000002607 001__ 2607
000002607 005__ 20220402123921.0
000002607 037__ $$aBELLE2-PTHESIS-2021-016
000002607 041__ $$aeng
000002607 100__ $$aMarcello Campajola
000002607 245__ $$aSearches for an invisible Z' and for the dark Higgsstrahlung A'h' process in μ⁺μ⁻ plus missing energy final states at Belle II
000002607 260__ $$aNapoli (IT)$$bUniversità di Napoli "Federico II"$$c2021
000002607 300__ $$amult. p
000002607 500__ $$aPresented on 24 06 2021
000002607 502__ $$aPhD$$bNapoli (IT), Università di Napoli "Federico II"$$c2021
000002607 520__ $$aThe 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^\prime$ boson (in the framework of a $L_{\mu}-L_{\tau}$ symmetry) produced radiatively by muons in the process $e^+e^-\to \mu^+\mu^- Z^\prime,\, Z^\prime\to \rm{invisible} $.  The only previous measurements in the same theoretical framework have been performed by the BaBar and CMS experiments for a $Z^\prime$ 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^\prime$ in the range $[5 \times 10^{-2}-1]$ have been placed for a $Z^\prime$ mass less than 6~GeV/c$^2$. As an extension of the above search, the existence of a LFV $Z^\prime$ boson  has been investigated  in the process $e^+e^-\to e^\pm \mu^\mp Z^\prime,\,Z^\prime\to \rm{invisible}$. 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^\prime$ and a dark Higgs $h^\prime$ boson via the so-called dark Higgstrahlung process $e^+e^- \to A^\prime h^\prime$ with $A^\prime\to \mu^+\mu^-$ and $h^\prime\to \rm{invisible}$. The analysis flow, optimized for the data collected by the experiment during 2019, corresponding to an integrated luminosity of $\sim9\,\rm{fb}^{-1}$, is described and the expected upper limits on the cross section and in  terms of the coupling constants product $\epsilon^2\times\alpha_D$ are provided. The only similar measurement was performed by the KLOE experiment, for $A^\prime$ masses up to $\simeq$ 1 GeV/c$^2$. Belle~II will produce a sizeable enlargement of the currently explored region. 
000002607 700__ $$aGuglielmo De Nardo$$edir.
000002607 700__ $$aFrancesco Di Capua$$edir.
000002607 8560_ $$fmacampajola@na.infn.it
000002607 8564_ $$uhttps://docs.belle2.org/record/2607/files/PhD_THESIS_CAMPAJOLA.pdf
000002607 980__ $$aTHESIS