25/10/2017 1
Leonid Serkin( INFN Gruppo Collegato di Udine and ICTP Trieste )
on behalf of the ATLAS Collaboration
Top quark as a window to new physics:
recent ATLAS results
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Impressive agreement between SM predictions and experimental measurements
Pro
duction c
ross-s
ection 6 orders of
magnitude!
Introduction
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Pro
duction c
ross-s
ection
Electroweak
Top
Higgs sectors
Introduction
Impressive agreement between SM predictions and experimental measurements in the
Electroweak, Top and Higgs sectors!
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I will focus on the Top sector measurements and searches of physics beyond the SM
Pro
duction c
ross-s
ection
Top&Co
Introduction
Cross-
section
< 1pb !
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ATLAS detector and LHC data
The ATLAS detector:
• Inner Detector precision tracking system
• ECAL/HCAL measure energies and positions of showers
• Muon Spectrometer, in combination with a toroidal
field, finds position and energy of muons
The datasets collected during the years 2015-2016 at
√s = 13 TeV of proton-proton collisions and corresponds to
3.2 - 13.2 fb-1 of integrated luminosity
New physics searches: strategy
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Strategy for new physics searches:
indirectly, by looking for non-standard properties (spin, CP, couplings…)
directly, by explicit search for BSM objects
In the case of direct searches, define selection based on signal signatures/acceptance
and background kinematics
Compare observed data to Standard Model background (Monte Carlo and data-driven)
and MC signal predictions
Split into control and signal regions, extrapolation checked before unblinding
(validation)
In the case of no evidence for new physics: set limits on cross-section times branching
ratio
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Top pair + ?
Extensions of the SM predict particles usually coupled preferentially to the top quark,
the heaviest fundamental particle in the SM
Possible new phenomena may enhance the SM cross-sections through the production of
heavy objects in association with a top-quark pair
High multiplicity searches: many b-jets, H, V and top quark decays
Some of the most complex analyses in ATLAS Exotics program
Profile likelihoods (shape fits) with O(100) nuisance parameters (syst. uncertainties)
and up to O(10) backgrounds
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MC Validation and Modelling
Monte Carlo validation and modelling important for background estimation strategies:
many BSM searches live in the extreme phase spaces
Harmonisation of ATLAS generation configurations, as well as recommmendations for
systematic uncertainties
Validation of the properties of simulated events from MC event
generators in order to ensure the correctness of generator
configurations and production samples used in physics analysesPseudorapidity
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We are interested in studying the final states containing (at least) one top-quark pair
Define selection based on signal signatures/acceptance and background kinematics
As an example: top-quark pair production with a single lepton final state
New physics searches: object selection
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exactly one central (|η| < 2.5)
reconstructed and isolated
electron or muon with pT > 30 GeV
Object selection
EPJC (2016) 76, 292
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jet
Object selection
JETM-2016-010
at least four central (|η| < 2.5) jets with pT > 25
GeV from calibrated topological clusters using
anti-kt (R=0.4) algorithm
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at least two of the selected jets have to be b-tagged by a
multivariate algorithm (<1% mistag) called b-jets
Object selection
ATL-PHYS-PUB-2016-012
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The top quark is the most strongly-coupled SM particle to the Higgs field
Indirect constraints on the top-Higgs Yukawa coupling come from loop diagrams with
top quarks which contribute to ggH production and H → γγ decays
ttH search
Associated production of a Higgs boson with a pair of top quarks (ttH) allows direct
probing of top-Higgs Yukawa couplings without assumption on new physics contribution
to loop diagrams
–
Any deviation could be a hint of new physics
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Final states in ttH
ttH process can be studied in a variety of final states, depending on the Higgs decay
mode and top-quark pair decay topology: γγ, WW/ZZ/ττ (multi-lepton) and bb
–
–
Top quark decays before it hadronises,
almost exclusively through t → Wb
W-boson decay modes characterise the
signature of top-quark pair final state
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ttH process can be studied in a variety of final states, depending on the Higgs decay
mode and top-quark pair decay topology: γγ, WW/ZZ/ττ (multi-lepton) and bb
–
• Clean signature thanks to excellent mass resolution, small branching ratio
(BR~0.23%)
• Looking for narrow signal peak in the di-photon invariant mass spectrum on top of a
smothly falling background
• Search dominated by statistical uncertainties
–
• ttH, H → γγ [ATLAS-CONF-2016-067]–
ttH and 2 photons
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ttH and many leptons
ttH process can be studied in a variety of final states, depending on the Higgs decay
mode and top-quark pair decay topology: γγ, WW/ZZ/ττ (multi-lepton) and bb
–
–
• ttH, H → WW/ZZ/ττ [ATLAS-CONF-2016-058]–
• (Semi)-leptonic final states with low background, but
with additional sensitivity to ttW/ttZ background
• Cut-and-count analysis in 4 categories, depending on
the number, charge and flavour of leptons (l): 2 same
sign (ss) l+0τhad , 2ssl+1τhad, 3l and 4l
• Main systematic uncertainty: fakes and charge mis-ID
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ttH and many b-jets
ttH process can be studied in a variety of final states, depending on the Higgs decay
mode and top-quark pair decay topology: γγ, WW/ZZ/ττ (multi-lepton) and bb
–
–
• ttH, H → bb [ATLAS-CONF-2016-080]– –
• Small signal on top of large background dominated by
production of tt+heavy flavour (HF) jets
• Events categorised according to jet and b-jet multiplicities:
6 control (CR) and 3 signal regions (SR) in 1l and 2 CR and 3
SR in 2l channels
• Analysis strategy uses 2-step multivariate technique:
reconstruction BDT and discrimination NN/BDT
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Combination of ttH
Combination of the three presented analysies:
95% CL observed (expected) limit: 3.0 x SM ( 2.1 x SM ) for mH = 125 GeV
Observed (expected) significance: 2.8σ ( 1.8σ )
A small excess is observed in the multilepton final state
Can that be coming from tt+W/Z background mismodelling?
