Measurement of top-quark properties with the ATLAS and CMS detectors at LHC - DESY Indico

Measurement of top-quark
properties with the ATLAS and
 CMS detectors at LHC
 European Physical Society conference on high energy physics 2021
 Online conference, July 26-30, 2021

 C. Escobar on behalf of the ATLAS and CMS Collaborations
 Instituto de Física Corpuscular (IFIC) - CSIC/UV

Contents
Large integrated luminosity collected by the ATLAS and CMS detectors during Run 2 at 13 TeV allows us to explore properties of the
top-quark…

Focus on newer results:
• CKM matrix elements measurement in t-channel (CMS, 35.9 fb-1, 13 TeV) — Phys. Lett. B 808 (2020) 135609
• Shape b fragmentation function in charmed mesons in tt̄ events (CMS, 35.9 fb-1, 13 TeV) — CMS-PAS-TOP-18-012
• Search for CP violating anomalous couplings in tt̄ events (CMS, 35.9 fb-1, 13 TeV) — CMS-PAS-TOP-18-007
• Search for CP violation in tt̄ events (CMS, 137 fb-1, 13 TeV) — CMS-PAS-TOP-20-005
• Top-quark polarisation in t-channel (ATLAS, 139 fb-1, 13 TeV) — ATLAS-CONF-2021-027
• AFB asymmetry in tt̄ events (CMS, 35.9 fb-1, 13 TeV) — JHEP 06 (2020) 146
• Universality of τ/μ lepton couplings in W boson in tt̄ events (ATLAS, 139 fb-1, 13 TeV) — Nature Phys. 17 (2021) 813

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 2

Phys. Lett. B 808 (2020) 135609
Measurement of the CKM matrix elements in t-channel at 13 TeV 35.9 fb-1, 13 TeV

First direct, model-independent measurement of the CKM matrix elements |Vtb|, |Vtd|, and |Vts| in t-channel production
• Both the production and decay vertices of the top quark
• Njets/Nbjets categories → tb vs tq
• topology/kinematics → MV discriminants
• Maximum likelihood t to: STbb (2j1t), STqb (3j1t), STbb (3j2t)

• Results in the SM hypothesis of CKM unitarity:
 |Vtb| slightly more precise than
 • |Vtb| > 0.970 (95% CL) ; |Vtd|2 + |Vts|2 < 0.057 (95% CL)
 Run-1 ATLAS+CMS combination
• Results in BSM scenario where heavier 4th generation, but SM BR: (JHEP 05 (2019) 088)
 • |Vtb| = 0.988 ± 0.051 ; |Vtd|2 + |Vts|2 = 0.06 ± 0.06
• Results in BSM scenario assuming total width from the mixing of the three families are negligible:
 • |Vtb| = 0.988 ± 0.024 ; |Vtd|2 + |Vts|2 = 0.06 ± 0.06
 • RΓ = 0.99 ± 0.42
• Dominant syst.: jets, PS modelling

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 3
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CMS-PAS-TOP-18-012
Shape b fragmentation function in charmed mesons at 13 TeV 35.9 fb-1, 13 TeV

First measurement of the shape parameter of the Lund-Bowler fragmentation function for b quarks in tt̄ events
• Lund–Bowler fragmentation model:

 • Universal: parameters a and b are determined from ts to data
 • Speci c to quark type: mass and fragmentation function shape parameter rq
• Using charm mesons produced inside b jets from tt̄ decays (single lepton and dilepton channels)
 •Samples of D0 and J/ψ mesons (reconstructed from the D0 → K±π∓ and J/ψ → µ+µ−) using charged particle track info.
• The reconstructed mesons are used as proxies for their parent B hadrons
• The distributions of the fraction of the total pT of the charged constituents of the jet carried by the charm meson, are tted to
 extract the value of the rb (mb xed to 4.78 GeV)

 • Simultaneous t (3 samples): rb = 0.858 ± 0.037 (stat.) ± 0.031 (syst.)

