Top quark mass measurements at CMS

Minsuk KimHelsinki Institute of Physics

23 November 2018Particle Physics day

Overview• Motivation

• Top Production and Mass Definition

• Object Definitions & Event Selection

• Kinematic Fit and Mass Extraction

• Potential Improvements

• CMS mt Measurements

• Summary

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Motivation

• Its precise value impacts directly on key predictions of the SM

• Production rates at LHC, size of quantum correction to electroweak processes, and coupling strength of top quark with Higgs boson

• Its value leads to a significant constraint on stability of the EW vacuum

• Theoretical predictions use the top-quark pole mass

• We need a good understanding of the measured top-quark mass w.r.t. the pole mass or a mass with a well defined renormalization scheme

• The mass of the top quark (mt) is one of the fundamental parameters of the Standard Model

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Motivation

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• Strong coupling 𝛼s and top-quark mass mt are two of the most fundamental ones

• Using the measured values of mt and mH, it seems that the current electroweak vacuum is meta-stable

• Quartic Higgs self-coupling (𝜆) depends heavily on mt, may turn negative at a scale lower than the Planck scale and induce vacuum instability

• Hope for greater accuracy in the experimental measurements to establish whether we are in the region that would be meta/unstable within the SM

• LHC jet measurements are key input, and Jet Energy Correction (JEC) is their fundamental uncertainty

Stability of SM vacuum

EPJC 77 (2017) 746

• We want ∆mt < 0.2 GeV for 3𝜎 confidence on vacuum meta-stability

• As the experimental sensitivity is entering the sub-GeV range, issues of theoretical interpretation become important

• To get there

• Assume theories can reduce mt(MC) vs. mt(pole) uncertainty (e.g. Hoang, Nason)

• Perform ultra-legacy re-reco and time-dependent MC to understand data

• Factorise main physics effects and constrain them with control regions

• Question

• How much data we need to constrain the dominant systematic uncertainties related to jet energy scale (JES) calibration and QCD modeling?

To get ultimate precision

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Top Quark Production

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• Top quark is the heaviest known elementary particle

• It decays before hadronization (lifetime τ = ~5×10-25 s)

• Main top decay: t → Wb

• Top quark pair production has 3 decay channels

• All-jets (46.2%) W → qq

• Lepton+jets (43.5%) W → 𝑙𝜈 or qq

• Dilepton (10.3%) W → 𝑙𝜈

• LHC is a top quark factory: 𝜎tt(14 TeV) = 800 pb, 2 ttbar events/sec

• Total cross section a factor of 100 larger at LHC than at Tevatron

Top quark mass definition• Decay before hadronizing ⇒ measure mt directly from decay products

• Different definitions of mt

• Most precise top mass measurements use kinematic reconstruction methods, determining the top mass parameter of a Monte Carlo event generator, mtMC, however mtMC (direct measurement) ≠ mtpole (indirect measurement)

• The uncertainty on the translation from the mtMC definition to a theoretically well defined short distance mass definition at low scale is currently estimated to be of the order of 1 GeV (arXiv:1405.4781, arXiv:1408.6080, Nucl. Phys. Proc. Suppl. 185 (2008) 220)

• Due to hadronization and parton shower dynamics, relating mtMC to a theory mass is difficult, but perhaps < 100 MeV (arXiv:1712.02796, arXiv:1608.01318)

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• mtMC (Monte Carlo mass) parameter measured from comparison to MC events with top-quark decay products• mtpole (pole mass) parameter is the classic rest mass entering the

top propagator (the pole is fixed order by order)• mtMS (running mass) parameter defined in a low-scale short

distance scheme (the pole is shifted at each order)

Object definitions

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Lepton + jets Channel

Estimated composition: 92.5% tt, 2% W+jets, 3.5% single top, 2% other!9

Precision measurement based on kinematic reconstruction where double-b-tagging and leptonic W decay reduce combinatorial background and allow to select ttbar events with high purity

Selection:

• Exactly 1 isolated e (𝜇) with pT > 34

(26) GeV, |𝜂| < 2.1 (2.4) and veto

additional e, 𝜇

• ≥ 4 jets with pT > 30 GeV, |𝜂| < 2.4

• 2 b-tagged jets

Kinematic fitSplit 3 permutation classes:• correct• wrong: flipped b-quarks, mistags• unmatched

Kinematic fit with constraints:• two untagged jets

mjj = 80.4 GeV• lepton and neutrino (MET)

m𝑙𝜈 = 80.4 GeV

• Combine with two b-tagged jetsm(jj,b1) = m(𝑙𝜈,b2)

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Constraining mjj=mW effectively removes light-quark-jet uncertainties, leaving b-jet corrections as the limiting uncertainties

Mass extraction: ideogram method• Ideogram method: modification of template

method using multiple permutations with different weights (suited with large datasets)

• To extract the maximum amount of mass information out of a tt candidate event

• Realized by constructing 2D likelihoods for each event (ideograms)

• To determine simultaneously mt and jet energy scale (JES) as the JES was found to result in the dominating systematic uncertainty in the previous top mass measurements (CMS-TOP-10-009)

• All selected permutations are taken into account and weighted by their fit probability

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High Luminosity2011 2012 2015-2018 nominal HL-LHC

CM energy (TeV) 7 8 13 14Cross section (pb) 167 246 806 951Luminosity (fb-1) 5 20 3/36/40/60 300 3000

<Pileup> 10 21 13/27/38/37 ~40 ~100

300 fb-1we are here!

