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Top quark mass and cross sections in ATLAS and CMS

Matteo M. Defranchis (CERN)on behalf of the ATLAS and CMS Collaborations

55th Rencontres de Moriond 2021 - QCD & High Energy Interactions

Matteo M. Defranchis (CERN)

Introduction

● At LHC, top quarks mainly produced in tt pairs● Inclusive production cross section can be calculated

up to NNLO+NNLL in QCD -> 5%○ Precision -> test of perturbative QCD○ New results in this talk (not included in the plot)

● Differential cross sections can be used to test fixed-order predictions and MC generators

○ Precision -> indirect search for new physics, e.g. in EFT framework (see next talk, by N. Tonon)

● Inclusive and differential measurements can be used to extract QCD parameters

○ Top quark mass, strong coupling constant, PDFs

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Interpretation of mt measurements

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DirectPRL 124 (2020) 202001

Direct measurements● Invariant mass of top quark

decay products● Depends on detail of MC

simulation

Indirect measurements● Measure observable sensitive

to mt (e.g. σtt)● Determine mt in well-defined

renorm. scheme (pole, MS)

Boosted: jet mass can be calculated in SCEFT -> can provide info on relation between mt

MC and mt PRD 77 (2008) 074010

New results not in this plot

Precision improved by factor 3 wrt previous measurement

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.202001

https://journals.aps.org/prd/abstract/10.1103/PhysRevD.77.074010


Inclusive/differential σtt at 13 TeV and mtpole

● σtt measured in eμ channel used to extract mtpole

using NNLO+NNLL predictions● Most precise σtt at 13 TeV (2.4%)

○ Improvements in lepton identification efficiency and integrated luminosity measurement

● mtpole precision limited by theory uncertainties

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mtpole = 173.1 ± 2.1 GeV

EPJC 80 (2020) 528

EPJC 80 (2020) 658● Simultaneous measurement of mt, αS, and PDFs from triple-differential cross section (combination with HERA data)

● Most precise determination of mtpole

https://link.springer.com/article/10.1140%2Fepjc%2Fs10052-020-7907-9



Inclusive σtt at 13 TeV in ATLAS and CMS

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Dileptonic final states with a hadronically decaying τ lepton

● Profile likelihood fit to lepton-MET transverse mass● Ratio to result of dileptonic analysis (EPJC 79 (2019) 368)

used to test lepton universality of W decays● Uncertainty of 6.8% limited by τh lepton identification

JHEP 02 (2020) 191

ℓ+jets channel full Run2 data set (139 fb-1)

● Profile likelihood fit to final-state distributions to constrain backgrounds and systematics

● Total uncertainty of 4.6%, in good agreement with NNLO+NNLL prediction

PLB 810 (2020) 135797


https://link.springer.com/article/10.1007%2FJHEP02%282020%29191

https://doi.org/10.1016/j.physletb.2020.135797


Inclusive σtt at 5.02 TeV in ATLAS and CMS

Small dataset collected by ATLAS and CMS in 2017

ATLAS: maximum-likelihood fit in dileptonic channels, in bins of b-tag multiplicity and mℓℓ -> 7.5% uncertainty

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CMS: event count in eμ channel (2017) combined with measurement in ℓ+jets channel (2015) -> 7.9% precision

● Both limited by statistical uncertainty● Good agreement with NNLO+NNLL prediction

CMS-PAS-TOP-20-004

ATLAS-CONF-2021-003

http://cds.cern.ch/record/2758333?ln=en



Single & double differential σtt at 13 TeV (all-jet)

Absolute and normalized σtt at parton and particle level compared to NLO MC or NNLO predictions, for many different variables (top, tt, jet-related)

● Double-differential -> study correlations● Mismodelling in Njets, pT

t and pTtt

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JHEP 01 (2021) 033

PRD 103 (2021) 052008

● Significant over-prediction of σtt at high pTt observed

in CMS analysis targeting the boosted regime ● Consistent with observation of CMS boosted jet mass

analysis PRL 124 (2020) 202001 and ATLAS measurements EPJC 79 (2019) 2018, PRD 98 (2018) 012003







Single & double differential σtt at 13 TeV (ℓ+jets)Combined fit of resolved and boosted topologyin ℓ+jets channel -> first of this kind

● Boosted top identified with NN approach● Profiled nuisance parameters

○ significant reduction of uncertainties● All calculations predict harder top pT

spectrum than observed in data○ Improved description with NNLO

● Inclusive σtt -> 3.1% uncertainty○ Most precise result in ℓ+jets channel

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ATLAS ℓ+jets (resolved+boosted) -> no generator found to describe distributions containing pT

tt EPJC 79 (2019) 1028

CMS-PAS-TOP-20-001




Running of the top quark mass at 13 TeV

In MS scheme, the value of mt depends on energy scale (same as for αS) -> running of mt(μm)

● Differential cross section measured in dilepton tt as a function of invariant mass of tt system -> μk = mtt

○ Profile maximum-likelihood unfolding reduces uncertainties almost by factor of 2

● Running extracted by comparing to NLO predictions with fixed or dynamic scales

● Result compatible with solution of QCD renormalization group equations (NLO -> 1 loop RGE)

