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Search for Electroweak Single Top-Quark Production using Neural Networks with 2.7 fb-1 of CDF II data |
Dominic Hirschbühl, Jan Lück, Thomas Müller, Adonis Papaikonomou, Manuel Renz, Wolfgang Wagner |
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KIT, Universität Karlsruhe |
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Abstract | |
Results | |
Event Selection | |
Neural Network Input Variables | |
Templates for Combined Search | |
Templates for Separate Search | |
Systematic Uncertainties | |
Expected Significance for Combined Search | |
Binned Likelihood Fit to Data for Combined Search | |
Binned Likelihood Fit to Data for Separate Search | |
Observed Significance for Combined Search | |
To download a plot in .eps format, left-click on the plot. To view a plot with full resolution in .gif format , right-click and select "View Image." |
Abstract |
We report on a search for electroweak single top-quark production with CDF II data corresponding to 2.7 fb-1 of integrated luminosity. We apply neural networks to construct discriminants that distinguish between single top-quark and background events. Two analyses are performed, assuming a top-quark mass of 175 GeV/c2. |
In the first one, we combine s- and t-channel events to one single top-quark signal under the assumption that the ratio of the two processes is given by the standard model (SM). The expected significance under the assumption of a SM cross-section is determined to be 5.0 σ (p-value of 0.26 x 10-6). A binned likelihood fit to the data measures a single top-quark production cross-section of 2.1-0.6+0.7 pb. The observed p-value is 103.94 x 10-6 which corresponds to a significance of 3.7 σ. |
In the second analysis, we separate the two single top--quark production modes, namely s- and t-channel. A binned likelihood fit done simultanously to two-dimensional and one-dimensional distributions of neural network outputs yields most probable values for the cross sections of 2.0-0.7+0.7 pb for the s-channel and 0.8-0.5+0.6 pb for the t-channel production mode. |
Event Selection | |
The CDF event selection exploits the kinematic features of the signal final state, which contains a top quark, a bottom quark, and possibly additional light quark jets. To reduce multijet backgrounds, the W boson originating from the top quark is required to decay leptonically. One therefore demands a single high-energetic electron (ET(e) > 20 GeV) or muon (PT(μ) > 20 GeV/c) and large missing transverse energy (MET > 25 GeV) from the undetected neutrino. |
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The backgrounds belong to the following categories: Wbb, Wcc, Wc, mistags (light quarks misidentified as heavy flavor jets), top pair production tt events (one lepton or two jets are lost due to detector acceptance), non-W (QCD multijet events where a jet is erroneously identified as a lepton), Z→ll and Diboson WW, WZ, and ZZ. We remove a large fraction of the backgrounds by demanding exactly two jets with ET > 20 GeV and |η| < 2.8 be present in the event. At least one of these two jets has to be tagged as a b-quark jet by using displaced vertex information from the silicon vertex detector (SVX). The non-W content of the selected dataset is further reduced by several requirements to MET, MET significance, transverse W boson mass, and several angles between the MET vector, lepton vectors and jet vectors. |
Neural Network Input Variables | ||
Using neural networks, kinematic or event shape variables are combined to a powerful discriminant. In the combined search we use four different networks in our analysis, one for the 2jet1tag category, one for 2jet2tag events, one for 3jet1tag events, and one for 3jets2tags. We devide each of the four categories into two separate channels, one containing triggered electrons and muons called Triggered Lepton Coverage (TLC), and the other containing muons from an Extended Muon Coverage (EMC) accepted through the MET + 2 jets trigger. For the separate search we include an additional network in the 2jet1tag category to build a 2D discriminant. This improves the apriori sensitivity for s-channel of about 15%. | ||
One of the variables is the output of the KIT flavor separator. The KIT flavor separator gives an additional handle to reduce the large background components where no real b quarks are contained, mistags and charm-backgrounds. Both of them amount to about 50% in the W+2 jets data sample even after imposing the requirement that one jet is identified by the secondary vertex tagger of CDF. The following plots show the 14 variables for the TLC 2jet1tag channel. The plots in the third column show the variables in the "0 Tag" sample (for cross-check).
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Expected Significance for Combined Search | To compute the significance of a potentially observed signal, we perform a hypothesis test. Two hypotheses are considered. The first one, H0, assumes that the single-top cross section is zero (β1 = 0) and is called the null hypothesis. The second hypothesis, H1, assumes that the single-top production cross section is the one predicted by the standard model (β1 = 1). The objective of our analysis is to observe single-top, that means to reject the null hypothesis. The hypothesis test is based on the Q-value, Q= -2(ln Lred(β1=1) - ln Lred(β1=0)) , where Lred(β1=1) is the value of the reduced likelihood function at the standard model prediction and Lred(β1=0) is the value of the reduced likelihood function for a single-top cross section of zero. Using the two ensemble tests the distribution of Q-values is determined for the case with single-top included at the standard model rate, q1, and for the case of zero single-top cross section, q0. The two Q-value distributions are shown below. In order to quantify the probability for the null hypothesis to be correct we define the p-value, often also named 1-CLb. To quantify the sensitivity of our analysis we define the expected p-value pexp = p(Q1med) where Q1med is the median of the Q-value distribution q1 for the hypothesis H1. The meaning of pexp is the following: Under the assumption that H1 is correct one expects to observe pexp with a probability of 50%. We find pexp = 0.26 x 10-6, including all systematic uncertainties. In other words, assuming the predicted single-top cross section, we expect, with a probability of 50%, to see at least that many single-top events that the observed excess over the background corresponds to a 5.0σ background fluctuation. |
Binned Likelihood Fit to Data for Separate Search | |
The templates for all channels are fitted simultaneously in all eight channel to the observed distributions using a binned likelihood function. The fit yields a s-channel single top-quark production cross section of 1.6-0.9+0.8 pb for the s-channel and 0.8-0.6+0.7 pb for the t-channel production mode. Below you find the resulting likelihood as a function of the s- and t-channel cross section.
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Observed Significance for Combined Search | |
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Our single top-quark results were approved (blessed) by CDF on Thursday 7/24/2008. |