Higher-order QCD predictions are used to extract the top quark mass, both in the pole and in the MS scheme, from the top quark pair production cross section measured in the dilepton final state. The analyzed dataset corresponds to an integrated luminosity of 1.14 fb−1 collected by the CMS experiment in 2011 in pp collisions at √s = 7TeV

Aldaya, M.

Lipka, K.

Naumann-Emme, S.

English

Aldaya, M

Lipka, K

Naumann-Emme, S

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Determination of the top quark mass from the t anti-t cross section measured by CMS at s**(1/2) = 7-TeV

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DOI 10.1393/ncc/i2012-11244-3Colloquia: TOP2011IL NUOVO CIMENTO Vol. 35 C, N. 3 Maggio-Giugno 2012Determination of the top quark mass from the tt¯ cross sectionmeasured by CMS at√s = 7TeVM. Aldaya, K. Lipka and S. Naumann-EmmeDeutsches Elektronen Synkrotron (DESY) - Germanyricevuto l’ 1 Marzo 2012pubblicato online il 4 Giugno 2012Summary. — Higher-order QCD predictions are used to extract the top quarkmass, both in the pole and in the MS scheme, from the top quark pair productioncross section measured in the dilepton final state. The analyzed dataset correspondsto an integrated luminosity of 1.14 fb−1 collected by the CMS experiment in 2011in pp collisions at√s = 7TeV.PACS 14.65.Ha – Top quarks.1. – IntroductionThe top quark mass (mt) is an important parameter of the Standard Model. Itsprecise measurement is one of the most relevant inputs to the global electroweak fitswhich provide constraints on the properties of the Higgs boson.Beyond leading-order (LO) Quantum Chromodynamics (QCD) predictions, mt de-pends on the renormalization scheme and its value can differ considerably for, e.g., polemass or MS mass definitions. It is therefore important to understand how to interpretthe experimental result in terms of the renormalization conventions.Direct measurements of the top quark mass from the reconstruction of the final statesin top-antitop (tt¯) decays rely highly on the detailed description of the correspondingsignal in Monte Carlo (MC) simulations. The quantity measured in the data is com-pared to the simulation and thus corresponds to the top quark mass definition used inthe MC generator (mMCt ). All currently used MC simulations contain only matrix ele-ments at fixed order (LO or next-to-leading order, NLO) QCD, while higher orders aresimulated by applying parton showers. Therefore, they are not precise enough to fixthe renormalization scheme, which leads to an uncertainty in the input top quark massdefinition.c© Societa` Italiana di Fisica 195196 M. ALDAYA, K. LIPKA and S. NAUMANN-EMMEWe extract the top quark mass by comparing the inclusive tt¯ production cross sec-tion, σtt¯, in the dilepton channel measured by CMS [1] to fully inclusive calculationsat higher-order QCD that involve an unambiguous definition of mt. This extractionprovides an important test of the mass scheme as applied in MC simulations and givescomplementary information, with different sensitivity to theoretical and experimentaluncertainties than the direct measurements of mMCt .2. – Results and conclusionsThe combined σtt¯ in dilepton decays [1] corresponding to an integrated luminosityof 1.14 fb−1 of collected data, σtt¯ = 169.90 ± 3.9(stat.) ± 16.3(syst.) ± 7.6(lumi.) pb, isused to determine the mass of the top quark through its comparison to different higher-order QCD predictions using the pole and the MS mass definitions. The dependenceof σtt¯ on the assumed value of mMCt via the signal acceptance is studied using differentsimulated samples of tt¯ events generated at different values of mMCt and parametrized ina polynomial form for each of the dilepton decay channels. The weighted average of theparametrizations is used to obtain the central value of mt(1).Three different approaches [2-4] to calculate the higher-order corrections to NLO cal-culations of tt¯ production (approximate NNLO) are used to extract mt. The top quarkmass is given in both the pole and MS definitions in the calculations [2, 