● No-running scenario excluded at > 95% CL

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PLB 803 (2020) 135263

CMS public web page

https://www.sciencedirect.com/science/article/pii/S0370269320300678?via%3Dihub

http://cms-results.web.cern.ch/cms-results/public-results/publications/TOP-19-007/index.html#AddFig


mt from single top t-channel at 13 TeV

Different phase space than tt -> improve combination

● Multivariate technique to increase signal purity● Functional fit to reconstructed ln(mt) distribution● Combined (ℓ±) and separate fits of ℓ+, ℓ- categories

○ Separate measurement of top and antitop quark mass-> stringent test of CPT invariance

10-> consistent with CPT conservation

CMS-PAS-TOP-19-009



mt from leptonic invariant mass mℓ𝜇 at 13 TeV

Invariant mass between ℓ from top decay and soft μ from b quark originating from same top quark is sensitive to mt

● Advantage: less sensitive to jet calibration than standard direct measurements -> beneficial for combinations

● b-fragmentation parameter rb (Bowler-Lund model) re-optimized via fit to LEP and SLC data

○ Found to be compatible with LHC data in dedicated study with dileptonic tt events (ATLAS-CONF-2020-050)

● Profile likelihood fit to the mℓ𝜇 distribution, separately for opposite-sign (OS) and same-sign leptons

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-> Main systematic uncertainty: B-hadron branching ratios

ATLAS-CONF-2019-046

http://cdsweb.cern.ch/record/2730444

http://cds.cern.ch/record/2693954


Summary and highlights● New measurements of σtt at 5.02 TeV by ATLAS and CMS,

reaching a precision of 7-8%○ Powerful test of perturbative QCD○ Based on small dataset -> still statistically limited

● New single/double-differential σtt at 13 TeV in CMS (ℓ+jets), simultaneously in boosted and resolved topology

○ First measurement of this kind○ Most precise inclusive σtt in ℓ+jets channel -> 3.1% precision

● New measurement of mt from single top t-channel (CMS)○ top/antitop mass difference and ratio used as test of CPT

● Precise determination of mt using inclusive / differential cross section measurements (up to sub-GeV precision)

● First investigation of the running of mt (CMS)● First measurement of mt from leptonic invariant mass (ATLAS)

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Thank you!


ATLAS inclusive 5.02 TeV

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ATLAS-CONF-2021-003



CMS inclusive 5.02 TeV

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CMS-PAS-TOP-20-004



mt from single top t-channel in CMS

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CMS-PAS-TOP-19-009



Single/double differential, resolved + boosted (CMS)

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CMS-PAS-TOP-20-001



Inclusive σtt at 13 TeV and mt in CMS

● Profile likelihood fit to multi-differential distributions in dilepton final states yields 4.2% uncertainty

○ Simultaneous fit with mtMC

○ Systematic uncertainties constrained in fiducial volume● mt and ɑS extracted using NNLO calculations in MS

scheme -> first result of this kind● mt and ɑS cannot be determined simultaneously

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EPJC 79 (2019) 368



mt from invariant mass of tt+1jet system at 8 TeV

Invariant mass of tt+1jet system sensitive to value of mt near the production threshold

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● Normalised differential cross section at 8 TeV unfolded to parton level (l+jets)

● Compared to dedicated NLO+PS predictions in pole and MS schemes

● Larger scale uncertainties in MSscheme due to threshold effects

JHEP 11 (2019) 150



mt, 𝛼S, and PDFs from 3D cross section at 13 TeV

normalized triple-differential 𝜎tt in dilepton-> simultaneously extract mt, 𝛼S, and PDFs

1. M(tt) -> sensitive to mt at threshold2. Njets -> determine 𝛼S3. y(tt) -> information about PDFs (gluon)

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N.B. result sensitive to threshold corrections, such as NLO Coulomb terms -> JHEP 06 (2020) 158

● Combined fit of 3D result and HERA data○ Significant improvement in gluon PDF○ Reduced correlation between g PDF and 𝛼S○ Most precise mt

pole result, to date

EPJC 80 (2020) 658




Modelling of b-fragmentation at 13 TeV

tt cross section (dilepton) measured as a function of variables sensitive to b-fragmentation

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-> only charged constituents are considered

● Unfolding performed via profile likelihood fit● Good agreement with state-of-the-art MC generators● Parameters obtained in the context of mℓ𝜇 analysis

(previous slide) in good agreement with LHC data○ rB = 1.05, with default Powheg+Pythia 𝛼S

FSR = 0.127

ATLAS-CONF-2020-050

http://cdsweb.cern.ch/record/2730444


mt from jet mass in boosted top decays at 13 TeVHadronic decays of top quarks with pT > 400 GeV can be merged in a single wide-area jet

● Peak of jet mass distribution sensitive to mt● Can be calculated from first principle (SCEFT)

● X-cone algorithm used in all-hadronic boosted tt○ Two step strategy: first two tops, then 3 decay products○ Smoothly interpolates between resolved and boosted○ Less sensitive to UE, PU, and soft radiation○ First time that it’s used at the LHC

● Factor 2 improvement in jet mass resolution with respect to standard boosted algorithms

● mt measured by comparing to Powheg+Pythia

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PRL 124 (2020) 202001
