The Tevatron proton-antiproton collider at Fermilab with its centre of mass
energy of 1.96 TeV allows for pair production of top quarks and the study of
top quark decay properties. This report reflects the current status of
measurements of the W boson helicity in top quark decays and the ratio of top
quark branching fractions as well as searches for neutral current top quark
decays and pair production of fourth generation t' quarks, performed by the D0
Collaboration utilising datasets of up to 5.4/fb.Comment: 5 pages. To be included in PANIC 2011 conference proceeding

Pleier, Marc-André

English

arXiv

The Tevatron proton-antiproton collider at Fermilab with its centre of mass energy of 1.96 TeV allows for pair production of top quarks and the study of top quark decay properties. This report reflects the current status of measurements of the W boson helicity in top quark decays and the ratio of top quark branching fractions as well as searches for neutral current top quark decays and pair production of fourth generation t' quarks, performed by the D0 Collaboration utilising datasets of up to 5.4 fb{sup -1}

Pleier, Marc-Andre

UNT (University of North Texas) Digital Library

arXiv:1108.5205v1  [hep-ex]  25 Aug 2011Searches for New Physics in Top Decays at D0Marc-André PleierBrookhaven National Laboratory, Physics Department,Omega Group, Building 510A, Upton, NY 11973On behalf of the D0 CollaborationAbstractThe Tevatron proton-antiproton collider at Fermilab with its centre ofmass energy of 1.96 TeV allows for pair production of top quarks and thestudy of top quark decay properties. This report reflects the current sta-tus of measurements of the W boson helicity in top quark decays and theratio of top quark branching fractions as well as searches for neutral cur-rent top quark decays and pair production of fourth generation t’ quarks,performed by the D0 Collaboration utilising datasets of up to 5.4 fb−1.1 IntroductionWith a mass of mt = 173.2 ± 0.9 GeV [1], the top quark is the most massivefundamental particle known to date. Top quarks are mainly produced in pairsvia the strong interaction, with a cross section of σtt̄ = 7.3+0.5−0.6 pb [2] in pp̄collisions at√s = 1.96 TeV. In the framework of the Standard Model (SM), topquarks decay almost exclusively into W bosons and b quarks. Consequently, topquark pair events contain a b and a b̄ quark from the tt decay, and the remainingfinal state to be observed depends on the decay mode of the two W bosons.The analyses presented here focus on two signatures: (i) events where bothW bosons decay leptonically, resulting in a final state containing two isolatedhigh-pT leptons, missing transverse energy 6ET corresponding to the two neu-trinos and two (b-) jets (dilepton events) and (ii) events where one W bosondecays leptonically, the other one hadronically, resulting in one isolated high-pTlepton, 6ET and four jets (lepton + jets events). As leptons only electrons andmuons are considered in these analysis channels, including those from leptonicτ decays.12 Searches for New Physics in Top Quark PairDecaysMeasurement of the W Boson HelicityTop quark decays in the SM proceed via the left-handed charged current weakinteraction, which results in a suppression of right-handed W bosons. Theexpected fractions of the three possible helicity states of the W boson are: lon-gitudinal fraction f0 ≈ 69%, left-handed fraction f− ≈ 31% and right-handedfraction f+ ≈ 10−3 [3]. The W boson helicity is reflected in the angular distri-bution cos θ∗ of its decay products, with θ∗ being the angle of the down-typedecay products of the W boson (charged lepton, d- or s-quark) in the W bosonrest frame relative to the top quark direction. Any observation of f+ at thepercent level would signal new physics beyond the SM.D0 measures the W boson helicity using a dataset corresponding to an inte-grated luminosity of 5.4 fb−1, using both lepton + jets and dilepton final statesof tt decays [4]. Since the measurement is still expected to be limited by sam-ple statistics, the event selection in each channel is optimised for tt acceptance,using multivariate likelihood discriminants based on event topology and objectkinematics. In the lepton + jets channel, the event kinematics is obtained fromthe best kinematic fit to the tt event hypothesis, which enables the calculation ofcos θ∗ for the leptonic W boson decay. The hadronic W boson decay is utilisedas well to evaluate | cos θ∗|, which does not permit discrimination between right-and left-handed W bosons, but still adds information to further constrain f0.In the dilepton events, mt = 172.5 GeV is assumed and cos θ∗ is obtained foreach top quark as the average over all possible solutions of the event kinematics,taking lepton and jet energy resolutions into account by varying the energieswithin their uncertainties.By comparing the observed angular distributions in data with templates ofbackground and tt signal with varying W boson helicity admixtures, f0 andf+ can be extracted in a simultaneous fit (f− = 1 − f+ − f0). This model-independent fit yields:f0 = 0.669±0.078(stat.)±0.065(syst.), f+ = 0.023±0.041(stat.)±0.034(syst.),in excellent agreement with the SM expectation.Search for Neutral Current Top DecaysTransitions between quarks of different flavour but identical charge (flavourchanging neutral currents, FCNC) are forbidden at lowest order and heavilysuppressed at higher orders in the SM. The decay t → Zq (with q = u, c) thusis expected to occur with a branching fraction of O(10−14) [5], well out of reachof experimental sensitivity. Any observation of FCNC decays would indicatephysics beyond the SM.D0 performs a search for tt → WbZq +X decays based on a dataset corre-sponding to an integrated luminosity of 4.1 fb−1, using the trilepton final state2ℓ′νℓℓ̄ + jets for the first time in this kind of search [6]. This signature bene-fits from low background contributions, but also suffers from limited statistics.After a basic preselection reflecting the decay signature, signal and background(mainly diboson and V + jets production) are separated using distributions ofjet multiplicity, scalar sum of the transverse momenta of leptons, jets and 6ET(HT ), and the reconstructed top quark mass from the t → Zq decay.All observed distributions are consistent with the SM background contribu-tions, and consequently 95% C.L. limits on FCNC decays of tt are derived:B(t → Zq) < 3.2%,with an expected limit of 3.8 %. This is currently the world’s best limit onB(t → Zq).Search for Fourth Generation t′ Quark Pair ProductionWhile a fourth generation of fermions with a light neutrino (mν < mZ/2)has been ruled out by electroweak precision data, the constraints on a fourthgeneration are much relaxed if the neutrino is sufficiently heavy. For example, afourth generation t′ quark mass of 400 GeV can be compatible with electroweakprecision measurements, and the mass splitting relative to the correspondingb′ quark is constrained to be small, leading to a preferred decay t′ → Wq(q = d, s, b) [7].Based on a dataset with an integrated luminosity of 5.3 fb−1, D0 searchesfor pair production of fourth generation up-type quarks (t′t̄′), assuming theintrinsic t′ width is smaller than the detector resolution and the decay alwaysproceeds via Wq [8]. Consequently, the t′t̄′ decay chain is identical to thatof the top quark pair production, and the lepton + jets final state is used forthe search with no b-jet identification applied. To separate the signal fromSM background processes, the observed events are compared with signal andbackground contributions in two-dimensional histograms of the scalar sum oflepton-pT , jet-pT and 6ET (HT ) versus t′ mass mfit from a kinematic fit to theevent hypothesis. Based on the observed agreement between data and the SMcontributions, 95% C.L. limits are derived on the t′t̄′ production cross sectionas a function of mt′ .While in the e + jets channel reasonable agreement between data and simu-lation is observed, in the µ + jets channel an excess of data over SM backgroundis visible around mfit = 325 GeV. However, the data are best described witha t′ t̄′ production cross section 3.2 ± 1.1 times the predicted cross section formt′ = 325 GeV. The probability of observing an excess of at least this sizefrom the SM background processes alone corresponds to 2.5 standard devia-tions. Combining the observations from both channels, t′t̄′ production