Measurements of the top quark mass from the LHC and the Tevatron

1 MB·

by  Oleg Brandt

Measurements of the top quark mass from the LHC
and the Tevatron
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Oleg Brandt∗

n
onbehalfoftheATLAS,CDF,CMS,andD0Collaborations
a
J UniversitätHeidelberg,Kirchhoff-InstitutfürPhysik,INF227,

9 69120Heidelberg,Germany
1 E-mail: [email protected]

]
x
e The mass of the top quark is a fundamental parameter of the standard model and has to be de-
-
p termined experimentally. In these proceedings, I review recent measurements of the top quark
√
e
mass in pp collisions at s=7, 8, and 13 TeV recorded by the ATLAS and CMS detectors at
h √
[ theLHC,andin pp¯collisionsat s=1.96TeVrecordedbytheCDFandD0experimentsatthe

1 Tevatron. The measurements are performed in final states containing two, one, and no charged
v leptons. Arelativeprecisionofdownto0.3%isattained. Inaddition,recentmeasurementsaim-
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8 ingtodeterminethetopquarkmassinthewell-definedpoleschemeusingbothinclusivett¯and
4 tt¯+1jetproductionarepresented.
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i
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r
a
9thInternationalWorkshopontheCKMUnitarityTriangle
28November-3December2016
TataInstituteforFundamentalResearch(TIFR),Mumbai,India
∗Speaker.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommons
Attribution-NonCommercial-NoDerivatives4.0InternationalLicense(CCBY-NC-ND4.0). http://pos.sissa.it/
TopquarkmassmeasurementsfromLHC+Tevatron OlegBrandt
1. Introduction
Sinceitsdiscovery[1,2],thedeterminationofthetopquarkmassm ,afundamentalparameter
t
of the standard model (SM), has been one of the main goals of the CERN Large Hadron Collider
(LHC)andoftheFermilabTevatronCollider. Indeed,m andmassesofW andHiggsbosonsarere-
t
latedthroughradiativecorrectionsthatprovideaconsistencycheckoftheSM[3,4]. Furthermore,
m dominantlyaffectsthestabilityoftheSMHiggspotential[4,5]. Withm =173.34±0.76GeV,
t t
aworld-averagecombinedprecisionof0.44%hasbeenachieved[6].
In the SM, the top quark decays to aW boson and a b quark nearly 100% of the time. Thus,
tt¯events are classified according toW boson decays as “dileptonic” ((cid:96)(cid:96)), “lepton+jets” ((cid:96)+jets),
or “all–jets”. Single top production contributes significantly at the LHC through the qg → q(cid:48)tb¯
process. In the following, I will present representative measurements in the three channels; a full
listingofm resultsfromtheLHCandtheTevatroncanbeaccessedthroughRefs.[7,8,9,10].
t
2. Standardmeasurementsofthetopquarkmass
The most precise single measurement of m in the (cid:96)(cid:96) channel is performed by the ATLAS
t
√
Collaboration using 20.2 fb−1 of pp collisions at s=8 TeV [11]. The selection requires two
isolatedleptons(eor µ)ofoppositecharge,missingtransversemomentumEmiss duetoneutrinos,
T
and≥2jets, whereatleastoneofwhichisidentifiedasoriginatingfromabquark(b-tagged). A
transversemomentum p >120GeVisrequiredfortheaverageofthetwo(cid:96)bsystemstoreduce
T,(cid:96)b
the dominant uncertainty from the jet energy scale (JES). The m is extracted with the “template
t
method”,whichinthiscasefitsthedistributionintheaverageinvariantmassofthe(cid:96)bsystemtothe
expectationsfromMonteCarlo(MC)simulationsfordifferentm ,showninFig.1(a). Thebestfit
t
todataisshowninFig.1(b),andresultsinm =172.99±0.41(stat)±0.74(syst)GeV.Tevatron’s
