Periodic Reporting for period 3 - InterLeptons (A search for new interactions at Belle II using leptons)
Reporting period: 2023-11-01 to 2025-04-30
During the first part of the project, the InterLeptons team was formed after a series of international selections of candidates. The team, consisting of 4 PhD students, one postdoc and the PI, performs and develop methods for the search new physics beyond the standard model of particle physics.
Since we know that the standard model fails to explain a number of observed phenomena (such as gravity, dark matter, dark energy, etc.), we know that new physics must exist. In the initial phase, we have set up the tools and prepared a few measurements that are related to potential physics beyond the standard model, in particular the search for a new Z' boson decaying invisibly and produced in association with muons at the Belle II experiment. The group also search for the breaking of the standard model via searching for tension between theoretical predictions and precision experimental measurements of quantities that are related to the lepton universality of the couplings of weak interactions. The overall objective of the InterLeptons project is to discover new physics related to dark matter or lepton universality, or severely constrain the parameter space and eventually rule out beyond standard model theories.
Over the years of the project, members of the team have participated, at various levels, in several searches for new physics at the Belle II experiment and in their phenomenological interpretation (rare decays, dark secotr searches, precision measurement in the tau sector etc.). Moreover, members of the team have developed their own analysis methodologies and published the algorithm for the general use of the scientific community.
Since we know that the standard model fails to explain a number of observed phenomena (such as gravity, dark matter, dark energy, etc.), we know that new physics must exist. In the initial phase, we have set up the tools and prepared a few measurements that are related to potential physics beyond the standard model, in particular the search for a new Z' boson decaying invisibly and produced in association with muons at the Belle II experiment. The group also search for the breaking of the standard model via searching for tension between theoretical predictions and precision experimental measurements of quantities that are related to the lepton universality of the couplings of weak interactions. The overall objective of the InterLeptons project is to discover new physics related to dark matter or lepton universality, or severely constrain the parameter space and eventually rule out beyond standard model theories.
Over the years of the project, members of the team have participated, at various levels, in several searches for new physics at the Belle II experiment and in their phenomenological interpretation (rare decays, dark secotr searches, precision measurement in the tau sector etc.). Moreover, members of the team have developed their own analysis methodologies and published the algorithm for the general use of the scientific community.
In the initial phase, we developed the tools needed to perform the searches. For example, for the search of an invisibly decaying Z' boson, we developed a neural network based on a minimization function typical of problems in particle physics. Thanks to this neural network, we can improve the sensitivity of our searches and simultaneously search for hundreds of mass hypotheses. In the first attempt, we haven't found a Z' boson and we have set upper limits to its (hypothetical) coupling. In parallel to this activity, we have prepared the measurement of lepton flavour universality using leptonic tau decays. We have defined two strategies and we are now implementing these on the data collected by the Belle II experiments, with results that will be available during the second period. Our searches and preliminary results have been presented to many leading conferences of field and published on leading peer-reviewed journals. We count ten publications and tens of contributions to conferences and outreach events. More recently, we performed the world's most precise test of lepton universality using tau decays. This measurement has allowed to exclude, or at least to lower the significance, of previous tensions that were observed in similar processes by previous experiments. An update of this measurement is ongoing. In this final part of the project, we are also finalising an analysis on rare B meson decays that involve a large amount of missing energy, the rare decay B->Knunu that might be enhanced by new forces or by low mass dark sector particles that would interfere with the decay. This analysis is in progress and will be finalised in the coming months.
Thanks to our analysis methodologies and developed tools we could set up world best upper limits to a variety of new physics models in relation to dark matter and dark sector physics. The main results will be available during the project's second phase.