In this paper a summary of some of the latest and most precise measurements in the top quark physics field performed by the ATLAS and CMS experiments are presented. These include the most precise measurements for t¯t and

single top cross-section as well as for the top mass, but also various measurements of top production and decay. Higgs and New Physics searches in the top quark Sector are not reported here

Lista, L.

Pinamonti, M.

English

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DOI 10.1393/ncc/i2014-11692-7Colloquia: IFAE 2013IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014Top physic results from ATLAS and CMSM. Pinamonti(1) and L. Lista(2)(1) SISSA, Trieste and INFN, Gruppo Collegato di Udine - Italy(2) INFN, Sezione di Napoli - Napoli, Italyricevuto l’1 Ottobre 2013Summary. — In this paper a summary of some of the latest and most precisemeasurements in the top quark physics field performed by the ATLAS and CMSexperiments are presented. These include the most precise measurements for tt¯ andsingle top cross-section as well as for the top mass, but also various measurements oftop production and decay. Higgs and New Physics searches in the top quark sectorare not reported here.PACS 29.85.Fj – Data analysis.1. – IntroductionThe top quark, discovered by the CDF and D0 experiments at the Tevatron 1.8TeVpp¯ collisions in 1995 [1], has been observed at CERN by the ATLAS [2] and CMS exper-iments [3] in 2010, in pp collisions at√s = 7TeV. Thanks to the excellent performanceof the Large Hadron Collider (LHC) and of the two detectors, millions of top quarkshave been produced in 7 and 8TeV collisions in 2010-2011 and 2012 respectively and col-lected by ATLAS and CMS. With this large dataset, top quark production mechanismsand properties have been measured with high precision, approaching or even beatingTeavtron precision.2. – Top pair productionAt the LHC the top quark is produced predominantly as tt¯ pairs via strong in-teraction. NNLO calculations are currently available, predicting a tt¯ cross section ofσtt¯ = 172.0+4.4−5.8(scales)+4.7−4.8(pdf) pb at 7TeV and σtt¯ = 245.8+6.2−8.4(scales)+6.2−6.4(pdf) pbat 8TeV [4]. Once produced, the top quark decays to a W boson and a b quark about100% of the times, according to the Standard Model (SM). Thus, top-quark pairs presentthree different final state topologies depending on the decays of the two W boson. Themost studied final states are the semi-leptonic and the dileptonic ones, where at leastone electron or muon is present in the final state, allowing a cleaner signature than ac© CERN on behalf of the ATLAS and CMS CollaborationsCreative Commons Attribution 3.0 License (http://creativecommons.org/licenses/by/3.0) 211212 M. PINAMONTI and L. LISTAfully hadronic one. The most precise measurements for the inclusive σtt¯ are performedin these channels, and rely on the first fraction of the 7TeV dataset, which is at presentbetter understood and less affected by systematics (e.g. the ones related to the pile-up).The best measurement by CMS [5] is performed in the dilepton channel, using 2.3 fb−1of integrated luminosity at 7TeV, selecting events with two opposite-signed, high-pT ,central and isolated electrons or muons and two or more reconstructed jets. A profilelikelihood ratio fit is then performed on the two-dimensional distribution of number ofjets and number of b-tagged jets, resulting in σtt¯ = 162± 2(stat)± 5(syst)± 4(lumi) pb.The most precise measurement by ATLAS [6] is instead performed in the single leptonchannel, requiring only one electron or muon and at least three jets in the final state.A Likelihood discriminant is then built combining four different kinematical variables todiscriminate the tt¯ signal from the main background coming from W+jets events. Like forCMS analysis, a profile likelihood ratio fit is performed to extract the final measurement,fitting symultaneously the Likelihood discriminant in three jet multiplicity regions perlepton flavour, and giving σtt¯ = 179± 4(stat)± 9(syst)± 7(lumi) pb.Other measurements at 7TeV have been performed by the two experiments consid-ering different final states, and a combination of the complementary results has beenproduced [7]. This combination, even if not including the most precise more recent re-sults, provides a good example for an efficient cooperation between the two experiments.The final result, σtt¯ = 173.3±2.3(stat)±9.8(syst) pb, shows a 7% gain in the experimentalprecision with respect to the most precise single measurement included there.Measurements at 8TeV are also available [8, 9], but despite the larger considereddataset and the higher expected cross section, the experimental measurements sufferfrom a higher systematic uncertainty coming form the higher amount of pile-up activity.The results, σtt¯ = 241 ± 2(stat) ± 31(syst) ± 9(lumi) pb from ATLAS and σtt¯ = 227 ±3(stat)±11(syst)±10(lumi) pb from CMS, obtained with a limited fraction of the 8TeVdataset, are anyway useful to test QCD calculations at different center of mass energies.Thanks to the large accumulated statistics, differential σtt¯ measurements were madepossible. The dependency of σtt¯ on various kinematical