can beexcluded at 95% C.L. formt′ < 285 GeV,with an expected exclusion below 320 GeV.3Measurement of B(t → Wb)/B(t → Wq)The ratio of top quark branching fractions R can be expressed via CKM matrixelements as R = B(t → Wb)/ Σq=d,s,bB(t → Wq)= |Vtb|2/ Σq=d,s,b|Vtq |2 andprovides therefore a measure of the relative size of |Vtb| compared to |Vtd| and|Vts|. Within the framework of the SM, assuming a unitary 3×3 CKM matrixand insignificance of non-W boson decay modes of the top quark, the aboveexpression simplifies to R = |Vtb|2 and R is strongly constrained from globalCKM fits: R = 0.99830+00006−00009 [9]. Deviations of R from unity could for examplebe a sign for the presence of a fourth heavy quark generation or non-SM topquark decay modes.D0 measures R in a dataset corresponding to an integrated luminosity of5.4 fb−1, using both the lepton + jets and dilepton tt decay channels [10].In the lepton + jets channels, the number of identified b-jets (0, 1, ≥2) isused together with a multivariate kinematic discriminant to separate the ttsignal from background in subsamples dominated by background in order toextract R simultaneously with the tt production cross section σtt̄, yielding R =0.95± 0.07 (stat.+ syst.) and σtt̄ = 7.90+0.79−0.67 (stat.+ syst.) pb. In the dileptonchannels, the lowest value of a neural network b-jet identification algorithmapplied to the leading jets in each event is used as a discriminant, yieldingR = 0.86 ± 0.05 (stat.+ syst.) and σtt̄ = 8.19+1.06−0.92 (stat.+ syst.) pb. Thesemeasurements allow the extraction of σtt̄ without assuming B(t → Wb) = 1 asusually done in tt cross section measurements. Combining both measurementsyields:R = 0.90± 0.04 (stat.+ syst.) and σtt̄ = 7.74+0.67−0.57 (stat.+ syst.) pb.The cross section measurement is in good agreement with the SM expectation,and the R measurement is within approximately 2.5 standard deviations of theSM expectation, being the most precise measurement to date.3 SummaryD0 is pursuing a wealth of top quark physics analyses that probe the validity ofthe SM, a few of which are presented in this report. So far, no evidence for newphysics phenomena has been found in top quark decays. The search continuesnow both at the Tevatron and the LHC, and will include updates to the analysespresented here.AcknowledgementsThe author would like to thank the organisers for the broad bandwidth of topicsaddressed and the fruitful collaborative atmosphere at PANIC 2011 and the D0collaboration and the staffs at Fermilab and collaborating institutions.4References[1] CDF, D0 Collaboration, The Tevatron Electroweak Working Group,“Combination of CDF and D0 results on the mass of the top quark usingup to 5.8 fb−1 of data,” arXiv:1107.5255 [hep-ex].[2] U. Langenfeld, S. Moch, and P. Uwer, “Measuring the running top-quarkmass,” Phys.Rev. D80 (2009) 054009, arXiv:0906.5273 [hep-ph].[3] A. Czarnecki, J. G. Korner, and J. H. Piclum, “Helicity fractions of Wbosons from top quark decays at NNLO in QCD,”Phys.Rev. D81 (2010) 111503, arXiv:1005.2625 [hep-ph].[4] D0 Collaboration, V. M. Abazov et al., “Measurement of the W bosonhelicity in top quark decays using 5.4 fb−1 of pp̄ collision data,”Phys.Rev. D83 (2011) 032009, arXiv:1011.6549 [hep-ex].[5] J. Aguilar-Saavedra, “Top flavor-changing neutral interactions:Theoretical expectations and experimental detection,” Acta Phys.Polon.B35 (2004) 2695–2710, arXiv:hep-ph/0409342 [hep-ph].[6] D0 Collaboration, V. M. Abazov et al., “Search for flavor changingneutral currents in decays of top quarks,”Phys.Lett. B701 (2011) 313–320, arXiv:1103.4574 [hep-ex].[7] G. D. Kribs, T. Plehn, M. Spannowsky, and T. M. Tait, “Fourgenerations and Higgs physics,” Phys.Rev. D76 (2007) 075016,arXiv:0706.3718 [hep-ph].[8] D0 Collaboration, V. M. Abazov et al., “Search for a Fourth Generationt′Quark in pp̄ Collisions at√s = 1.96 TeV,”Phys. Rev. Lett. 107 (2011) 082001, 1104.4522.[9] Particle Data Group Collaboration, K. Nakamura et al., “Review ofParticle Physics,” Journal of Physics G 37 (2010) 075021.[10] D0 Collaboration, V. M. Abazov et al., “Precision measurement of theratio B(t → Wb)/B(t → Wq),” arXiv:1106.5436 [hep-ex]. Accepted inPRL.5

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