Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://qfthep.sinp.msu.ru/talks2013/Romano_TopPhysicsAtAtlas.pdf Äàòà èçìåíåíèÿ: Sat Jun 22 19:18:22 2013 Äàòà èíäåêñèðîâàíèÿ: Thu Feb 27 20:31:43 2014 Êîäèðîâêà:


Top Quark physics
0 Why? 0 Precise tests of the Standard Model and verification of pQCD 0 Yukawa coupling with the Higgs ~1Important role in the EWSB breaking 0 Privileged window to search for new physics 0 Top quark studies in ATLAS presented in this talk 0 Top pair cross section 0 Top pair differential cross section Production mechanisms 0 Single top cross section 0 Top-quark mass measurement Intrinsic property 0 Other top analyses in ATLAS 0 Spin correlation Phys. Rev. Lett. 108, 212001 (2012) 0 W helicity JHEP 1206 (2012) 088 0 Top pair associated with heavy flavor arXiv:1304.6386 0 Heavy resonances decaying in top-antitop (see talk by F. Fassi) 0 FNCN in top decays JHEP 1209 (2012) 139 0 Top pair charge asymmetry Eur.Phys.J. C72 (2012) 2039
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Cross section measurements
: · allows a direct measurement of · can put constraints on SM parameters · current statistics allow the study of differential spectra : · Sensintive to electroweak physics involving Wtb vertex · Sensitive to the pdf of the valence quarks

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Top pair production
Production mechanisms at LHC
0 Gluon-gluon fusion (~85%) 0 Quark-antiquark annihilation

Decays
0 (~100%)

Top pair final states

~33%

~66%
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Top pair cross section @ 7 TeV
Cross section summary at 7 TeV ATLAS-CONF-2012-024 The measurements share several common sources of systematic uncertainty A likelihood is defined for each channel The full combination is implemented as a product of the individual likelihoods The achieved precision is already better than the uncertainties on the aNNLO predictions
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Top pair cross section @ 8 TeV
0 Lepton+jets channel 0 Template fit method
ATLAS-CONF-2012-149
= TeV, = . fb- 0 Cut-based event selection (3 jets, 1 lepton and T )

0 Likelihood discriminant based on 2 variables
0 and = -8 , being the aplanarity 0 Evaluated from simulations of the signal and

the W+jets background

0

is measured through a max-likelihood fit of the in data and the templates from MC


( = TeV) = ± ± ± () pb

= 238
0 Main systematics:

+22 -24

pb

0 JES 0 Signal modeling (Hard scattering/IFSR/PDF)

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Top pair differential cross section
Eur. Phys. J. C (2013) 73, 2261 = 7 TeV, = 2.05 fb-1


0 Total



measurements show very good agreement with the SM
1

0 New physics phenomena can still affect the shape of 0 Top-antitop relative differential cross section 0 Relative measurement more precise

where = , T, and

than the absolute cancellation of correlated systematics

0 Cut-based analysis in the l+jets channel 0 system reconstructed via

a kinematic likelihood fit.
and b-tag info

0 Input: lepton and jets 4-momenta, 0 Fixed parameters: W and top masses and

decays amplitudes

0 A cut on ln() is applied improvement

in the truth-reco correlation

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Top pair differential cross section
Eur. Phys. J. C (2013) 73, 2261 = 7 TeV, = 4.7 fb Reconstructed spectra
-1

Unfolding
Simple matrix inversion

Parton level cross section

No significant deviation from SM predictions is observed

No significant deviations from the SM predictions are observed
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Single top cross section
Cross section summary at 7 TeV
t-chan Wt s-chan

Measurements at 7 TeV: · Cross section for all channels · measured for the first time (3.3 level) · Upper limit for the s-channel · Single top/antitop t-channel ratio Measurements at 8 TeV: · Cross section in t-channel
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Single top t-channel cross section (8 TeV)
0 A multivariate Neural Network (NN) discriminant
ATLAS-CONF-2012-132 = 8 TeV, = 5.8 fb-1

trained with the most-sensitive variables
0 Two exclusive samples used: 2 jets and 3 jets 0 Contributions from signal and background

evaluated via simulations
0 Lepton + 2(3) jets channel, 1-btag

0

-

extracted via a maximum-likelihood

fit of the NN output in the data
-

( = TeV) = ± ± ± () pb ( = 8 TeV) = 87. 8 -
+3 . 4 -1 . 9

pb

0 Dominating uncertainties: JES, b-tag efficiency and normalization
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Top mass measurements
· Free parameter in the SM
· High mass strong coupling ( 1) with the Higgs field

· Top quark decays before hadronizations
Unique possibility to measure the mass of a `bare' quark

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Top quark mass
Most precise measurement in ATLAS
0 3D template fit in the lepton+jets channel 0 Parameters: , global jet energy scale factor (JSF) and bJet energy scale factor (bJSF) 0 Simulated distributions:
, , and (ratio of the sum of the of the bjets from the top and light jets from the W) ,

ATLAS-CONF-2013-046 = 7 TeV, = 4.7 fb
-1

0 Templates built by varying the fit parameters in Monte Carlo

= linear dependency for signal and bg = linear dependency for signal only


,



JSF

bJSF



,







0 Probability density functions for each parameter evaluted by fitting each template

distribution

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Top quark mass
0 Lepton+jets channel 0 kinematics reconstructed by a fit maximizing an event likelihood

and



,

,

,

0 is not fixed in the fit

0 Signal and background PDFs are used in an unbinned likelihood fit to the data for all

events:

