The Tevatron experiments D0 and CDF have found evidence for single top quark
production, based on datasets between 0.9fb-1 and 2.2fb^-1. Several different
multivariate techniques are used to extract the single top quark signal out of
the large backgrounds. The cross section measurements are also used to provide
the first direct measurement of the CKM matrix element |Vtb|.Comment: 6 pages, proceedings for the Rencontres de Moriond EW 200

Schwienhorst, Reinhard

English

arXiv

The Tevatron experiments D0 and CDF have found evidence for single top quark production, based on datasets between 0.9 fb{sup -1} and 2.2 fb{sup -1}. Several different multivariate techniques are used to extract the single top quark signal out of the large backgrounds. The cross section measurements are also used to provide the first direct measurement of the CKM matrix element |V{sub tb}|

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Single top quark production at the TevatronR. SchwienhorstaMichigan State University, 3234BPS, East Lansing, MI 48824, USAThe Tevatron experiments D0 and CDF have found evidence for single top quark production,based on datasets between 0.9 fb−1 and 2.2 fb−1. Several different multivariate techniques areused to extract the single top quark signal out of the large backgrounds. The cross sectionmeasurements are also used to provide the first direct measurement of the CKMmatrix element|Vtb|.1 IntroductionEvidence for single top quark production at the Tevatron and a first direct measurement ofthe CKM matrix element |Vtb| was first reported by the D0 collaboration1. In contrast to topquark pair production through the strong interaction, which was observed in 1995 2,3, singletop quarks are produced via the weak interaction. The Feynman diagrams for standard model(SM) s-channel (tb) and t-channel (tqb) single top quark production are shown in Fig. 1. Thereis third production mode, associated production of a top quark and a W boson, but its crosssection is so small that it will not be considered further. The SM cross section for the s-channelprocess pp¯→tb¯+X, t¯b+X is 0.88± 0.14 pb at NLO for mtop = 175 GeV4,5. At the same orderand mass, the cross section for the t-channel process pp¯→tqb¯+X, t¯q¯b+X is 1.98 ± 0.30 pb 4,6.Measuring the single top quark production cross section provides a direct measurement ofthe CKM matrix element |Vtb|. The single top quark final state also allows for studies of the topquark polarization, and it is sensitive to many models of new physics, for example flavor changingneutral currents via the gluon 7 or heavy new bosons W ′ that only couple to quarks 8. The s-channel process is also an important background to Higgs searches in the associated productionmode, and the advanced analysis techniques used in the single top searches will be applicableto Higgs searches as well.aOn behalf of the D0 and CDF collaborations.FERMILAB-CONF-08-129-EqW+b¯tq¯′Wq q′tb(b)Figure 1: Feynman diagrams for s-channel (left) and t-channel (right) single top quark production at the Tevatron.The D0 collaboration has updated two of their analysis methods using a dataset of 0.9 fb−1.The updated results, including a combination of different methods are presented below. TheCDF collaboration has analyzed a dataset of 2.2 fb−1 and significantly improved the sensitivityto single top quark production. These new results are presented below.2 D0 results2.1 Event selectionThe D0 analysis selects electron+jets and muon+jets events in 0.9 fb−1 of data with the followingrequirements: One high-pT lepton (electron (pT > 15 GeV ) or muon (pT > 18 GeV )), missingtransverse energy 6ET > 15GeV , and between two and four jets with jet pT > 15 GeV andjet 1 pT > 25GeV , at least one is tagged with a neural-network based b-tagging algorithm.Additional cuts remove fake-lepton background events. Events are collected by lepton+jetstrigger requirements.The number of events observed in data and expected from the background model and SMsignal is shown in Table 1. The largest sources of systematic uncertainty are the backgroundnormalization, jet energy scale, as well as b-tag and trigger modelling.Table 1: Numbers of events expected by D0 in 0.9 fb−1 for electron and muon, 1 b-tag and 2 b-tag channelscombined.2 jets 3 jets 4 jetss-channel 16±3 7±2 2±1t-channel 20±4 12±3 4±1tt¯ 59±14 134±32 155±36W+jets 531±129 248±64 70±20Multijets 96±19 77±15 29±6Total background 686±131 460±75 253±42Data 697 455 246Table 1 shows that after selection cuts, the expected SM single top signal is small comparedto the background sum, and in fact the signal is significantly smaller than the backgrounduncertainty. Thus, more advanced techniques are required to extract the signal.2.2 Multivariate techniquesThe D0 analysis employs three different multivariate techniques to extract the single top quarksignal out of the large backgrounds. The boosted decision tree (BDT) analysis has not changedsince the publication of evidence for single top quark production7. Here we focus on the Bayesianneutral network analysis and the matrix element analysis, both of which have