The large data samples of thousands of top events collected at the Tevatron experiments CDF and D{O} allow for a variety of measurements to analyze the properties of the top quark. Guided by the question ''Is the top quark observed at the Tevatron really the top quark of the standard model,'' we present Tevatron analyses studying the top production mechanism including resonant t{bar t} production, the V -A structure of the t {yields} Wb decay vertex, the charge of the top quark, and single-top production via flavor-changing neutral currents

Husemann, Ulrich

U., /Yale

UNT Digital Library

MEASUREMENTS OF TOP PROPERTIES AT THE TEVATRONU. HUSEMANN on behalf of the CDF and DØ CollaborationsDepartment of Physics, Yale University, P.O. Box 208120, New Haven, CT 06520–8120, USAThe large data samples of thousands of top events collected at the Tevatron experimentsCDF and DØ allow for a variety of measurements to analyze the properties of the top quark.Guided by the question “Is the top quark observed at the Tevatron really the top quark ofthe standard model,” we present Tevatron analyses studying the top production mechanismincluding resonant tt production, the V −A structure of the t→Wb decay vertex, the chargeof the top quark, and single-top production via flavor-changing neutral currents.Keywords: Hadron Collider, Tevatron, Heavy Quark Production, Top Quark PropertiesFERMILAB-CONF-07-209-E1 IntroductionAt the Tevatron collider at Fermi National Accelerator Laboratory, protons and antiprotons arecollided at a center-of-mass energy of 1.96 TeV. The Tevatron provides the highest energiescurrently available at a hadron collider and is the only collider with sufficient energy to producetop quarks. The two multi-purpose experiments at the Tevatron, CDF II 1 and DØ 2, havestudied production and decays of top quarks in great detail. The focus of this article will be onmeasurements of top properties at CDF and DØ (other than mass and cross section). The eventselection for top properties measurements is built on the experience gained in the mass andcross section analyses in the “lepton+jets” channel (tt→WbWb→ `νb qqb) and the “dilepton”channel (tt → WbWb → `νb `νb). Also the background composition, with W production inassociation with jets as the dominant source, and systematic uncertainties, mainly coming fromdetermining the jet energy scale, are similar to those in the top mass and cross section analyses.2 Top Pair Production2.1 Fraction of tt Pairs Produced via Gluon-Gluon FusionIn the standard model (SM), the production of tt pairs at the Tevatron is dominated by theprocess of qq annihilation. The contribution of the gg fusion channel to the tt productioncross section amounts to 15± 5%, where the large uncertainty is due to poor knowledge of thegluon parton distribution function inside the proton 3,4. The CDF collaboration has developedtwo complementary methods to measure the fraction of tt pairs produced via gg fusion. Withdatasets of up to 1 fb−1 of integrated luminosity, both methods are dominated by statisticaluncertainties and are therefore expected to improve with more data.One method 5 utilizes an artificial neural network (NN) to distinguish the processes qq → tt,gg → tt, and qq/gg →W+jets. The NN input comprises the velocity and angle of the top quark,and the angles of the three decay products in the off-diagonal spin basis. From the resultingNN discriminant, an upper limit on the gg fraction is derived via a Feldman-Cousins methodthat includes systematic uncertainties. As shown in Fig. 1a, the measurement yields a limit ofσ(gg → tt)/σ(pp→ tt) < 0.51 at 95% C.L.fFitter C–0.4  0 0.4 0.8 1.2True gg Fraction00.20.40.60.81Feldman Cousins68% C.L.95% C.L.Best ValueCDF Run II (Preliminary)–1Luminosity 695 pbMeasured Value = –0.275Limit at 68% CL: < 0.23Limit at 95% CL: < 0.51(a) tracks10 20 30 40048121620 1 fb≈ L dt ∫CDF Run II PreliminaryTagged W+4 jet dataNo-gluon + gluon-richNo-gluon contributionGluon-rich contributionNumber of low pTNumber of events–10(b)Figure 1: Measurements of the gg fraction in tt production. (a) Feldman-Cousins band obtainedfor neural network discriminant method. (b) Distribution of low-pT tracks fitted by gluon-richand no-gluon templates in W+4-jet data.The other method 6 is based on the observation that the number of tracks with small trans-verse momenta pT in the range of 0.3–2.9GeV/c is strongly correlated with the number of gluonsin the event. The correlation is calibrated using gluon-rich and gluon-free control samples inthe data, and then extrapolated to the top-rich sample of W+4 jets. Gluon-rich and no-gluontemplates are fitted to the distribution of low-pT tracks in the data, as depicted in Fig. 1b, toextract a gg fraction of σ(gg → tt)/σ(pp→ tt) = 0.01± 0.16(stat.)