The standard model of particle physics (SM) is a theoretical framework that describes fundamental interactions and constituents of matter. Although very successful in predicting phenomena, the SM cannot be considered as a complete description of nature at the fundamental level due to the fact that there are phenomena observed in nature that are not accounted for by the underlying theory, such as dark matter, dark energy, or neutrino masses, to name a few. Our team at HEPHY is searching for physics beyond the SM by strongly contributing to different experimental activities either directly looking for the production of new particles and forces or by looking for deviations from SM predictions in high precision measurements. These research activities are complemented by technical activities such as the development statistical (multivariate) methods of analysis, the implementation and study of particle identification algorithms, the monitoring and study of the Belle II trigger system.

Project leader: Gianluca Inguglia

Search for dark matter particles and mediators

Since the first experimental observation of anomalous dispersion velocities in the Coma Cluster that indicated the existence of non-luminous matter, dark matter has intrigued and puzzled generations of scientists. There is today a very wide consensus on the existence of dark matter but what the real nature of dark matter is remain a mystery. The Belle II team at HEPHY tries to solve this mystery.
In particular the team is focusing on the search of low mass dark matter and mediators using data collected by the Belle II detector containing leptons and missing energy in the final state.
The main efforts are now towards (but not limited to):

- the search for muonic and invisible decays of the Z’ boson, a hypothetical dark force that might mediate the interaction between ordinary and dark matter

- the search for dark Higgsstrahlung, a process in which a low mass dark Higgs boson h’ is radiated off from a massive dark photon A’

- the search for invisible decays of bottomonium states such as the Y(1S)

- the search of invisible decays of lepton flavor violating mediators

New activities are promptly discussed and planned based on experimental results and on on theoretical developments.

Tau physics and the search for lepton flavor universality violation

in recent years experiments worldwide have reported tensions arising from anomalies observed in the decays of B mesons and in properties of leptons. Most of these tensions strongly point towards new interactions which treat, unlike the SM, leptons of different generations or flavors differently. Such kind of interactions are said to violate lepton flavor universality (LFUV), an accidental symmetry within the SM, and might also be the link between ordinary and dark matter (DM). The team is focussing on the following

- implementation of new algorithms, based on neural networks, for the identification of tau leptons

- measurement of the branching fractions of leptonic tau decay 

- precision tests of LFU in leptonic tau decays

The searches, motivated by the reported flavor anomalies, are also an indipendent search of physics beyond the SM. The team largely benefit by the collaboration with international partners in the field of the theoretical physics and phenomenology that allows a solid interpretation of the results.

Technical activities

Searches for new physics at Belle II in final state events with low particle multiplicity and missing energy depend first on the capability of the detector to collect this events and make them available to the analysts. At the same time, while preserving precious data that might contain undiscovered processes, background noise must be rejected. This is done via what we call a level 1 trigger (L1). Our team is therefore involved in the monitoring and study of the efficiency of the low multiplicity triggers to guarantee that high quality data are stored and made available for analyses, in the specific we are responsible for the central drift chamber (CDC) based triggers. Reconstructed particles (electrons, muons, pions, ...) need to be identified via so called particle identification algorithms (PID) based on global likelihoods. Since PID is a probabilistic assignment of fundamental importance in the searches of new physics, the possibility that a particle is wrongly identified (misidentified) needs to be addressed and understood, because this will contaminate and pollute a measurement. Our team is involved in the study pion identification and study how pions can contaminate muon samples, of interest for our searches of new physics.