Luigi Corona ; Francesco Forti ; Katsuro Nakamura

2022
University of Pisa Pisa

Abstract: In this thesis we investigate the possibility of detecting these new particles in the mass range 3.6-10 GeV/c2 through their decays in a τ+τ− pair in e+e− → µ+µ−τ+τ− events. In particular we search for the visible decay of a Z’ dark boson into a τ+τ− pair with the data collected by the Belle II detector in the first half of 2020, corresponding to 54.7 fb-1. Belle II is a luminosity-frontier experiment installed at the SuperKEKB collider, which is a second generation B-factory that collides 7 GeV e- and 4 GeV e+ mostly at the energy in the center of mass frame of 10.58 GeV. The Z’ is a dark vector boson with a coupling only to the second and third generation of leptons introduced by the Lμ-Lτ model that can both explain the observed deviations of some measurements from Standard Model prediction and solve the dark matter puzzle. The Z’ can decay both invisibly, in neutrinos or dark matter, if a dark matter particle kinematically accessible exists, or visibly, in a μ+μ- or a τ+τ− pair. Constraint on the Lμ-Lτ model from the search for a Z’ → μ+μ- (performed at BaBar, Belle and CMS) and Z’ → invisible (performed at Belle II) exist. The work presented in the thesis is the first search for a Z’ → τ+τ−. Although the analysis of this particular channel results to be more complicated compared, for example, to the search for the Z' → μ+μ- decay and does not improve the existing constraints on the Lμ-Lτ model, it results to be very interesting because mainly for the following reasons: it provides a unique measurement that can be interpreted in other dark sector models that specifically enhance the tau-coupling with respect to the lighter leptons: like leptophilic dark scalar and axion-like particles with a coupling to leptons and a negligible coupling to photons; it demostrates the feasibility of dark sector analysis involving taus. In the thesis we show the sensitivity to the cross section of the process e+e- → μ+μ-Z', Z' -> τ+τ− at 90% CL estimated from simulation assuming 80 fb-1 of data and the measurement obtained from the unboxing of the 10% of the dataset, in which we did not observe any excess compatible with signal events.

Note: Presented on 18 07 2022
Note: PhD