We present the first measurement of σ (gg → ttbar) ⁄ σ (ppbar → ttbar) using the low pT track multiplicity in lepton+jet channel to separate out gg initial states. We show that the average number of low pT tracks scales with the gluon content of the sample. We take advantage of the fact that the gluon composition of the gluon rich fraction of the Standard Model ttbar processes is close to that of the gluon-rich fraction of dijet samples with relatively high leading jet ET values, and that the W+0 jet sample is dominated by qqbar initial states. We extract the gluon rich fraction and measure σ (gg → ttbar) ⁄ σ (ppbar → ttbar). We find a value of 0.25 ± 0.24(stat) ± 0.10(syst) for σ (gg → ttbar) ⁄ σ (ppbar → ttbar) using 330 pb-1 of data.
CDF8416 presents the first measurement of σ(gg → ttbar) ⁄ σ(ppbar → ttbar) using 330 pb-1 (Public Conference Note)
CDF8271 presents the first measurement of σ(gg → ttbar) ⁄ σ(ppbar → ttbar) using 330 pb-1 (CDF only, password required)
There is a clear correlation between the average number of gluons and the average number of low pT charged particles present in a given sample. The average number of low pT charged particles is measured using different data sample (y-axis) while the average number of gluons in the sample is what we find using Monte Carlo (MC) samples (x-axis). Here, we consider a gluon in our calculation if it is part of the hard scattering Matrix Elements (ME). Any gluon that is radaited from the partons in the ME as a result of branching through the MC generator will not be counted regardless of its momentum.
 Using three W and three dijet samples, we find the correlation between the average low pT track multiplicity and the average number of gluons. <Ng> is predicted using MC for each sample and <Ntrk> is measured using data. Different samples are shown with different markers and colors. The red corresponds to dijet samples and the blue represents W samples. Solid circles, squares and triangles are used to distinguish different subsamples as specified in the legend. |
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 The values used in the correlation plot is shown in the table. |
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 The average number of gluons as found using the correlation fit for the other calibration samples. The values are in good agreement with the expected <Ng> in the samples. |
Now that we sttled this correlation exists, we can use data samples with non or little gluon content and samples with high gluon content to define no-gluon and gluon-rich low pT track multiplicity distributions and later on use their normalized parameterizations in a simple likelihood fit with 2 free parameters to find the fraction of gluon-rich events or the average number of gluons in any given data sample.
We use the W+0 jet sample to extract our no-gluon distribution and the dijet sample with ET of 80-100 GeV to extract the gluon-rich distribution. We subtract the qq → qq fraction of the dijet sample using the W+0 jet distribution and then use this subtracted distribution, we subtract the gluon-rich contribution from the W+0 jet distribution and iterate one more time to get the final distributions. The process for finding the distributions (left) and the comparison between first and second iteration as well as the parameterization (right) are shown in the two following plots.
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The following 2 plots show the fit to dijet sample with ET of 140-160 GeV (left) and the W+1 jet data sample (right). The solid black line is the fit to the distribution, the red shows the gluon-rich contribution and the blue is the no-gluon fraction. One can find the average number of gluons present in this sample by myltiplying the gluon-rich fraction by the average number of gluons of the gluon-rich distribution (2.37). In cases where the gluon-rich component of a sample has about 2 gluons on average, one can simply use the fraction as the fraction of gluon rich events in the sample. This is the case for the ttbar events and as such we simply use the fraction for our final calculation.
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 The average number of gluons as found using the distribution fit. All data and fitted uncertainties are statistical.
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We can get the gluon-rich fraction of events in our W+4 jet b-tagged sample using the fit. This fraction can be written as:
fgW+4jet = fsfgttbar + fbkgfgbkg
where fs is the signal fraction and fbkg is the fraction of backgroundand in the W+4 jet b-tagged sample. fgtt is the fraction of gluon-rich events in ttbar events and fgbkg is the fraction of gluon-rich events in the background. We use a similar selection criteria as used in SecVtx MII tight cross section measurement except that we do not require an HT of at least 200 GeV and we only consider events with at least 4 jets. We take the background fraction from the cross section measurements, ~10%, get fs as (1-fbkg) and measure fgW+4jet using the track multiplicity distribution. Therefore, if we know fgbkg we can measure fgttbar.
There is no clear answer on how to estimate fgbkg and as such we use 2 different methods to estimate this value and take the difference between the two values as the systematic uncertainty. The first method is to measure fg in W+1, 2 and 3 jet(s) with no b-tag samples. Thiese samples are more likely to be background and then extrapolate to 4 and 5 jet bins and take the average of the 4th and 5th bin fg as fgbkg. The second method is to find fg in W+1, 2 and 3 jets with at least 1 b-tag. The W+2 and W+3 jets with at least 1 b-tagged jet have some ttbar events included, using estimated ttbar fractions in these samples we make a correction to fg and then average over the 3 values. Using the first method, we get a value of 0.65 ± 0.06 and using the second method we find a systematic uncertainty of ±0.14.
 This plot shows the linear fit used to extrapolate the no tag gluon rich fraction to estimate the gluon rich fraction of the $\ttbar$ in the tagged 4 or more jet bin.
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 The fit result for the tagged W+4 or more jet sample. The two components of the fit (gluon rich and no-gluon) contributions are also shown.
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 Systematic uncertainties
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First Measurement of σ(gg → ttbar)⁄σ(ppbar → ttbar) in ppbar Collisions at ECM of 1.96 TeV, presented in Canadian Associations of Physicists Congress, St. Catharines, ON, Canada, June 13, 2006
Toward a Measurement of σ(gg → ttbar)⁄σ(ppbar → ttbar) in ppbar Collisions at ECM of 1.96 TeV, presented in American Physical Society Meeting, Dallas, TX, USA, April 23, 2006