Probing the tWb structure in t- channel single top-quark production using the ATLAS detector at the LHC

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Probing the tWb structure in t- channel single top-quark

production using the ATLAS detector at the LHC

Galo Gonzalvo Rodríguez

IFIC (CSIC-UVEG)

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I N T R O D U C T I O N

• The top-quark is the heaviest fundamental particle in the SM.

• At the LHC, it is produced via tt production ̄

(dominant) mediated by the strong interaction and via single top-quark production mediated by the electroweak interaction.

• Top quarks are only polarised in single top-quark events (in tt production, top quarks are produced ̄ unpolarised because of parity conservation in QCD).

• The t-channel is the dominant process in single top- quark production.

• In the t-channel at LO, as a consequence of the V-A form of the tWb vertex, single top quarks are

produced with their spin completely aligned along the direction of the down-type quarks (Phys.

Rev.D55(1997) 7249).

• Since the top-quark lifetime is shorter than the

depolarisation timescale (10^-21 s), the top-quark spin information is directly transferred to its decay

products.

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I N T R O D U C T I O N

• The distributions of the top-quark decay products are sensitive to its polarisation (Phys. Rev. D53 (1996) 4886):

• Using a proper coordinate system, as proposed in (Phys. Rev. D89 (2014) 114009) one can obtain the differential angular distributions associated with the three different polarisation components {PX,PY,PZ}.

• These differential angular distributions are distorted by detector inefficiencies and acceptance. To correct this, the detector effects are unfolded and the measured angular differential distributions can be compared to theoretical predictions.

• In this analysis, the unfolding is produced at particle level and hence the comparisons are made within a fiducial region.

• Deviations from SM predictions can give hints of physics beyond the SM (the measurement can be interpreted within an EFT framework in terms of tWb anomalous couplings or Wilson coefficients).

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αl: spin analysing power associated with the final state charged lepton (~1 at LO) Pi: Top-quark degree of polarisation in a given direction i

z axis: along the direction of the spectator jet in the top quark reference frame y axis: perpendicular to the direction of the incoming light quark and the spectator jet x axis: perpendicular to the z and y axis

1 d

dcos✓_`i = 1

2 (1 + ↵`Pi cos✓`i) with i = x, y, z

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S I G N A L A N D C O N T R O L R E G I O N S

• This analysis is performed at 13 TeV using the full Run2 dataset, corresponding to 2015-2018 (~140.5 fb^-1)

• Pre-selection (PR):

• Exactly one tight charged lepton (electron or muon).

• pT > 30 GeV, |η| < 2.5.

• Exactly 2 jets, exactly one b-tagged:

• Spectator jet: pT > 30 GeV, |η| < 4.5; b-jet: pT > 30 GeV, |η| < 2.5 (b- tagging WP 60%).

✴ pT > 35 GeV in transition region (2.75 < |η| < 3.5) to prevent mismodelling

• MET > 35 GeV

• MTW > 60 GeV

• Additional multijet rejecting cut

• Signal region:

• mlb < 153 GeV

• 134 < mtop < 206 GeV

• Trapezoidal cut: a = 9, b = 6

• mj,top > 280 GeV

• HT > 170 GeV

• The fiducial region at particle level is   defined applying the same requirements   used to define the signal region to  

particle level objects.

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Control regions:

• W+jets CR: PR-SR

• tt CR: Same as PR, but with 2 b-jets ̄ and no light jets.

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M U LT I J E T B A C K G R O U N D E S T I M AT I O N

• Multijet is a non-negligible background coming from fake and non-prompt

leptons.

• Shape of the multijet contributions being modelled using:

• Jet-electron model and generic

simulated dijet events in the electron channel (ATLAS-CONF-2014-058)

• Data-driven anti-muon model in the muon channel.  

(ATLAS-CONF-2014-058)

• Multijet normalisation (for central

electrons, forward electrons and muons) extracted from a likelihood fit to the data of the MET distribution in regions

enhanced in multijet (by removing the MET and multijet rejecting cut).

• Top-quark processes and W+jets

floating in their theoretical constraints (6% and 20%)

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O V E R A L L N O R M A L I Z AT I O N C O N S T R A I N T S

• A data-driven method, based on a maximum-likelihood fit of the numbers of data events obtained in the tt and W+jets control regions and in the signal region is applied to constrain simultaneously ̄ the overall normalizations of the W+jets and top-quark background (tt, single top Wt and s-̄

channel) contributions and the t-channel signal.

• In this fit, the three fitted parameters are constrained within their theoretical normalisation uncertainties (4% t-channel, 6% tt, 20% W+jets). ̄

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A N G U L A R D I S T R I B U T I O N S I N T H E A N D W + J E T S C O N T R O L R E G I O N S

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Achieved a good modelling of the backgrounds

t t ̄

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U N F O L D E D D I F F E R E N T I A L M E A S U R E M E N T S : M E T H O D

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• Since we are still blinded to real data, the strategy is being tested with an Asimov dataset*.

• The three angular distributions are provided in the signal region.

• The different contributing backgrounds are substracted to the Asimov dataset so that we only stay with events expected to be t-channel signal events.

• Finally this t-channel signal events are unfolded in order to correct for the acceptance and detector effects.

• Differential angular distributions are provided unfolded at particle level.

• These differential distributions can be compared within the same fiducial region with theoretical predictions at particle level.

*The one dataset in which all observed quantities are set equal to their expected values.