Measuring tt+W/Z
Search for deviations in the top coupling to gauge bosons: tt + W/Z
• Cut-and-count analysis: 2 same sign leptons, 3
leptons outside Z window, 3 leptons with 2
of them inside the Z window, and 4 leptons
• Fit in 9 signal and 2 control regions
• Observed (expected) significance:
3.9σ (3.4σ) over the B-only hypothesis for ttZ
2.2σ (1.0σ) over the B-only hypothesis for ttW
• No visible excess
number of jets MET [GeV]
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tt+many jets: search for 4tops/VLQ/HBSM
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• Target several signal scenarios with tt+many jets final states:
4-top quarks production (SM, 2UED-RPP, EFT)
Vector-like quarks (VLQ) pair production (TT→HtHt, HtZt,
HtWb, ZtZt, ZtWb)
BSM Higgs: ttH/A (→tt), bbH/A(→tt), tbH+(→tb)
• Orthogonal 1-lepton and 0-lepton channels split by number of
(b)-jets and reclustered jets, effective mass used in signal
discrimination
ATLAS-CONF-2016-104
tt+many jets: search for 4tops/VLQ/HBSM
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• Total of 8 (12) search regions
and 6 (9) validation regions in
1(0)-lepton channel
• regions with ≥ 6 (7) jets are
search regions and with
exactly 5(6) jets are
validation regions
• Different phase-spaces covered
by each channel
• Complex top-quark pair
background model (low to high
signal purity)
ATLAS-CONF-2016-104
Fit
tt+many jets: search for 4tops/VLQ/HBSM
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• Model-independent limits on 2D BR plane with
good coverage obtained from combination
• Nice complimentarity of 0-lepton and 1-lepton
analyses
• Limits on T quark mass reach: 1 – 1.2 TeV
ATLAS-CONF-2016-104
• Extended Higgs sector in 2HDM model, H/A to top-quarks pair decay is
dominant for mH/A > 2mtop
• Type-II 2HDM model:
probing masses from 400 GeV to 1 TeV
• Sensitivity in HBSM ttH(tt): excluding tan β < 0.2
• First limits on bbH(tt)
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Conclusions
Recent results from the ATLAS Collaboration on searches for new phenomena
through the production of heavy objects in association with a top-quark pair
Pushing the frontiers of complexity in the searches in order to increase the
sensitivity
Extensive experimental program in ATLAS, many new searches continue to provide
stringent constraints on masses, couplings and cross-section in several models
No new physics found yet, but looking forward for new and exciting searches!
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Conclusions
Recent results from the ATLAS Collaboration on searches for new phenomena
through the production of heavy objects in association with a top-quark pair
Pushing the frontiers of complexity in the searches in order to increase the
sensitivity
Extensive experimental program in ATLAS, many new searches continue to provide
stringent constraints on masses, couplings and cross-section in several models
No new physics found yet, but looking forward for new and exciting searches!
Muchas gracias por su atención!
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Syst uncert
Simultaneous profile likelihood fit to nine
regions with systematic uncertainties
implemented in the fit as additional terms in the
binned likelihood:
Vector-like quarks
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• Chiral 4th quark generation excluded by Higgs boson
measurements
• Vector-like quarks (VLQ):
spin ½, coloured, charged
LH and RH coupling to charged currents
• Arise in little Higgs, composite Higgs, extra dimensions,
and GUTs models, naturalness would require mQ ≤ 2 TeV
• Production:
• strong pair production (less model dependent)
• single via EW (mixing angles model dependent)
• Decay: usually assume only
coupling to 3rd generation:
VLQ → t/b + Z/H/W