The top quark mass measurement with ATLAS in the lepton plus jets channel is
summarized from the perspective of the early data. Using the invariant mass of
the three jets arising from the hadronic side as the estimator of the top quark
mass, a precision of the order of 1 to 3.5 GeV on the top quark mass
measurement should be achievable, assuming a jet energy scale uncertainty of 1
to 5%.Comment: Poster session at ICHEP08, Philadelphia, USA, July 2008. 3 pages,
  LaTeX, 3 eps figure

Marzin, Antoine

English

arXiv

The top quark mass measurement with ATLAS in the lepton plus jets channel is summarized in the perspective of the early data. Using the invariant mass of the three jets arising from the hadronic side as the estimator of the top quark mass, a precision of the order of 1 to 3.5 GeV on the top quark mass measurement should be achievable, assuming a jet energy scale uncertainty of 1 to 5%.The top quark mass measurement with ATLAS in the lepton plus jets channel is summarized in the perspective of the early data. Using the invariant mass of the three jets arising from the hadronic side as the estimator of the top quark mass, a precision of the order of 1 to 3.5 GeV on the top quark mass measurement should be achievable, assuming a jet energy scale uncertainty of 1 to 5%

CERN Document Server

arXiv:0809.3692v3  [hep-ex]  24 Aug 200934th InternationalConferenceon High Energy Physics,Philadelphia,2008Top quark mass measurement with ATLASAntoine Marzin (on behalf of the ATLAS collaboration)CEA-Saclay, IRFU, SPP, F-91191 Gif-sur-Yvette, FranceThe top quark m ass m easurem ent with ATLA S in the lepton (‘ = e,) plus jets channelis sum m arized from theperspective ofthe rstfb  1 ofdata taking. U sing the invariantm ass ofthe three jets arising from the hadronic sideas the estim ator ofthe top quark m ass,a precision ofthe orderof1 to 3.5 G eV on the top quark m assm easurem entshould be achievable with 1 fb  1 ofcollected data,assum ing a jetenergy scale uncertainty of1 to 5 % .1. IntroductionW ith a m assclose to the scale ofthe electroweak sym m etry breaking,the top quark m assM top isa particularlyim portant param eter ofthe Standard M odel(SM ).For instance,we can test the consistency ofany theory (SMand itsextensions)and probe Physicsbeyond the SM by com paring the m easurem entsofthe electroweak precisionobservables with the theoreticalpredictions ofthe m odelunder consideration. As a distinctive feature,the topquark m assentersquadratically in the W boson m assM W while the dependence on the Higgsboson m assM H isonly logarithm ic. Consequently,accurate m easurem ents ofboth M W and M top are required to provide stringenttheoreticalconstraints on M H in order to test the consistency ofthe SM in the hypothesis ofa SM Higgs bosondiscovery.At the LHC,top quarks willbe produced m ainly in tt pair through the hard process gg ! tt (90% ) andqq ! tt(10% )with a crosssection atthe nextto leading orderof833 pb. Therefore,about800 000 ttpairswillbe produced with the integrated lum inosity of1 fb  1 expected in 2009. This huge statistic available at the LHCwillallow usto perform a directtop quark m assm easurem entby determ ining the peak ofthe invariantm assoftheparticlesarising from the top quark decay.W ith a width   ’ 1.5 G eV,roughly an orderofm agnitude greaterthatQ C D ,thetop quark hassuch a shortlifetim e thatitdecaysbeforehadronizing and alm ostexclusively through them ode t ! W b. Isum m arize here the m easurem entofM top in the lepton (‘ = e,) plus jets channel,where oneofthe W bosonsdecayshadronically while the otherone decaysleptonically (the analysisis detailed in [1]). Thischannelis the m ostprom ising one since it constitutes the best com prom ise between a high branching ratio and ahigh signaloverbackground ratio,the lepton ofthe nalstateallowing a good rejection ofthe Q CD background.2. Event selection and backgroundThe m ajorsource ofphysicalbackground (sum m arized in Table 1)are single top eventsand W + jetsproductionwith leptonicW boson decay.An analysisperform ed on fastsim ulation hasshown thatbackgroundsfrom Q CD andbbproduction arenegligibleafterthe leptonic cuts(isolated lepton,pT , =E T ).Allparticlesform ing thettnalstatearerequired to liewithin jj 2:5.In orderto reducetheQ CD backgroundand the tt fully hadronic and di-lepton processes,we require =E T higher than 20 G eV and exactly one isolatedlepton (‘ = e,)with pT  20 G eV. To reduce the W + jetsbackground,exactly 2 b-tagged jetsare required withpT  40 G eV. Finally,we require atleasttwo lightjetswith pT  40 G eV. Thiscutleadsto an im portantlossofeciency since only 34 % ofW bosonswith hadronic decay have both jetspassing thisrequirem ent,butjetsbelow40 G eV are known to be less precisely calibrated. M oreover,30 % ofsignalevents have m ore than two light jetsbecause ofinitialand nalstate radiation (ISR/FSR);therefore,we require atleasttwo lightjets(and notexactlytwo).The eciency ofthe selection with respect to l+ jets events is 5 % and,starting from a signaloverbackgroundratio very unfavorable,the successivecutsallow usto reach a signaloverbackground ratio of10 atthisstep ofthereconstruction.134th InternationalConferenceon High Energy Physics,Philadelphia,2008Table I:Num berofrem aining eventsforthe m ain backgroundsto the sem