Top-quark production at LHC Javier Fernández For the ATLAS and CMS Collaborations 27 th Rencontres...
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Transcript of Top-quark production at LHC Javier Fernández For the ATLAS and CMS Collaborations 27 th Rencontres...
Top-quark production at LHC
Javier Fernández
For the ATLAS and CMS Collaborations
27th Rencontres de Blois on “Particle Physics and Cosmology”Blois 2/6/2015
Top quark(s) production• Top pair production at the LHC through gg (>84%) and qq
• Single-top production (at LO)– t-, Wt-, s-channel
• LHC Run 1: per experiment (approximately)– 6M top-quark pair events (1/second)– 2M t-channel single-top events– 150k s-channel single-top events– 10k tt+W/Z– 3k tt+Higgs– 20 tttt
• Sensitive to new physics & test of perturbative QCD• Can constrain modeling (PDF, ISR/FSR)• Important background to many Higgs and BSM searches
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Top quark decay
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• Decays ~100% to W-boson and b-quark → |Vtb| ~ unity
• Final state topology depends on W decay
Vtb
t→Wb, W→jets or lepton + v
Learning about top quark• ATLAS TOP Public Page:
– https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResults
• CMS TOP Public Page:– https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsTOP
• Every topic deserves a dedicated long presentation:
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Differential
CMS PAS TOP-12-027ATLAS-CONF-2014-057ATLAS-CONF-2014-057CMS PAS TOP‐14‐004CMS PAS TOP-12-028
CERN‐PH-EP-2014‐222JHEP 11 (2014) 154
ATLAS-CONF-2012-130
MultitopCross sections
tt+V (V=g,W,Z) covered in next talk by Kelly Beernaert
tt + cc/bb
CMS PAS TOP-12-024Phys.Rev. D89 (2014) 072012
arXiv:1411.5621
Top pair inclusive cross section• Very precise QCD prediction to NNLO with
NNLL corrections available since 2013• All measurements are systematics limited• Best results from dilepton analyses• All measurements agree with predictions• Experimental uncertainties challenge
theoretical ones!
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8TeV
Theoretical uncertainty 5.5%
Experimental uncertainty 3.5%
~3.9% including LHC beam energy uncertaintity
Inclusive top-pair production: Summary
• Excellent progress both on the theory and experiment sides!
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Theory band represents uncertainties due to
renormalisation and factorisation scale,
parton density functions and the strong coupling
LHC WG : https://twiki.cern.ch/twiki/bin/view/LHCPhysics/TopLHCWG
Single top inclusive production
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Upper limit √s [TeV]
t-channel
Wt production
s-channel
Different processes sensitive to new physics
Top pair differential measurement• Differential cross sections as a function of top and top decay products kinematic.
– Measurements performed in both fiducial (visible) and full phase space– Probe perturbative QCD, test and tune MC models with measurements
• Top quark full kinematic reconstruction necessary and background subtraction• Unfolding techniques used (correcting bin to bin migration):
– Account for acceptance and detector effects (resolution, efficiency, etc.)– Correct observed distributions to particle or parton-level– Compare with generators and/or calculations to unfolded data distribution
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Kin. Reconstruction
▪ Background subtraction▪ Unfolding
Diff. cross sectionExample for l+jets
@8TeV
Top pair differential measurement
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Fiducial (a.k.a. visible) particle-level differential cross-section:
• Experimentally directly accessible region
• Correct for detector response only (resolutions and efficiency)
• Minimally model dependent
Full phase-space extrapolation:• Model dependent
Parton-level:• Remove hadronization effects• Model dependent
Fiducial Detector to
Particle level
Particle to Parton level
Top pair differential measurement tests
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Generators
Global event
variables and syst.
shifts
PDFs
NLO
Top pair differential: pseudo-top• Pseudo-top and tt built from objects directly related to
particle-level observables (leptons, jets, ETmiss)• Strongly correlated with top quark at parton level,
reduced model dependence• Data/MC comparisons sensitive to PDF sets and models• Main systematics: b-tagging, jet energy, ISR/IFR parton
shower
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Extending measurement
from particle-level to construct top proxy (pseudo-
top) from particle level objects
Top pair differential: boosted
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First boosted (lepton+jets) top pair differential cross-section w.r.t. hadronic top:- top ID with jet substructure techniques - Jet reconstruction algorithms up to TeV range- anti-kt jet with R=1.0, pT
t > 300GeV• Main systematics: large-R jets energy scale (15-30%)• NLO and LO MC normalized to NNLO+NNLL QCD predictions overestimate data (at higher pT). Better description by Powheg+Herwig at parton level• Modeling uncertainty much larger at parton level!
Fiducial Particle level
Full phase-space Parton level
tt+jets: fiducial differential vs njets
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ATLAS l+jets @7TeV CMS dilep @8TeV
Total systematics on jet multiplicity: 10 to 30%(background modeling and jet energy scale)• Data discriminate models and validate choice of scale• POWHEG+PYTHIA with tuning of hard radiationbest overall description (accuracy and consistency)
MadGraph+Pythia6 showing generally good agreement with the data
Normalization reduces Syst. Unc.
