Top Dilepton Cross Section with 5.1 fb-1 using  the Standard DIL Selection

 

Authors :

Chang-Seong Moon, Soo-Bong Kim
Seoul National University

Monica Tecchio, Alexei Varganov
University of Michigan

Andrew Beretvas
FNAL

 

Abstract :

We report on a measurement of the ttbar cross section using 5.1 fb-1 of data collected by CDF II. We use ttbar dilepton events reconstructed with the Dilepton selection assuming Mtop = 172.5 GeV/c2. Signal selection and background estimates are based on the methods developed previously for 2.8 fb-1 measurement. We measure


σpretag = 7.40 ± 0.58stat  ± 0.63syst ± 0.45lumi pb.

for 343 signal candidate events in pre-tag sample and using one tight SecVtx b-tag we measure

σbtag = 7.25 ± 0.66stat  ± 0.47syst ± 0.44lumi pb.

for 137 signal candidate events.

where the first uncertainty is statistical, the second is the convolution of acceptance and background systematics and the third is due to the uncertainty in  the luminosity measurement.


The analysis and results were blessed at the 01-APR-2010 top meeting and have been documented in CDF Public Conference Note.

 

Analysis :

The DIL selection for the 5.1 fb-1 is based on the GEN6 DIL selection of 2.8 fb-1 analysis.

  1. 2 leptons requirement : First lepton should be isolated electron of ET, or muon with PT, greater than 20 GeV are selected. a second electron of ET, or muon of PT, greater than 20 GeV is also required using looser identification cuts and no requirement on isolation. Events with more than two leptons in the final state are rejected.

  2. At least 2 jets : ttbar candidate events is obtained by requierd at least 2 jets with ET >15 GeV and |η| < 2.5 .

  3. MET cut : Missing transverse energy is required missing ET > 25 GeV.

  4. Lcut : If any lepton or jet is closer than 20o from the missing ET direction, Missing ET should be ET > 50 GeV.

  5. Z-veto : This cut removes possible ee/mm contamination from Z events in the 76-106 GeV invariant mass window. In previous top DIL cross-section analysis, we were using a cut on Jet Significance, which is now replaced with a cut on Missing ET Significance. Optimization studies show that we can slightly improve the S/B by rejecting 25% of the Z background while maintaning over 95% of the signal.

  6. Dilepton invariant mass cut : This cut requires Mll > 5 GeV. It removes possible J/ψ → μμ events which are not considered background source.

  7. COT exit radius cut : This cut is made to ensure that each reconstructed muon in our sample has a high efficiency for triggering the event and that each track through enough COT layers to be well-reconstructed and in a region well-described by the Monte Carlo. We require ρCOT > 140 cm.

  8. Summed transverse energy HT > 200 GeV and the two leptons to be of opposite charge.

  9. b jet tagging : At least 1 Tight SecVtx b tagging is required for b-tagged events. We take the Tight SecVtx b-tagging scale factor 0.95 ± 0.04 from CDF hight pT b-tag group.

  10. Only Stubless CMIO muons are allowed. Stubbed CMIO muons pointing to Miniskirt/Keystone CMX regions are now considered as CMX (trigger) muons. The CMX fiduciality is now solely defined according to the Muon Fiducial tool, i.e. no extra cuts are applied to the muon (eta,phi)  direction as a way to separate Arches CMX muons from Miniskirt/Keystone CMX muons.

  11. Jet are "truly" corrected up to L5.

  12. Muon momentum correction(Curvature corrections) and plug energy corrections are applied by TopEventModule while creating the TopNtuples. This eliminate the need to apply these correction at ntuple level before calculating quantities like MET and HT which are used by the top DIL selection

 

Denominator :

  1. The signal acceptance is calculated using "ttop25". The final raw MC acceptance for ttbar events with OBSV vertex generated not further away than 60 cm from CDF nominal z=0 and which have the two W's decaying leptonically at GENP level, are 0.756 ± 0.004% for pre-tagged events and 0.461 ± 0.003% for b-tagged events.

  2. The raw acceptance is corrected for trigger efficiency and lepton ID scale factors. See Section 4 note of CDF 9635 and tables there in for a summary of the efficiency factors used for each dilepton pair and their final correction. The corrected acceptance for each dilepton pair is multiplied by its relative luminosity before being all added together in the cross-section denominator.

 

Numerator :

  1. This is the difference between the observed number of candidate and the background prediction

  2. The data candidates are obtained by running over the bhel0x and bhmu0x (x=0d/0h/0i/mi/mj/mk/mm) data samples and imposing version 31 of the good run list.

  3. Fake rates have been calculate using 5.1 fb-1 of QCD data.

 

Results :

The following table contains a summary of pre-tagged background estimates for each jet bin before the HT and Opposite Sign requirements are applied and in the 2 jet bin after applying only the HT cut. The last column contains the candidate events with all cuts applied. This table also shows the total background expectation, the ttbar expectation for the cross section which is 7.4 pb, their sum (labeled as "Total SM expectation") and the number of candidate events in 5.1 fb-1 of data.

 

The following table shows the total number of background, SM expectation and 5.1 fb-1 data candidate events, divided by lepton flavor contribution.

 

The next table contains b-tagged background estimates for 1 jet control sample and the signal candidate events after the HT and Opposite Sign cut. This table also shows the total Standard Model expectation and the number of candidate events in 4.8 fb-1 of data after b-tagging.

 

The following table shows the total number of background, SM expectation and 4.8 fb-1 data candidate events after b-tagging, divided by lepton flavor contribution.

 

The following plot Final_plot_5.1fb_pretag.eps and Final_plot_4.8fb_btag.eps present the background and signal composition of pre-tagged 5.1 fb-1 and b-tagged 4.8 fb-1 DIL events vs jet multiplicity after the Z-veto, Missing ET and L-cut have been applied.

Jet Multiplicity spectrum for pre-tagged events 1 jet and signal b-tagged events



Selected kinematic variables for background and top signal predictions before b-tagging, overlaid to data for the lepton transverse energy spectrum, the dilepton invariant mass, the missing ET, HT, Number of jets spectrum and Jet ET distributions in 5.1 fb-1 top DIL pre-tagged candidate events. The corresponding eps files are found here: Lepton PT, Dilepton invariant Mass, Missing ET, HT, Number of jets spectrum, Jet ET





Selected kinematic variables for background and top signal predictions after b-tagging, overlaid to data for the lepton transverse energy spectrum, the dilepton invariant mass, the missing ET, HT, Number of jets spectrum and Jet ET   distributions in 4.8 fb-1 top DIL b-tagged candidate events. The corresponding eps files are found here: Lepton PT, Dilepton invariant Mass, Missing ET, HT, Number of jets spectrum, Jet ET



 

Systematic uncertainty :

Systematics for pre-tagged events Systematics for b-tagged events

 

Further detail :

More information can be found at the documentation for this analysis, or by contacting any one of the good people listed at the top of this page.