ttH-> ttbb Search at CDF

Search for ttH at CDF

Authors: Stan Lai and Pekka K. Sinervo

University of Toronto

Summary

The analysis uses 320 pb-1 of data and searches for a signature WWbbbb. In summary, we require a high pT identified electron or muon candidate, 5 or more jets, Missing Transverse Energy, and 3 or more b-tags.

Details about the event selection can be found here

ttH Acceptance

Acceptance for ttH is estimated using pythia MC, and then corrected for lepton id efficiency and b-tagging efficiency.

The systematic uncertainties for the acceptance are shown:

Backgrounds

There are three backgrounds that we consider. They are:

Mistag Background - backgrounds where a light quark jet is misidentified as a b-jet.
QCD Background - multijet events where a jet fakes a lepton.
Irreducible Background - events with the same signature as ttH (ttbb, ttcc, and also tt and ttjj where there is a W->cs decay.

All three evaluation rely on parametrizing the probability that a light quark jet is misidentified as a b-jet. Below are plots that show the good predictivity of this parametrization (from jet data) to the observed data (from lepton data).

The event tally for the signal region is shown below:

Result

We set 95% C.L. limits on the ttH production cross section (as a function of Higgs mass).

For a Higgs mass of 115 GeV, this translates into a limit on cross section times BR(H->bb) of 660 fb. This is a factor of 168 above the SM prediction.

The plot of the limit set as a function of Higgs mass is shown below:

Event and Distributions

Here are plots of the event observed and the a priori distributions as a function of certain kinematic variables.