Evidence for a $B_s^0 \pi^\pm$ State

We report evidence of a narrow structure, $X(5568)$, in the decay sequence $X(5568) \rightarrow B_s^0 \pi^{\pm}$, $B_s^0 \rightarrow J/\psi \phi$, $J/\psi\rightarrow \mu^+ \mu^-$, $\phi \rightarrow K^+K^-$. 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 $m = 5567.8 \pm 2.9 {\rm \thinspace (stat)} ^{+0.9}_{-1.9} {\rm \thinspace (syst)}$ MeV/$c^2$ and $\Gamma = 21.9 \pm 6.4 {\rm \thinspace (stat)} ^{+5.0}_{-2.5} {\rm \thinspace (syst)}$ MeV/$c^2$. If the decay is $X(5568) \rightarrow B_s^* \pi^{\pm} \rightarrow B_s^0 \gamma \pi^{\pm}$ with an unseen $\gamma$, $m(X(5568))$ will be shifted up by $m(B_s^*) - m(B_s^0) \sim 49$ MeV/$c^2$. This measurement is based on $10.4~\rm{fb^{-1}}$ of $p \bar p$ collision data at $\sqrt{s}$ = 1.96 TeV collected by the D0 experiment at the Fermilab Tevatron collider.