In this paper we will review the most recent experimental results obtained at Tevatron (Fermilab) on top and electroweak physics

Amerio, S.

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DOI 10.1393/ncc/i2011-10747-7Colloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010Top, W and Z: Experimental resultsS. AmerioINFN, Sezione di Padova - via Marzolo 8, 35131 Padova, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online il 18 Gennaio 2011)Summary. — In this paper we will review the most recent experimental resultsobtained at Tevatron (Fermilab) on top and electroweak physics.PACS 14.65.Ha – Top quarks.PACS 14.70.Fm – W bosons.PACS 14.70.Hp – Z bosons.1. – IntroductionTop and electroweak physics are two of the main research fields at the Tevatronaccelerator at Fermilab: the precise knowledge of t, W and Z properties is a powerfultest of the Standard Model (SM) and can open a window to new physics phenomena.In some cases, like diboson associated production, final states involving these particlesrepresent an irreducible background to Higgs searches. The Tevatron, with more than7.2 fb−1 of data collected per experiment and the perspective of acquiring 5 fb−1 moreby the end of 2011, is currently the ideal place where these particles can be studied indetail. In this paper we will review the most recent CDF and D0 results on t, W and Zphysics, obtained with data samples ranging from 0.36 to 5 fb−1.2. – TopThe t-quark, discovered at Tevatron in 1995 [1, 2], can be considered special amongthe 6 quarks: it is the heaviest and according to the SM it decays with a branching ratio(BR) of almost 100% in a W -boson and a b-quark without hadronizing; as a consequence,its properties can be measured directly from its decay products.At Tevatron, where pp¯ interactions occur at a center-of-mass energy√s = 1.96TeV, itis mainly produced in tt¯ pairs via strong interaction through qq¯ annihilation (BR ∼ 85%)or g-g fusion (BR ∼ 15%). Top can also be produced singly via electroweak interaction,though with a cross section much smaller than pair production.At CDF and D0 experiments tt¯ pairs are observed and studied in all possible finalstates: dileptonic, when both W ’s decay into leptons, lepton + jets, where one W de-cays leptonically and the other into jets of particles, and all hadronic, where both W ’sc© Societa` Italiana di Fisica 4546 S. AMERIOFig. 1. – Tevatron t mass combination (March 2009).decay hadronically. The golden channel for t studies is the lepton+jets, which has thehighest branching ratio (38%) and a clean signature thanks to the leptonic decay of theW -boson.2.1. Top mass. – The t-quark mass is a fundamental parameter of the SM and itsprecise knowledge is necessary to calculate physics observables involving t-quark quantumloops to high precision. In combination with a precise W mass determination, it helpsto constrain the Higgs boson mass.The t mass has been measured at both CDF and D0 experiments in all final states andwith different techniques, the Matrix Element [3] and the Template methods [4] beingthe most used. In fig. 1 we report the most recent combination of CDF and D0 results,together with all the values entering the combination. The measurements are in goodagreement and their combined value is mt = 173.1 ± 0.6 (stat) ± 1.1 (syst)GeV/c2 [5].The t mass is now known with a relative precision of 0.75%, limited by the systematicuncertainties which are dominated by the jet energy scale uncertainty. This systematicis expected to improve as larger data samples are collected since the jet energy scalecan be constrained in-situ using W → qq¯ decays. Both collaborations expect to reach aprecision of 1GeV/c2 each.Recently CDF has performed the single most precise top-quark mass measurementever done: the analysis is performed in the lepton+jets channel on a sample of 4.8 fb−1applying the Matrix Element technique. Key ingredients of the measurement are the useof a Neural Network discriminant to better distinguish the tt¯ signal from the backgroundand an increased acceptance on t events thanks to the inclusion of new muon categories.The result is mt = 172.8±1.3GeV/c2: this measurement alone has a precision comparableto the world average [6] and will be soon included in the Tevatron combination.TOP, W AND Z: EXPERIMENTAL RESULTS 47A very interesting measurement related to the t mass is the determination of the massdifference between t and t¯, a direct test of the CPT theorem. D0 has performed the firstmeasurement of this quantity in the lepton+jets channel on 1 fb−1 of data, using thecharge of the lepton to infer if the quark is a t or t¯. The measured mass difference is3.8± 3.7GeV/c2 [7], consistent with the equality of t and t¯ masses.2.2. tt¯ spin correlations. – Due to its short lifetime, t quark decays weakly before anyhadronization process takes effect, thus enabling its spin information to be transmitted toits decay products. SM tt¯ pair production shows a characteristic spin correlation whichcan be modified by new