Belle Physics

Our group participates in the Belle experiment at KEK in Japan. The Belle experiment is dedicated to high-precision studies of b-flavoured meson decays. Such decays allow us to perform precision measurements of Standard Model parameters and the search for beyond-Standard Model physics via loop effects. Our group analyzes semi-leptonic and photonic decays (b --> s gamma).

Lepton Flavor Universality

In the Standard Model of Particle Physics there are three flavor generations of leptons and quarks. Particles from different generations are identical except for their masses. The strength of the weak interaction is assumed to be the same for all flavors in the leptonic sector of the Standard Model. This postulate is termed as Lepton Flavor Universality. Recent experimental observations indicate that certain branching fractions of electroweak B meson decays deviate from the Standard Model expectations. Confirmation of such deviations will pave the way for the discovery of new particles such as leptoquarks or multiple Higgs bosons. We are involved in the measurement of the following quantities at Belle II.

R(D*) Ratio

The experimentally measured value of R(D*) is one of the most prominent anomalies in particle physics in the last decade. The averaged value of all experimental measurements of the combined ratio for the D* and D mesons show a deviation of more than 3σ from the Standard model prediction. Our group is strongly involved in measuring the R(D*) value and increasing its precision in different kinematic scenarios and final states.

R(X) Ratio

Similar to the R(D*) ratio, R(X) is another excellent LFU test. The non-explicit reconstruction of the final state allows for a measurement with higher statistics while most of the theoretical uncertainties associated with the complicated hadronic X system cancel out in the measured ratio. Our group is leading the efforts of measuring R(X) for the first time in history at a B-factory, with all the previous measurements coming from the experiments at LEP in the early 2000s.

Rare decays

Rare decays like the leptonic B meson decays such as B → μ ν are highly CKM- and helicity-suppressed. In a two-body decay like B → μ ν, the muon momentum is exactly known in the rest frame of the signal-side B meson. By boosting the signal-side muon into that frame, a better signal resolution and improved sensitivity can thus be achieved compared to the center-of-mass frame. This decay can also be used to studied to provide an outlook on the search for sterile neutrinos, a particle beyond the standard model.

Angular observables in semileptonic B decays

The B → D* l ν decay is mediated in the SM via W -boson exchange. Due to the spin of the final state D∗ meson, much of the properties of the V − A coupling and the spin of the virtual W boson is encoded in angular distributions.  Recent measurements of such observables like the difference of the differential forward-backward asymmetry ΔA(fb) between electrons and muons,  show deviations with respect to the SM expectation that exceed 4σ. Our group is leading the endeavors within Belle II of confirming or ruling out such discrepancies. Furthermore we are pursuing measurements of experimentally unexplored angular observables, which can test the SM with respect to various beyond SM scenarios.

Machine Learning

A large variety of tasks, in different reconstruction stages, are carried out  by Machine Learning algorithms in the majority of modern High Energy Physics experiments. In Belle II we also profit from the advancements in certain subfields of Machine Learning. Our group is involved in the development, commission and calibration of algorithms to maximize the performance of the Belle II detector.

B-meson tagging

B mesons are produced in pairs in lepton colliders that operate as B-factories. Due to the clean experimental environment of a lepton collider, if one of the two B-mesons is fully reconstructed then the second B-meson decay of interest, can be kinematically constrained. This technique is termed as B-meson tagging. The large number of possible B meson decays makes the tagging of the first B-meson non-trivial and the use of multivariate techniques essential. In Belle II we use a Boosted Decision Tree based algorithm called the Full Event Interpretation for B-tagging. Our group plays a key role in the calibration and improvement of the Full Event Interpretation as well as in the development of alternative algorithms which make use of modern Deep Learning techniques.

Particle Identification

The Belle II Physics program relies heavily on efficient particle identification. Combining the information from all the different subsystems of the Belle II detector, in order to infer the particle species of a certain signal inside the detector has a great impact on any physics result. Our group is involved in the development of techniques aiming to improve the particle identification performance using different Machine Learning techniques such as Neural Networks or Boosted Decision Trees.

Precision Measurements of CKM Matrix Elements

In the Standard Model, the Cabibbo-Kobayashi-Maskawa (CKM) matrix is the only source of charge-paritiy violating interactions. Such are needed to explain the matter dominance of today's universe and are only possible if matter and antimatter exhibit different properties. Our group is world leading in the determination of the CKM matrix elements Vub and Vcb, whose size describe the coupling strength of the weak interaction between beauty and charm or beauty and up quarks. The ratio of |Vub|/|Vcb| directly constrains the allowed amount of of matter-antimatter asymmetry. Intriguingly different methods lead to different amounts! We are on the forefront to explore and substantiate these differences, to develop a better understanding of the matter-antimatter symmetry breaking meachnisms of the quark sector. 


Besides the experimental study of beauty quarks, we are also involved in exploring the phenomenlogy of beauty hadron decays. This we do frequently in collaboration with other world leading scientist. We produced one of the most precise predictions on the ratios R(D/D*), and provided predictions on the dynamics of many semileptonic decay processes, also such that are studied at the LHC with LHCb. 

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