properties of the tt¯ system,of individual tops or their decay products are measured at 7TeV by both ATLAS [10]and CMS [11] and at 8TeV by CMS [12]. They provide useful tests for perturbativeQCD calculations and can be used to constrain new physics models. In addition, thedependency of the σtt¯ on the number of additional jets is obtained [13,14], providing testsfor Monte Carlo (MC) predictions for non-perturbative QCD including the emission ofinitial and final state radiation. No significant deviation from the shapes predicted bythe SM-based MC simulations are found in any of the considered distributions.The isolation of associated production of a tt¯ pair with either a vector boson or a pairof b-jets, useful to control important backgrounds in the searches for the top-associatedHiggs production, has also been made possible by the large available statistics. Byselecting events with at least three (or two same-sign) leptons in the final state, tt¯+Z ismeasured to be σtt¯Z = 0.30+0.14−0.11(stat)+0.04−0.02(syst) pb by CMS [15] and σtt¯Z = 0.28+1.57−0.14 pbby ATLAS [16], with a NLO theoretical prediction of σtt¯Z = 0.14 pb [17] (no theoreticaluncertainty was available at the time this report was written). CMS also measures att¯ + W and an inclusive tt¯ + W/Z cross sections, σtt¯W = 0.28+0.14−0.12(stat)± 0.04(syst) pband σtt¯V = 0.45+0.17−0.15(stat)+0.06−0.05(syst) pb respectively, to be compared with the theoreticalpredictions at NLO [18] of σtt¯W = 0.17+0.03−0.05 pb and σtt¯V = 0.308 pb. ATLAS reports ameasured tt¯+γ cross section of σtt¯γ = 2.0±0.5(stat)±0.7(syst)±0.8(lumi) pb [19], witha theoretical prediction of σtt¯γ = 2.1± 0.4 pb [20]. The tt¯ + bb¯ cross section is measuredTOP PHYSIC RESULTS FROM ATLAS AND CMS 213by CMS [21] with the result, σtt¯bb¯σtt¯jj = 3.6± 1.1(stat)± 0.9(syst)%, being somehow higherthan the fractions predicted by various LO and NLO MC generators.Another important measurement in the context of the tt¯ production is the so called tt¯Charge Asymmetry, defined as AC =N(Δ|y|>0)−N(Δ|y|<0)N(Δ|y|>0)+N(Δ|y|<0) , where Δ|y| = |yt| − |yt¯|. Thisquantity is predicted to be very close to 0 in the SM (AC = 0.0115 ± 0.0006) [22], butmany models which are able to explain the anomalous tt¯ Forward-Backward asymmetrymeasured at the Tevatron [23] also predict higher values for AC . Both ATLAS and CMSmeasured AC considering single lepton and dilepton tt¯ final states, finding no significantdeviation from the SM prediction. ATLAS single lepton and dilepton results combinedgive AC = 0.029± 0.018(stat)± 0.014(syst) [24], while CMS best measurement (in singlelepton channel) is AC = 0.004 ± 0.010(stat) ± 0.011(syst) [25]. Both experiments alsoreported differential measurements of AC as a function of the invariant mass (ATLASand CMS), pT and y (CMS only) of the tt¯ system.3. – Single top productionBeside the tt¯ production, top quarks can be produced singly, via weak interaction.The main single top production mode at the LHC is the t-channel, with the associatedWt and the s-channel production having lower cross section. Precise measurements ofthe single top production cross section in the different production modes are importantto extract direct |Vtb| measurements and to constrain new physics anomalous couplings.For the t-channel, at 7TeV ATLAS measures σt = 83± 4(stat)± 20(syst) pb [26], whileCMS measures σt = 67± 4(stat)± 5(syst)± 1(lumi) pb [27]. At 8TeV ATLAS measuresσt = 95± 2(stat)± 18(syst) pb [28], while CMS measures σt = 80± 6(stat)± 11(syst)±4(lumi) pb [29]. All these values agree with the approximated NNLO predictions [30] ofσt = 64.6 ± 2.4 pb and σt = 87.8 ± 2.4 pb at 7 and 8TeV respectively, and are used toderive the limit |Vtb| > 0.92 at 95% confidence level.Being the LHC a pp collider, the t-channel production is expected to be charge asym-metric, with the number of tops being approximately twice the number of anti-tops. Themeasurements of the ratio Rt = σt/σt¯ by ATLAS [31] and CMS [32] agree with theSM prediction within the experimental error. Once sufficient precision is reached, thismeasurement could be used to constrain proton PDF’s at the LHC energies.First results are available for Wt and s-channels as well. With a dilepton eventselection and a dedicated multi-variate discriminant against tt¯ events, both ATLAS andCMS measure the Wt cross section with a ∼ 30% precision: ATLAS measures σWt =16.8 ± 2.9(stat) ± 4.9(syst) pb [33], while CMS measures σWt = 16+5−4 pb [34], bothmeasurements agreeing with the SM prediction. For s-channel, suffering from a lowercross section and being characterized by final states very close to the t-channel ones,no observation is obtained yet, even if ATLAS set an upper limit of 5.8 times the SMprediction [35].4. – Top quark massDue to its large mass, the top quark gives large contributions to electroweak radiativecorrections. Together with precision electroweak measurements, the top quark mass canbe used to derive constraints on the masses of the Higgs boson and new Physics particles.The best top mass measurements at the LHC are performed focusing on tt¯ eventswith single lepton final states, being the best compromise between having a clean signal214 