!12Precision achievable with 300-3000 fb-1 ?

Possible Improvementsin the understanding of systematic uncertainties for standard techniques

unc

erta

inty

[GeV

]to

pm

0

0.2

0.4

0.6

0.8

1

1.2 Present -1 30 fb 13 TeV

-1 300 fb 14 TeV

-1 3000 fb 14 TeV

CMS preliminary projectionTotal Stat+iJESb-JES UE,CRdJES others

,ME-PS2Q

Std. methods

full NLO

move to 3D fit

dedicated UE studies

differential measurements

by using lepton+jets measurement at 7 TeV with 5 fb-1, JHEP 12 (2012) 105, mt = 173.49 ± 0.43 (stat) ± 0.98 (syst) GeV (results are comparable in precision to those from Tevatron)

0.2 GeV

• CMS estimated ultimate precision achievable with 30 fb-1, 300 fb-1 & 3000 fb-1 of data at 13-14 TeV

• Done in 2013 based on optimistic assumptions with large data

• Promising methods to reduce errors

• Particle-level studies with data: improve tuning of the UE event

• 3D fitting methods: fit both the light-jet and b-jet energy scale (JES, bJES) in-situ, with mt

• Differential mt studies: compare different CR models and check for different MC generators

• Full NLO tools: improve description of ISR and FSR

5 fb-1 7 TeV

Extrapolation to high lumi. (FTR-13-017)

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CMS mt Measurementsin the understanding of systematic uncertainties for standard techniques

unc

erta

inty

[GeV

]to

pm

0

0.2

0.4

0.6

0.8

1

1.2 Present -1 30 fb 13 TeV

-1 300 fb 14 TeV

-1 3000 fb 14 TeV

CMS preliminary projectionTotal Stat+iJESb-JES UE,CRdJES others

,ME-PS2Q

Std. methods

0.48 GeV (Run1)

5 fb-1 7 TeV

EPJC 78 (2018) 891

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Lepton + jets channel mt (GeV)2012 8 TeV 19.7 fb-1 172.35 ± 0.16 ± 0.48 2017 13 TeV 35.9 fb-1 172.25 ± 0.08 ± 0.62

0.2 GeV

Possible with ~150 fb-1 of

data in Run2 ?

First goal!

Extrapolation to high lumi. (FTR-13-017)

JEC calibrations

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Slide from M. Voutilainen

Wishlist for Run 2• Improvements to analysis:

• NLL with nuisances to make analysis more similar to others

• get optimal hybrid weight automatically

• Improve selection and/or split in categories

• Add more observables, measurements to reduce Flavor uncertainties

• External improvements:

• High statistics MC samples

• As low JEC/JER uncertainties as possible allowing to focus on modelling

• Second Parton Shower generator

• More CR models to understand spread

• Understand ME generator difference, more gen studies

• Supporting measurements to constrain models (e.g. b fragmentation)!16

Slide from H. Stadie

Summary• Precise measurements of mt are fundamental to provide inputs to test

the self-consistency of the SM

• Presented prospect and latest CMS results of mt

• For the first time mt measured at √s = 13 TeV at CMS

• With ultimate precision achievable at LHC, the relation between mt definition of the experimental analysis and mtpole is becoming relevant

• mt measurements dominated by systematic uncertainties

• First goal with the challenge to bring systematic uncertainties down

• Better than Run 1 with new 2016 JEC/flavor uncertainty or 2017

Backup

ATLAS+CMS Preliminary

ATLAS: 172.51 ± 0.27 ± 0.42

CMS: 172.44 ± 0.13 ± 0.47

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Mass extraction: ideogram method

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Slide from LHCtopWG open meeting, 20.11.2018

Calibration

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Slide from LHCtopWG open meeting, 20.11.2018

Mass extraction: hybrid approach

Slide from LHCtopWG open meeting, 20.11.2018

Systematic uncertainties

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Lepton + jets channel

Slide from LHCtopWG open meeting, 20.11.2018

Systematic uncertaintiesAll-jets channel

Slide from LHCtopWG open meeting, 20.11.2018

Color reconnection in ttbar• CR affects the reconstruction of the top quark system

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JEC uncertainties

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Slide from M. Voutilainen

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Slide from M. Voutilainen

JEC: current status

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JEC: current statusSlide from M. Voutilainen