Recent CMS results on Top quark physics

Hongbo Liao
Institute of High Energy Physics, CAS, China
On behalf of CMS collaborations

20th Lomonosov, 19-25 August, 2021

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top-quark
The top has several features that make it a very interesting particle:

 Heaviest particle discovered till now-- mt =173.34土 0.27(stat) 土0.71(syst) GeV
 Decays before hadronization-- Give access to the physics of a “free” quark Intensively couples to the Higgs boson
NThe LHC
is a top factory and allows:eLHCw

Over 200MLHC
top quark pairs in LHC Run II 13 TeV dataphysics

 Precise measurements of top pairs and single top production
 Observation of rare processes involving top
 Use the top quark as a “tool” to study the SM
02/09/2020-F.Fabbri-QCD@LHC-X

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tt̅ cross-section
Precise measurements of tt̅ production:

 Provide a stringent test of our level of understanding of QCD
 Allow to extract several SM parameters: αS, mtop
 Allow to improve the description of parton distribution functions3

Differential and double-differential σ(tt̅ ) in lepton + jets arXiv:2108.02803
 High precision measurement of differential and doubledifferential cross sections
 For the first time the full spectra of differential crosssections are determined-- combine of resolved and boosted tt̅ topologies

Most of the predictions are in good agreement with the measurement, except: M(tt̅ ) vs. pT(th) and pT(tt̅ ) vs. pT(th) shows largest disagreements.
 At particle level add. jets vs. kinematic observable are difficult to describe by NLO.

Inclusive cross section: 791 ± 1 (stat.) ± 21 (syst.) ± 14 (lumi.) pb
 most precise measurement in lepton + jets channel
 Dominanted by: JES and b-tagging
4𝒕𝒕̅𝒄𝒄/𝒃𝒃 and 𝒕𝒕̅𝒋𝒋 production• Test the state-of-art predictions at NLO• Irreducible background to 𝑡𝑡𝐻, 𝐻 → 𝑏𝑏
• 𝒕𝒕𝒃𝒃̅ and 𝒕𝒕𝒋𝒋̅ measurement•••

PLB 820 (2021) 136565
JHEP 07 (2020) 125

𝜎𝑡𝑡̅𝑏𝑏 and 𝜎𝑡𝑡̅𝑏𝑏 /𝜎 𝑡𝑡 ̅ j j extracted simultaneously
from a 2D discriminant
PowhegPythia8 and MG_aMC@NLO+Pythia8
provide the best description

̅ 𝒄- production
First measurement of 𝒕𝒕𝒄• Simultaneous extraction of 𝜎𝑡𝑡̅𝑏𝑏 , 𝜎 𝑡𝑡 ̅ 𝑐 𝑐̅ and 𝜎𝑡𝑡̅𝐿𝐿
using a template fit procedure

Precision dominatedby : MC modelling,JES, c-tagging

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Rare processes
Precise measurement of t(t̅ )+Xproduction:
 Provide a stringent test also ofelectroweak processes Access to several coupling (tγ,tW,tH)
sensitive to new physics effects

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tW production in ℓ+jets

CMS-PAS-TOP-20-002

 Categories based on jet multiplicity and 1 b-tagged jet:
2J1T (W+Jets). 3J1T (tw Signal region) and 4J1T (ttbar)
 Data-driven background One BDT is trained per lepton flavor in signal (3J1T)
region and evaluation in all regions

Simultaneous ML fit performed in all categories using
BDT discriminants
 Dominant uncertainty:
Background estimation, JES and modeling

Measured (expected) signal strength:
μ = 1.24 ± 0.18 (1.00 ± 0.17)

Cross section:
σtW = 89 ± 4 (stat.) ± 12 (syst.) pb

σSM =72 ± 4 pb

 Observed (expected) significance is7.4 (6.8) standard deviationsFirst observation of tW production in ℓ+jets7

Inclusive and differential tZq
 Full run2 dataset
 3 leptons with improved lepton MVA
 constraining nonprompt background multiclass NN or BDT
Spin asymmetry:
Inclusive tZq cross-section:

CMS-PAS-TOP-20-010

Differential tZq cross-section:

Agreement with SM prediction:
Improvement 30%
w.r.t.earlier measurements𝜎l−0.003l−0.005tZq(𝑙 +𝑡)−𝑡+5.9+4.4+3.0

Partial tZq cross-sections:First time!

