The mass of the top quark (m{sub top}) is a fundamental parameter of the standard model (SM). Currently, its most precise measurements are performed by the CDF and D0 collaborations at the Fermilab Tevatron p{bar p} collider at a centre-of-mass energy of {radical}s = 1.96 TeV. We review the most recent of those measurements, performed on data samples of up to 8.7 fb{sup -1} of integrated luminosity. The Tevatron combination using up to 5.8 fb{sup -1} of data results in a preliminary world average top quark mass of m{sub top} = 173.2 {+-} 0.9 GeV. This corresponds to a relative precision of about 0.54%. We conclude with an outlook of anticipated precision the final measurement of m{sub top} at the Tevatron

/Gottingen U., II. Phys. Inst.

Brandt, Oleg

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MEASUREMENTS OF THE TOP QUARK MASS AT THE TEVATRONO. Brandt on behalf of the CDF and D0 CollaborationsII. Physikalisches Institut, Friedrich-Hund-Platz 1,Göttingen, GermanyThe mass of the top quark (mtop) is a fundamental parameter of the standard model (SM).Currently, its most precise measurements are performed by the CDF and D0 collaborations atthe Fermilab Tevatron pp̄ collider at a centre-of-mass energy of√s = 1.96 TeV. We review themost recent of those measurements, performed on data samples of up to 8.7 fb−1 of integratedluminosity. The Tevatron combination using up to 5.8 fb−1 of data results in a preliminaryworld average top quark mass of mtop = 173.2 ± 0.9 GeV. This corresponds to a relativeprecision of about 0.54%. We conclude with an outlook of anticipated precision the finalmeasurement of mtop at the Tevatron.PACS 14.65.Ha – Top quarks.1 IntroductionThe pair-production of the top quark was discovered in 1995 by the CDF and D0 experiments 1at the Fermilab Tevatron proton-antiproton collider. Observation of the electroweak productionof single top quarks was presented only two years ago 2. The large top quark mass and theresulting Yukawa coupling of almost unity indicates that the top quark could play a crucial rolein electroweak symmetry breaking. Precise measurements of the properties of the top quarkprovide a crucial test of the consistency of the SM and could hint at physics beyond the SM.In the following, we review measurements of the top quark mass at the Tevatron, which is afundamental parameter of the SM. Its precise knowledge, together with the mass of the W boson(mW ), provides an important constraint on the mass of the postulated SM Higgs boson. This isillustrated in the mtop,mW plane in Fig. 1, which includes the recent, most precise measurementsof mW3. A detailed review of measurements of the top quark mass is provided in Ref. 4. Recentmeasurements of properties of the top quark other than mtop at the Tevatron are reviewed inRef. 5. The full listing of top quark measurements at the Tevatron is available at public webpages 6,7.At the Tevatron, top quarks are mostly produced in pairs via the strong interaction. By theend of Tevatron operation, about 10 fb−1 of integrated luminosity per experiment were recordedby CDF and DØ, which corresponds to about 80k produced tt̄ pairs. In the framework of theSM, the top quark decays to a W boson and a b quark nearly 100% of the time, resulting in aW+W−bb̄ final state from top quark pair production. Thus, tt̄ events are classified accordingto the W boson decay channels as “dileptonic”, “all–jets”, or “lepton+jets”. More details onthe channels and their experimental challenges can be found in Ref. 8, while the electroweakproduction of single top quarks is reviewed in Ref. 9.arXiv:1204.0919v1  [hep-ex]  4 Apr 201280.380.480.5155 175 195LHC excludedmH [GeV]114 300 600 1000mt  [GeV]mW  [GeV] 68% CL∆αLEP1 and SLDLEP2 and TevatronMarch 2012(a) (b)Figure 1: (a) The constraint on mass of the SM Higgs boson from direct mtop and mW measurements inthe mtop,mW plane9. The red ellipsis indicates the 68% CL contour. (b) The anticipated precision on mtopmeasurements at D0 and the Tevatron combination versus integrated luminosity.2 Direct measurements of the top quark mass in `+ jets final statesD0’s most precise measurement of mtop is performed in `+ 4 jets final state using the so-calledmatrix