Evidence for s-channel single top quark production in pp¯ collisions at s=1.96 TeV

We present measurements of the cross sections for the two main production modes of single top quarks in pp¯ collisions at s=1.96 TeV in the Run II data collected with the D0 detector at the Fermilab Tevatron Collider, corresponding to an integrated luminosity of 9.7 fb−1. The s-channel cross section is measured to be σ(pp¯→tb+X)=1.10−0.31+0.33 pb with no assumptions on the value of the t-channel cross section. Similarly, the t-channel cross section is measured to be σ(pp¯→tqb+X)=3.07−0.49+0.54 pb. We also measure the s+t combined cross section as σ(pp¯→tb+X,tqb+X)=4.11−0.55+0.60 pb and set a lower limit on the CKM matrix element |Vtb|>0.92 at 95% C.L., assuming mt=172.5 GeV. The probability to measure a cross section for the s channel at the observed value or higher in the absence of signal is 1.0×10−4, corresponding to a significance of 3.7 standard deviations.