Search for Heavy Top t'->Wq In Lepton Plus Jets Events
in 2.3 fb-1

J. Conway, D. Cox, R. Erbacher, W. Johnson, A. Ivanov, A. Lister, T. Schwarz
University of California, Davis

limit
 		Upper limit, at 95% CL, on the production rate for t' as a function of t' mass (red). The purple curve is a theoretical cross section. The blue band represents +/-1 standard deviation expectation limit (light blue corresponds to +/- 2 standard deviation)

(EPS)

CDF public conference note 9234
pdf

             Public Page for previous iteration (w/ 760 pb-1 )

CDF Public Note for previous iteration  

Summary Slide
Summary Slide
(ppt)





Event Selection
We search for the t' in events which meet several selection criteria
  • one and only one high-pT (pT > 20 GeV/ c) isolated electron or muon
  • large missing transverse energy (> 20 GeV)
  • at least four energetic jets (ET > 20 GeV after corrections for detector effects)
To reduce the QCD background in our sample we apply three additional requirements.

       Leading jet ET > 60 GeV
 To remove mis-measured muons with very high pT we also require that the Delta phi between the muon and the Missing Transverse Energy be < 3.05.

The Delta phi cuts were optimized with respect to top pair production. The cut on leading jet ET removes an additional 60% of QCD background while only removing a few percent of the signal.

The dominant contributing backgrounds after these cuts are from electroweak processes as well as top pair production. Electroweak process are dominated by W + jets. We assume the mass of the top quark to be 175 GeV. Other backgrounds include Z + jets, WW + jets, WZ + jets and single top, all of which have a smaller rate than W + jets. Moreover the other backgrounds are found to have similar kinematic distributions to W + jets and so are modeled as one using the W + jets model. The QCD background is modeled using a sample of data where the lepton ID cuts have been reversed.

Using these event selection criteria we observe a total of 1,118 events of which 667 contain an electron and 551 contain a muon.

Analsysis Method
We utilize the fact that the t' decay chain in the regime of interest is identical to the one of the top quark, the t' mass is reconstructed in the same way as is done in the top quark mass measurement analyses. We use the template method for top quark mass reconstruction based on the best -fit to the kinematic properties of final top decay products. For each event there are 4!/2 = 12 combinations of assigning 4 jets to partons. In addition, there are two solutions to account for the unknown z-component of the neutrino momentum. After minimization of the expression, the combination with the lowest is selected and the value of the top (t') mass is declared to be the reconstructed mass Mreco of top (or t' respectively).

We use the observed distributions of the Mreco and total transverse energy (HT) in the event to distinguish the t' signal from backgrounds by fitting it to a combination of t' signal, top electroweak background, and QCD background shapes.

We use a binned in HT and Mreco likelihood fit to extract the t' signal and/or set an upper limit on its production rate. We calculate the likelihood as a function of the t' cross section and use Bayes' Theorem to convert it into a posterior density in the t' cross section. We can then use this posterior density to set an upper limit on the production rate of t'.

The production rate for W + jets is a free parameter in the fit. Other parameters, such as the top pair production cross section, lepton ID, data/MC scale factors and integrated luminosity are related to systematic errors and treated in the likelihood as nuisance parameters constrained within their expected (normal) distributions. We adopt the profiling method for dealing with these parameters, i.e. the likelihood is maximized with respect to the nuisance parameters.


Systematic Errors
Jet Energy Scale
The sensitivity to t' depends on knowing accurately the distribution of (HT, Mreco) in data. One of the largest sources of uncertaity comes from the jet energy scale. Jets in the data and Monte Carlo (MC) are corrected for various effects leaving some residual uncertainty. This uncertainty results in possible shifts in the HT and Mreco distributions for both new physics and standard model templates. We take this effect into account by generating templates with energies of all jets shifted upwards by one standard deviation and downwards respectively. We then use a template morphing technique that was developed in 2005 for a previous version of this analysis.
W + jets Q2 Scale

The effect of the choice of appropriate Q2 scale for W + jets production is evaluated by measuring the resulting change in the measured t' cross section given that t' exists. The scale is varied to twice and half its nominal value and the expected change in the measured cross section is then interpreted as the uncertainty on the t' cross section itself. We measure teh shift as a function of the t' cross section by drawing pseudoexperiments from shifted templates and fitting them to the nominal distribution. The resulting shift is fitted to a linear function of the t' cross-section and is incorporated into the likelihood as an additive parameter to the t' cross section. Where the additive parameter ois constrained by a gaussian with a width, that is half of the largest of the upwards or downwards shifts for each mass of the t'. The observed offsets and slopes for the linear fits can be seen below.
ISR and FSR

The systematic error associated with the inital- and final-state radiation was determined by generating some samples with more ISR and more FSR and some samples with less ISR and less FSR. We refer to these samples as IFSR more and IFSR less. We generated samples for t' with masses of 250, 300 and 350 GeV which brackets the region where we expect to be able to place our exlcusion limit. The resulting effect is treated in a similar way to the Q2 systematic. Templates are made for each of these mass points. Pseudoexperiments are then thrown with the shifted top and t' IFSR samples, where the shift is set to be the same for top and t'. We then fit the obtained cross-section shift using a linear function of the t' cross-section. We add the resulting shifts in quadrature with the Q2 error in the likelihood. The shifts can be seen below.
mt'  Q2
IFSR
(GeV) offset slope offset
 slope
180 0.61 0.016
0.125
0.026
200 0.72 0.018
0.125
0.024
220 0.48 0.025
0.125
0.022
240 0.36
0.022
0.110
0.020
260 0.20 0.027
0.080
0.018
280 0.12
0.028
0.060
0.017
300 0.093
0.022
0.035
0.014
320 0.072
0.021
0.025
0.011
340
 0.055
 0.016
 0.015
 0.009
360
 0.043
 0.014
 0.010
 0.008
380
 0.33
 0.011
 0.007
 0.007
400
 0.025
 0.011
 0.005
 0.006
450
 0.015
 0.007
 0.004
 0.005
500
 0.013
 0.006
 0.003
 0.004

