ttV and ttbb at LHC · ttV and ttbb at LHC SM@LHC 2019 Zürich Thorsten Kuhl On behalf of Atlas &...
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tt V and tt bb at LHC SM@LHC 2019 Zürich Thorsten Kuhl On behalf of Atlas & CMS 24.04.2019
Transcript of ttV and ttbb at LHC · ttV and ttbb at LHC SM@LHC 2019 Zürich Thorsten Kuhl On behalf of Atlas &...
ttV and ttbb at LHCSM@LHC 2019 Zürich
Thorsten Kuhl
On behalf of Atlas & CMS24.04.2019
Page 2SM@LHC | ttV and ttbb | Thorsten Kuhl, 24.04.2019
tt+X at the LHCProduction of ttZ, ttW, ttγ and ttbb
● Rare processes in the SM
– Several orders suppressed vs tt
● Test of QCD predictions
– Multiple scale problem
– Matching/Merging: shower vs matrix element
● Important background for searches with multiple leptons and/or b-jets (ttH)
● Deviation of tV coupling from SM → indication of new physics
– vectorquarks, 4/3 charged quarks, anomalous di-pole moment
● Concentrate on new results at 13TeV
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PUBNOTES/ATL-PHYS-PUB-2018-034/
Page 3SM@LHC | ttV and ttbb | Thorsten Kuhl, 24.04.2019
tt+W/Z/γ
Production of ttZ, ttW, ttγ● Rare Cross section in the order of 1 pb-1
→ three order of magnitude suppressed vs tt● tt+γ: need to defined well phase space for separation vs “QCD”-photons (π0) and
FSR from leptons
● Publications:
Z,γ
Z,γZ,γ
Atlas● ttW+ttZ: arXiv:1901.03584, 36.1fb-1
● ttγ: arXiv: 1812.01697, 36.1 fb-1
(differential)
CMS● ttW+ttZ: JHEP 08 (2018) 011, 35.9fb-1
● ttZ: CMS-PAS-TOP-18-009, 77.5fb-1
(differential)● ttγ: JHEP 10 (2017) 006, 19.7fb-1@8TeV
Page 4SM@LHC | ttV and ttbb | Thorsten Kuhl, 24.04.2019
tt+W/Z
General measurement strategy:
● Split data in regions of lepton multiplicity/charge/flavor and bjet multiplicity → separation of signal and background processes
● Use cuts or multivariate method (depending on S/B) to enrich signal further
● simultaneous fit of tt+W and tt+Z
ttW: same sign leptons
ttZ: three lepton selection
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tt+W/Z
Results:● Consistent picture between Atlas and CMS● Agreement of theory (NLO fixed order) with data at 1σ level● Precision: experiment: ~22% for tt+W and ~13% for tt+Z
theory: 13% 11%
Theory: arXiv: 1610.07922 [hep-ph]
Page 6SM@LHC | ttV and ttbb | Thorsten Kuhl, 24.04.2019
tt+Z differentialNew improved measurement from CMS for tt+Z• 3 and 4 lepton channel using twice the luminosity, similar strategy• Improvements: more inclusive trigger, better lepton reconstruction• Measured cross section:
→ uncertainty smaller then uncertainty of NLO prediction
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tt+Z differential First measurement with unfolded differential distribution
● cos(θZ*) and p
T(Z)
● Generator describes shapes of observable reasonable well
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tt+Z differential: AC and EFT interpretationAnomalous coupling • Scan of the EW dipole plane• No deviation vs SMEffective field theory• Large improvement of sensitivity by using differential distributions• Well in agreement with SM expectation
Anomalous dipole moment
Anomalous current
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tt+γ Measurement in lepton+jet and di-lepton channel:
● Selection of well isolated photons in di-top events● use full event shape to separate signal events from background,
fit contributions of different processes
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tt+γ: cross section measurements
Fiducial measurement of tt+γ production
• Measurements agree well with theory predictions
• Theory error is factor 2.5 bigger then experimental precision → need more precise theory calculations
• di-lepton channel: lower stats but smaller systematic→ promising for future measurements
CMS: 8 TeV
Theory: Phys. Rev. D 83 (2011) 074013
Photon part of the fiducial volume definition for Atlas: p
T>20 GeV, |η|<2.37, less then 10% had.
Energy around photon in ΔR=0.1, ΔR(l,γ)>1.0
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tt+γ: differential distributions Momentum of the first photon• Reasonable agreement with MG5_aMC: generator use hard
photons in the hard (ME) calculations
– Results not very dependent on ISR and shower modeling• Powheg+Pythia8: setup with photons from parton shower only
(FSR, decays) → cannot describe tt+γ process
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tt+bb measurements
Complicated final state:• multiple scales involved• origin of b-quarks: PDF, hard interaction, parton shower• processes contribute to tt+bb: gluons splitting, tt+X (X=Z,H) decaying in bb
→ fiducial measurements are essential and need care in interpretationVarious generator setups used:• Fixed-Order tt @NLO: easy to used/tune, additional b’s only from parton shower• (NLO-)Matrix Element: (Massless) b-quarks in ME and shower, matched/merged • 4-F tt+bb calculation: Massive b-quarks in ME, missing b’s from later emissions→ Effort to use best of each (Frank Siegerts talk)
CMS: 2.3 fb-1, Phys. Lett. B 776 (2018) 355, di-lepton channel onlyAtlas: 36.1 fb-1, JHEP 04 (2019) 46, di-lepton and lepton+jets
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tt+bb: CMSFirst measurement by CMS with early 13 TeV data• Di-lepton channel • Handful events with 4 b-tags• Fair agreement between data and MC• Extract x-section and fraction of b-jets in sample (see later)
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tt+b(b): AtlasDifferential measurement of ttbb• Using di-lepton (3+ b-jets) and lepton+jet (4 b-jets) channel• Experimental problem: generator (powheg+pythia8) does not describe rates of
different flavors well; use data driven estimate to subtract tt+c/light
– Improves generator modeling of kinematics before unfolding as well
more b-like
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tt+b(b) total cross section
Results consistent between Atlas and CMS:• Measured x-sections higher then
NLO-ME calculations• ttX X=(Z,H) contribution
– ~2-3% in 3 b-jet
– ~8% in 4 b-jet
(much smaller then data/MC difference)
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tt+bb:(3b di-lepton selection)
Statistics only sufficient to unfold up to the third b-jet • Fraction of tt+bb not that well described in tt fixed-Order NLO (b only in shower)• Sherpa tt+jets ME@NLO (massless b-quarks) does reasonably well• Third b-jet kinematics described reasonably by all generator setups
Page 17SM@LHC | ttV and ttbb | Thorsten Kuhl, 24.04.2019
tt+bb: lepton+jet channel distributions
Access to the additional b-quarks system ● Fair agreement for all generator setups● Needs more statistics to get separation power
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Summary
tt+X are rare final states ● Probes of perturbative QCD on various scales ● Important background for many searches● 36-78 fb-1 are analysed, improvements expected with full run-2 dataset
tt+V final states observed with 13 TeV data ● measurements getting already more precise then NLO calculation● moving to differential measurements● interpretation in EFT and anomalous couplings are available● no significant deviations from Standard Model seen
tt+bb a probe to study b quark production in ME and PS● Fixed-order NLO generator (b-pairs from showers) describe kinematic
distribution well, but cannot describe fraction of b-quarks (too low)● Very active development in 4-flavor tt+bb-ME generators @NLO, cross
sections too low as well, something missing?● Issue of matching/merging of different parts of generation (PDF, ME, shower)