 Shape fragmentation function: xb

 Signi cantly improves the experimental precision on the shape of
 the fragmentation function (xB)
European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 4
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CMS-PAS-TOP-18-007
Search for CP violating anomalous couplings at 13 TeV 35.9 fb-1, 13 TeV

Measurement of CP asymmetries and chromoelectric dipole moment of the top quark
• In the SM, CP violation in tt̄ production and decay is predicted to be very small
• Spin information directly transferred to the decay products → CP violation manifests itself in altered properties of top-quark decay
 products correlations
• Interaction via chromoeletric dipole moment (CEDM) of the top quark → BSM source of CP violation in tg vertex
• CP-odd observables (O1, O3) from 4-mom. in tt̄ (dilepton) events → Asymmetry → CEDM → CP violation
 4x4 matrices: O1 = ε (pt, pt̄ , pl+, pl-) ; O3 = ε (pb, p ̅ pl+, pl-) being ε the Levi-Civita tensor
 •
• Kinematic reconstruction of decay topology in tt̄ (dilepton) events
• Ai and σtt̄ measured simultaneously from a maximum likelihood t points = MC samples with different dtG values

 CP-odd CEDM

 Asymmetry:

• Set limits to anomalous couplings
 More details in “Results on rare and BSM top
• Dominant unc.: Stat. still relevant. MC modelling (colour recon., UE, ME-PS matching) quark interactions" by Reza Goldouzian

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 5
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CMS-PAS-TOP-20-005
 Search for CP violation in tt̄ events at 13 TeV 137 fb-1, 13 TeV

 Measurement of CP asymmetries in tt̄ events
 • T-odd observables (O3, O6, O12, O14) from 4-mom. in tt̄ (lepton+jets) events → Non-zero asymmetry → CP violation

 • Kinematic reconstruction of decay topology in tt̄ (dilepton) events
 • t and t̄ candidates are reconstructed using a χ2 sorting algorithm
 • Template t of distributions with W+jets background control region
 • Background contribution in the signal region estimated with a t to m(lb) of the leptonically decaying top quark

 1 e, ≥ 4 jets ( 2 b jets ) 137fb-1 (13 TeV) 1 e, ≥ 4 jets ( 2 b jets ) 137fb-1 (13 TeV) 1 e, ≥ 4 jets ( 2 b jets ) 137fb-1 (13 TeV) 1 e, ≥ 4 jets ( 2 b jets ) 137fb-1 (13 TeV)
Events / 0.04

 Events / 0.04

 Events / 0.04

 Events / 0.04
 8
 10
 DY+jets Single t VV DY+jets Single t VV DY+jets Single t VV DY+jets Single t VV
 107 CMS 107 CMS CMS 107 CMS
 W+jets tt dileptonic tt semileptonic W+jets tt dileptonic tt semileptonic 107 W+jets tt dileptonic tt semileptonic W+jets tt dileptonic tt semileptonic
 106 Preliminary Data Stat. ⊕ Syst. Syst. Preliminary Data Stat. ⊕ Syst. Syst. Preliminary Data Stat. ⊕ Syst. Syst. Preliminary Data Stat. ⊕ Syst. Syst.
 dtG=3 106 dtG=3 106 dtG=3 106 dtG=3
 5
 10
 105 105 105
 104
 104 104 104
 103
 103
 102 103 103
Data/MC

 Data/MC

 Data/MC

 Data/MC
 1.5 1.5 1.5 1.5

 1 1 1 1

 0.5 0.5 0.5 0.5
BSM/SM

 BSM/SM

 BSM/SM

 BSM/SM
 1.5 1.2 1.2 1.2
 1 1 1 1
 (d = −3) / (d = 0) (d = −3) / (d = 0) (d = −3) / (d = 0) (d = −3) / (d = 0)
 tG tG tG tG tG tG tG tG

 0.5
 (d
 tG
 = +3) / (d
 tG
 = 0) 0.8 (d
 tG
 = +3) / (d
 tG
 = 0) 0.8 (d
 tG
 = +3) / (d
 tG
 = 0) 0.8 (d
 tG
 = +3) / (d
 tG
 = 0)

 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1
 O3 / m3t [GeV] O6 / m3t [GeV] O12 / m3t [GeV] O14 / m3t [GeV]