± 0.07(syst.).2.2 Resonant Production of tt PairsSeveral extensions of the SM predict tt production from the decay of heavy particles. In oneparticular model 7, the heavy particle is assumed to be a narrow Z ′ resonance that couplesstrongly only to third generation quarks and does not couple to leptons (“leptophobic Z ′”).The CDF collaboration has studied the production of a Z ′-like heavy tt resonance 8. The ttinvariant mass Mtt is reconstructed with the help of a kinematic fitter developed for top massmeasurements. The Mtt distribution in the data is compared to the expectation for SM tt pro-duction and non-tt background. The background includes W+jets events, QCD multijet events,and diboson production (WW , WZ, ZZ), and was estimated with a method that combinesinput from Monte Carlo simulations and data control samples. The data are compatible withSM tt production, so that a limit on the Z ′ production cross section can be derived, see Fig. 2a.From the comparison with the cross section prediction for leptophobic Z ′, CDF obtains a limitof MZ′ > 725 GeV/c2 at 95% C.L.3 Helicity of W Bosons from Top DecaysDue to its small lifetime of less than 10−24 s, the top quark decays before it hadronizes. As aconsequence, the complete spin information is transferred to its daughter particles. In the SM,top quarks decay to a W boson and a b quark almost exclusively. The spin-1 W boson has threepossible helicity states: longitudinal, left-handed, and right-handed. The V −A structure of thetWb vertex in the SM does not allow for a right-handed state, so that any sizable admixture ofright-handed W ’s would be a sign of new physics. The longitudinal fraction of the W helicityin the SM is determined by the Yukawa coupling of the top quark and amounts to f0 ≈ 0.7.Both CDF and DØ have measured the right-handed helicity fraction f+ of W bosons fromtop decays 9,10,11,12. The analyses use different techniques to reconstruct the observable cos θ∗,the angle between the top boost direction and the charged lepton in the W rest frame, or theMass of Z'-Like Resonance (GeV/c2)500 600 700 800 900 σZ’→ t! (pb)02468CDF Run II Preliminary, L=955 pb–1Expected Limit at 95% C.L.Expected Limit at 95% C.L. ± 1σ Topcolor Theoretical Cross SectionObserved limit at 95% C.L.Leptophobic Z’excluded at 95% C.L.(a)+fright-handed fraction –0.1 0 0.1W Helicity 0.10± 0.06  0.08±–0.02+0.06–0.03 0.10±–0.06L+J & Dilepton: DØL+J & Dilepton: CDFLepton+Jets: CDFL+J 2D: CDF(955 pb–1, prel.)–0.07(955 pb–1, prel.)(750 pb–1, PRL98 072001)(370 pb–1, PRD75 031102) (f0 fixed to 0.7)(f0 fixed to 0.7)(f0 fixed to 0.7)(f0 = 0.74 ± 0.25 ± 0.06)(b)Figure 2: (a) Limit on the production cross section of leptophobic Z ′ bosons. The shadedarea is excluded at 95% C.L. (b) Summary of W helicity measurements. Shown are fits to theright-handed helicity fraction f+ in different analyses.correlated quantity M`b, the invariant mass of the charged lepton and the b jet. Due to thelimited size of the data samples, most analyses 9,10,11 check the consistency of f0 with the SM,assuming f+ = 0, and fix the value to f0 = 0.7 before f+ is measured. With 1 fb−1 of data, asimultaneous fit to f0 and f+ becomes feasible 12. As shown in Fig. 2b, all measurements arecompatible with the SM prediction of f+ = 0.4 Top ChargeIn the SM, the top quark and the bottom quark form a left-handed isospin doublet with charges(2/3 e,−1/3 e)L, where e is the elementary charge. Fits to electroweak precision data can beimproved using a theoretical model with a top mass of 270 GeV/c2 and an exotic right-handedquark doublet with charges (−1/3 e,−4/3 e)R that