Periodic Reporting for period 1 - TAULEPGAMMA (Search for the Lepton Flavour Violating Decay Tau to Lepton + Gamma at the Belle II Experiment)
Période du rapport: 2021-06-01 au 2023-05-31
The Standard Model (SM) of elementary particle interactions is one of the best-tested theories in physics. The discovery of the Higgs boson was one of the big breakthroughs in physics of the past decade and the last missing piece of the SM. However, despite its success in describing fundamental forces, there is abundant evidence of phenomena not covered by this model, such as the existence of Dark Matter.
In the SM, lepton flavour (LF) is empirically conserved: electrons, muons and tau are produced or decay in association with their respective neutrinos, such that the total number in each family remains the same. The tau can thus decay into a tau neutrino, a muon, and a muon antineutrino; with the antiparticle counted as (-1). Meanwhile the decay τ → μγ , where a muon is produced alongside a photon, is forbidden.
The observation of neutrino oscillations, through which the three neutrinos can transform into each other, was the first direct observation of physics outside the SM, proving lepton flavor violation (LFV) is possible. However so far no such phenomenon has been observed with charged particles. Direct observation of charged LFV decays would constitute an enormous breakthrough in the field and an unambiguous, exciting proof of the existence of new physics.
This project aims to exploit the unique data set collected by the Belle II experiment at the SuperKEKB electron-positron collider in Tsukuba, Japan, to perform a high-precision measurement of LFV decays of tau. Belle II, which has started full operation in 2019, has so far observed over 900 million tau decays that can be exploited to search for LFV decays of tau leptons with world-leading precision.
This action produced a first important measurement of LFV using this data, set the basis for the measurement of a second LFV decay, and developed techniques broadly applicable to all tau research taking place at Belle II.
In the SM, lepton flavour (LF) is empirically conserved: electrons, muons and tau are produced or decay in association with their respective neutrinos, such that the total number in each family remains the same. The tau can thus decay into a tau neutrino, a muon, and a muon antineutrino; with the antiparticle counted as (-1). Meanwhile the decay τ → μγ , where a muon is produced alongside a photon, is forbidden.
The observation of neutrino oscillations, through which the three neutrinos can transform into each other, was the first direct observation of physics outside the SM, proving lepton flavor violation (LFV) is possible. However so far no such phenomenon has been observed with charged particles. Direct observation of charged LFV decays would constitute an enormous breakthrough in the field and an unambiguous, exciting proof of the existence of new physics.
This project aims to exploit the unique data set collected by the Belle II experiment at the SuperKEKB electron-positron collider in Tsukuba, Japan, to perform a high-precision measurement of LFV decays of tau. Belle II, which has started full operation in 2019, has so far observed over 900 million tau decays that can be exploited to search for LFV decays of tau leptons with world-leading precision.
This action produced a first important measurement of LFV using this data, set the basis for the measurement of a second LFV decay, and developed techniques broadly applicable to all tau research taking place at Belle II.
This project searched for charged LFV in a model-independent fashion by studying two very important processes: τ → lγ, where the tau decays into a lighter lepton (muon or electron) and a photon; and τ → la, where the photon is replaced by an unknown, invisible particle.
τ → lγ is known as a “golden mode”, one of the simplest decays that can be studied in order to gain information on the nature of LFV. TAULEPGAMMA developed machine learning techniques to isolate this rare process and maximise the experimental reach of a future, large-scale analysis.
Meanwhile, the unique properties of the Belle II detector such as large angular coverage, good hermeticity, and high momentum and energy resolution make it extremely effective in the study of processes with invisible particles. Together with the sizeable amount of data collected, this allowed to search for the τ → la process with unprecedented precision; this decay had not been investigated in almost 30 years. The results were published in Physical Review Letters and set new bounds, up to 14 times more stringent, on the existence of LFV invisible particles. This was the first tau physics output of Belle II.
τ → lγ is known as a “golden mode”, one of the simplest decays that can be studied in order to gain information on the nature of LFV. TAULEPGAMMA developed machine learning techniques to isolate this rare process and maximise the experimental reach of a future, large-scale analysis.
Meanwhile, the unique properties of the Belle II detector such as large angular coverage, good hermeticity, and high momentum and energy resolution make it extremely effective in the study of processes with invisible particles. Together with the sizeable amount of data collected, this allowed to search for the τ → la process with unprecedented precision; this decay had not been investigated in almost 30 years. The results were published in Physical Review Letters and set new bounds, up to 14 times more stringent, on the existence of LFV invisible particles. This was the first tau physics output of Belle II.
The bounds placed on the existence of τ → la advance our knowledge of charged LFV, informing the development of future theories which aim to incorporate it within the SM. Additionally, the invisible “a” might be Dark Matter - an elusive form of matter which has so far only been observed indirectly. As such, these limits also constrain its nature - giving clues on what Dark Matter might exactly be.
This measurement sets the stage for future studies in Belle II by defining procedures and methods used in the search of tau LFV. This will begin from the search for τ → lγ, where the machine learning techniques developed during this project will play a key role, and will continue to be relevant throughout Belle II’s lifetime, enabling world-leading measurements that exploit this experiment’s unique data set.
This measurement sets the stage for future studies in Belle II by defining procedures and methods used in the search of tau LFV. This will begin from the search for τ → lγ, where the machine learning techniques developed during this project will play a key role, and will continue to be relevant throughout Belle II’s lifetime, enabling world-leading measurements that exploit this experiment’s unique data set.