production mechanisms such as Z ′ bosons or Kaluza-Klein gluons.The spin correlation coefficient k can be defined as k = (NS−NO)/(NS+NO) where NSand NO are the number of tt¯ pairs with parallel and antiparallel spin, respectively. CDFmeasures this coefficient in the lepton+jets channel on 4.3 fb−1 of data in the helicitybasis. The result is k = 0.60 ± 0.50 (stat) ± 0.16 (syst) [8] to be compared with a SMexpected value of kHSM = 0.40 in the same basis. D0 performs the same measurement inthe dilepton channel on 4.2 fb−1 of data in the beam axis basis: the correlation coefficientis measured to be k = −0.17+0.65−0.53 (stat + syst) [9], to be compared with a SM expectedvalue of kBASM = 0.78.2.3. Top-quark charge. – Exotic models of a fourth generation of quarks predict thatthe t-quark decays to a W− and a b-quark, hence having a charge of −4/3, rather than toa W+ and a b quark as predicted by the SM. CDF has recently performed a measurementof the t charge in the lepton plus jets channel in 2.7 fb−1. Two b-jets are required to bepresent in the event: one is identified looking for tracks displaced with respect to theprimary vertex; the second looking for soft leptons from the semileptonic decay of theb-quark. The charge of the two W ’s and two b-quarks is determined for each data eventusing a kinematic fitter to select the best jet to parton assignment compatible with thet-quark decay. CDF observes 29 events consistent with the SM and 16 events consistentwith a −4/3 charge t-quark. This results in a 95% confidence level exclusion of the −4/3charge hypothesis [10].2.4. Top-quark width. – In the SM the theoretical t-quark lifetime is very short, of theorder of 5 · 10−25 s, which corresponds to a decay width of 1.5GeV. Deviations from thepredicted value could signal contributions from decays to non-SM particles, for examplet → H+b. A t-quark width of 1.5GeV is out of reach of current experiments and directmeasurements can only result in upper limits. Recently D0 explored a new technique andmeasured the t-quark width indirectly on 2.3 fb−1 as a function of the theoretical andexperimental single-top t-channel cross sections, the theoretical t width and the measuredbranching ratio of t → Wb. The result, Γt = 2.05+0.57−0.52 GeV [11], is the first indirect andmost precise determination of the t-quark width.2.5. Single top. – The search for t-quark produced singly via the electroweak inter-action is very challenging due to the small cross section and a less distinctive signaturewith respect to tt¯ pair production. Nevertheless the reasons for studying single t-quarkare compelling: its production cross section is proportional to the square of the CKMmatrix element |Vtb| and it is sensitive to various new physics phenomena. Both CDFand D0 have searched for this signal using sophisticated multivariate techniques (neuralnetworks, boosted decision trees, matrix element and log likelihood methods). In 2009both collaborations claimed the observation of single t at the 5.0 standard deviations48 S. AMERIOFig. 2. – W mass world average, along with all the measurements entering the combination.level of significance. Recently, two CDF measurements and one from D0 have been com-bined in order to reduce the statistical error on the cross section and provide a strongerconstraint on |Vtb|. The results are σ = 2.76+0.58−0.47 pb (s-channel and t-channel combined)and |Vtb| = 0.88± 0.07: combining the three measurements the uncertainty on the crosssection is reduced from 22% to 19% while for |Vtb| the error is reduced from 14% (CDF)and 11% (D0) to 8% [12].2.6. Search for new physics in the top sector . – Among the different searches for newphysics in the top sector we mention the search for a heavy t, the t′, predicted by LittleHiggs models and fourth generation theories. According to those theories, this new quarkis pair-produced strongly, it has a mass greater than the t and it decays promptly to aW -boson and lighter quarks. CDF has searched for t′t¯′ pairs in 4.6 fb−1 of data in thelepton+jets final state. A 2-dimensional fit to the reconstructed transverse mass of t′ andthe scalar sum of all transverse energies of the event showed no evidence for t′t¯′ signal.A t′ quark with mass below 335GeV/c2 has been excluded at 95%CL, given a t massMt = 172.5GeV [13].3. – W mass and widthThe precise measurements of W -boson mass and width are of particular importance asa powerful test of the SM and to constrain the SM Higgs boson mass. The measurementsare performed in the channel where the W decays leptonically, using a template methodapplied to the transverse mass and the transverse momenta of the lepton and the neutrino.A very accurate measurement of lepton momentum and hadronic recoil (to estimate theneutrino momentum) is critical to improve the precision of the measurements which iscurrently limited by the systematic uncertainties on the lepton energy scale and on theparton distribution functions (PDFs). The best and most recent single measurementof the W mass has