i-leptonic signalafterthe successive cuts.Process Num berofevents 1 isolated lepton  4 jets 2 b-jets(L = 1 fb  1) pT  20 G eV pT  40 G eV pT  40 G eVand =E T  20G eVSignal‘= e, + jets 313200 132380 43370 15780W boson backgrounds 9.5  105154100 9450 200all-jets(top pairs) 466 480 1 020 560 160di-lepton (top pairs) 52 500 16 470 2 050 720single top:t,W tand schannels 91 810 33 470 2 011 5053. Hadronic W boson mass reconstructionFortherem aining eventswith m orethan two lightjetsin thenalstate,wehaveto nd thetwo jetsarising fromthe hadronically-decaying W boson.The hadronicW boson candidatesareselected in a m asswindow of 30 G eVaround thepeak valueofthedijetsdistribution ofeventsreconstructed with exactly two lightjets(Fig.1).W ethenperform an eventby event2 m inim ization foreach possible way ofpairing two lightjetsand choose the pairwiththe sm allest2 asthe hadronicW boson candidate.The 2 form ula containsa term which constrainsthe invariantm assofthe two jetsM jj to the W boson m assand width from the ParticleData G roup asfollows:2 =(M jj(1;2)  MPD GW )2( PD GW)2+(E j1(1  1))22E 1+(E j2(1  2))22E 2:The m ain asset ofthis m ethod is to reduce the system atic uncertainty due to the light jet energy scale since weperform an event by event in-situ rescaling oflight jets by constraining the jet energies to their m easured valuewithin the jet energy resolution E . Further on,only events with a hadronic W boson candidate within a m asswindow of 2  PD GM W ( PD GM W= 2.1 G eV)arekept. [GeV]jjM0 50 100 150 200 250Events/2 GeV05010015020050 Signal Physics backgroundCombinatorial backgroundATLAS-11 fbFigure1:Invariantm assoflightjetpairsforeventswithonly two lightjets. minimization [GeV]2χ after WM50 60 70 80 90 100 110Events/0.5 GeV0200400600800100012001400160018002000Signal Physics backgroundCombinatorial backgroundATLAS-11 fbFigure2:HadronicW boson m assafterthe2m inim iza-tion.4. Top quark mass determination with the hadronic sideO nce the hadronic W boson candidate is selected,we have to choose am ong the two b-jets the right one to beassociated with the hadronic W boson in order to reconstruct the top quark m ass. Severals m ethods have beeninvestigated and weobtain a betterpurity by picking the b-jetwhich isthe closestto the hadronicW boson.234th InternationalConferenceon High Energy Physics,Philadelphia,2008The naleciency ofthe reconstruction with respect to l+ jets events is 2.22  0.03 % with a top purity of40.2  0.8 % .Thereconstructed top quark m assdistribution isshown on Fig.3;thetisthesum ofa G aussian forthe signaland a third degree polynom ialforthe com binatorialbackground. Fora generated m assof175 G eV,wereconstructM top = 175.0  0.2 G eV with a width equalto 11.6  0.2 G eV. [GeV]jjbM50 100 150 200 250 300 350 400Events/4 GeV0100200300400500 Signal Combinatorial background Physics background ATLAS -11 fbFigure 3:Thehadronictop quark m ass,twith a G aus-sian and a third orderpolynom ial,scaled to 1 fb  1.System atic uncertainty Shifton M topLightjetenergy scale 0.2 G eV /%b jetenergy scale 0.7 G eV /%ISR/FSR ’ 0.3 G eVb quark fragm entation  0.1 G eVBackground negligibleM ethod 0.1 to 0.2 G eVTable II:System atic uncertaintieson the top quarkm assm easurem entin the sem i-leptonic channel.5. Systematic uncertaintiesThestatisticaluncertainty on M top willbequickly negligibleattheLHC;with 1 few fb  1 ofintegrated lum inosity,the experim entalerror on M top willbe dom inated by the system atic uncertainties sum m arized in Table 2. SinceM top ism easured asthe invariantm assofthree jets,the jetenergy scale constitutesthe m ain source ofsystem aticuncertainties. Its incidence on the top quark m ass m easurem ent is estim ated by m ultiplying separately the lightjet and b-jet m om enta by a rescaling factor without m odifying =E T . The resulting top quark m ass being linearlydependenton therescaling factor,wecan expresstherelated uncertainty asa shifton M top by percentofjetenergyscale m iscalibration. Note the reduced dependence on the lightjetenergy scale com pared to the b-jetenergy scalethanksto the W m assconstraintin the2 m inim ization.6. Conclusion and perspectivesThe statisticaluncertainty being negligible at the LHC,the error on the top quark m ass m easurem ent willbedom inated by thesystem aticuncertaintiesand a precision oftheorderof1 to 3.5 G eV on M top should beachievablewith 1fb  1 ofcollected data,assum ingajetenergyscaleuncertaintyof1to5% .Severalstudieshavebeen perform edusingarelaxed requirem enton theb-taggingtoincreasetheeciency duringthecom m issioningphaseofdatataking.Even with oneorno b-tagged jet,westillhavea good signaloverbackground ratio and,using a kinem aticton theentire nalstate in orderto decrease the com binatorialbackground,we should reach a precision on M top ofabout5 G eV for100 pb  1 assum ing a jetenergy scale uncertainty of5 % .The developm entofotherm ethodsforthe topquark m assm easurem entwith ATLAS are in progressin the three dierentchannels:the m atrix elem entm ethod,the tem plate m ethod and the b decay length m ethod which exploitsthe correlation between M top and the b quarkm om entum when the top quark decaysatrest.References[1]ATLAS Collaboration,Expected Perform anceofthe ATLAS Experim ent,Detector,Triggerand Physics,CERN-O PEN-2008-020,G eneva,2008,to appear.3

