Campajola, Marcello (2021) Searches for an invisible Z' and for the dark Higgsstrahlung A'h' process in μ⁺μ⁻ plus missing energy final states at Belle II. [Tesi di dottorato]

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Abstract

The work presented in this thesis concerns two searches for dark sector mediators in e^+e^- annihilations at the center-of-mass energy of 10.58 GeV with the Belle II experiment. Two different processes have been investigated in the same final state, consisting of two muons plus missing energy, by using the first data collected by the experiment so far. The first search presented in this work investigates an invisibly decaying Z' boson (in the framework of a L_µ - L_τ symmetry) produced radiatively by muons in the process e^+e^- → μ^+μ^-Z'; Z'→ invisible. The only previous measurements in the same theoretical framework have been performed by the BaBar and CMS experiments for a Z' decaying to muons, while no results for an invisible decay have been reported before. For this measure, the data-set collected by Belle II during the so-called Phase 2 commissioning run in 2018 has been used, corresponding to an integrated luminosity of 276 pb^-1. No anomalies have been observed in data, and upper limits on the coupling constant g' in the range [5 × 10^2 − 1] have been placed for a Z' mass less than 6 GeV/c^2. As an extension of the above search, the existence of a LFV Z' boson has been investigated in the process e^+e^- → e^±μ^∓Z'; Z'→ invisible. Even in that case, no anomalies have been observed, and model independent upper limits on the cross section (times efficiency) have been computed. The search for a different process is also presented, consisting in the simultaneous production of a dark photon A' and a dark Higgs h' boson via the so-called dark Higgstrahlung process e^+e^- → A'h'; A'→ μ^+μ^-, h'→ invisible. The only similar measurement was performed by the KLOE experiment, for A' masses up to ~ 1 GeV/c^2. Therefore, Belle II would be able to produce a sizeable enlargement of the explored region. The analysis flow, optimized for the data collected by the experiment during 2019, corresponding to an integrated luminosity of ~ 9 fb^-1, is described and the expected upper limits on the cross section and in terms of the coupling constants product є × α_D are provided.

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