Thesis BELLE2-PTHESIS-2023-008

Search for a visibly decaying Z' dark boson at Belle II in e^+e^- --> mu^+mu^-mu^+mu^- events.

Martina Laurenza ; Enrico Graziani

Roma Tre University Roma

Abstract: The Standard Model (SM) of fundamental particles and interactions is a highly predictive theory. Despite this, it cannot be considered as a complete description of the nature, because of the many observed phenomena which are not taken into account. Among these phenomena not predicted by the SM, there are observations which could be explained through the presence of Dark Matter (DM). Many theoretical models postulate extensions of the SM with the aim of including the phenomenology associated to DM and predict the observed relic abundance. One of the simplest ways to extend the SM is by adding an extra $U(1)^{\prime}$ gauge group to the theory. An additional gauge boson, the \zprime\ , would rise and couple to both SM and undiscovered particles such as dark matter constituents~\cite{He:1990pn,He:1991qd,Pospelov:2008zw,Shuve:2014doa}. The work described here considers the visible decay of a \zprime\ boson in the framework of the so-called \lmultau\ model~\cite{Shuve:2014doa, Altmannshofer:2016jzy}, which gauges the difference between the muonic and tauonic lepton numbers. Under a \lmultau\ symmetry, the \zprime\ boson would couple only to $\mu$, $\tau$ and the respective $\nu_\mu$ and $\nu_\tau$ neutrinos among the SM particles, with a coupling constant $g^{\prime}$.\\ Two theoretical models based on the \lmultau\ symmetry address the DM topic. They are identical from the mediator point of view (the \zprime\ ) and differ as far as DM candidates are concerned: these are sterile neutrinos in one case~\cite{Shuve:2014doa} and light Dirac fermions in the other~\cite{Pospelov:2008zw}. \\ Two additional benefits of the \lmultau\ model are its capability of explaining the observed discrepancy of the muon anomalous magnetic moment $(g-2)_\mu$ with respect to the SM prediction and of solving the flavour anomalies measured by the LHCb experiment. \BelleII operates at the SuperKEKB electron-positron collider~\cite{superkekb} at the KEK laboratory in Tsukuba, Japan. \BelleII\ is recording data since April 2018, mainly at the center-of-mass energy of the $\Upsilon$(4S) resonance peak. Up to now the total integrated luminosity collected by the \BelleII is 267.9 fb$^{-1}$. The analysis aims at the investigation of the visible decay of the \zprime\ into a muon pair in the reaction $e^+e^-\to\mu^+\mu^-$\zprime\ (\zprime\ $\to\mu^+\mu^-)$, in order to discover it or to set an upper limit on the coupling constant $g^{\prime}$. The same search was performed by \babar\ ~\cite{lees2016search} and the Belle experiments, with $\sim$500\invfb and $\sim$640\invfb, respectively. The work introduced here is based on a target luminosity of 54\invfb\ and was performed without looking at the data in the signal region. The entire 267.9\invfb\ \BelleII\ data-set was not used because of the different reconstruction and trigger configurations, that would have required more time to be taken into account in the analysis. The final goal is to demonstrate that it is possible to set competitive or better limits with respect to the experiments results employing a different background reduction approach. It will be shown later that such a luminosity is not sufficient to get a sensitivity better than those of \babar\ and Belle. For this reason, the analysis has not obtained the \BelleII collaboration approval yet. Nevertheless, explicitly for this thesis, the permission to look at the 10\% of the target luminosity has been granted, with the prescriptions that the performances of the analysis and the partial results obtained on real data in the signal region cannot be shown elsewhere. The projections of the g$^{\prime}$ coupling constant sensitivity using 54\invfb and 200\invfb are shown. With the latter we can set more restrictive limits on the coupling constant with respect to both \babar\ and Belle, for \zprime\ masses larger than 1 GeV/$c^{2}$, with much less luminosity.

Note: Presented on 01 07 2022
Note: PhD

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 Record created 2023-04-25, last modified 2023-04-25

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