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U N F O L D E D D I F F E R E N T I A L M E A S U R E M E N T S : M E T H O D

• An iterative Bayesian approach implemented in RooUnfold is used with additional correction factors.

• The correction factors and unfolding matrices are computed using the NLO Powheg+Pythia8 sample.

• 4 bins used (ensure > 70% of events in diagonal elements of the migration matrix).

• Convergence, closure and linearity tests have been performed.

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U N F O L D E D D I F F E R E N T I A L M E A S U R E M E N T S : T H E O R Y

• An iterative Bayesian approach implemented in RooUnfold is used with additional correction factors.

• The correction factors and unfolding matrices are computed using the NLO Powheg+Pythia8 sample.

• 4 bins used (ensure > 70% of events in diagonal elements of the migration matrix).

• Convergence, closure and linearity tests have been performed.

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C O N V E R G E N C E T E S T S T O F I X T H E N U M B E R O F I T E R AT I O N S

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Convergence test:

• The unfolding procedure is considered to have converged when the absolute change between two successive steps becomes negligible (α10^-4 difference between successive iterations for all samples).

• Different generators are considered:

• Powheg + Pythia8

• Protos + Pythia8

• aMc@NLO + Pythia8

• One scalar quantity, known as the forward-backward asymmetry, is considered for each angle in order to check the convergence of each angular distribution.

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U N F O L D E D D I F F E R E N T I A L M E A S U R E M E N T S : T E S T S

Closure test:

• Baseline Powheg+Pythia8 (FS) sample is split into two

subsets of same size: one sub-set is used to determine the migration matrix and the corrections applied to unfold the angular distributions built from the second sub-set of

events.

• These unfolded distributions are compared with the actual particle level distribution from the second subset to check the performance of the unfolding algorithm

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Linearity test:

• Tests performed by applying the nominal unfolding

corrections to the available Protos samples with different anomalous couplings (BSM effects considered in terms of these anomalous couplings).

• The angular distributions predicted by these simulated samples are unfolded using the resolution and efficiency corrections calculated with the Protos SM sample.

• The extracted histograms are then compared to the values derived from the particle-level distributions of each

sample.

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E X P E C T E D R E S U LT S F O R U N F O L D E D A N G U L A R D I S T R I B U T I O N S

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θX

cos

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Data/Pred.

0.5 0.75 1 1.25 1.5

θ/dσ d⋅σ1/

0.1 0.2 0.3 0.4 0.5

0.6 ATLAS Internal = 13 TeV , 140.5 fb-1

s

Data (expected) Stat. Unc.

Stat. + Sys. Unc.

POWHEG + Py8 POWHEG+H7 aMC@NLO+Py8 Protos

POWHEG+Py8 AFII

θY

cos

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Data/Pred.

0.5 0.75 1 1.25 1.5

θ/dσ d⋅σ1/

0.1 0.2 0.3 0.4 0.5 0.6

s

POWHEG+Py8 AFII

θl

cos

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Data/Pred.

0.5 0.75 1 1.25 1.5

θ/dσ d⋅σ1/

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

s

POWHEG+Py8 AFII

θl

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8cos 1

Fractional uncertainty [%]

-100 -80 -60 -40 -20 0 20 40 60 80 100

b-tagging leptons_SF Modelling others Jets ATLAS Internal

= 13 TeV , 140.5 fb-1

s

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θY

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8cos 1

-100 -80 -60 -40 -20 0 20 40 60 80 100

= 13 TeV , 140.5 fb-1

s

Comparisons with different MC predictions at particle level in the fiducial region

θX

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8cos 1

-100 -80 -60 -40 -20 0 20 40 60 80 100

= 13 TeV , 140.5 fb-1

s

Breakdown of uncertainty sources: Mainly dominated by jet energy resolution.

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E X P E C T E D R E S U LT S F O R U N F O L D E D A N G U L A R D I S T R I B U T I O N S

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Unfolded differential measurements for top and antitop quarks

• In pp collisions, both the production cross-section and the polarisation of top quarks and top antiquarks differ owing to the predominance of u-type quarks in the proton.

• This analysis provides a straight comparison between the differential cross section for top quarks and top antiquarks to check for any deviations from the theoretical predictions which could be related with new physics.

θX

cos

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 θ/dσ d⋅σ1/ 1

Top (expected) Antitop (expected) Stat. + Sys. Unc.

ATLAS Internal = 13 TeV , 140.5 fb-1

s

θY

cos

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 θ/dσ d⋅σ1/ 1

s

θl

cos

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 θ/dσ d⋅σ1/ 1

s

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S U M M A R Y A N D P L A N S

• The goals and the strategy of this analysis for measuring the top-quark differential angular distributions is fully determined.

• Differential angular measurements for angles θx,θy,θz unfolded at particle level comparing expected data (still unblinded) with different theoretical predictions provided by various MC generators in a fiducial region have been provided.

• Measurements can be carried out separately for top quarks and antiquarks.

• Results based on full Run 2 dataset (~140.5 fb^-1).

• Planning to go unblinded very soon!

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BACKUP

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B E Y O N D T H E S TA N D A R D M O D E L : A N O M A L O U S C O U P L I N G S

• The most general effective tWb interaction arising from a minimal set of dimension-six effective operators is (Nucl.Phys. B812 (2019)):

• Given the current limits (JHEP 04 (2017) 124), the following anomalous couplings samples have been studied in this analysis:

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