In general, observe good agreement
between measurement and prediction.
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Rare top production: 𝒕𝒕̅𝐕

arXiv: 2107.01508
𝒕𝒕̅𝛾 production

JHEP 03 (2020) 056𝒕𝒕̅𝑍 production
differentially Targets
3 or 4 isolated lepton channel with 𝑍 to 𝑙 + 𝑙 - Measured in lepton+jets channel

 Inclusive cross section already systematic limited

 Precision limited by MC modelling Differential cross sections measured
in several kinematic observables
 Good agreement with SM prediction

 Dominated by signal/background MC modelling Differential cross sections are measured fist time

𝑙–5Z𝒕𝒕̅𝑍

𝒕𝒕𝛾+ 𝒕𝑊𝛾

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top properties
Now at LHC is possible to reach unprecedent precisions for the property
measurements10/3
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M easurement of the yt
 Measure the Yukawa (yt) coupling in tt̅ production.
 Exploit the large effect that the radiation of a virtual H bosonhas on tt̅ differential distributions
 tt̅ predictions for different values of yt obtained as event-based multiplicative corrections using HATHOR: Applied on POWHEG predictions
 The comparison with an additive approach is taken as uncertainty
 Event collected in the diletopn channel, >=2bjets
 Variables used based on partial system reconstruction: M(l++l-+2bjets) and 𝛥ybl : requires the correct matching of b and lCompatible with the result in l+jets
PRD 102 (2020) 092013

PRD 100 (2019) 072002
11tt̅ forward-backward asymmetry

JHEP 06 (2020) 146



NLO interference terms in tt̅ production from qq initial state creates a forward-backward asymmetry.



Quantity never measured before @LHC, where the charge asymmetry is measured as a proxyUse variables sensitive to the difference between qq, qg and gg initial state to build templates and separate the qq
c* =cosθ*Extract AFB and anomalous chromoelectric and chromomagnetic dipole moments

 Events collected in the l+jets channel
 Both resolved and boosted topologies.
 Profile likelihood-fit to the 3D template to extract
AFB and the anomalous moments separately

forward-backward asymmetry
chromomagnetic moments

anomalous chromoelectric

12Consistent with the SM and previous CMS results /3
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W polarization in ATLAS and CMSJHEP 08 (2020) 051

 Combination of the Wboson polarization in top
quark decays, on Run1(8 TeV, 20fb-1)data.
 Wboson polarization determined by the V-A
structure of the tWb vertex

 Combination of the polarization fractions from 4 measurements

 Combination Improvement > 20% wrtthe most precise
measurement
 Measurement used to set limits on the anomalous
coupling in the tWb vertex

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Mass measurement in single top events

CMS-PAS-TOP-19-009

 BDT discriminator and cut optimization
 Data-driven QCD is subtracted from the data Simultaneous ML ﬁt using y =ln(mt) distributions in μand e ﬁnal states, validated in control region

F(y) = ft−ch Ft−ch(y ; y0) + fTop FTop(y ; y0) + fEWKFEWK(y) y0, ft−ch, fTop and fEWK are allowed to float during the fit

mass results:
Sub GeV precision

Masses ratio and difference (a check for CPT Invariance)

LHCP 2021

Precision limited byLatest Results on: Single
JES and modellingTop

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Search for new physics
Tool to search for new physics:
 Many BSM models are expected to involve top quarks
 Possible to perform direct searches for new
resonances and FCNC
 Use the precise measurements to set a limit on newoperators in an EFT framework

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Searc h for FCNC in the top sector Flavor changing neutral currents (FCNC) allow for transitions between quarksof different flavor but same electric charge
 F C N C processes are highly suppressed in the SM d u e to the GIM mechanism Small contributions a p p e a r at one loop level

 Many extensions of the SM predict the presence of F C N C a n d give rise todetectable F C N C amplitude

Any evidence ofFCNC will indicate
the existence of new
physics

Aguilar-Saavedra, ACTA Phys. Pol. B 35(2004)

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Searc h for FCNC tHq interaction by H→γγCMS-PAS-TOP-20-007

 Signal modeling: effective Lagrangian Production & d e c a y Signal regions: 2 photons, 100 < mγγ< 180 G e V lep tonic: ≥1 je t, ≥1ℓ
 h ad ro nic: ≥3 je t, ≥1 b -jet

 Strategy
 8 BDTs: (u, c) × (lep, had) × (res, non-res bkg)
 7 c a te g o rie s d e fine d b y BDTscore 14 m γγ distributions to fittHutHc

 Dominant uncertainties:b-tagging a n d γ identification
 D ata compatible with abs enc e of signal
 Upper limits on the signal cross sections aretranslated to the strength of the tqH
anomalous couplings a n d relatedbranching fractions

 Upper limits

 B(t→Hu) < 1.9×10-4 (exp. 3.1×10-4)
 B(t→Hc ) < 7.3×10-4 (exp. 5.1×10-4)