t
most precise single measurement in the (cid:96)(cid:96) channel of m =173.32±1.36(stat)±0.85(syst) GeV
t
√
isperformedbytheD0Collaborationusing9.7fb−1 of pp¯collisionsat s=1.96TeV[12].
Themostprecisesinglemeasurementofm fromtheTevatronisperformedbytheD0Collab-
t
oration using 9.7 fb−1 of data in the (cid:96)+jets channel [13] with a “matrix element (ME) method”.
This approach determines the probability of observing a given event under both the tt¯signal and
(a)
(b)
Figure 1: (a) Expected dependence of the m distribution of processes involving top quarks on m from
√ (cid:96)b t
Monte Carlo simulations at s=8 TeV with the ATLAS detector [11]. (b) The distribution in m in
√ (cid:96)b
20.3fb−1ofdataat s=8TeVwiththeATLASdetector. Thepredictionscorrespondtothebest-fitvalues.
2
TopquarkmassmeasurementsfromLHC+Tevatron OlegBrandt
background hypotheses, as a function of m . This probability is calculated ab initio using the re-
t
spective MEs of the tt¯signal and dominantW+jets background, taking into account effects from
parton showering (PS), hadronisation, and finite detector resolution. This selection requires the
presence of one isolated lepton, Emiss, and exactly four jets with at least one b-tag. A new JES
T
calibration from exclusive γ+jet, Z+jet, and dijet events is applied to account for differences in
detector response to jets originating from a gluon, a b quark, and u,d,s, or c quarks. The over-
all JES k is calibrated in situ by constraining the reconstructed invariant mass of the hadron-
JES
ically decaying W boson to M = 80.4 GeV. The likelihood over all candidate events is max-
W
imised in (m ,k ) as shown in Fig. 2 (a), and m =174.98±0.58(stat+JES)±0.49(syst) GeV
t JES t
is obtained. The most precise m result from the CDF Collaboration in the (cid:96)+jets channel of
t
m =172.85±0.71(stat+JES)±0.85(syst)GeV[14]isobtainedwiththetemplatemethod.
t
The most precise single measurement of m from the LHC is performed by the CMS Collab-
t
√
oration using 19.7 fb−1 of data at s=8 TeV in the (cid:96)+jets channel [15]. The analysis uses a
similar selection to the D0 result and applies the “ideogramm method” to extract m . Similar to
t
the ME method, this approach calculates the probability to observe a given event as a function of
(m ,k ). However, this probability is not calculated ab initio, but is obtained from MC simu-
t JES
lations, in analogy to the template method. The final result of m =172.35±0.16(stat+JES)±
t
0.48(syst) GeV is represented in Fig. 2 (b). The most precise m result from the ATLAS Col-
t
√
laboration is obtained with the template method using 4.7 fb−1 of data at s=7 TeV and reads
m =172.33±0.75(stat+JES)±1.02(syst)GeV[16].
t
(b)
√
Figure2: (a)Thelikelihoodin(m,k )in9.7fb−1 of pp¯ collisionsat s=1.96TeVrecordedwiththe
t JES
D0detector[13]. Fittedcontoursofequalprobabilityareoverlaidassolidlines. Themaximumismarked
√
with a cross. (b) Same as (a), but in 19.5 fb−1 of pp collisions at s=8 TeV recorded with the CMS
detector[15]. Thecentralresultcorrespondsto“Hybrid”,andk isdenotedas“JSF”.
JES
The all-jets channel is particularly challenging due to very high background from QCD mul-
tijets. Tevatron’s most precise single m result in this channel comes from the CDF Collaboration
t
using 9.3 fb−1 of data [17]. A neural network and b-tagging enhance the signal-to-background
