Top Cross Section with Displaced Vertex b-Tagging


Melissa Franklin, Sebastian Grinstein, Joao Guimaraes da Costa, Daniel Sherman
Harvard University/CDF


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Analysis Summary
We present of measurement of the top pair production cross section in the lepton+jets decay channel. We initially select events kinematically consistent with this decay channel, requiring a lepton (electron or muon) with transverse momentum greater than 20 GeV/c, three or more jets with transverse energy greater than 20 GeV, and a missing transverse energy (to account for the non-interacting neutrino) greater than 30 GeV. To reject background, we additionally require the scalar sum of the transverse energy of the jets, neutrino, and lepton to exceed 250 GeV.

We further isolate top signal events by requiring at least one jet to be identified as a b-jet; Top events will in general have two bottom quark jets in the final state, while the largest backgrounds (W+jets) will only have even one heavy flavor jet roughly 20% of the time. We use a displaced-vertex tagging algorithm, SecVtx, which explicitly reconstructs decay vertices inside jets. The algorithm has an efficiency of roughly 40% per b-jet with a false positive rate of 0.5%. In tagged events, which have a purity of nearly 90%, we measure a cross section of:
8.2 ± 0.5 (stat) ± 0.8 (syst) ± 0.5 (lum) pb

We cross check the results in a sample of events with two or more tagged jets. Here, we loosen the total transverse energy cut to 200 GeV, and we use an alternate tuning of SecVtx with a b-jet efficiency of 48% and a false positive rate of 1.2%. In such events, we measure:
8.8 ± 0.8 (stat) ± 1.2 (syst) ± 0.5 (lum) pb

Additional Documentation (postscript)

Expected and observed number of events sorted by jet multiplicity requiring at least one tight tag. The top contribution is normalized to the measured value of 8.2 pb. Expected and observed number of events sorted by jet multiplicity requiring at least two loose tags. The top contribution is normalized to the measured value of 8.8 pb.


Kinematic Plots
The expected (solid histogram) and observed (points) Ht (left) and missing Et (right) distribution in single-tagged events. The top contribution is normalized to the measured value of 8.2 pb.
The expected (solid histogram) and observed (points) W transverse mass (left) and lepton pT (rigmt) distribution in single-tagged events. The top contribution is normalized to the measured value of 8.2 pb.
The expected (solid histogram) and observed (points) leading jet Et (left) and pseudo-rapidity (right) distribution in single-tagged events. The top contribution is normalized to the measured value of 8.2 pb.
The expected (solid histogram) and observed (points) second jet Et (left) and pseudo-rapidity (right) distribution in single-tagged events. The top contribution is normalized to the measured value of 8.2 pb.
The expected (solid histogram) and observed (points) third jet Et (left) and pseudo-rapidity (right) distribution in single-tagged events. The top contribution is normalized to the measured value of 8.2 pb.
The expected (solid histogram) and observed (points) tag mass (left) and two-dimensional displacement (right) distribution in single-tagged events. The top contribution is normalized to the measured value of 8.2 pb.
The expected (solid histogram) and observed (points) tag displacement significance (left) and pseudo-ctau (right) distribution in single-tagged events. The top contribution is normalized to the measured value of 8.2 pb.