17Searc h for FCNC tHq interaction by H→bb CMS-PAS-TOP-19-002
 Production & d e c a y Sig na l re g io n: 1ℓ , ≥3 je t, ≥2 b -jet
 A d e e p neural network is used to associate the reconstructedobjects to the matrix-element partonic final state
 BDTs are used to distinguish the signal from the background event
 All bjet-jet categories are combined
 No significant excess with respect to the SM backgroundexpectations: 95% C L limits a re se t o n the xs, c o up lings a nd BRs
 Significant improve with respect to the early run-2 searchUpper limits:

 B(t→Hu) < 7.9 ×10-4(exp: 11×10-4)
 B(t→Hc) < 9 .4×10-4(exp: 8.6×10-4)

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EFT interpretation of t/tt̅ +leptons

JHEP 03 (2021) 095

 Considering at the same time the yields from several signals: tt̅ H,tt̅ Z(ll),tt̅ W(lnu),tZq,tHq
 Divide the event collected in regions categorized on jets and b-jets multiplicity and lepton flavour and charge Obtained a total of 35 independent regions used to constraint the EFT operators
 Investigate 16 EFT operators simultaneously Only dimension 6 operators: 4 fermion operators, boson-quark and quark gluon operators included Interference among operators and with the SM considered Obtain 1D and 2D profiled and individual limits from likelihood fit
 Main systematics: Theory (𝜇R,F), modelling, JES, lepton ID and isolation

Good agreement of all WCs with the SM prediction
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EFT search in ≥3L final states

CMS-PAS-TOP-21-001

 Full Run II Luminosity 138/fb with main processes: tZq/ttZ/tWZ• Leptonically decaying top + Z boson candidate

 5 operators: weak dipole moment interactions, left- and right-handed top quark vector couplings

 Extensive use of MVAs-- discriminate SM processes :-- separate SM/BSM events• signal extraction with 1D, 2D, and 5D likelihood fit• Systematics: theory and NP lepton dominate• Better limits than eariler results from thettZ cross section measurement• Agreement within 2σ in general
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EFT search in ttɣ

arXiv: 2107.01508

 Full Run II luminosity 137 fb-1
 1lepton channel, binned in lepton flavor
 Interpretation in ctZ (weak dipole moment) Best current limits.part

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Search for CLFV interactions

CMS-PAS-TOP-19-006

 In the SM , le p to n fla vo r is c o nserved in a ll intera c tions
 Many new physics models predict sizable CLFV (neutrino mass, multi-Higgs doublet models,…) If the new p hysic s resp o nsib le fo r the C LFV is a t sc a les b eyo nd what the LHC c a n directly probe, theSM Lagrangian c a n b e extended by dimension-6 operators

 Search for CLFV in e𝜇 final state [137 fb-1]

Pro d uc tio n

D ec a y Production & d e c a y Signal: CLFV vector, scalar a n d tensor
 BDTis use d to disc rimina te sig na l fro m BG events
 Data consistent with SM expectation
 Upp e r limits are se t at 95%C L22

Search for CP Violation

CMS-PAS-TOP-20-005

 C P vio la tio n in SM is insufficient to d e sc rib e the matter-antimatter asymmetry of the universe
 In the SM , C PV in the p ro d u ctio n a nd d e c a y o f to p quark pairs is predicted to b e very small
 Simple C P o d d observables
 chromo-electric dipole moment (CEDM) of top quark in top pair production induces C P V
 Lep to n + je ts fina l sta te s [137 fb -1]
 Observables; O 3 , O 6 , O 12 a n d O 14
 Top quark a n d antiquark candidates arereconstructed using a χ2 sorting algorithm
 The background contribution in the signal region isestima te d fro m a fit to the mass d istrib ution M lb

 There is no significant evidence of C P V in e a c h observable Consistent with the SM prediction23SummaryLHC is a top factory and CMS is exploiting the large sample collected maximally: Reducing the uncertainties on inclusive measurements Performing differential measurements-- in challenging phase-space and channels and as a function of several variables-- providing a complete description of the tt̅ kinematic-- providing stringent test of QCD and electroweak predictions
 Searching for very rare processes
 Measuring with increasing precision t(t̅ )+Xand including differential distributions
 First observation of tW production in ℓ+jets
 Measuring the top properties and couplings with innovative techniques
 Use top events to test the fundamental bases of the SM Setting constraints to the existence of new physics: FCNC, EFT, CLFV, CPV…

Several exciting new measurements have been presented,
but stay tuned since more results are coming…..
Thanks for your attention!!!

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References LHCTopWG: https://twiki.cern.ch/twiki/bin/view/LHCPhysics/LHCTopWG
 ATLAS: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResults
 CMS: https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsTOP

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