M. PINAMONTI and L. LISTAand being able to fully reconstruct at least one of the two top quarks trough its hadronicdecay. As in the case of the tt¯ total cross section, the best precision is obtained by bothATLAS [36] and CMS [37] by considering only 7TeV data, where systematic uncertaintiesare more under control. In ATLAS analysis, a so called 2D Template Method is used.An event-by-event kinematical likelihood fit is performed to reconstruct the tt¯ system bysolving the combinatorial issue of assigning jets and b-jets to the hadronically decayingW and the two tops. A two-dimensional histogram is then filled with the reconstructedvalues for top and W masses and a template fit is applied on it, by using different MCtemplates for different top mass hypotheses. While the W mass is fixed in the fit, boththe top mass (mt) and a jet energy scale (JES) overall factor are fitted simultaneously,performing a sort of in situ JES calibration thanks to the W mass constraint. Theresult is mt = 174.5±0.6(stat)±2.3(syst) GeV. CMS analysis applies the same principleof a simultaneous mt and JES fit, but uses the so called Ideogram Method. Each jetpermutation in each event in data is weighted according to its probability of havingreconstructed the tt¯ decay correctly. An analytical expression for a global Likelihoodfunction is then maximized, providing a pair of best fit values for mt and JES. Theresult is mt = 173.5± 0.4(stat + jes)± 1.0(syst) GeV.Also for top mass measurement ATLAS and CMS exercised a combination of someof their early results [38], resulting in mt = 173.3 ± 0.5(stat) ± 1.3(syst) GeV. Amore recent CMS combination [39], including dilepton and all-hadronic final statesin addition to the measurement in single lepton channel reported above, gives mt =173.4± 0.4(stat)± 0.9(syst) GeV, reaching Tevatron experiment precision (mt = 173.2±0.6(stat)± 0.8(syst) GeV [40]).CMS experiment also reported a differential top mass measurement [41], testing theMC description of the dependence of the top mass measurement in tt¯ single lepton eventson various kinematical properties of the final state.5. – Top decayConcerning top quark decay, two recent results from ATLAS and CMS are presented.CMS measured the top quark relative branching ratio R = BR(Wb)/BR(Wq) (withq = d, s, b) [42], which can be used to extract an indirect constrain on |Vtb| (under theassumption of unitary CKM matrix) or probe new physics scenarios like fourth gen-eration quarks or charged Higgs boson from top decay. The measurement is focusedon a tt¯ dilepton event selection, but takes into account also the fraction of single topevents entering the selection. A maximum likelihood fit is applied on the distributionof b-tagged jet multiplicity resulting in R = 1.023+0.036−0.034, corresponding to a 95% con-fidence level limit |Vtb| > 0.972 (the most stringent experimental constrain on Vtb atpresent).An important measurement to probe the V -A coupling of the weak charge is the Wpolarization measurement in top decay. By looking at the t → W (→ ν)b decay in tt¯events in 7TeV data, ATLAS and CMS Collaborations measure the angle between theb-jet from top and the lepton from W in the top rest frame. From this distribution, thefraction of right-handed, left-handed and non-polarized W are extracted. The resultsfrom ATLAS with single lepton and dilepton final states were combined with the CMSsingle muon measurement [43], finding a result in good agreement with SM predictions.CMS also measured the same quantity selection single top final states [44], again inagreement with theoretical predictions, even if with larger uncertainty.TOP PHYSIC RESULTS FROM ATLAS AND CMS 2156. – SummaryAfter 18 years from top quark discovery, top physics is still a fascinating topic in ex-perimental high energy physics. Many measurements, like the total tt¯ cross section andthe top mass, are now precision measurements, reaching and even exceeding Teavtronprecision, while single top and differential measurements are on the way too, benefitingfrom the recent large increase in statistics. Many of the presented measurements are notyet fully exploiting the potential of the LHC: they use only a fraction of the availablestatistics and are already dominated by systematics mainly related to detector perfor-mances and MC simulation, uncertainties which are expected to become smaller withthe increasing understanding of the relatively young multi-TeV-energy physics.REFERENCES[1] CDF and D0 Collaboration, Phys. Rev. Lett., 74 (1995) 2626; Phys. Rev. Lett., 74(1995) 2632.[2] ATLAS Collaboration, EPJC, 71 (2011) 1577.[3] CMS Collaboration, Phys. Lett. B, 695 (2010) 424.[4] Czakon M., Fiedler P. and Mitov A., arXiv:1303.6254 [hep-ph] (2013).[5] CMS Collaboration, JHEP, 11 (2012) 067.[6] ATLAS Collaboration, ATLAS-CONF-2011-121 (2011).[7] ATLAS and CMS Collaboration, ATLAS-CONF-2012-134, CMS-PAS-TOP-12-003(2012).[8] ATLAS Collaboration, ATLAS-CONF-2012-149 (2012).[9] CMS Collaboration, CMS PAS TOP-12-006, CMS PAS TOP-12-007 (2012).[10] ATLAS Collaboration, Eur. Phys. J. 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Top physic results from ATLAS and CMS

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