element (ME) method in 3.6 fb−1 of data 11. This technique was pioneered by DØ inRun I of the Tevatron 12, and it calculates the probability that a given event, characterised by aset of measured observables x, comes from the tt̄ production given an mtop hypothesis, or from abackground process: Pevt(x) ∝ fPsig(x,mtop)+(1−f)Pbgr. The dependence on mtop is explicitlyintroduced by calculating Psig using the differential cross section dσ(y,mtop) ∝ |Mtt̄|2(mtop),where Mtt̄ is the leading order (LO) matrix element for tt̄ production:Psig(x,mtop, kJES) =1σobservedtt̄·∫W (x, y, kJES) dσ(y,mtop) .Since dσ(y,mtop) is defined for a set of parton-level observables y, the transfer functionW (x, y, kJES)is used to map them to the reconstruction-level set x. This accounts for detector resolutionsand acceptance cuts, and introduces explicitly the dependence on the jet energy scale (JES)via an overall scaling factor kJES. The uncertainty on the JES, which is almost fully correlatedwith mtop, is around 2% or larger. Therefore, an in situ calibration is performed by requiringthat the mass of the dijet system assigned to the parton pair from the hadronically decayingW boson be mjj = 80.4 GeV. Thus, mtop and kJES are extracted simultaneously. This reducesthe uncertainty from the JES to about 0.5%, decreasing with the number of selected tt̄ events.The measurement is performed in events with four jets, resulting in 24 possible jet-parton as-signments. All 24 assignments are summed over, weighted according to the consistency of agiven assignment with the b-tagging information. Pbgr is calculated using the VECBOS matrixelement for W + 4 jets production. Generally, the ME technique offers a superior statistical sen-sitivity as it uses the full topological and kinematic information in the event in form of 4-vectors.The drawback of this method is the high computational demand.D0 measures mtop = 174.9 ± 0.8 (stat) ± 0.8 (JES) ± 1.0 (syst) GeV, corresponding toa relative uncertainty of 0.9%. The dominant systematic uncertainties are from modeling ofunderlying event activity and hadronisation, as well as the colour reconnection effects. On thedetector modeling side, diffential uncertainties on the JES which are compatible with the overallkJES value from in situ calibration, and the difference between the JES for light and b-quark jetsare dominant. This picture is representative for all mtop measurements in ` + jets final statesshown here.CDF employs the ME technique similar to that used at D0 to measure mtop on a datasetcorresponding to 5.6 fb−1and finds mtop = 173.0 ± 0.7 (stat) ± 0.6 (JES) ± 0.9 (syst) GeV 13.Most notable differences from the D0 measurement are: (i) background events present in the datasample are accounted for on average rather than on an event-by-event basis using a likelihoodbased on a neural network output, (ii) the contribution of “mismeasured” signal events, whereone of the jets cannot be matched to a parton, is reduced with a cut on the aforementionedlikelihood.Currently, the world’s best single measurement of mtop is performed by CDF in `+ jets finalstates using the so-called template method to analyse the full dataset of 8.7 fb−1 14. The basicidea of the template method is to construct “templates”, i.e. distributions in a set of variables x,which are sensitive to mtop, for different mass hypotheses, and extract mtop by matching themto the distribution found in data, e.g. via a maximum likelihood fit. CDF minimises a χ2-likefunction to kinematically reconstruct the event for jet-parton assignments consistent with the b-tagging information. To extract mtop and calibrate the JES in-situ, three-dimensional templatesare defined in the fitted mtop of the best jet-parton assignment, the fitted mtop of the second-bestassignment, and the fitted invariant mass of the dijet system from the hadronically decaying Wboson. CDF finds mtop = 172.9 ± 0.7 (stat)± 0.8 (syst) GeV.3 Direct measurement of the top quark mass in all-hadronic final statesThe third most statistically significant contribution to the current Tevatron average of mtopcomes from a measurement in 6 ≤ Njets ≤ 8 final states by CDF using 5.8 fb−1 of data 15. Themain challenge is the high