Thesis BELLE2-MTHESIS-2023-020

Search for Y (10753) → ππhb(1P) at Belle II

Federico Testa ; Stefano Spataro ; Umberto Tamponi

Torino University Torino

Abstract: During the scan of the energies above the Υ(4S), the Belle experiment found a resonance structure at around 10753 MeV in the cross section of e+e− → ππΥ(nS), with n = 1, 2, 3. This structure was identified as an exotic state and it was named Y (10753); since it was produced directly in the e+e− collisions, the quantum numbers JPC = 1−− were assigned. In November 2021 Belle II took new data with increased luminosity at four energies around the Y (10753), between the energy points already inspected by Belle. In this work it is presented a blind analysis aimed to measure the Y (10753) → ππ hb(1P) cross section via the inclusive reconstruction of the hb(1P). This measurement will help to investigate the nature of the Y (10753), and to study if transitions to intermediate exotic states do occur, which should lead to an increase in the production cross section. The analysis will be performed by studying the Mmiss(ππ) spectrum in search of a peak in the hb(1P) mass region and measuring the yield of the signal events. The yields will be measured for four energy points (10657, 10706, 10745, 10805 MeV) to study the behaviour of the cross section around the Y (10753). The tracks selection for the pion candidates have been performed by Monte Carlo simulations of e+e− → ππhb(1P) and background events. The efficiency for the obtained cuts has been validated on two control channels, D∗+ → π+D0 → π+K−π+ and D∗+ → π+D0 → π+K−π+π0, with the data collected by Belle II at the four points of the scan. A strategy to correct the eventual data-simulation discrepancy for the track selection is developed, according to the values of pt of the tracks . A multi variate analysis is performed in order to reject continuum events from e+e− → qq process, using a stochastic gradient boosted decision tree (SGBDT) already trained for the Υ(4S) → ηhb(1P) analysis. In chapter 1 the phenomenological and theoretical aspects of the heavy quarkonium systems are discussed, alongside a presentation of some experimental results in regard of the exotic Zb(10610) and Zb(10650) charged state in the bottomonium and the Y (10753) state. In chapter 2 an overview of the Belle II detector and the SuperKEKB collider is given, together with a presentation of the software for the event reconstruction and MC simulation. Chapter 3 gives an outline of the structure of the analysis, in chapter 4 the study on the MC samples is presented and the cuts for the tracks selection are reported. Chapter 5 presents the validation of the efficiency for the tracks selection on the two control channels. The strategy to correct for tracks selection and reconstruction is presented in chapter 6. Suppression of continuum events e+e− → qq using a multivariate analysis is presented in chapter 7. An introductory study of the signal and background shape for the fit is presented in chapter 8. In chapter 9 a summary of the results of this analysis will be drawn.

Note: Presented on 04 04 2023
Note: MSc

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 Record created 2023-05-26, last modified 2023-05-26

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