ratio from 10−3 to about 1. The correct assignment of jets to partons is determined by min-
imising a χ2, which accounts for consistency of the two dijet systems with m , consistency
W
of the two jjb systems with each other, and consistency of the individual fitted jet momenta
with measured ones, within experimental resolutions. The measured value is m = 175.07±
t
1.19(stat+JES)±1.55(syst) GeV. The most precise result in the all-jets channel at the LHC of
m =172.32±0.25(stat+JES)±0.59(syst)GeVcomesfromtheCMSCollaboration[15].
t
3
TopquarkmassmeasurementsfromLHC+Tevatron OlegBrandt
Anoverviewofrecentm measurementsattheLHC[18]isgiveninFig.3. Acombinationof
t
m measurementsfromRunIandIIoftheTevatronconsideringstatisticalandsystematiccorrela-
t
tionsyieldsm =174.30±0.35(stat)±0.34(syst)GeV[19].
t
ATLAS+CMS Preliminary LHCtop WG mtop summary, s = 7-8 TeV Aug 2016
World Comb. Mar 2014, [7]
stat
total uncertainty total stat
mtop = 173.34 ± 0.76 (0.36 ± 0.67) GeV mtop ± total (stat ± syst) s Ref.
ATLAS, l+jets (*) 172.31 ± 1.55 (0.75 ± 1.35) 7 TeV [1]
ATLAS, dilepton (*) 173.09 ± 1.63 (0.64 ± 1.50) 7 TeV [2]
CMS, l+jets 173.49 ± 1.06 (0.43 ± 0.97) 7 TeV [3]
CMS, dilepton 172.50 ± 1.52 (0.43 ± 1.46) 7 TeV [4]
CMS, all jets 173.49 ± 1.41 (0.69 ± 1.23) 7 TeV [5]
LHC comb. (Sep 2013) 173.29 ± 0.95 (0.35 ± 0.88) 7 TeV [6]
World comb. (Mar 2014) 173.34 ± 0.76 (0.36 ± 0.67) 1.96-7 TeV [7]
ATLAS, l+jets 172.33 ± 1.27 (0.75 ± 1.02) 7 TeV [8]
ATLAS, dilepton 173.79 ± 1.41 (0.54 ± 1.30) 7 TeV [8]
ATLAS, all jets 175.1 ± 1.8 (1.4 ± 1.2) 7 TeV [9]
ATLAS, single top 172.2 ± 2.1 (0.7 ± 2.0) 8 TeV [10]
ATLAS, dilepton 172.99 ± 0.85 (0.41 ± 0.74) 8 TeV [11]
ATLAS, all jets 173.80 ± 1.15 (0.55 ± 1.01) 8 TeV [12]
ATLAS comb. (lJ+ujentes ,2 d01il6.) 172.84 ± 0.70 (0.34 ± 0.61) 7+8 TeV [11]
CMS, l+jets 172.35 ± 0.51 (0.16 ± 0.48) 8 TeV [13]
CMS, dilepton 172.82 ± 1.23 (0.19 ± 1.22) 8 TeV [13]
CMS, all jets 172.32 ± 0.64 (0.25 ± 0.59) 8 TeV [13]
CMS, single top 172.60 ± 1.22 (0.77 ± 0.95) 8 TeV [14]
CMS comb. (Sep 2015) 172.44 ± 0.48 (0.13 ± 0.47) 7+8 TeV [13]
[1] ATLAS-CONF-2013-046 [6] ATLAS-CONF-2013-102 [11] arXiv:1606.02179
(*) Superseded by results [[23]] AJHTELAP S1-2C (O2N01F2-2) 011035-077 [[78]] aEruXri.vP:h1y4s0.3J..4C47257 (2015) 330 [[1123]] APhTyLsA.SR-eCvO.DN9F3- 2(2001166-0) 60472004
shown below the line [4] Eur.Phys.J.C72 (2012) 2202 [9] Eur.Phys.J.C75 (2015) 158 [14] CMS-PAS-TOP-15-001
[5] Eur.Phys.J.C74 (2014) 2758 [10] ATLAS-CONF-2014-055
165 170 175 180 185
m [GeV]
top
Figure3: Overviewofrecentm measurementsattheLHC[18].Referencestotheindividualmeasurements
t
aregivenatthebottomoftheFigure.
3. Measurementsofthetopquarkmassinthepolescheme
The standard measurements of m from Sect. 2 are experimentally the most precise ones.
t
However, they extract an m parameter as implemented in MC generators, which is related to the
t
polemassschemedefinitionmpole intheSMLagrangianwithinanuncertaintyof≤1GeV[20].
t √
The first LHC result on m at s = 13 TeV is an extraction of mpole from σ performed
t t tt¯
by CMS in the (cid:96)+jets channel using 2.3 fb−1 of data [21]. This analysis exploits the depen-
dence of σ on mpole, which is now known with ≈3% precision at NNLO with NNLL correc-
tt¯ t
tions [22]. The input measurement of σ achieves a relative uncertainty of ≈4% by constraining
tt¯
the dominant W+jets background through sidebands in low jet and b-tag multiplicities, and us-