Top quark mass measurement with ATLAS

The top quark mass measurement with ATLAS in the lepton plus jets channel is summarized from the perspective of the first data taking at a center of mass energy of 14 TeV. Using the invariant mass of the three jets arising from the hadronic top quark decay as the estimator of the top quark mass, a precision of the order of 1 to 3.5 GeV on the top quark mass measurement should be achievable with 1 fb^{-1} of collected data, assuming a jet energy scale uncertainty of 1 to 5 %

Marzin, A

ATL-PHYS-PROC-2009-08330August2009Top quark mass measurement with ATLASAntoine Marzin on behalf of the ATLAS collaboration∗CEA-Saclay, IRFU, SPP, F-91191 Gif-sur-Yvette, FranceE-mail: antoine.marzin@cea.frThe top quark mass measurement with ATLAS in the lepton (ℓ = e,µ) plus jets channel is sum-marized from the perspective of the first fb−1 of data taking at a center of mass energy of 14 TeV.Using the invariant mass of the three jets arising from the hadronic top quark decay as the es-timator of the top quark mass, a precision of the order of 1 to 3.5 GeV on the top quark massmeasurement should be achievable with 1 fb−1 of collected data, assuming a jet energy scaleuncertainty of 1 to 5 %.The 2009 Europhysics Conference on High Energy Physics,July 16 - 22 2009Krakow, Poland∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/Top quark mass measurement with ATLAS Antoine Marzin on behalf of the ATLAS collaboration1. IntroductionWith a mass close to the scale of the electroweak symmetry breaking, the top quark mass mtopis a particularly important parameter of the Standard Model (SM). We can test the consistency ofany theory (SM and its extensions) and probe Physics beyond the SM by comparing the measure-ments of the electroweak precision observables with the theoretical predictions of the model underconsideration. For instance, accurate measurements of both MW and mtop are required to providestringent theoretical constraints on the SM Higgs boson mass.At the LHC, top quarks will be produced with a cross section at the next to leading orderof 833 ± 100 pb at a center of mass energy of 14 TeV. The huge statistics available at the LHCwill allow us to perform a direct top quark mass measurement by determining the peak of theinvariant mass of the particles arising from the top quark decay. With a width Γ ≃ 1.5 GeV,roughly an order of magnitude greater that ΛQCD, the top quark has such a short lifetime that itdecays before hadronizing and almost exclusively through the mode t → Wb. I summarize herethe measurement of mtop in the lepton (ℓ = e,µ) plus jets channel, where one of the W bosons decayshadronically while the other one decays leptonically [1]. This channel is the most promising onesince it constitutes the best compromise between a high branching ratio and a high signal overbackground ratio, the lepton of the final state allowing a good rejection of the QCD background.2. Top quark mass recontructionEvent selection and background The major source of physical background are t ¯t events in fullyhadronic and dilepton channels, single top events and W+jets production with leptonic W bosondecay. In order to maximize the signal over background ratio, we apply the following cuts:• /ET≥ 20 GeV and exactly 1 isolated lepton (e,µ) with |η | ≤ 2.5 and pT ≥ 20 GeV. These cutsreject fully hadronic and dilepton t ¯t events, and QCD background events.