Michel Bertemes ; Gianluca Inguglia ; Christoph Schwanda

2021
Institut für Hochenergiephysik (HEPHY) Vienna

Abstract: This thesis presents two different searches for dark sector mediators in final state events consisting of a pair of muons and large missing energy. Both searches are performed using data collected at the Belle II experiment which is located at the SuperKEKB $e^+e^-$-accelerator in Tsukuba, Japan. A first analysis is looking for the simultaneous production of a dark photon $A'$ and a dark Higgs $h'$ in the so-called Dark Higgsstrahlung process $e^+e^- \to A'h'$; $A'\to \mu^+\mu^-$ and $h'\to \mathrm{invisible} $. 90\% CL upper limits on the product of mixing parameter times dark coupling constant $\epsilon^2\times\alpha_D$ are set in a fully unexplored region, greatly extending the mass range investigated by the KLOE experiment. Another project focuses on the first search for the invisible decay of a $Z'$ in the process $e^+e^-\to \mu^+\mu^- Z'$. This neutral boson belongs to an Abelian symmetry indicated as $L_{\mu}-L_{\tau}$ and may explain different flavor anomalies. Upper limits are set at the 90\% CL on the $g'$ coupling constant. Finally, the performance of the Belle II trigger system in identifying the studied low multiplicity events is evaluated.

Note: Presented on 20 10 2021
Note: PhD