level of the background contribution from QCD multijet production:the S : B ratio is about 1 : 1200 after a multijet trigger requirement. Therefore, a discriminationvariable DNN is constructed with a multilayered neural network (NN). Beyond typical kinematicand topological variables, also jet shape variables which provide discrimination between quarkand gluon jets, are used as inputs. To enhance the purity of the sample and to reduce thenumber of combinatoric possibilities, b tagging is applied. For each jet–parton assignment, a χ2is constructed which accounts for: the consistency of the two dijet pairs with the reconstructedmW , the consistency of the jjb combinations with the reconstructed mtop, and the consistencyof the individual fitted jet momenta with the measured ones, within experimental resolutions.The final sample for top mass extraction is defined by DNN > 0.97 (0.84) for events with1 (≥ 2) b tags, yielding a signal to background ratio of 1 : 3 (1 : 1). The measured value ismtop = 172.5 ± 1.4 (stat)± 1.4 (syst) GeV. Beyond systematic uncertainties relevant in `+ jetsfinal states, potential biases from the data-driven background model pose a notable contributionto the total uncertainty.4 Direct measurement of the top quark mass in dilepton final statesThe world’s most precise measurement of mtop in dilepton final states is performed by D0 using5.4 fb−1 of data 16. Leaving mtop as a free parameter, dilepton final states are kinematicallyunderconstrained by one degree of freedom, and the so-called neutrino weighting algorithm isapplied for kinematic reconstruction. It postulates distributions in rapidities of the neutrino andthe antineutrino, and calculates a weight, which depends on the consistency of the reconstructed~p νν̄T ≡ ~p νT + ~p ν̄T with the measured missing transverse momentum /pT vector, versus mtop. D0uses the first and second moment of this weight distribution to define templates and extractmtop. To reduce the systematic uncertainty, the in situ JES calibration in `+ jets final states11is applied, accounting for differences in jet multiplicity, luminosity, and detector ageing. Aftercalibration and all corrections, mtop = 174.0 ± 2.4 (stat)± 1.4 (syst) GeV is found.Top quark pole mass (GeV)150 160 170 180 190 (pb)ttσ2468101214+X)t t→p(pσMeasured σMeasured dependence of Approximate NNLONLO+NNLL (pb)ttσ-1DØ, L=5.3 fb (a)  (GeV)tm170 175 180 (GeV)tm170175180(a) -1DØ 3.6 fbe+jets(b)  (GeV)tm170 175 180 (GeV)tm170175180(b) -1DØ 3.6 fb+jetsµ(c)Figure 2: (a) σtt̄ measured by D0 using 5.3 fb−1 (black line) and theoretical NLO+NNLL 17 (green solid line)and approximate NNLO 18 (red solid line) predictions as a function of mpoletop . The experimental acceptancecorrection assumes mMCtop = mpoletop . The gray band corresponds to the total uncertainty on measured σtt̄. Thedashed lines indicate theoretical uncertainties from the choice of scales and parton distribution functions. (b) mtand mt̄ measured by D0 directly and independently using 3.6 fb−1in e+ jets final states. The solid, dashed, anddash-dotted lines represent the 1, 2, and 3 SD contours. (c) same as (b) but for µ+ jets.5 Measurement of mtop from the tt̄ production cross-sectionThe tt̄ production cross section (σtt̄) is correlated to mtop. This can be used to extract mtop bycomparing the measured σtt̄ to the most complete to–date, fully inclusive theoretical predictions,assuming the validity of the SM. Such calcualtions offer the advantage of using mass definitionsin well-defined renormalisation schemes like mMStop or mpoletop . In contrast, the main methods usingkinematic fits utilise the mass definition in MC generators mMCtop , which cannot be translated intomMStop or mpoletop in a straightforward way. D0 uses 5.3 fb−1 of data to measure σtt̄ and extractsmtop17 using theoretical calculations for σtt̄ like the next-to-leading order (NLO) calculationwith next-to-leading logarithmic (NLL) terms resummed to all orders 18, an approximate NNLOcalculation 19, and others. For this, a correction is derived to account for the weak dependenceof measured σtt̄ on mMCtop . The results for mpoletop are presented in Fig. 2, and can be summarisedas follows: mpoletop = 163.0+5.1−4.6 