Single top t-channel: differential• Differential cross sections as functions of top pT and rapidity
– Comparison with MC after showering/hadronization using different modeling for b-quarks
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CMS-PAS-TOP-14-004
𝜎 (𝑡 𝑡 𝑏𝑏 )𝜎 (𝑡 𝑡 𝑗𝑗 )
=0.022±0.004 (𝑠𝑡𝑎𝑡 )±0.005 (𝑠𝑦𝑠𝑡 )𝑎𝑡 𝐽𝑒𝑡 𝑝𝑇>40𝐺𝑒𝑉
tt+cc/bb
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Dilepton @7TeV CMS PAS TOP-12-024
Dilepton @7TeV 2D fit: (jet pT, vertex mass) ATLAS Phys.Rev. D89 (2014) 072012
MADGRAPH/POWHEG: 0.013/0.014 ± 0.002
Dilepton CMS@8TeV: Fit to b-tag discriminator performed on signal plus background categories. Dominant systematic: mistag efficiency
ALPGEN+HERWIG = 3.4%POWHEG+HERWIG = 5.2%
arXiv:1411.5621
Predictions systematically lower than observed ratios
𝜎 (𝑡 𝑡 𝑏𝑏 )𝜎 (𝑡 𝑡 𝑗𝑗 )
=[3.6±1.1 (𝑠𝑡𝑎𝑡 )±0.9 (𝑠𝑦𝑠 )]%
𝑅𝐻𝐹=𝜎 (𝑡 𝑡+𝑏+𝑋 𝑜𝑟 𝑡 𝑡+𝑐+𝑋 )
𝜎 (𝑡 𝑡+ 𝑗 )=𝜎 (𝑡 𝑡+𝐻𝐹 )𝜎 (𝑡 𝑡+ 𝑗 )
=[6.2±1.1 (𝑠𝑡𝑎𝑡 )±1.8 (𝑠𝑦𝑠𝑡 )]%
Multi-top: tttt
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• σ(tttt) SM ≈ 1 fb @ 8 TeV (MG)• Many BSM models predict enhancement of this
cross section (SUSY squark/gluino decays)
• No significant excess observed over SM expectations
• Cross section limit:• Observed: 32 fb (25 x SM)• Expected: 32 ± 17 fb
• Will be of interest for Run 2• 4-top production cross section
~9-15 times larger
ATLAS 7TeV result (same sign dilepton) produced 95% CL limit on (exotic) four top quarks from contact interaction: <61 fb (obs.) < 90fb (exp.)
ATLAS-CONF-2012-130CERN- PH- EP-2014-222
JHEP 11 (2014) 154
Conclusions/Summary• Rich and mature top physics program @ Run 1
– Single top measured in its 3 signatures– Inclusive and differential cross sections– Study of Multitop & tt+HF
• Precise and extended cross section :– Theoretical calculations challenged by precision of
experimental data (in an hadronic collider)• Top community eager to look at new LHC data
– A factor of 2 more top-pair events in 2015!!
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Fiducial particle level vs
Full phase-space Parton level
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~10 x Fiducial Particle level(solid marker
types)
Full phase-space Parton level
(empty marker types)
Note:different scales!
Fiducial parton levelvs
Fiducial Pseudo-top
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Note:different scales due to change to Pseudo-top (quality reco cuts)
Top pair inclusive mtop=173.34• Very precise QCD prediction to NNLO with
NNLL corrections available since 2013• All measurements are systematics limited• Best results from dilepton analyses• All measurements agree with predictions• Experimental uncertainties challenge
theoretical ones!
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8TeV
Theoretical uncertainty 5.5%
Experimental uncertainty 3.5%
~3.9% including LHC beam energy uncertaintity
Inclusive Cross Sections 7TeV• All final states investigated (except
tautau)• Similar event selection in dilepton
and lepton+jets modes– All hadronic: at least 6 high-pT jets, at
least 2 b-tagged– Tau+jets: at least 3 high-pT jets (>1 b-
tagged) + tau jet; fed into ANN
• Measurements from likelihood fits• Data-driven estimates for main
backgrounds• Good agreement between data and
predictions• CMS dilepton (4.2%) more precise
than LHC combination!
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32
The detectors
Coverage
Tracking |h|<2.5
2.52.4
ECAL |h|<3.0 & Tracking |h|<2.5
mm
Electromagnetic Calorimeter |h|<3.2
Reminder: CMS detector
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TrackerHigh granularity Silicon detector. | |<h 2.5Pixels: sensors 100x150 μm2. 66M channels.Strips: pitch 80-183 μm. 9.6M channelsSolenoidSuperconduting solenoid in NbTi. High magnetic field (3.8 T) surrounds the tracker and calorimeters. Inside the muon chambers the magnetic field value is 1.8 T
ECALPbWO4 crystals |h|<3.High segmentation: 0.0174x0.0174 in ηxΦ.
Muon SystemDT, RPC, CSC. Coverage up to |η|<2.4.
HCALMade of brass alloy and iron.
tt+g
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• Sensitive to top charge and top-photon couplings• Background to tt+H, H→gg
ATLAS l+jets@7TeVObservation + fiducial cross sectionPhys. Rev. D 91, 072007 (2015)
CMS m+jets@8TeVCMS-PAS-TOP-13-011
In agreement with NLO predictions
tt+W,Z
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Eur. Phys. J. C 74 (2014) 3060CMS-TOP-14-021
Channels: 2L(SS,OS),3L Channels: 2L(SS,OS), 3L, 4L
Limits on the vector and axial couplings of the Z boson to the top quark, and on dimension-six
operators parameterizing new physics
ATLAS-CONF-2014-038
Channels Expected (fb) Observed (fb)
ttZ: OS+3L+4L
ttW: SS+3L