QCD Background
The QCD background shape is modled from a sample of data in which the electron cuts have been reversed. The QCD normalization is obtained by fitting the background (electroweak, top, and QCD) distributions to the data with the Missing Transverse Energy removed and then computing how much remains after all cuts are applied, as most of the QCD is expected to be found at low Missing Transverse Energy.

Cutting very hard on the leading jet  removes most of the QCD background which makes our fit rather insensitive to the QCD modeling. The relative normalization uncertainty is taken to be 50% as was done in the kinematic cross section analysis and the original 190 pb-1 analysis, due to our lack of confidence in our model and normalization method. With our QCD veto cuts it turns out to change the fit by a negligble amount whether we constrain QCD or let if loat. The uncertaintiy is represented by a Gaussian-constrained parameter in the likelihood. The QCD background has a negligible effect on the t' limit.
Integrated Luminosity

The integrated luminosity is taken to be 5.9% and is represented by an additional gaussian-constrained parameter multiplying all contributions except for the QCD background, which is normalized from data.
Lepton ID

We have two components for lepton ID, the efficiencies for individual electrons and muons and the uncertainty on the lepton ID efficiency data / MC scale factor.

To account for the efficincies for individual electrons and muons we multiply each lepton type by the associated efficiency and gaussian constrain it within the error on the efficiency. To account for the lepton ID efficiency data/MC scale factor, which is of 2%, and taken as correlated across lepton types, we add it in quadrature with the luminosity error, which is also correlated across lepton types, and include it with a gaussian constraint into the likelihood.
PDF Uncertainty

The Parton Distribution Functions (PDFs) are not precisely known, and this uncertainty leads to a corresponding uncertainty in the predictd cross sections, as well as the acceptance. This effect is evaluated on both the top and t' MC samples. The method consists in re-weighting the existing MC samples by the relative PDF weights given the parton momentum fractions and Q2 of the generated interaction.

The final PDF uncertainties are given for each t' mass point as well as for top below. A common conservative systematic error is added in quadrature to all other multiplicative factors and it is taken as 1.1% for all templates.
mass
 positive
uncertainty
 negative
uncertainty
top
175
 +0.007
 -0.008
tprime
180
 +0.007
 -0.008
200
 +0.004
 -0.005
220
 +0.005
 -0.005
240
 +0.003
 -0.003
260
 +0.003
 -0.003
280
 +0.002
 -0.003
300
 +0.001
 -0.003
320
 +0.001
 -0.002
340
 +0.002
 -0.002
360
 +0.003
 -0.002
380
 +0.002
 -0.002
400
 +0.005
 -0.002
450
 +0.004
 -0.005
500
 +0.015
 -0.013

Theory Uncertainty

The theory uncertainty in the t' cross section is about 10%, mainly due to uncertainty in PDFs (~ 7%). The other effect comes from uncertainty in the choice of the scale. We take theory uncertainty in the top pair cross section as fully correlated with the one of t' pair and introduce it into the likelihood as a single nuissance parameter.

Results and Conclusions
We tested the sensitivity of our method by drawing pseudoexperiments from standard model distributions, i.e. assuming no t' contribution. The ranges of expected 95% confidence level upper limits with one and two standard deviation bandwidths are shown above along with the associated upper limit on the t' mass. Of course this was given that the true top mass is 175 GeV and our measurement of the top mass may have been affected by the presence of a higher mass t' and thus we should treat these conclusions with care. We checked the coverage of our 95% confidence level limit method for one t' mass, 300 GeV, as a function of the signal cross section. The method over-covers by a few percent for all cross-sections.

Other results follow.

Distributions of HT (left) and Mreco (right) showing result of the no signal fit. The normalizations of the various sources and distortions of kinematic distributions due to systematic effects are those corresponding to the maximum likelihood when the cross section for t' is set to its 95% CL upper limit.
Ht-nosig
 mrec-nosig
(EPS)
 (EPS)
merged_194323_9830702
(EPS)
(jpg)
COT View Only
 RZ View Only
(EPS)
(gif)
 (EPS)
(gif)

merged_199187_607250
(EPS)
(jpg)
COT View Only
 RZ View Only
(EPS)
(gif)
 (EPS)
(gif)


merged_199983_1321408
(EPS)
(jpg)
COT View Only
 RZ View Only
(EPS)
(gif)
 (EPS)
(gif)


merged_231294_5485839
(EPS)
(jpg)
COT View Only
 RZ View Only
(EPS)
(gif)
 (EPS)
(gif)
merged_232062_1502182
(EPS)
(jpg)
COT View Only
 RZ View Only
(EPS)
(Full-Size gif)
 (EPS)
(Full-Size gif)

merged_237972_15304994
(EPS)
(jpg)
COT View Only
 RZ View Only
(EPS)
(Full-Size gif)		 (EPS)
(Full-Size gif)

merged_244720_1368195
(EPS)
(jpg)
COT View Only
 RZ View Only
(EPS)
(Full-Size gif)		 (EPS)
(Full-Size gif)