Measuring the sign of the top charge using the top decay products

Veronique Boisvert1, Kevin McFarland1
Zeynep Gunay Unalan2, Veronica Sorin2, Kirsten Tollefson2
Yen-Chu Chen3,Jaroslav Antos4
Peter Bednar5, Stanislav Tokar5,Andy Beretvas6

1 University of Rochester, 2Michigan State University, 3Academia Sinica
4 Slovak Academy of Science,5Comenius University Bratislava,6FNAL

Public Conference Note

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Since the discovery of the top quark, CDF has measured several properties of those events to confirm that the top quark has the properties expected in the standard model (SM) , as yet undone is measuring the top charge. Determining whether the top decays into a W+ and a bottom quark while the anti-top quark decays to a W- and an anti-bottom quark would ensure indirectly that the charge of the top quark is indeed +2/3 as is the charge of the top quark in the standard model. If these events were found to have an object decaying to a W- and a bottom quark, the charge of this object would be -4/3 and would not correspond to the standard model top quark. Such a proposal has been put forward by D.Chang, W. Chang and E. Ma (see references). .
We measured the sign of the top charge using the products of the top decay in t -> Wb. Three are three main components to this measurement: determining the charge of the W (using the charge of the lepton), getting the flavor of the b-jet and finally pairing the W with the b jet to ensure W and the b jet come from the same top decay branch. Using 1.5fb-1 of data ( Dilepton(DIL) and Lepton_Jets channel (L+J) ) and defining the probability of incorrectly rejecting the SM to be 1%, we found the result to be consistent with the SM, while excluding the Exotic quark hypothesis (XM) with 87% confidence.

The gif version of the plots can be obtained with a right-click and "save image as",the eps version by clicking on the eps link beside the plot.

Pairing: To find the right association between the lepton and b jet, we make use of the invariant mass of the lepton b-jet pair, Mlb 2 in Dilepton channel and we use top mass &chi2 fitter in Lepton+Jets channel.See the public note for the details. How often each method gives the right pairing is the purity of pairing,ppairing.
ppairing       95% (DIL)       86% (L+J)
Efficiency       39% (DIL)       53% (L+J)
Jet Charge: On the right is the Weighted jet charge algorithm which uses the charge of the tracks associated to the jet weighted by their momentum projection on the jet axis.This algorithm has been optimized to determine the flavor of b jets in high Pt enviroment. How often this algorithm gives the right flavor in MC is the purity of Jet charge, pJQ.
pJQ       61% (DIL)       61% (L+J)
Efficiency       86.3% (DIL)       98% (L+J)

MC performance can not be relied on therefore purity of jet charge was calibrated in data as explained below.
Combining right pairing with the Jet charge information, we get
N+ = number of SM like events with top charge +2/3
N- = number of XM like events with top charge -4/3
Calibration of Jet Charge Algorithm in Data

Performance of the Jet Charge (JQ) algorithm is calibrated using dijet data on selected b-bbar events where one of b's decay semileptonically to a muon. The plot shows the observed purity calculated as the fraction of the total events for which the muon and the JQ of the away jet have opposite sign. Red points corresponds to the corrected purity which takes into account the amount of non b-bbar events present in the sample, secondary decays and mixing.

Scale Factor between the data corrected purity and the JQ purity calculated on b-jets selected from a Heavy Flavor enriched MC (Pythia) sample. Shown is the constant fit.
SFJQ = 1.01 ± 0.01 (stat.) ± 0.02 (sys.)

Systematic Uncertainties on SF.

The expected number of background and signal events after event selection and pairing requirements. The numbers are obtained by multiplying the predictions (using a top cross section of 6.7pb) by the corresponding total efficiency (pairing efficiency times jet Q efficiency).

The expected SM like and Exotic Model (XM) like events for background and signal. These numbers are obtained as the product of the expected number of events shown in previous table and their corresponding purities.

eps Expected number of signal and background pairs and their purities. There are two pairs per event for each event containing a top and and anti-top.
Getting Signal Purity

Ps = fnonb SFnonb pnonb + (1-fnonbSFnonb)(ppair pJQ SFJQ + (1-ppair)(1-pJQSFJQ))
Above is the definition of Signal Purity, Ps.The measured jet charge purity in MC is corrected by the SFJQ. The measured fraction of nonb in MC,fnonb, is corrected by the mistag rate, SFnonb, between data and MC.

