Evidence for a Bs0π± State

We report evidence for a narrow structure, X (5568 ), in the decay sequence X (5568 )→B_s⁰π^±, B_s⁰→J /ψ ϕ , J /ψ →μ⁺μ^-, ϕ →K⁺K^-. This is evidence for the first instance of a hadronic state with valence quarks of four different flavors. The mass and natural width of this state are measured to be m =5567.8 ±2.9 (stat₎^{-1.9 +0.9}(syst ) MeV /c² and Γ =21.9 ±6.4 (stat₎^{-2.5 +5.0}(syst ) MeV /c² . If the decay is X (5568 )→B_s^*π^±→B_s⁰γ π^± with an unseen γ , m (X (5568 )) will be shifted up by m (B_s^*)-m (B_s⁰)∼49 MeV /c² . This measurement is based on 10.4 fb^-1 of p p ¯ collision data at √{s }=1.96 TeV collected by the D0 experiment at the Fermilab Tevatron collider.