Evidence for s-channel single top quark production in pbar{p} collisions at sqrt{s} = 1.96 TeV

We present measurements of the cross sections for the two main production modes of single top quarks in $p\bar{p}$ collisions at $\sqrt{s} = 1.96$ 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 $\sigma(p\bar{p} \to tb+X) = 1.10^{+0.33}_{-0.31}$ pb with no assumptions on the value of the t-channel cross section. Similarly, the t-channel cross section is measured to be $\sigma(p\bar{p} \to tqb+X) = 3.07^{+0.54}_{-0.49}$ pb. We also measure the $s+t$ combined cross section as $\sigma(p\bar{p} \to tb+X, tqb+X) = 4.11^{+0.60}_{-0.55}$ pb and set a lower limit on the CKM matrix element $|V_{tb}| > 0.92$ at $95\%$ C.L., assuming $m_t=172.5$ GeV. The probability to measure a cross section for the s channel at the observed value or higher in the absence of signal is $1.0\times 10^{-4}$, corresponding to a significance of 3.7 standard deviations.