4], while itis defined as a pole mass in the prediction [3]. The calculations are performed usingthe MSTW08NNLO [5] parton density functions (PDF). The uncertainty of the approx.NNLO calculations includes the error due to variation of renormalization and factoriza-tion scales (μr and μf , respectively), the uncertainty on the PDF added in quadratureand the uncertainty due to variation of the strong coupling constant (αS(MZ)) in thePDF [6].The top quark mass and its uncertainty are determined using a joint-likelihood ap-proach [7, 8]. The probability density function fexp(σtt¯|mt) is constructed from a Gaus-sian distribution with the measured cross section as its mean value, using a third-orderpolynomial to parametrize the mass dependence, and the total experimental uncertaintyas the width of this Gaussian. The probability density function fth(σtt¯|mt) is constructedfrom a Gaussian centered on the predicted cross section, with the standard deviationadjusted to the uncertainty of the prediction. For this purpose, the predictions wereparametrized asσtt¯(mt) =1m4t(a + b ·mt + c ·m2t + d ·m3t).Finally, mt is obtained as the maximum of a combined uncorrelated likelihood con-structed asL(mt) =∫fexp(σtt¯|mt)fth(σtt¯|mt)dσtt¯.Asymmetric uncertainties on mt are determined from the 68.3% area around thatvalue. To estimate the uncertainty arising from the parametrizations for the measured σtt¯(1) Hereafter, mt denotes the top quark mass in both the pole and the MS definition.DETERMINATION OF THE TOP QUARK MASS FROM THE tt¯ CROSS SECTION ETC. 197mtpole (GeV)σ t t_  (pb)CMS Preliminary, √s=7 TeV,  L=1.14 fb-1approx. NNLO × MSTW08NNLO:Langenfeld et al.KidonakisAhrens et al.Measured cross section dependence on mtMCMeasured cross sectionCross section corrected for mtpole (Kidonakis)0200400600140 150 160 170 180 190Fig. 1. – Comparison of the predicted σtt¯(mpolet ) for the different calculations and the measuredcross section (open square) as a function of mpolet . The dependence of the measured σtt¯ on mMCtis shown as a light shaded band. Different approximate NNLO predictions are given as differentlyhatched bands. The measured σtt¯ corrected for the mpolet dependence at the determined valueof mpolet using the calculation [3] is also depicted (filled circle).determined for each dilepton channel, the analysis is repeated for all three parametriza-tions and the spread of the resulting mass values is taken as an uncertainty, which isadded in quadrature to the uncertainty obtained from the shape of the original joint like-lihood. To evaluate the uncertainty by defining mMCt = mpolet , a shift of mMCt by ±1GeVis applied and the resulting shift of the extracted mass is also added quadratically. Infig. 1, the measured cross section together with its dependence on the mMCt assumptionand approx. NNLO predictions are presented. As an example, the value of the measuredcross section corrected for the extracted mpolet using the calculation [3] is also depicted.The values of the top pole mass obtained using the three approx. NNLO predictions aresummarized in table I. In fig. 2 (left), the same values are compared to similar mea-surements by the ATLAS [8] and D0 [7] Collaborations. These results are in very goodagreement. They are consistently lower than the average of the direct measurements atTevatron [9], which is also shown.The theoretical calculations [2, 4] are also available as a function of mMSt . For themeasured cross section, it is again assumed that mMCt ≡ mpolet and this is then translatedinto mMSt using the relation at the three-loop level [10,11]. The extracted values of mMStare also listed in table I and compared in fig. 2 (right) to those obtained by D0 [7].Both the experimental and the theoretical uncertainty contribute similarly to theuncertainty of the top quark mass determination, with the dominant uncertainty on theapprox. NNLO calculations being the variation of the αS(MZ) in the PDF, so far notaccounted for by the previous analyses [7, 8].Alternatively, the top quark mass was obtained using the calculations [2] and [4]with HERAPDF15NNLO [12,13], taking into account the experimental PDF uncertainty,variation of μr and μf and of the αS(MZ) value. The resulting values for mpolet and mMSt198 M. ALDAYA, K. LIPKA and S. NAUMANN-EMMETable I. – Top quark mass in the pole and MS definition as obtained using the measured σtt¯and approximate NNLO calculations with MSTW08NNLO and HERAPDF15NNLO PDFs.Approx. NNLO×MSTW08NNLO Approx. NNLO×HERAPDF15NNLOmpolet /GeV mMSt /GeV mpolet /GeV mMSt /GeVLangenfeld et al. [2] 170.3+7.3−6.7 163.1+6.8−6.1 171.7+6.8−6.0 164.3+6.5−5.7Kidonakis [3] 170.0+7.6−7.1 – – –Ahrens et al. [4] 167.6+7.6−7.1 159.8+7.3−6.8 169.1+6.7−5.9 161.0+6.8−6.1are also listed in table I. They are higher by 1.2–1.5GeV than the results obtained withMSTW08NNLO due to the different values of αS(MZ) that are used in the respectivePDF sets. The uncertainties on the extracted masses are slightly smaller when usingHERAPDF15NNLO.To summarize, we extract the top quark pole and MS mass by comparing the measuredσtt¯ with different higher-order QCD calculations [14]. The measurement was performedby the CMS Collaboration in tt¯ decays with dileptonic final states using 1.14 fb−1 ofcollected data at√s = 7TeV. The results are in very good agreement with similarmeasurements by D0 and ATLAS and provide the first determination of the top quarkpole and MS mass at CMS. The uncertainty of the results is dominated by the systematicerror of the measured σtt¯ and by the PDF uncertainty in the theory, including thevariation of αS(MZ) in the PDF.Top quark pole mass from cross sectionCMS Preliminary, √s=7 TeV, L=1.14 fb-1value ± theo ⊗ exp ± αs(mz)167.6 +6.8 +3.3-6.1 -3.6170.0 +6.6 +3.7-5.8 -4.0170.3 +6.2 +3.8-5.3 -4.0Tevatron direct measurement (July 2011)D0 (L=5.3 fb-1) approx. NNLO ⊗ MSTW08NNLOLangenfeld et al.KidonakisAhrens et al.173.2 +0.9-0.9166.7 +5.2-4.5167.5 +5.2-4.7163 +5.1-4.6ATLAS (Prel., L=35 pb-1) approx. NNLO ⊗ MSTW08NNLOAhrens et al.KidonakisLangenfeld et al.162.2 +8-7.6166.2 +7.8-7.4166.4 +7.8-7.3CMS (Prel., L=1.14 fb-1) approx. NNLO ⊗ MSTW08NNLOAhrens et al.KidonakisLangenfeld et al.mtpole (GeV)140 150 160 170 180 190Top quark MS mass from cross sectionCMS Preliminary, √s=7 TeV, L=1.14 fb-1value ± theo ⊗ exp ± αs(mz)159.8+6.3 +3.6-6.2 -2.7163.1+5.7 +3.7-4.8 -3.7D0 (L=5.3 fb-1) approx. NNLO ⊗ MSTW08NNLOLangenfeld et al.Ahrens et al.160.0+4.8-4.3154.5+5.0-4.3CMS (Prel., L=1.14 fb-1) approx. NNLO ⊗ MSTW08NNLOAhrens et al.Langenfeld et al.mtMS (GeV)⎯           .140 150 160 170 180 190Fig. 2. – Top quark pole (left) and MS mass (right) extracted from the measured σtt¯ comparedto results from [7, 8] and the direct top quark mass world average [9]. The inner error bar onthe CMS result includes the experimental, PDF and scale variation uncertainty, the outer onecorresponds to the uncertainty including variation of αS(MZ) in the PDF.DETERMINATION OF THE TOP QUARK MASS FROM THE tt¯ CROSS SECTION ETC. 199REFERENCES[1] CMS Collaboration, CMS-PAS-TOP-11-005.[2] Langenfeld U., Moch S. and Uwer P., Phys. Rev. D, 80 (2009) 054009.[3] Kidonakis N., Phys. Rev. D, 82 (2010) 114030.[4] Ahrens V. et al., JHEP, 09 (2010) 097.[5] Martin A. D., Stirling W. J., Thorne R. S. et al., Eur. Phys. J. C, 63 (2009) 189.[6] MSTW2008nnlo90cl asmzrange, http://projects.hepforge.org/lhapdf/pdfsets.[7] D0 Collaboration, Phys. Lett. B, 703 (2011) 422.[8] ATLAS Collaboration, ATLAS-CONF-2011-054.[9] Tevatron Electroweak Working Group, arXiv:1107.525.[10] Melnikov K. and Ritbergen T. v., Phys. Lett. B, 482 (2000) 99.[11] Bethke S., Eur. Phys. J. C, 64 (2009) 689.[12] H1 and ZEUS Collaborations, https://www.desy.de/h1zeus/combined results/herapdftable/.[13] HERAPDF15NNLO ALPHAS, http://projects.hepforge.org/lhapdf/pdfsets.[14] CMS Collaboration, CMS-PAS-TOP-11-008.

Determination of the top quark mass from the t¯t cross section measured by CMS at √s = 7TeV

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Determination of the top quark mass from the t¯t cross section measured by CMS at √s = 7TeV

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