000003217 001__ 3217
000003217 005__ 20220915100259.0
000003217 037__ $$aBELLE2-PTHESIS-2022-007
000003217 041__ $$aeng
000003217 100__ $$aLuigi Corona
000003217 245__ $$aSearch for the visible ττ decay of a Z′ dark boson in µµττ final state with Belle II
000003217 260__ $$aPisa$$bUniversity of Pisa$$c2022
000003217 300__ $$a200
000003217 500__ $$aPresented on 18 07 2022
000003217 502__ $$aPhD$$bPisa, University of Pisa$$c2022
000003217 520__ $$aIn 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.
000003217 700__ $$aFrancesco Forti$$edir.
000003217 700__ $$aKatsuro Nakamura$$edir.
000003217 8560_ $$fluigi.corona@desy.de
000003217 8564_ $$uhttps://docs.belle2.org/record/3217/files/BELLE2-PTHESIS-2022-007.pdf
000003217 980__ $$aTHESIS