Authors:
Shabnaz Pashapour, Pekka K. Sinervo
University of Toronto


Abstract

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 0.07 ± 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.

Document

Analysis

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) 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.








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 calculations 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.
left-click to bring up the EPS version of the image
right-click to save the GIF version of the image


The values used in the correlation plot is shown in the table.
left-click to bring up the EPS version of the table
right-click to save the GIF version of the table


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.
left-click to bring up the EPS version of the table
right-click to save the GIF version of the table


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 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 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.


left-click to bring up the EPS version of the image
right-click to save the GIF version of the image



left-click to bring up the EPS version of the image
right-click to save the GIF version of the image



The fraction of gluon-rich events as found using the distribution fit. All data uncertainties are statistical. The uncertainties on MC predictions are both statistical and systematical.
left-click to bring up the EPS version of the table
right-click to save the GIF version of the table


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:

fgW+4jet = fsfgttbar + fbfgbkg

where fs is the signal fraction and fb is the fraction of background in the W+4 or more 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, ~13%, get fs as (1-fb) and measure fgW+4jet using the track multiplicity distribution. Therefore, if we know fgbkg we can measure fgttbar.

To estimate fgbkg, 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:

fgbkg = fbLFfgLF + fbHFfgHF

where fbLF, fbHF are the fraction of LF and HF background, and fgLF and fgHF 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).


The gluon fractions found by the fit for W+1, 2 and 3 jet in both tagged and no-tag samples, as well as the extrapolated values are shown.
left-click to bring up the EPS version of the image
right-click to save the GIF version of the image



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.
left-click to bring up the EPS version of the image
right-click to save the GIF version of the image



Systematic uncertainties affecting track multiplicities and fgbkg
left-click to bring up the EPS version of the table
right-click to save the GIF version of the table



Systematic uncertainties due to fg, fgbkg and fb
left-click to bring up the EPS version of the table
right-click to save the GIF version of the table



Systematic uncertainties due to fgttbar and acceptances
left-click to bring up the EPS version of the table
right-click to save the GIF version of the table


Results

Most Recent Presentations



last updated 22 April 2008