements of many variables and to cope with the higher luminosity of the superKEKB collider. In this thesis the sensitivity of the B0 ! K∗0 m+ m− decay at the new experimental setup, with the upgraded Belle II detector and superKEKB collider, is analyzed using data from Monte Carlo simulations of the Belle II group. The studies show that, even for the integrated luminosity of the Belle dataset, a higher amount of signal events and better statistical significance can be expected of the new setup. Even more so for a final integrated luminosity, which is 50 times higher than that of the previous experiment. Zusammenfassung Obwohl das Standardmodell der Teilchen experimentell gut bestätigt ist, gibt es einige be- obachtete Phänomene, die nicht allein durch das Standardmodell erklärt werden können. Des Weiteren haben kürzlich durchgeführte Auswertungen experimenteller Daten Ergebnisse her- vorgebracht, die mit einer Signifikanz von bis zu 4s − 5s von den Vorhersagen des Standard- modells abweichen. Der Zerfall B0 ! K∗0 l+ l− ist einer derjenigen, bei neue Physik gefunden werden könnte. Der Belle Detektor wird nun aufgerüstet, um genauere Messungen vieler Varia- blen erreichen und der erhöhten Luminosität des SuperKEKB Beschleunigers gerecht werden zu können. In dieser Arbeit wird die Empfindlichkeit des neuen Aufbaus des Experiments, für den B0 ! K∗0 m+ m− Zerfall, mit dem verbesserten Belle II Detektor und dem superKEKB Be- schleuniger, unter Verwendung von Daten aus Monte Carlo Simulationen der Belle II Gruppe, analysiert. Die Arbeit zeigt, dass mit dem neuen Aufbau, selbst bei einer integrierten Luminosi- tät, die der des Belle Datensatzes entspricht, eine höhere Anzahl an Signalereignissen und eine bessere statistische Signifikanz zu erwarten ist. Um so mehr sollte dies für eine letztendliche, integrierte Luminosität der Fall sein, die 50 mal so hoch ist, wie die des vorherigen Experiments. Contents 1. Introduction 9 1.1. Motivation ..................................... 9 1.2. Theory ....................................... 10 2. Analysis Setup 13 2.1. The Belle II Experiment ............................. 13 2.1.1. Vertex Detector .............................. 14 2.1.2. Central Drift Chamber .......................... 15 2.1.3. Particle Identification ........................... 16 2.1.4. Electromagnetic Calorimeter ....................... 17 0 m 2.1.5. KL and Detector ............................ 17 2.2. Belle II Analysis Software Framework ...................... 18 2.2.1. The Analysis Script ............................ 19 3. Analysis 21 3.1. Generation, Simulation and Reconstruction ................... 21 3.2. Classifier and variable tests ............................ 27 3.2.1. Testing variables ............................. 27 3.2.2. Testing different classifiers ........................ 37 3.3. Results ....................................... 39 4. Conclusion and Outlook 43 A. Appendix: additional plots 47 7 1. Introduction 1.1. Motivation Over the last years the Belle and BaBar experiments, as well as LHCb experiment conducted measurements with special focus on CP violation in the quark sector, mostly in the b-flavor sector. The results of those measurements revealed some deviations to the Standard Model's predictions. One of the observables deviating is the ratio of the branching fraction of the two decays B ! K∗ m+ m− and B ! K∗ e+ e− [7], which is expected to be close to 1 in the Stan- dard Model. Measurements of this branching ratio by the LHCb combined with the previously measured branching fraction RK result in a deviation of 3:5s from the Standard Model [7]. In the Standard Model lepton flavor is supposed to be universal, however the deviations observed might hint at a New Physics scenario where lepton flavor universality is violated, since most of the deviations from the Standard Model are observed in decays including m or t leptons while the same decays including electrons seem to behave Standard Model like. Although the deviations may be correlated and can potentially be explained by a single theory, there are many theories that might explain those anomalies and up to now the results do not hint to one specific New Physics scenario, but allow several different theories. Combining all the deviations from the Standard Model that were observed at LHCb, BaBar and Belle a significance of up to 4s - 5s is reached [6]. For measurements with high statistics it is to be expected, that a few data points deviate with a significance of 3s or more, due to statistical fluctuation. Therefore in particle