 More details in:
 • “Results on rare and BSM top quark interactions" by Reza Goldouzian
 • “Standard Model Measurements” by Paolo Azzurri

 European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
 Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 6
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CMS-PAS-TOP-20-005
Search for CP violation in tt̄ events at 13 TeV 137 fb-1, 13 TeV

Measurement of CP asymmetries in tt̄ events
• T-odd observables (O3, O6, O12, O14) from 4-mom. in tt̄ (lepton+jets) events → Non-zero asymmetry → CP violation
 1 e, ≥ 4 jets ( 2 b jets ) 137fb-1 (13 TeV) 1 lep, ≥ 4 jets ( 2 b jets )
 Events / 0.04

 20

 [%]
 DY+jets Single t VV
 107 CMS
 W+jets tt dileptonic tt semileptonic
 106 Preliminary Data Stat. ⊕ Syst. Syst. CMS

 CP
 dtG=3 15 Simulation Preliminary
 Asymmetry:
 105

 Output asymmetry A
 104

 10 3
 10
 102
 5
 Data/MC

 1.5

 1 0
 0.5 A CP
 BSM/SM

 1.5
 −5
 1
 A'CP
 (d = −3) / (d = 0)
 tG tG

 (d = +3) / (d = 0)
 0.5
 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4
 tG

 0.6
 tG

 0.8 1 −10 Fit to ACP
 O3 / m3t [GeV]
 Fit to A'CP
 −15

• Measured effective asymmetries A’CP are affected by dilution effects due for −20
 −20 −15 −10 −5 0 5 10 15 20
 Input asymmetry A in O14 [%]
 example to the mis-assignment of the quark/antiquark CP

 1 lep, ≥ 4 jets ( 2 b jets ) 137fb-1 (13 TeV)
 1

 Asymmetry [%]
 0.8
 CMS A'CP in e+jets A'CP in µ +jets

 Preliminary A'CP in lepton+jets (8 TeV) A'CP in lepton+jets
 0.6

 0.4

 0.2

 0

• All measures asymmetries consistent with zero indicating no evidence for −0.2

 −0.4
 CP violation
 −0.6
• Dominant unc.: W+HF modelling and the ME-PS matching
 −0.8

 −1
 O3 O6 O12 O14

 Results reduce the uncertainties by a factor 3 compared with the 8 TeV results
 More details in:
 • “Results on rare and BSM top quark interactions" by Reza Goldouzian
 • “Standard Model Measurements” by Paolo Azzurri

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 7

ATLAS-CONF-2021-027
Polarisation at 13 TeV 139 fb-1, 13 TeV
 z'

 cosθlz’
 Direct measurement of the top-quark polarisation components (t-channel) Top-quark rest frame

 θlz’
 • Highly polarised top quarks in t-channel due to V-A nature θ lx’ pTlepton
 x’
 • Pro le likelihood t in 4 regions: 2 signal regions (top & antitop) + 2 control regions

 SR: Octant variable Q: Slice the phase space depending on the sign of cos θj
 θly’
 CRs (tt̄ and W+jets): 2-bin splitting based on lepton charge cos
 lx’
 y’ sθ θ
 co
 • 6 simulated Protos+Pythia8 templates with fully polarised states are used in the t (Px’,y’,z’ = ±1)
 ly’

 • 6 POI (Pxt, Pyt , Pzt, Pxt̄ , Pyt̄ , Pzt̄ ) + 3 normalisations (tt̄ , W+jets & t-channel signal)
Events

 ATLAS Preliminary Data t-channel 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

 25000 s = 13 TeV, 139 fb-1 tt, tW, s-ch W+jets
 Signal Region
 Parameter Extracted value (stat.)
 Z+jets, VV others
 top quark Multijet Uncertainty t-channel norm. +1.045 ± 0.022 ( ± 0.006)
 20000 Post-Fit
 W +jets norm. +1.148 ± 0.027 ( ± 0.005)

 First measurement ever!
 15000 tt̄ norm. +1.005 ± 0.016 ( ± 0.004)
 Pxt0 +0.01 ± 0.18 ( ± 0.02)
 10000
 Pxt̄0 0.02 ± 0.20 ( ± 0.03)
 5000 Pyt0 0.029 ± 0.027 ( ± 0.011)
 Pyt̄0 0.007 ± 0.051 ( ± 0.017)
 0
 Pzt0
Data / Pred.