• At least 2 light jet and exactly 2 b-tagged jets with |η | ≤ 2.5 and pT ≥ 40 GeV. The b-taggingleads to a significant loss of efficiency but is required to reject the W+jets background anddecrease the combinatoric background, i.e. events where one of the selected jets does notmatch a quark from the hadronic top decay.The efficiency of the selection with respect to all ℓ(e,µ) + jets events is 5 % and the successivecuts allow us to reach a signal over background ratio of 10 at this step of the selection.Hadronic W boson mass reconstruction In order to find the two jets arising from the hadronically-decaying W boson, we perform an event by event χ2 minimization for each possible way of pairingtwo light jets, allowing the jet energy to vary within the jet energy resolution σE :χ2 = (Mjj(α1,α2)−MPDGW )2(ΓPDGW )2+(Ej1(1−α1))2σ2E1+(Ej2(1−α2))2σ2E2.The free parameters to be determined are the jet energy scale correction factors α and the light jetpair with the lowest χ2 after minimization is kept as the hadronic W boson candidate. The mainasset of this method is the event by event in-situ rescaling of light jets based on the W boson massconstraint which reduces the systematic uncertainty due to the light jet energy scale.2Top quark mass measurement with ATLAS Antoine Marzin on behalf of the ATLAS collaborationTop quark mass determination with the hadronic side To reconstruct the top quark mass, wehave to choose among the two b-jets the right one to be associated with the hadronic W boson. Theb-jet the closest to the hadronic W boson is selected. The final efficiency of the reconstruction withrespect to ℓ(e,µ) + jets events is 2.22± 0.03 % with a top purity of 40.2± 0.8 %. The reconstructedtop quark mass distribution is shown in Fig. 1; the fit is the sum of a Gaussian for the signal and athird degree polynomial for the combinatorial background. For a generated mass of 175 GeV, wereconstruct mtop = 175.0 ± 0.2 GeV with a width equal to 11.6 ± 0.2 GeV. [GeV]jjbM50 100 150 200 250 300 350 400Events/4 GeV0100200300400500 Signal Combinatorial background Physics background ATLAS -11 fbFigure 1: Hadronic top quark mass (L =1 fb−1).Systematic uncertainty Shift on mtopLight jet energy scale 0.2 GeV/%b jet energy scale 0.7 GeV/%ISR/FSR ≃ 0.3 GeVb quark fragmentation ≤ 0.1 GeVBackground negligibleMethod 0.1 to 0.2 GeVTable I: Systematic uncertainties.3. Systematic uncertaintiesThe statistical uncertainty on mtop will be quickly negligible at the LHC; with 1 fb−1 of inte-grated luminosity, the experimental error on mtop will be dominated by the systematic uncertaintiessummarized in Table I. Since mtop is measured as the invariant mass of three jets, the jet energyscale constitutes the main source of systematic uncertainties. Its effect on the top quark massmeasurement has been estimated by multiplying separately the light jet and b-jet momenta by arescaling factor. The resulting top quark mass being linearly dependent on the rescaling factor, wecan express the related uncertainty as a shift on mtop by percent of jet energy scale miscalibration.4. Conclusion and perspectivesThe statistical uncertainty being negligible at the LHC, the error on the top quark mass mea-surement will be dominated by the systematic uncertainties and a precision of the order of 1 to3.5 GeV on mtop should be achievable with 1 fb−1 of collected data at a center of mass energy of14 TeV, assuming a jet energy scale uncertainty of 1 to 5 %. The development of other methodsfor the top quark mass measurement with ATLAS are in progress in the three channels of t ¯t decay:the matrix element method, the template method and the b decay length method which exploits thecorrelation between mtop and the b quark momentum when the top quark decays at rest.References[1] Atlas Collaboration, Expected Performance of the ATLAS Experiment Detector, Trigger, Physics,arXiv:0901.0512v1, pages 978-1002 (2009)3

Top quark mass measurement with ATLAS

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