000002464 001__ 2464
000002464 005__ 20210625074851.0
000002464 037__ $$aBELLE2-MTHESIS-2021-019
000002464 041__ $$aeng
000002464 100__ $$aStefano Moneta
000002464 245__ $$aA novel method for tau-lepton lifetime measurement with early Belle II data
000002464 260__ $$aPisa$$bUniversità di Pisa$$c2020
000002464 300__ $$amult. p
000002464 500__ $$aPresented on 29 10 2020
000002464 502__ $$aMSc$$bPisa, Università di Pisa$$c2020
000002464 520__ $$aThe Belle II experiment, operating at the SuperKEKB electron-positron collider, is going to collect 50/ab of integrated luminosity by 2030, a factor 50 more than its predecessors. The large cross section for tau pair production at the Y(4S) resonance energy, will yield to about 46 billion produced tau pairs, making Belle II, besides a B-factory, a tau-factory as well.  The tau-lepton is a powerful laboratory for both Standard Model (SM) and beyond SM measurements. A precise tau lifetime measurement, in particular, can contribute to the test of lepton flavor universality within the SM. Also, lifetime measurements are very effective tools to monitor detector performance and alignment. Furthermore, they are challenging in case of a particle such as tau-lepton, whose lifetime uncertainty is below 0.2% at present. To achieve a competitive precision a very good understanding of detector performance is needed. At Belle II, tau pairs have an expected mean decay length of about 250 microns.  In this thesis, a new method for tau lifetime measurement is studied. The method aims to fully reconstruct e^+e^- -> tau^+tau^- events, where both taus decay with one neutrino only, i.e. hadronically. For this class of events, which potentially represents 42% of the total tau pair decays, tau four-momenta can be reconstructed up to a two-fold ambiguity, just measuring the momenta of all hadronic final states and requiring four-momentum conservation.  Thanks to the nano-beam collision scheme adopted by SuperKEKB machine, the beam spot size at the interaction point is much smaller than in any of the previous collider experiments. The primary vertex position on the transverse plane can hence be treated as a fixed constraint.  Using the information on the impact parameter of reconstructed tracks, the vertex relations can be included in a unique system of equations together with the kinematic relationships. The system can be numerically solved through a chi-square-like minimization where the tau decay lengths and momenta, as well as the primary vertex position along the beam direction, are considered as free parameters.  The method proposed is tested for the 1-prong vs 3-prong decay topology, where the vertex position of the 3-prong tau can be independently fitted. A selection of these events in Belle II had been developed on Monte Carlo and applied on 34.6/fb data, collected during 2019 and early 2020 runs. The full event reconstruction method gives a proper decay time resolution 65 fs, obtained for the 3-prong side. In addition, the interaction point along the beam direction (z direction) is also reconstructed with a resolution of 20 microns, thus making possible a precise event per event monitoring of the beam spot position. 
000002464 700__ $$aFrancesco Forti$$edir.
000002464 700__ $$aEugenio Paoloni$$edir.
000002464 8560_ $$fstefano.moneta@pg.infn.it
000002464 8564_ $$uhttps://docs.belle2.org/record/2464/files/BELLE2-MTHESIS-2021-019.pdf
000002464 980__ $$aTHESIS