Observation of $s$-Channel Production of Single Top Quarks at the Tevatron

We report the first observation of single-top-quark production in the $s$ channel through the combination of the CDF and D0 measurements of the cross section in proton-antiproton collisions at a center-of-mass energy of 1.96 TeV. The data correspond to total integrated luminosities of up to $9.7{\mathrm{fb}}^{-1}$ per experiment. The measured cross section is ${\sigma }_s=1.29_{-0.24}^{+0.26}\mathrm{pb}$. The probability of observing a statistical fluctuation of the background to a cross section of the observed size or larger is $1.8\times {10}^{-10}$, corresponding to a significance of 6.3 standard deviations for the presence of an $s$-channel contribution to the production of single-top quarks.

Figure 1

Dominant Feynman diagrams for (a) $s$-channel and (b) $t$-channel single-top-quark production at the Tevatron.

Figure 2

Distribution of the discriminant histograms, summed for bins with similar signal-to-background ratio ($s/b$). The expected sum of the backgrounds is shown by the unfilled histogram, and the uncertainty of the background is represented by the gray shaded band. The expected $s$-channel signal contribution is shown by a filled blue histogram.

Figure 3

The background-subtracted distribution of the discriminant histograms, summed for bins with similar signal-to-background ratio ($s/b$). The background and $s$-channel signal (blue filled histogram) have been normalized to the most likely values returned from the likelihood fit and the uncertainty on the background uncertainty has also been constrained by the data (gray shaded band).

Figure 4

The posterior probability distribution for the combination of the CDF and D0 analysis channels compared with the $\mathrm{NLO}+\mathrm{NNLL}$ theoretical prediction [31].

Figure 5

Measured single-top-quark $s$-channel production cross sections from each of the individual analyses and various combinations of these analyses compared with the $\mathrm{NLO}+\mathrm{NNLL}$ theoretical prediction [31].

Figure 6

Log-likelihood ratios using an asymptotic approximation for the background-only (solid green line) and SM-signal-plus-background (dashed blue) hypotheses from the combined measurement.