Search for an invisibly decaying boson at Belle II in plus missing energy final states

Belle II実験でのに加え欠損エネルギーがある終状態を用いた見えない崩壊をするボゾンの探索

足立 一郎*; 谷田 聖   ; Belle II Collaboration*; 他358名*

Adachi, Ichiro*; Tanida, Kiyoshi; Belle II Collaboration*; 358 of others*

Theories beyond the standard model often predict the existence of an additional neutral boson, the . 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 in the process and of a lepton-flavor-violating in . 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 theory, into upper limits on the coupling constant at the level of - for GeV/.