Periodic Reporting for period 2 - NEPAL (NEw Physics searches with tAu Leptons)
Período documentado: 2021-04-01 hasta 2022-09-30
If the Standard Model (SM) of particle physics succeeds to describe the behaviour of fundamental constituents of matter and their interactions observed experimentally, it is unable to resolve the most important riddles of our time such as the nature of the dark matter or the origin of the matter-antimatter asymmetry of the Universe. The search for a physics beyond the SM, and its related new physics particles, has therefore became the principal quest of particle physicists. A very powerful way to search for these new particles is to look for heavy flavour decays that are rare or forbidden in the SM, especially, b to s transitions and lepton flavour violating decays. In this domain, final states involving electrons and muons have been and are still extensively studied by several experiments while channels involving tau leptons are much less known because of their challenging reconstruction. This is due to the short tau lifetime and the fact that it decays into neutrinos, which escape detection. The interest of decays involving tau leptons is also dramatically reinforced by the recent anomalies reported in tests of lepton flavour universality violation and rare B decays, suggesting a special role of the third family. In particular, lepton flavour violating tau decays and rare B decays into tau could be just below the current experimental limits.
The first objective of this project is to exploit the tau pair events recorded in the clean environment of an e+e- machine by the new Belle II experiment to obtain the world best results on lepton flavour violating tau decays. The second one is to improve the search for B lepton flavour violating decays with tau lepton in the final state. These first two categories of decays are forbidden in the SM and any discovery would be an indisputable sign of physics beyond the SM. The third objective is to improve the experimental knowledge of the b to s tau+ tau- transitions, which are allowed in the SM but could very much enhanced thanks to the presence of new particles.
Overall this project constitutes a unique strike force to test the presence of new physics coupling to the third generation obtaining the world’s most precise measurements.
The first objective of this project is to exploit the tau pair events recorded in the clean environment of an e+e- machine by the new Belle II experiment to obtain the world best results on lepton flavour violating tau decays. The second one is to improve the search for B lepton flavour violating decays with tau lepton in the final state. These first two categories of decays are forbidden in the SM and any discovery would be an indisputable sign of physics beyond the SM. The third objective is to improve the experimental knowledge of the b to s tau+ tau- transitions, which are allowed in the SM but could very much enhanced thanks to the presence of new particles.
Overall this project constitutes a unique strike force to test the presence of new physics coupling to the third generation obtaining the world’s most precise measurements.
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/) 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.
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/) 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.
Unfortunately, the SuperKEKB accelerator did not perform as foreseen originally in the project and the amount of recorded data (called luminosity), has been limited due to large beam backgrounds. Belle II will have recorded by summer 2022 about half of the Belle statistics and it will be thus very challenging to improve the existing constraints on tau and B LFV searches already published by Belle.
Nevertheless, the inclusive tau reconstruction can help a lot and we are now quantifying this improvement. The first public results are expected in the coming months. The last part of the project, dedicated to b to s τ+ τ- transitions can also improve the current state of the art since very few experimental results have been published up to now.
Nevertheless, the inclusive tau reconstruction can help a lot and we are now quantifying this improvement. The first public results are expected in the coming months. The last part of the project, dedicated to b to s τ+ τ- transitions can also improve the current state of the art since very few experimental results have been published up to now.