Observation of wrong-sign tri-muon events in 250 GeV muon-nucleon interactions

Two trimuon events of the type μ+N→μ+μ+μ+X and one μ+N→μ+μ−μ−X have been observed in 250 GeV μ+ -iron interactions. The expected rate from the decay of hadrons (charm, π, K) is about one event. Other processes that may contribute are both beauty production and charm production with D0−D0 mixing. The data give upper limits of 12×10−36 cm2 for the cross section of beauty pair production by muons, and 20% for D0 ↔ D0 transitions due to mixing, both at 90% confidence level.