The work performed since the beginning of the project has been mainly focused on the first objective, which is the search for lepton flavour violating (LFV) tau decays. Four decays have been studied so far: τ → ℓφ, τ → ℓρ, τ → ℓKs and τ → ℓℓℓ, where ℓ can be an electron or a muon. At Belle II, and Belle before, τ leptons are produced in pairs. Historically, all searches for tau LFV decays were performed reconstructing both tau leptons: the signal one, and the opposite one (named tag tau) that decays into a SM channel with a known probability. In this project, we are using a more inclusive selection, getting rid of the tag tau reconstruction. This allows to gain up to a factor of 2 in signal efficiency. The price to pay is a large background increase but it can be mitigated thanks to machine learning techniques and additional discriminating variables corresponding to global event characteristics. The gain in sensitivity is being investigated and the first public results are expected in the coming months. The searches for LFV decays of B meson into tau leptons has started six months ago, with the aim of improving the tagging efficiency.
In addition to the data analyses, a large part of the work in this first period has been dedicated to technical activities that are necessary to ensure the good performances of the experiment. A six-month service task is also required by the Belle II collaboration to sign the publications. The team efforts have concentrated into two topics:
- Tracking performances: the tracking consists in the reconstruction of charged particles trajectories from the hits deposited in the different detector layers. We performed a measurement of the fake track rate at Belle II for which we invented a new ‘tag-and-probe’ method. We also worked on the reduction of the fake track rate based on the timing information of the hits used in the track reconstruction. This is in good progress and the new timing variables will be used in the future software release, allowing a reduction of the fake track rate by a factor of two.
- Silicon vertex detector (SVD) performances: we implemented a new method to compute the spatial resolution of the clusters from the vertex detector using the region where two ladders of the SVD overlap. A post doctorate researcher of the team also had the responsibility role of ‘SVD run coordinator’ during the run taken at fall 2021.
We are also participating to the operation of the experiment and data taking with various type of shifts : remote control room, SVD remote operation, data production.
Finally, we have organized MasterClass events (
http://physicsmasterclasses.org/(s’ouvre dans une nouvelle fenêtre)) with high school classes. The event, which is one day long, consists of an introduction to particle physics and the Belle II experiment, and a hands-on session where the students can analyze Belle II data. In 2022, they could also come to the laboratory and make a virtual reality tour of the experiment.