ing the difference in dσ/dm dependence between signal and background. The final result is
(cid:96)b
mpole=173.3+2.3(stat+syst)+1.6(theo)GeV.
t −2.0 −1.1
Themostprecisempole measurementisperformedbytheATLASCollaborationinthe(cid:96)+jets
t √
channelusing4.6fb−1ofdataat s=7TeV[23]. Thempoleisextractedfromfromtheproduction
t
cross section of a tt¯ system in association with a jet σ , since the radiation rate of a high-
tt¯+1jet
p gluon off the tt¯ system is proportional to mpole. More precisely, the differential production
T t
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TopquarkmassmeasurementsfromLHC+Tevatron OlegBrandt
crosssectionR(mpole,ρ )≡1/σ ·dσ /dρ iscomparedtoNLOcalculations[24],where
t s tt¯+1jet tt¯+1jet s
√
ρ ≡2m / s ,andthearbitraryconstantm issetto170GeVinthisanalysis. Theselection
s 0 tt¯+1jet 0
is similar to other analyses in the (cid:96)+jets channel discussed in Sect. 2, and the correct jet-parton
assignmentisdeterminedthroughaχ2 kinematicfit. Toreducethetotaluncertainty, p >50GeV
T
isrequiredfortheextrajet. Thedistributioninρ iscorrectedfordetector,PS,hadronisationeffects,
s
andthepresenceofbackground. TheresultingdistributionatpartonlevelisgiveninFig.4(a). The
finalresultreadsmpole=173.1±1.50(stat)±1.43(syst)+0.93(theo)GeV.
t −0.49
The second most precise mpole measurement is performed by the D0 Collaboration in the
t
(cid:96)+jets channel using 9.7 fb−1 of data [25]. This analysis extracts mpole by relating measured
t
dσ /dm (m )anddσ /dp (m )torecentNNLOandNLOcalculations[26]. Differentialcross
tt¯ tt¯ t tt¯ T,t/t¯ t
sectionsallowforamorecompleteuseofkinematicinformation,andthusanotablyhigherstatisti-
calprecisionthanthempole extractionfromaninclusiveσ measurement. Theselectionissimilar
t tt¯
toRef.[13],andthecorrectjet-partonassignmentisidentifiedthrougha χ2 kinematicfit. There-
sultingdistributionsarecorrectedfordetector,PS,hadronisationeffects,andthepresenceofback-
ground to obtain dσ /dm (m ) and dσ /dp (m ), which are then directly compared to theory
tt¯ tt¯ t tt¯ T,t/t¯ t
calculationstoextractmpole. Thefinalresultreadsmpole=169.1±2.5(stat+syst)±1.5(theo)GeV.
t t
(a) (b)
√
Figure4: (a)ThedistributionR≡1/σtt¯+1jet·dσtt¯+1jet/dρsin ppcollisionsat s=7TeVwiththeATLAS
det√ector[23], comparedtoNLOpredictions[24]. (b)Thedistributionofdσtt¯/dpT,t/t¯(mt)in pp¯ collisions
at s=1.96 TeV with the D0 detector [25], compared to NNLO predictions [26]. Both distributions are
shownatpartonlevel,aftercorrectionsfordetector,PS,andhadronisationeffects.
4. Conclusions
I presented recent measurements of the top quark mass, a fundamental parameter of the
SM. The most precise single measurements at the LHC and the Tevatron of respectively m =
t
172.35±0.16(stat+JES)±0.48(syst)GeVandm =174.98±0.58(stat+JES)±0.49(syst)GeV
t
areperformedbytheCMSandD0Collaborationsinthe(cid:96)+jetschannel,correspondingtoarelative
precisionof0.30%and0.43%. Theprecisionofm measurementsinthepoleschemeisimproved
t
to1.3%duetotheadventofnewtheorycalculationsandexperimentalapproaches.
I would like to thank my colleagues from the ATLAS, CDF, CMS, and D0 experiments for
theirhelpinpreparingthisarticle,thestaffsatCERNandFermilabtogetherwiththeircollaborating
institutions,aswellastherelevantfundingagencies.
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TopquarkmassmeasurementsfromLHC+Tevatron OlegBrandt
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