Combined Measurement of Single Top Quark Production Cross Section in 1.96 TeV p anti-p Collisions

Matteo Cremonesi, Tom Junk, Ben Wu  [Contact]

CDF Public Note


The single top quark production cross section measurement in events with with an imbalance in the total transverse momentum, b-tagged jets, and no identified leptons, is combined with a previously reported CDF result obtained from events with an imbalance in total transverse momentum, b-tagged jets, and exactly one identified lepton. An s- and t-channel single top quark cross section of 3.02 +0.49 -0.48 (stat+syst) pb and a 95% confidence level lower limit of Vtb > 0.84 are measured from the combination. A t-channel single top quark production cross section, considering the s-channel production as a background constrained to the theoretical prediction, is measured to be 1.65 +0.38 -0.36 (stat+syst) pb.

+ Introduction

The observation of single top quark production at the Tevatron was a significant achievement for the CDF and D0 experiments. With these observations, the cross section for electroweak-produced top quarks at a hadron collider was measured for the first time, and due to the direct coupling of the b quark with the singly-produced top quark, an upper limit on the Cabibbo-Kobayashi-Maskawa (CKM) matrix element magnitude Vtb could be placed. The standard model (SM) prediction for the combined s- and t-channel single top quark production cross section has been calculated to next-to-next-to-leading order. Assuming CP conservation, the SM calculation is 3.15 +- 0.36, excluding the contribution from the tW production mode, which is expected to be negligible in this final state. The primary sensitivity to measuring this quantity is usually obtained from events where the W decays leptonically to a lepton-neutrino pair alongside a pair of jets, one of which is b-tagged or identified as likely having originated from a bottom quark. This sample of events provides a distinct signature against backgrounds produced by the strong force (QCD multijet or MJ background), which contain no leptons. A complementary method in measuring the single top quark cross section is through events in the final state that contain two or three jets and significant imbalance in the total transverse momentum (missing transverse energy, MET), which results from the leptonic decay of the W boson, where the lepton has been lost due to reconstruction or acceptance effects. The results of these analyses are combined to obtain a more precise measurement of the single top cross section and to place a lower limit on the CKM matrix element magnitude Vtb.

+ Combination

The two searches are combined by taking the product of their likelihoods and simultaneously varying the correlated uncertainties. The results are shown below.

σs+t = 3.02 +0.49 -0.48 pb

σt = 1.65 +0.38 -0.36 pb