Authors:

Shabnaz Pashapour, Pekka K. Sinervo

University of Toronto

We present the first measurement of σ (gg → ttbar) ⁄ σ (ppbar → ttbar) using the low p_{T} track multiplicity in lepton+jet channel to separate out gg initial states. We show that the average number of low p_{T} 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 E_{T} 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 0.07 ± 0.14(stat) ± 0.07(syst), corresponding to an upper limit of 0.33 at 95% confidence level including both statistical and systematic uncertainties, for σ (gg → ttbar) ⁄ σ (ppbar → ttbar) using 0.95 fb^{-1} of data.

Public Conference Note The first measurement of σ(gg → ttbar) ⁄ σ(ppbar → ttbar) using 0.95 fb

^{-1}of data

There is a clear correlation between the average number of gluons and the average number of low p_{T} charged particles present in a given sample. The average number of low p_{T} 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) calculations (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.

Now that we settled 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 p_{T} 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 E_{T} 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 distributions converge after the first iteration. The process for finding the distributions (left) and the comparison between the no-gluon and gluon-rich parameterizations (right) are shown in the two following plots.

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We can get the gluon-rich fraction of events in our W+4 or more jet b-tagged sample using the fit. This fraction can be written as:

f_{g}^{W+4jet} = f_{s}f_{g}^{ttbar} + f_{b}f_{g}^{bkg}

where f_{s} is the signal fraction and f_{b} is the fraction of background in the W+4 or more jet b-tagged sample. f_{g}^{tt} is the fraction of gluon-rich events in ttbar events and f_{g}^{bkg} 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 H_{T} 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, ~13%, get f_{s} as (1-f_{b}) and measure f_{g}^{W+4jet} using the track multiplicity distribution. Therefore, if we know f_{g}^{bkg} we can measure f_{g}^{ttbar}.

To estimate f_{g}^{bkg}, we extrapolate the gluon-rich fraction in the 4 or more jet bin from the W+1, 2 and 3 jet gluon-rich fractions. We use both tagged and no-tag events, representing HF and LF backgrounds, respectively. We then use:

f_{g}^{bkg} = f_{b}^{LF}f_{g}^{LF} + f_{b}^{HF}f_{g}^{HF}

where f_{b}^{LF}, f_{b}^{HF} are the fraction of LF and HF background, and f_{g}^{LF} and f_{g}^{HF} are the gluon-rich fraction in LF and HF background, respectively.
We find a value of 0.53 ± 0.09 (calculation) ± 0.06 (nonW background variation).

- gluon-rich fraction measured from W+4 or more jet tagged sample, f
_{g}^{W+4jet}= 0.15 ± 0.14 (stat) ± 0.07 (syst) - gluon-rich fraction in ttbar candidates, f
_{g}^{ttbar}= 0.09 ± 0.16 (stat) ± 0.08 (syst) - σ(gg → ttbar)⁄σ(ppbar → ttbar) = 0.07 ± 0.14 (stat) ± 0.07 (syst)

First Measurement of σ(gg → ttbar)⁄σ(ppbar → ttbar) in ppbar Collisions at E

_{CM}of 1.96 TeV, presented in Canadian Associations of Physicists Congress, Saskatoon, SA, Canada, June 19, 2007Toward a Measurement of σ(gg → ttbar)⁄σ(ppbar → ttbar) in ppbar Collisions at E

_{CM}of 1.96 TeV, presented in American Physical Society Meeting, Jacksonville, FL, USA, April 15, 2007

last updated 22 April 2008