Search for an Invisibly Decaying Z^ ' Boson at Belle II in e^ + e^ - →μ^ + μ^ - (e^ ± μ^ ∓ ) Plus Missing Energy Final States.
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| Abstract | :  Theories beyond the standard model often predict the existence of an additional neutral boson, the Z^{'}. Using data collected by the Belle II experiment during 2018 at the SuperKEKB collider, we perform the first searches for the invisible decay of a Z^{'} in the process e^{+}e^{-}→μ^{+}μ^{-}Z^{'} and of a lepton-flavor-violating Z^{'} in e^{+}e^{-}→e^{±}μ^{∓}Z^{'}. We do not find any excess of events and set 90% credibility level upper limits on the cross sections of these processes. We translate the former, in the framework of an L_{μ}-L_{τ} theory, into upper limits on the Z^{'} coupling constant at the level of 5×10^{-2}-1 for M_{Z^{'}}≤6 GeV/c^{2}. | 
| Year of Publication | :  2020 | 
| Journal | :  Physical review letters | 
| Volume | :  124 | 
| Issue | :  14 | 
| Number of Pages | :  141801 | 
| Date Published | :  2020 | 
| ISSN Number | :  0031-9007 | 
| URL | :  https://link.aps.org/abstract/PRL/v124/p141801 | 
| DOI | :  10.1103/PhysRevLett.124.141801 | 
| Short Title | :  Phys Rev Lett | 
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