been made by D0 on 1 fb−1 of data in the electron channel. Theresult is MW = 80.402 ± 0.043GeV/c2 [14]. Combining this measurement with theCDF result Tevatron obtains the value MW = 80.376± 0.031GeV/c2 [15] (fig. 2), moreTOP, W AND Z: EXPERIMENTAL RESULTS 49precise than LEP2 direct measurement. Both CDF and D0 are analyzing more data andworking to further reduce the systematic error: larger Z → ll (l = e/μ) samples willhelp to improve the lepton momentum determination, while ongoing measurements of Wcharge asymmetry and Z rapidity will allow a better knowledge of the PDFs. Tevatronexpects to reduce the total uncertainty on the W mass down to 25MeV. As far asthe W width is concerned, both experiments have produced measurements consistentwith each other and with the SM: CDF measures ΓW = 2.032 ± 0.073GeV [16] and D0ΓW = 2.034±0.072GeV [17]. In this case both measurements are separately more precisethan the LEP2 determination and combined with Run I results give a Tevatron averageΓW = 2.046± 0.049GeV [18].4. – Diboson productionThe diboson production is sensitive to new physics through anomalous trilinear gaugecouplings (aTGCs): the presence of anomalous couplings would increase the dibosonproduction cross sections and alter the production kinematics, so any deviation from theexpected values could indicate the presence of physics beyond the SM. Moreover, thediboson production is also an important and irreducible background to the search of SMHiggs boson and SUSY.D0 searches for Zγ production, sensitive to ZZγ and Zγγ aTGCs, in 3.6 fb−1 of datain the channel Z → νν. It observes 51 signal events with a predicted background of 17.3and measures a cross section value σ = 39±9 (stat+syst)±2 (lumi) fb for photons withtransverse energy greater than 90GeV [19]. This measurement, which corresponds to astatistical significance of 5.1 σ and is in good agreement with the theoretical value (σth =39± 4 fb) represents the first observation of Zγ → ννγ at Tevatron. Another interestingdiboson state is WW , which is the main background to H → WW searches, as well as afinal state sensitive to WWγ and WWZ aTGCs. At Tevatron it has first been observedin the final state with two leptons: thanks to the clean signature and the high statistics,CDF measures a cross section σ(pp → WW ) = 11.5±2.1(stat+syst)±0.7 (lumi) pb [20]and D0 measures σ(pp → WW ) = 12.1 ± 0.9 (stat) +1.6−1.4(syst) pb [21], both compatiblewith the theoretical value σNLO = 11.7 ± 0.7 pb. D0 combines this measurement withother searches for Wγ → lνγ, WZ → lνll and W (W/Z) → lν+jets and obtains the firsthigh-statistics combination of limits on WWZ and γWW aTGCs across different dibosonproduction mechanisms [22]. Both CDF and D0 are working on a future combination ofresults on 5 fb−1 of data to improve the sensitivity to be competitive with LEP.Finally, in the recent past new challenging signatures have started being explored:CDF looks for a very general final state characterized by missing transverse energy andjets; this search is sensitive to both lνqq and ννqq final states and has acceptance onWW , WZ and ZZ production. In 3.5 fb−1 of data, CDF extracts 1516 events with astatistical significance of 5.3 σ, making this measurement the first observation of thisdiboson final state at a hadron collider [23].REFERENCES[1] The CDF Collaboration, Phys. Rev. Lett., 74 (1995) 2626.[2] The D0 Collaboration, Phys. Rev. Lett., 74 (1995) 2632.[3] Fiedler F. et al., arXiv:1003.1316v1 [hep-ex].[4] The CDF Collaboration, Phys. Rev. D, 73 (2006) 032003.50 S. AMERIO[5] Tevatron Electroweak Working Group for the CDF and D0 Collaborations,arXiv:0903.2503 [hep-ex].[6] The CDF Collaboration, CDF Conference Note 10077 (2010).[7] Abazov V. M. et al. (The D0 Collaboration), Phys. Rev. Lett., 103 (2009) 132001.[8] The CDF Collaboration, CDF Conference Note 10048 (2010).[9] The D0 Collaboration, D0 Conference Note T84 (2009).[10] The CDF Collaboration, CDF Conference Note 9939 (2010).[11] The D0 Collaboration, D0 Conference Note 6034 (2010).[12] Tevatron Electroweak Working Group for the CDF and D0 Collaborations,arXiv:0908.2171v1 [hep-ex].[13] The CDF Collaboration, CDF Conference Note 10110 (2010).[14] The D0 Collaboration, Phys. Rev. Lett., 103 (2009) 141801.[15] Tevatron Electroweak Working Group for the CDF and D0 Collaborations,arXiv:0908.1374v1 [hep-ex].[16] The CDF Collaboration, Phys. Rev. Lett., 100 (2008) 071801.[17] The D0 Collaboration, Phys. Rev. Lett., 103 (2009) 231802.[18] Tevatron Electroweak Working Group for the CDF and D0 Collaborations,arXiv:1003.2826v1 [hep-ex].[19] The D0 Collaboration, arXiv:0902.2157 [hep-ex].[20] The CDF Collaboration, arXiv:0912:4500 [hep-ex].[21] The D0 Collaboration, arXiv:0904.0673 [hep-ex].[22] The D0 Collaboration, arXiv:0907.4952 [hep-ex].[23] The CDF Collaboration, arXiv:0905.4714 [hep-ex].

Top, W and Z: Experimental results

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Top, W and Z: Experimental results

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