GeV and mpoletop = 167.5+5.2−4.7 GeV for Ref.18 and 19, respectively.6 Measurements of the mass difference between the t and t̄ quarksThe invariance under CPT transformations is a fundamental property of the SM. mparticle 6=mantiparticle would constitute a violation of CPT , and has been tested extensively in the chargedlepton sector. Given its short decay time, the top quark offers a possiblity to test mt = mt̄ atthe percent level, which is unique in the quark sector. D0 applies the ME technique to measuremt and mt̄ directly and independently using 3.6 fb−1 of data, and finds ∆m ≡ mt − mt̄ =0.8 ± 1.8 GeV 20, in agreement with the SM prediction. The results are illustrated in Fig. 2.With 0.5 GeV, the systematic uncertainty on ∆m is much smaller than that on mtop dueto cancellations in the difference, and is dominated by the uncertainty on the difference incalorimeter response to b and b̄ quark jets. CDF uses a template-based method and a kinematicreconstruction similar to that in Ref. 14 to measure ∆m directly given the constraint mt+mt̄2 ≡172.5 GeV from 8.7 fb−1 of data, and finds ∆m = −2.0± 1.3 GeV 21.7 Tevatron combination and outlookCurrently, the world’s most precise measurements of mtop are performed by CDF and D0 col-laborations in `+ jets final states. The preliminary Tevatron combination using up to 5.8 fb−1of data results in mtop = 173.2± 0.9 GeV 22, corresponding to a relative uncertainty of 0.54%.With about 10.5 fb−1 recorded, the precision on mtop is expected to further improve, espe-cially at D0, where only 3.6 fb−1 are used in the flagship measurement in ` + jets final states.This applies not only to the statistical uncertainty, but also to several systematic uncertaintiesdue to the limited size of calibration samples, like e.g. some components of the JES. Moreover,efforts are underway to better understand systematic uncertainties from the modeling of tt̄ sig-nal, in particular the dominating uncertainty from different hadronisation and underlying eventmodels. We look forward to exciting updates of mtop measurements presented here.With uncertainties approaching O(GeV) at the LHC23, we strongly advocate to start prepa-rations towards the first world-wide combination of the measurements of the top quark massincluding ATLAS and CMS results.AcknowledgmentsI would like to thank my collaborators from the CDF and D0 experiments for their help inpreparing this article. I also thank the staffs at Fermilab and collaborating institutions, as wellas the CDF and D0 funding agencies.References1. F. Abe et. al. (CDF Coll.), Phys. Rev. Lett. 74, 2626 (1995), S. Abachi et. al. (D0 Coll.),Phys. Rev. Lett. 74, 2632 (1995).2. T. Aaltonen et. al. (CDF Coll.), Phys. Rev. Lett. 103, 092001 (2009), V. M. Abazov et.al. (D0 Coll.), Phys. Rev. Lett. 103, 092002 (2009).3. R. Lopes de Sà, these proceedings.4. A. B. Galtieri et. al., arXiv:1109.2163 [hep-ex] (2011).5. D. Mietlicki, these proceedings; A. Lister, these proceedings.6. http://www-cdf.fnal.gov/physics/new/top/public_mass.html7. http://www-d0.fnal.gov/Run2Physics/WWW/results/top.htm,http://www-d0.fnal.gov/Run2Physics/WWW/documents/Run2Results.htm.8. I. Aracena, these proceedings.9. B. Wu, these proceedings; R. G. Suarez, these proceedings.10. http://lepewwg.web.cern.ch/LEPEWWG/.11. V. M. Abazov et. al. (D0 Coll.), Phys. Rev. D 84, 032004 (2011).12. V. M. Abazov et. al. (D0 Coll.), Nature 429, 638 (2004).13. T. Aaltonen et. al. (CDF Coll.), Phys. Rev. Lett. 105, 252001 (2010).14. T. Aaltonen et. al. (CDF Coll.), CDF Conf. Note 10761 (2012)15. T. Aaltonen et. al. (CDF Coll.), FERMILAB-PUB-11-668-E, submitted toPhys. Rev. Lett., arXiv:1112.4891 [hep-ex] (2011).16. V. M. Abazov et. al. (D0 Coll.), Fermilab-Pub-12/020-E, submitted to Phys. Rev. Lett.,arXiv:1201.5172 [hep-ex] (2012).17. V. M. Abazov et. al. (D0 Coll.), Phys. Lett. B 703, 422 (2011).18. V. Ahrens et. al., J. High Energy Phys. 1009 (2010) 097, Nucl. Phys. B (Proc. 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Measurements of the top quark mass at the Tevatron

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Measurements of the top quark mass at the Tevatron

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