 1.02
 +0.91 ± 0.10 ( ± 0.02)
 1 Pzt̄0 0.79 ± 0.16 ( ± 0.03)
 0.98
 0.96 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

 0 1 2 3 4 5 6 7
 Uncertainty source Pxt0 Pxt̄0 Pyt0 Pyt̄0 Pzt0 Pzt̄0
 Q+
 Modelling
 Modelling (t-channel) ±0.037 ±0.051 ±0.010 ±0.015 ±0.061 ±0.061
 • Systematics: Mainly dominated by jet energy resolution Modelling (tt̄) ±0.016 ±0.021 ±0.004 ±0.016 ±0.003 ±0.016
 Modelling (other) ±0.013 ±0.031 ±0.003 ±0.006 ±0.026 ±0.043
 • In agreement with SM NLO Powheg+Pythia8 generator predictions Experimental
 ±0.045 ±0.048 ±0.005 ±0.007 ±0.033 ±0.025
 Pxt = 0.040 ± 0.012 (stat.), Pzt = 1.024 ± 0.015 (stat.)
 Jet energy scale
 Jet energy resolution ±0.166 ±0.185 ±0.021 ±0.040 ±0.070 ±0.130
 Pxt̄ = -0.070 ± 0.016 (stat.), Pzt̄ = -0.967 ± 0.020 (stat.) Jet flavour tagging ±0.004 ±0.002

ATLAS-CONF-2021-027
Polarisation at 13 TeV 139 fb-1, 13 TeV

Normalised differential cross-section measurements (t-channel)
• As a function of the 3 angular distributions (cos θlx’, cos θly’, cos θlz’) sensitive to new physics effects in the tWb vertex
 z'
 Top-quark rest frame

 θlz’
 θlx’ pTlepton

 lx'
 ly'
 x’

 σ ⋅ dcos θ
 σ ⋅ dcos θ
 ATLAS Preliminary Data ATLAS Preliminary Data
 1.2

 dσ
 dσ
 Stat. Unc. Stat. Unc.
 s = 13 TeV , 139 fb-1 Stat.+Sys. Unc. 0.9 s = 13 TeV , 139 fb-1 Stat.+Sys. Unc.
 Powheg-Box+Pythia8 Powheg-Box+Pythia8

 1
 1
 Protos+Pythia8 Protos+Pythia8
 1 0.8
 MG5_aMC@NLO+Pythia8 MG5_aMC@NLO+Pythia8
 θly’
 Powheg-Box+Herwig7 Powheg-Box+Herwig7
 y’ 0.7
 0.8
• The normalisations of the W+jets and top-quark
 0.6
 backgrounds and the t-channel signal are 0.6
 0.5
 constrained with a maximum likelihood t to the
 0.4
 data in the signal and control regions 0.4

• After background subtraction the distributions are 0.3
 0.2
 unfolded (iterative Bayesian approach) to the

 Pred./Data
 Pred./Data
 1.2 1.2
 particle-level in a ducial region 1 1
 0.8 0.8
• Systematics: Mainly dominated by jet energy
 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1
 resolution, jet energy scale and t-channel modelling cos θly' cos θlx'

The results are interpreted in an EFT context to set limits on Wilson coef cients
A pro le likelihood t is performed including morphing templates to simulate BSM effects

 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

 CtW CitW
 68% CL 95% CL 68% CL 95% CL Best limits for CitW so far
 All terms [-0.2, 0.9] [-0.7, 1.5] [-0.5, -0.1] [-0.7, 0.2]
 Order 1/⇤4 [-0.2, 0.9] [-0.7, 1.5] [-0.5, -0.1] [-0.7, 0.2] from high-energy
 Order 1/⇤2 [-0.2, 1.0] [-0.7, 1.7] [-0.5, -0.1] [-0.8, 0.2] experiments!