(

,

,

,

, | , , , )

0 Results in the 1 btag and 2btag samples

are in good agreement 0 First time implementation of an measurement with simultaneous constraint on , JES and bJES
0 Systematic uncertainties reduced by 40% (at the

cost of small contributions to the total stat error)

= . ± . + + ± . () GeV
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Top quark mass
Top mass summary
Most precise measurement by ATLAS: = 172.31 ± 0.75 ± 1.35 GeV ATLAS-CONF-2013-046 l+jets channel, =4.7fb
-1

Precision on measurement at LHC is constantly improving and getting closer to the precision achieved at Tevatron

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Summary
0 Physics of the top quark can answer fundamental questions. 0 So far all results agree with SM predictions
0 Most of the measurements are limited by systematics.

0 Top analyses in ATLAS presented in this talk 0 Top pair cross section 0 Single top cross section 0 Top pair differential cross section 0 Top-quark mass measurement
0 Additional results can be found at the ATLAS public page

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/ 0 Stay tuned for more results with data collected in 2012 at 8 TeV, as well as for

more refined studies at 7 TeV
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ATLAS & CMS top mass combination
ATLAS-CONF-2012-095 and CMS-PAS-TOP-12-001 = 7 TeV, 4.9 fb
-1

Statistical combination performed using the Best Linear Unbiased Estimator (BLUE) method LHC measurement suffer of greater systematic uncertainty respect to the result from Tevatron

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Common object definitions
0 Details can vary among the different analyses 0 Jets:

0 B-tagging via a Neural network based algorithm (MV1) with average efficiency

0 Reconstructed from topological clusters using the anti-kt algorithm ( = 0.4) 0 T > 25 GeV, || <2.5

of 70% and light jet rejection factor ~140 0 Electrons:
0 EM cluster with track matched 0 Isolation in tracker and calorimeter 0 T > 25 GeV, ||<1.37 or 1.52 < || <2.47

0 Muons:

0 Missing transverse energy

0 Tracks in inner detector and muon spectrometer 0 Isolation in tracker and calorimeter 0 T > 20 GeV, || <2.5 0 Vector sum of energy deposits in calorimeters, with corrections based on the

associated reconstructed object

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Reconstruction of the system via kinematic likelihood fit
0 The tt system reconstruction is performed trough a kinematic fit

using a maximum likelihood approach

0 The likelihood assesses the compatibility of the event with a

typical ttbar pair 0 The algorithm is fed with the 4 or 5 reconstructed highest-pt jets (and their b-tag info), the lepton and the 0 The output is the permutation of the four jets, lepton and that maximizes the likelihood
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From the detector-level spectra to the cross section measurement
The `detector-level' spectra are linked to the `parton level'cross section via =


+

Where 0 is the number of observed data events in the bin j. 0 L is the luminosity 0 is the number of background events in the bin i. 0 is the branching ratio 0 is the `migration matrix'
0 is the efficiency of the selection
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Jet multiplicity in top­antitop final states
0 Useful to constrain models of initial and final state

radiation (ISR/FSR) 0 Provides a test of perturbative QCD 0 Single-lepton channel unfolding

0 Four jet thresholds: (25, 40, 60, and 80 GeV)

0 Results are corrected for all detector effects through
0 Reconstructed level particle level

0 Measurement is limited by systematic uncertainties, 0 background modelling (at lower jet multiplicities) 0 jet energy scale (at higher jet multiplicities)
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Jet multiplicity in top­antitop final states
0 MC@NLO modelling predicts a

lower jet multiplicity spectrum and softer jets HERWIG or PYTHIA and POWHEG + PYTHIA are consistent with the data

> 25 GeV

> 40 GeV

0 Predictions from ALPGEN +

> 60 GeV

> 80 GeV

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=

Single top/antitop t-chan ratio
ATLAS-CONF-2012-056 = 7 TeV, = 4.7 fb
-1

0 Very sensitive to the ratio of the PDF of the valence quark in

the high x regime

0 Smaller uncertainties because of error cancelations

0 Sensitive to new physics effects 0 Same analysis technique used in the measurement
The measurement is in agreement with the predictions from different PDF sets and is dominated by systematic uncertainties

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resonances
0 resonances searches @ 7 TeV have been performed in the lepton+jets and

full hadronic channels (arXiv:1305.2756) 0 First measurement @ 8 TeV in the lepton+jets channel 0 Exploits both traditional `resolved' jet analysis and a large-radius jet substructure analysis 0 No significant deviation from the prediction 0 Upper cross section limits are given for two benchmark models ATLAS-CONF-2013-052 = 8 TeV, = 5.8 fb
-1

0 95% C.L. exclusion regions: Leptophobic Z' [0.5, 1.8] TeV; KK gluon [0.5, 2] TeV

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Reconstruction of the system in the resonances searches
0 `Small' radius jet: anti-kt, = 0.4, T > 25 GeV, < 2.5 0 `Large' radius jet: anti-kt, = 1.0 T > 300 GeV, < 2.0 0 `Resolved' technique
0 2 algorithm is used to select the best assignment of jets to the hadronically and

semileptonically decaying top quarks 0 Neutrino built from the missing transverse energy ( assigned using the W mass constraint

0 `Large jet substructure' technique
0 `Large' jet tagged as the hadronic top 0 Leptonic top built from the lepton and the neutrino (leptonic W) and the

remaining small radius jet

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