physics a significance of at least 5s is needed to call a result a new discovery. Statistically 99.9998% of all data are to be found within 5s of the “true value”. Therefore even in very large data samples it is very improbable to observe a data point outside the 5s region just due to statistical fluctuation. To get a better idea of what these results mean more precise measurements and better statistics are needed [6, 7, 8]. With the upgrade of the accelerator from KEKB to SuperKEKB a higher luminosity will be achieved at the Belle II experiment resulting in a data set that is estimated to reach 50 times the data of the Belle data set, leading to much better statistics. While the Belle II detector was also upgraded in order to yield measurements of higher precision, the raised luminosity of Su- perKEKB compared to KEKB resulted in a raised background level, making the upgrade of the detector inevitable. In the course of upgrading the experiment the software framework was rebuilt completely. This sensitivity study employs the new software framework as well as mul- tivariate analysis in order to estimate the efficiency and expected number of B0 ! K0∗ m+ m− 9 decays at the new experimental setup and compare the results to those of the Belle experiment. 1.2. Theory According to the Standard Model of particle physics all matter is composed of quarks and leptons, where both types of particles are fermions and are divided into three so called generations, which can be seen in Figure 1.1 together with all the other particles of the Standard Model. In this theory each generation contains two types of particles. In the case of leptons for electrons, muons (m) and taus (t), there exist corresponding neutrinos with a flavor named after their part- ner. While electrons, muons and taus carry a charge1 of 1 (depending on whether the particle or the antiparticle is referred to), the neutrinos are electrically neutral. The quark pairs making up the three quark generations are called up and down, charm and strange as well as top and bot- 2 tom, where the first named quark of each pair carries an electric charge of + 3 , while the second − 1 carries a charge of 3 . For each of those particles there exists a corresponding antiparticle with opposite quantum numbers. Figure 1.1.: Visualization of the particles included in the Standard Model of particle physics. the figure is taken from [14]. Furthermore the Standard Model states that there exist bosons of spin 1, which mediate in- teractions between particles. There is the photon (g) for the electromagnetic force, the gluon (g) for the strong force and the three bosons mediating the weak force (W ; Z0). However up to now there is no tested theory that is able to describe gravitation on the scale of elementary particles. Thus, there is only one more particle in the Standard Model called the Higgs boson 1In this thesis the charge of all particles are presented in units of the electron charge 10 (H0), a particle of spin zero, which is responsible for the particles' inertia. Of all the mediator particles of the Standard Model only the W boson carries an electric charge. The decay of interest in this thesis is a very rare one as it requires a bottom quark to turn into a strange quark. However the only interaction in the Standard Model which allows for a change of flavor between the initial and final state is the weak interaction, more specifically an interaction mediated by the W boson. Since the charge has to be conserved either two W-bosons or a W-boson and a Z-boson or a photon are needed for a process with the desired initial and final states. The corresponding Feynman diagrams are illustrated in Figure 1.2. These processes are suppressed, however, since W and Z bosons have a high mass, which reduces the allowed phase space compared to other processes. Figure 1.2.: Lowest order penguin (top) and box (bottom) Feynman diagrams allowed in the Standard Model for B0 ! K∗0 l+ l− . The probability for a transition from one quark to another is described by the Cabbibo-Kobayashi- Maskawa (CKM) matrix. If there were no transitions across generations this would be an identity matrix. In reality, however, the CKM matrix is a unitary but not an identity matrix. The mag- nitude of the entries on the main diagonal marking transitions within the generation are still the biggest, while those of the entries marking a transition to the next generation are smaller and those describing transitions across two generations are the smallest.

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