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 9
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JHEP 06 (2020) 146
Asymmetry AFB at 13 TeV 35.9 fb-1, 13 TeV

Measurement of the AFB and the anomalous chromoelectric (dt) and chromomagnetic (μt) moments in tt̄ events
• qq̅ → AFB

 c* = cos θ*

 → A(1)FB ≈ AFB →

• tt̄ (l+jets) measurement: resolved, intermediate, boosted
 • reconstructed through a kinematic t of the decay products to tt̄ hypotheses t
• templates f(c*,|xF|, mtt̄ , ql) tted to data
 • limits on top-quark anomalous dipole moments:

 A(1)FB = 0.048 +0.095 +0.020
 −0.087 (stat.) −0.029 (syst.)

 µt = − 0.024 +0.013 +0.016
 −0.009 (stat.) −0.011 (syst.)

 Limit is placed on the magnitude of |dt| < 0.03 at 95% CL

 • A(1)FB stat-limited, dt and μt syst-limited
 • A(1)FB measurement as well as dt and μt (μt = dt = 0) consistent with the SM expectation

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 10
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Nature Phys. 17 (2021) 813
W→ τ/μ ratio at 13 TeV 139 fb-1, 13 TeV

Measurement of the universality of the couplings of the µ and τ generations to the W-boson in tt̄ events
• The universality of the lepton couplings to the EW gauge boson l(l = e, µ, τ) is a fundamental axiom of the SM
• R(τ/µ) = BR(W→ τ)/BR(W→ μ)
 • Previously measured at LEP at 2.7σ from SM
• Measurement of the BR ratio using dilepton tt̄ events
• A tag and probe analysis is performed probing whether a µ comes from a prompt decay or via an intermediate τ
 • Softer pT spectrum
 • Displacement of the decay vertex → Impact parameter (|d0|) discriminant
 • low d0 : likely W→μ
 • high d0 : likely W→τ→μ
• d0 calibrated using Z→μμ
 mµµ distribution which is used to
• had→μ bkg. from same-charge dilepton events
 extract the Z→μμ normalisation

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 11
 
 

Nature Phys. 17 (2021) 813
W→ τ/μ ratio at 13 TeV 139 fb-1, 13 TeV

Measurement of the universality of the couplings of the µ and τ generations to the W-boson in tt̄ events
 • A pro le likelihood t is performed to extract R(τ/µ) in “3×8 pT and d0 bins” for each eµ and µµ

 channels e.g.

R(τ/µ) = 0.992 ± 0.007 (stat.) ± 0.011 (syst.) = 0.992 ± 0.013
• Syst. Limited. Dominant syst.: detector and modelling
• The measurement is in good agreement with the SM and more
 precise than the LEP measurement

 The most precise measurement to date (by a factor 2) and the SM prevails

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 12
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Summary
• LHC = precision laboratory for top quark (and decay products)

• ATLAS and CMS have performed many top-quark properties measurements
 • Unfortunately not all shown in this talk

• Investigation of SM & BSM
 • Measure many top-quark properties with high precision and great detail
 • Now also able to see subtle higher-order effects in top-quark properties
 • Several results still stat-limited, so these can improve easily already in LHC Run-3!
 • Bottleneck: systematics from jets and signal modelling

More information:
 • LHCtopWG: https://lpcc.web.cern.ch/lhc-top-wg-wg-top-physics-lhc
 • ATLAS Top Physics results: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResults
 • CMS Top Physics results: https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsTOP

Anyhow… more results are in the pipeline → Stay tuned!!!

 European Physical Society conference on high energy physics 2021
 Online conference, July 26-30, 2021

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 13

BACKUP

Introduction

The top quark is the heaviest known elementary particle.

At hadron colliders, top quarks are produced:
• Predominantly in pairs (tt̄ ) via the avour-conserving strong interaction.

• Alternatively, singly through the electroweak interaction.

The top quark was discovered by CDF/D0 at Tevatron in 1995 in tt̄ events.
• The single top-quark production was discovered in 2009 by CDF/D0 and observed in 2011 by ATLAS/CMS.

 Why is interesting?
 • Unique quark:
 • Decays before hadronising.
 • Most of its properties can be directly measured.
 • Allows to test pQCD at NNLO precision ( xed-order).

