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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' e+e→μ+μZ′ and of a lepton-flavor-violating Z' in e+e→e±μZ′. 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 Lμ−Lτ theory, into upper limits on the Z' coupling constant at the level of 5 × 10−2 − 1 for MZ' ≤ 6 GeV/c2.

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Adachi, I., Ahlburg, P., Aihara, H., Akopov, N., Aloisio, A., Anh Ky, N., Asner, D. M., Atmacan, H., Aushev, T., Aushev, V., Aziz, T., Babu, V., Baehr, S., Bambade, P., Banerjee, Sw., Bansal, V., Barrett, M., Baudot, J., Becker, J., … Zhukova, V. I. (2020). Search for an invisibly decaying . Physical Review Letters, 124(14).

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