Summary of Systematic Uncertainties.


124 SM like pairs and 101 XM like pairs have been observed in data.
f+ = fraction of pairs with top charge +2/3
Using a Profile Likelihood, and the above nuisance parameters, the Log Likelihood curve for the observed N+ and N- . The minimum of this curve is at a value of 0.87.
Distribution of the fraction of SM like pairs (f+) assuming either the Exotic or the Standard Model. Indicated is the measured f+ value of 0.87 which corresponds to a p-value of 0.31 .
p-value distribution for the SM assuming the XM is true. Indicated is the a priori alpha value of 1% (probability of incorrectly rejecting the SM if SM is true) and its corresponding beta value or Power of Test (probability of rejecting the SM if XM is true).
Distribution of the fraction of SM like pairs (f+) assuming either the Exotic or the Standard Model where indicated in blue is the range of f+ values with corresponding p-value less than or equal to the a priori chosen alpha value of 1%. The f+ value corresponding to 1% is 0.32.
On the left is the plot showing the Feldman Cousins bands. The different colors indicate different confidence limits (yellow corresponds to 68%, green to 90% and red is the 95% CL). Given the measured f+ value of 0.87, we obtained the following limits :
f+>0.4 @95% CL
f+>0.46 @90% CL
f+>0.6 @68% CL
Bayes Factor = P(N+ | SM) / P(N+ | XM) = odds of SM versus XM
2.Ln(BF) Evidence Against H0
0-2 Not worth than a bare mention
2-6 Positive
6-10 Strong
>10 Very Strong
Observed 2*Ln(BF) = 12.01.
Based on Bayes Scale, 12.01 means "data favors very strongly SM over XM".
On the left is the plot showing the probability for both SM and XM with given total number of 225 (124 SM + 101 XM) events. Pcomb is the combined purity taking the background into account. Given that the probability of XM and SM with respect to the total number of events can be calculated. If we present &alpha = 0.01 then the boundary for us to claim SM is true is shown by the green vertical line. One can read from the plot that it is 115 or so. Since we observe 125 SM Top pairs in data ( that exceeds the 115 ), the CDF data supports SM strongly based on the 2.Ln(BF) = 12.01.
Product of the W charge and the associated jet charge for Data and MC ("+2/3 Q" corresponds to the SM signal MC distribution for WQ*JQ). A negative value corresponds to a SM like pair.

For speakers who are likely to mention also the D0 result if they are covering Tevatron results

Since CDF and D0 do not calculate the confidence limits in the same way a direct comparison of their results is not possible. What can be compared is their sensitivity. On the left is the &beta versus &alpha plot. The first two blue points are the &beta at 1% corresponding to CDF's limit and &beta at 5%. The third is the value of &beta (99.9%) at &alpha of 50% which approximately corresponds to the D0 quoted sensitivity of 91.2%.

The following number CAN NOT appear on an official conference slide, it is just provided as extra information in case the speaker gets a question

D0 reports a p-value under the exotic quark model hypothesis, this value is 7.8%. They turn this p-value into a confidence limit of 92.2% of excluding the exotic quark hypothesis. CDF computes their confidence limit in a different way and use the SM as their null hypothesis, but nonetheless their p-value under the exotic quark model hypothesis is 0.2%. "

Results in 1.5fb-1
Observed f+=0.87
p-value =0.31  
Since the p-value under the SM hypothesis is 0.31, this is greater than the a priori chosen value of &alpha 0.01, so we exclude the exotic quark model with 87%. confidence. Based on the Bayes Factor value, we conclude that the data favors very strongly the standard model top quark hypothesis over the exotic quark hypothesis.


Pairing Related Plots

The left plot shows the &chi2 distribution after L+J cuts and double tagging. The right plot is the distribution of the maximum value from the 4 Mlb 2 reconstructed per event. A cut at 21000Gev/c2 is applied to select the final candidate sample.
Jet Charge Related Plots
(number and transverse momentum of the tracks per jet that were used to perform the JQ calculation)






  • "Alternative Interpretation of the Tevatron Top Events", D. Chang et, al., hep-ph/9810531.

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