been re-optimized.BNN output0 0.2 0.4 0.6 0.8 1Event Yield / 0.05050100150Event Yield / 0.05 -1DØ 0.9 fbtb+tqbttbarW+jetsfake-leptonBNN output0.6 0.7 0.8 0.9 1Event Yield / 0.0510210Event Yield / 0.05Figure 2: Comparison between data and background sum for the Bayesian neural network output. Shown isthe full distribution (left), and the high-discriminant region (right). The signal has been normalized to the SMexpectation.In a conventional neural network, the network parameters and weights are determined in anoptimization (training) procedure. Rather than optimizing for these network parameters onceand then fixing them, the optimal network configuration can be obtained as an average overmany different values for the network parameters. In this Bayesian procedure, an integrationover all of the possible network parameter space is performed. The network architecture is fixed,and the weight of each set of parameters is obtained through a Bayesian integration. The finalnetwork discriminant is then the weighted average over all the individual networks. Fig. 2 showsthe output of the BNN for the D0 data.The Matrix element analysis starts from the Feynman diagrams for the single top quarkprocesses and uses transfer functions to relate the parton level quark-level information to thereconstructed jet and other information. Matrix elements for the single top quark signal aswell as the W+jets backgrounds are included. For 3-jet events, a top pair matrix element isalso included. For each event, an integration over the phase space is performed, employing thetransfer functions to compute the probability for this particular event to arise from a specificmatrix element. A likelihood function is then formed as the ratio of the signal and signal plusbackground probabilities.2.3 D0 summaryThe cross section is measured as the peak of the Bayesian posterior probability density, shownin Fig. 3 for the ME analysis. The three different methods measure the following cross sectionsfor the sum of s- and t-channel:σobs (pp¯ → tb+X, tqb+X) = 4.9+1.4−1.4 pb (DT)= 4.4+1.6−1.4 pb (BNN)= 4.8+1.6−1.4 pb (ME).The measured cross sections are consistent with each other and above the SM expectation.The decision tree analysis has also measured the s- and t-channel cross sections separately,σobs (pp¯ → tb+X) = 1.0 ± 0.9 pbσobs (pp¯ → tqb+X) = 4.2+1.8−1.4 pb,where the standard model cross section is used for the single top process not being measured.Removing the constraint of the standard model ratio allows to form the posterior probabilitydensity as a function of both the tb and tqb cross sections. This model-independent posterior isshown in Fig. 3 (right)for the DT analysis, using the tb+tqb discriminant. The most probablevalue corresponds to cross sections of σ(tb) = 0.9 pb and σ(tqb) = 3.8 pb. Also shown are pb-1.4+1.6 = 2.8 expσ  pb-1.4+1.6 = 4.8 obsσ Matrix Elementstb+tqb cross section [pb]Posterior Prob. Density  [pb 1]D     0.9 fb 10.30.20.100      1      2      3      4      5      6      7      8      9     10s-channel cross section [pb]t-channel cross section [pb]D    0.9 fb 1SM1 std. dev.2 std. dev.3 std. dev.1098765432100     1     2     3     4     5     6     7     8     9    10Figure 3: Posterior probability density for the matrix element analysis as a function of the sum of s-channel andt-channel cross sections (left), and for the BDT analysis as a function of both the s-channel and t-channel crosssections (right).the one, two, and three standard deviation contours. While this result favors a higher value forthe t-channel contribution than the SM expectation, the difference is not statistically significant.Several models of new physics that are also consistent with this result are shown in Ref.9. Theseupdated results have recently been published 10.3 CDF results3.1 Event selectionThe CDF analysis selects electron+jets and muon+jets events in 2.2 fb−1 of data with thefollowing requirements: One high-pT lepton (pT > 20 GeV ), 6ET > 25GeV , and two or three jetswith jet pT > 20 GeV , at least one of which is tagged by a displaced vertex tagging algorithm.Additional cuts remove fake-lepton background events. Events are collected by single-leptontrigger requirements. The matrix element analysis uses additional triggers in the muon channelto increase the acceptance.The number of events observed in data and expected from the background model and SMsignal is shown in Table 2. The largest sources of systematic uncertainty are the backgroundnormalization, jet energy scale, and b-tag modelling. Again, it is clear that a advanced analysisTable 2: Numbers of events expected by CDF in 2.2 fb−1 for electron and muon, 1 b-tag and 2 b-tag channelscombined.2 jets 3 jetss-channel 41±6 14±2t-channel 62±9 18±3tt¯ 146±21 339±48W+bottom 462±139 141±43W+charm 395±122 109±34W+light 340±56 102±17Z+jets 27±4 11±2diboson 63±6 22±2Multijets 60±24 21±9Total background 1492±269 755±91Data 1535 752techniques are required to extract the signal.tchan 2jetL0 0.2 0.4 0.6 0.8 1Events10210310Events s-channelt-channelWCC/WCttbarWbbW+LFz+jetsDibosonnon-W-1CDF Run II Preliminary, 