 • Constrains proton PDFs, S, top-quark pole mass.
 • Window to New Physics.
 • Main background in plenty of BSM searches.
 • The tWb vertex can be studied at production and the decay (e.g. direct
 measurement of CKM element |Vtb|)

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 15
 
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Introduction
The top quark is the heaviest known elementary particle.

At hadron colliders, top quarks are produced:
• Predominantly in pairs (tt̄ ) via the avour-conserving strong interaction.

 10% (13 TeV, pp) 90% (13 TeV, pp)

• Alternatively, singly through the electroweak interaction.

 t-channel (~73% at LHC) tW (~24% at LHC) s-channel (~3% at LHC)

The top quark was discovered by CDF/D0 at Tevatron in 1995 in tt̄ events.
• The single top-quark production was discovered in 2009 by CDF/D0 and observed in 2011 by ATLAS/CMS.

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 16
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ATLAS and CMS detectors

General purpose detectors at the LHC

 ATLAS
 • Length: 44 m, diameter: 25 m
 • Mass: ~7.0 ktons
 • Two magnet elds:
 • Solenoid (ID): 2 T
 • Toroid (Muon System): 2-8 Tm

 CMS
 • Length: 21 m, diameter: 15 m
 • Mass: ~ 12.5 ktons
 • Solenoid: 4 T

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 17
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Phys. Rev. D 100 (2019) 072002 36
Polarisation and spin correlations at 13 TeV 35.9 fb-1, 13 TeV
 n
 io
 ct
Measurement of the top quark polarization and tt̄ spin correlations rk
 dire

 ua
• Top-quark not polarised (at LO) in tt̄ production in SM (parity invariant), but spins of t and t̄ correlated p-
 q
 to
• tt̄ dilepton (two oppositely charged leptons) nal state
• kinematic reconstruction event
• A set of parton-level unfolded normalised differential cross-section distributions, sensitive to each of
 tt̄ CM frame
 the independent coef cients of the spin-dependent parts of the tt̄ production density matrix, is
 measured for the rst time at 13 TeV
• Spin density matrix → 6 polarisation + 9 spin correlations + spin corr. trace + 2 lab-frame angular correlations
• Dominant uncertainties: stat. relevant, syst: jets and modelling (scales, fragm., ...)

• Set limits on anomalous couplings: EFT
 approach, 10 Wilson coef cients

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 18
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Eur. Phys. J. C 80 (2020) 370
Polarisation at 13 TeV 35.9 fb-1, 13 TeV

Differential cross-section measurement (t-channel)
• Highly polarised top quarks in t-channel due to V-A nature

• Maximum-likelihood t performed to the BDTs and the mT(W) distributions: signal region + 2 control regions

 SR: Exactly 2 jets. Exactly 1 b-tagged (2j1b)
 CRs (tt̄ and W+jets): 2j0b, 3j2b
• Two BDTs are used:

 BDT (t-ch.): Discriminates signal from the backgrounds
 BDT (tt̄ /W+jets): Additional sensitivity to the main contributing backgrounds
• Split by lepton avour and lepton charge

• The signal distributions (top-quark pT and |y|, lepton pT and |y|, W-boson pT and cos θ*pol) are unfolded to Top-quark rest frame

 particle and parton levels and the results are compared with theoretical predictions

 • Spin asymmetry Al (sensitive to the top quark polarisation) is extracted from
 the parton-level unfolded differential measurement:

 • Al = 0.440 ± 0.070 (σr ≈ 16%)
 • In agreement with SM NLO Powheg generator predictions (0.436)
 • Dominant systematics: Dependence with top-quark mass, tt̄ parton shower
 and colour reconnection

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 19
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Top-quark polarisation at 13 TeV
At the LHC (pp collisions), the top quark…
• Detectable: due to its large mass, the top-quark decays before hadronisation to an on-shell W boson. Top-quark spin directly

 transferred to its decay products (lifetime (10-23 s) < spin decorr. time (10-21 s)) → almost full spin info about top quark in the charged
 lepton from top-quark decay → accessible via angular distributions (in top-quark rest frame)
• QCD production: Top-quark not polarised (at LO) in tt̄ production in SM (parity invariant), but spins of t and t̄ correlated
• EW production: highly polarised top quarks due to V-A nature
 • Top-quark polarisation (being P its magnitude) can only be measured in single top-quark events
 • In the t-channel at LO single top quarks are produced with their spin aligned along the direction of the down-type quarks