mixes with right-handed b quarks 13. In thismodel, the exotic replacement for the top quark has a charge of −4/3 e.The CDF collaboration has performed a measurement to test if the top charge is 2/3 e or−4/3 e 14. The measurement consists of three steps. The W charge is obtained via the chargeof the lepton in the decay W → `ν. To reconstruct the top from the decay t → Wb, the Wis paired with a b jet, using a kinematic fitter (lepton+jets channel) or a cut on M`b (dileptonchannel). Finally, the observable “jet charge,” a weighted sum of the charge of all tracks thatform a jet, is employed to measure the flavor of the b jet. The product of W charge and jetcharge is used to distinguish the exotic model from the SM, see Fig. 3a.CDF has tested the consistency of the data with the SM and the exotic model with ahypothesis test, with the null hypothesis that the SM is correct. If the exotic model werecorrect, 81% of all measurements would return p-values below 0.01 (probability to incorrectlyreject the SM, chosen a priori). The measured p-value of 0.35 shows that the data are consistentwith the SM, and the exotic model is excluded at 81% C.L.5 Single-Top Production via Flavor-Changing Neutral CurrentsFlavor-changing neutral currents (FCNC) in the top quark sector are heavily suppressed in theSM. For example, the branching fraction B(t → gc) is approximately 5 × 10−12, far below thereach of present and future hadron collider experiments 15. Any FCNC signal at the Tevatronwould be a sign of physics beyond the SM.–1 –0.5 0 0.5 1051015202530CDF Run II preliminary L+J, L=695 pb–1DataSM tt SignalW+HFQCDDibosonMistagSingle TopEventsW Charge × Jet ChargeStandardModel-LikeExotic Model-Like(a)0 0.01 0.02 0.03 0.0400.0010.0020.003 95% CL90% CL68% CL–1DØ 230 pb( c / )2 (TeV–2)g(u/)2(TeV–2)g(b)Figure 3: (a) Top charge measurement via jet charge. The data are compared to the predictionsof the SM for tt signal and background. (b) Exclusion contours for the couplings (κ/Λ)2 of theflavor-changing neutral current vertices tgu and tgc.The DØ collaboration has searched for FCNC in the production of single top quarks 16. Inthe presence of the processes ug → t and cg → t, the single top production rate is enhanced.DØ has deployed a NN to discriminate the kinematics of the FCNC signal from tt, W+jets, andQCD multijet backgrounds. The data agrees very well with the SM prediction, so that an upperlimit on the FCNC couplings tgu and tgc could be derived. Fig. 3b shows the upper limit as afunction of the strengths (κ/Λ)2 of the tgu and tgc coupling, where κ is the coupling parameterin the Lagrangian, and Λ is a generic new physics scale. The DØ measurement improves uponprevious measurements, prominently by the HERA experiments, by a factor of 3–11.AcknowledgmentsIt is a pleasure to thank the organizers of Moriond QCD for creating a very relaxing andinspiring atmosphere at the workshop. This material is based upon work supported by the U.S.Department of Energy under Award Number DE-FG02-92ER40704.References1. D. Acosta et al. Phys. Rev., D71:032001, 2005.2. V. M. Abazov et al. Nucl. Instrum. Meth., A565:463–537, 2006.3. M. Cacciari et al. J. High Energy Phys., 04:068, 2004.4. N. Kidonakis and R. Vogt. Phys. Rev., D68:114014, 2003.5. CDF Collaboration. CDF Public Note 8725, 2007.6. CDF Collaboration. CDF Public Note 8724, 2007.7. C. T. Hill and S. J. Parke. Phys. Rev., D49:4454–4462, 1994.8. CDF Collaboration. CDF Public Note 8675, 2007.9. V. M. Abazov et al. Phys. Rev., D75:031102, 2007.10. A. Abulencia et al. Phys. Rev. Lett., 98:072001, 2007.11. CDF Collaboration. CDF Public Note 8368, 2006.12. CDF Collaboration. CDF Public Note 8380, 2006.13. D. Chang, W.-F. Chang, and E. Ma. Phys. Rev., D59:091503, 1999.14. CDF Collaboration. CDF Public Note 8782, 2007.15. J. A. Aguilar-Saavedra. Acta Phys. Polon., B35:2695–2710, 2004.16. V. M. Abazov et al. FERMILAB-PUB-07-031-E, submitted to Phys. Rev. Lett., 2007.

Measurements of Top Properties at the Tevatron

https://digital.library.unt.edu/ark:/67531/metadc882086/m2/1/high_res_d/909172.pdf

Measurements of Top Properties at the Tevatron

Abstract

Similar works

Full text

Available Versions

UNT Digital Library