2.2 fbNN Output-1 -0.5 0 0.5 1Candidate Events050100150200Candidate Events-1CDF II Preliminary 2.2 fbMC normalized to fit values W+2/3 Jets0.4 0.6 0.8 101020304050lightη •Q  -2 0 2Candidate Events0102030Candidate EventsW+2Jets 1Tag -1CDF II Preliminary 2.2 fbMC normalized to fit values single topttc+WcbWbWcMistagsDibosonZ+jetsdataFigure 4: Comparison between data and background sum for the t-channel likelihood discriminant (left), theneural network discriminant (center), and the light quark jet pseudorapidity in the high-discriminant region forthe neural network analysis (right). The signal has been normalized to the SM expectation.3.2 CDF Likelihood FunctionA multivariate likelihood is built from several kinematic variables that each separate the singletop quark signal from the backgrounds. One special variable is a specially developed b-taggingneural network that aids in separating b-quark jets from light quark and c-quark jets. Anadditional special variable is a kinematic solver using constraints from the W boson mass andthe top quark mass to determine if an event is well reconstructed. Another special variable isthe t-channel matrix element, which uses the kinematic information provided by the kinematicsolver. The likelihood discriminant for the t-channel likelihood is shown in Fig. 4 (left).The measured cross section is obtained as the peak of a Bayesian posterior probability. Thelikelihood analysis measures a cross section of σ(tb+tqb) = 1.8+0.9−0.8 pb, below the SM expectation.3.3 CDF Neural NetworkSeveral kinematic variables as well as the b-tagging neural network output are combined in aneural network. Four different networks are built with 10-14 variables each, trained separatelyfor 2-jet and 3-jet as well as 1-tag and 2-tag events. The full neural network output distributionis shown in Fig. 4 (center), and the signal region is shown in Fig. 4 (right). The neural networkanalysis measures a cross section of σ(tb + tqb) = 2.0+0.9−0.8 pb, below the SM expectation butconsistent with the SM within uncertainties.3.4 CDF Matrix ElementThe matrix element analysis uses the same approach as described above, but also includes a toppair matrix element in the 2-jet bin. The matrix element for top quark pair events has more finalstate particles than the single top process, and these additional particles have to be integratedout. This is done by integrating over the kinematics of the hadronically decaying W -boson in alepton+jets top pair event.The Bayesian posterior probability density for the Matrix element analysis is shown in Fig. 5,showing the measured cross section and the measurement uncertainty. The mesured cross sectionis σ(tb+ tqb) = 2.2+0.8−0.7 pb, again below the SM expectation but consistent with the SM withinuncertainties. The CKM matrix element |Vtb| is also extracted from the posterior probabilityand a lower limit is found to be |Vtb| > 0.59 at the 95% confidence level.Event Probability Discriminant0 0.2 0.4 0.6 0.8 1Candidate Events050100150200250300Candidate EventsSingle topb-likec-likeMistagstt-barNormalized to Prediction-1CDF Run II Preliminary, L=2.2 fb0.7 0.75 0.8 0.85 0.9 0.95 10204060Single Top Cross Section [pb]0 2 4 6 8 10Posterior Probability Density68% pb -0.7+0.8 = 2.2Single Topσ-1CDF Run II Preliminary, L=2.2 fbPosterior Probability DensityFigure 5: Data-background comparison for the matrix element discriminant (left) and Bayesian posterior densitydistribution observed by the Matrix element analysis.4 SummaryBoth Tevatron experiments have found better than 3 sigma evidence for single top quark pro-duction and have made the first direct measurement of the CKM matrix element |Vtb| usingadvanced multivariate techniques. The CKM matrix element |Vtb| can be measured to betterthan 15%. Further improvements to the analyses are in progress and both experiments areworking towards observation of single top quark production at the 5 sigma level.AcknowledgmentsWe thank the Fermilab staff and the technical staffs of the participating institutions for theirvital contributions.References1. V.M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 98, 181802 (2007).2. F. Abe et al. (CDF Collaboration), Phys. Rev. Lett. 74, 2626 (1995).3. S. Abachi et al. (D0 Collaboration), Phys. Rev. Lett. 74, 2632 (1995).4. Z. Sullivan, Phys. Rev. D 70, 114012 (2004).5. Q. H. Cao, R. Schwienhorst and C. P. Yuan, Phys. Rev. D 71, 054023 (2005) [arXiv:hep-ph/0409040].6. Q. H. Cao, R. Schwienhorst, J. A. Benitez, R. Brock and C. P. Yuan, Phys. Rev. D 72,094027 (2005) [arXiv:hep-ph/0504230].7. V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 99, 191802 (2007) [arXiv:hep-ex/0702005].8. V. M. Abazov et al. (D0 Collaboration), accepted by Phys. Rev. Lett, arXiv:0803.3256[hep-ex].9. T. Tait and C.P. Yuan, Phys. Rev. D 63, 014018 (2001).10. V. M. Abazov et al. (D0 Collaboration), accepted by Phys. Rev. D, arXiv:0803.0739[hep-ex].

Single top quark production at the Tevatron

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Single top quark production at the Tevatron

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