 Since the valence u-quark density of the proton is
 about twice as high as the valence down-quark The degree of the polarisation in each axis depends on the
 density, the degree of polarisation is different for mixture of the leading and subleading processes
 top quarks and top antiquarks
 top-quark rest frame
 Phys. Rev. D89 (2014) 114009
 dominant sub-process
 Polarised along spectator

 Polarised along the beam
 quark

 axis

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 20

ATLAS-CONF-2019-026
Asymmetry AC at 13 TeV 139 fb-1, 13 TeV

Measurement of the tt̄ charge asymmetry
• tt̄ symmetric at LO
• Higher orders → asymmetry (interference between tt̄ production processes causes

 arXiv:1207.0331
 asymmetry in t and t̄ direction in the hadron colliders) → AC
• This measurement is essential to test QCD higher order effect
• tt̄ (l+jets) measurement (unfolded) combining both the resolved (BDT reconstruction) and
 boosted (top-quark tagging) topologies of top quark decays
• Bayesian unfolding procedure is used to infer the asymmetry at parton level
• AC measured inclusively and differentially as a function of the mtt̄ and ßztt̄
• In agreement with the NNLO QCD + NLO EW predictions
 where

• Systematic uncertainties are pro led as nuisance parameters
 • Stat-limited. Dominant syst.: signal and W+jets modelling, pTmiss, and jets
• Inclusive measurement: AC = 0.0060 ± 0.0011 (stat.) ± 0.0010 (syst.)
 • 4σ from zero → First evidence for charge asymmetry in pp collisions!

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 21
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ATLAS-CONF-2019-026
 Asymmetry AC at 13 TeV 139 fb-1, 13 TeV

 The results are interpreted in an EFT context to set limits on Wilson coef cients
 • The inclusive and mtt̄ (per bin) measurements are interpreted in the EFT framework
 • Derived limits on the linear combination of Wilson coef cients for dimension-six operators
 • AC at hadron colliders is sensitive to seven four-fermion operators in the Warsaw basis, which is reduced to four by using a

 avour-speci c linear combination
 • This four operators are further reduced by the assumption of equal couplings to up- and down-type quarks is valid in

 models where the couplings are avour-universal
 • The tt̄ production cross-section at hadron colliders is sensitive to the linear combination C+ = C1 + C2 , while the charge

 asymmetry is affected by the difference C− = C1 − C2

 The measured data provide considerably tighter bounds than the
 combination of previous ATLAS and CMS measurements

 European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
 Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 22
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Eur. Phys. J. C 80 (2020) 754
Spin correlations at 13 TeV 36.1 fb-1, 13 TeV

Measurement of the tt̄ spin correlations
• Spin information directly transferred to the decay products → Angle between the leptons sensitive to spin correlations
 • Top-quark not polarised (at LO) in tt̄ production in SM (parity invariant), but spins of t and t̄ correlated
• tt̄ dilepton (opposite-sign eµ channel) nal state (≥2j, ≥1b)
• Results are unfolded to both parton and particle level
• The observed spin correlation is slightly higher than the generator predictions

 xspin/nospin; cross-sections under
 the SM spin/no-spin hypothesis

• fSM increases as a function of mtt̄
 • Due to larger uncertainties, none of the results

 deviate substantially from the SM expectation

Compare with various SM predictions
• Higher-order calculations appear to reduce the tension but

 still do not agree fully
• NLO in the strong and weak gauge couplings agrees better

 with the data but large scale uncertainties
• NLO expansion with µR = µF = mt leads to comparable results

 but again with signi cant scale uncertainties
• NNLO prediction using the same expansion does not agree

European Physical Society Conference on High Energy Physics (EPS-HEP) 2021 Carlos Escobar
Online conference, July 26-30, 2021 Instituto de Física Corpuscular (IFIC) - CSIC/UV 23
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