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14Top quark properties in ATLASRuth Laura Sandbach

X-SILAFAE-2014, Medellin, Colombia

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Introduction• Top quark:

• Heaviest known elementary particle• Shortest lifetime of any quark (3.29+0.90

-0.63) x 10-25s• Can be studied as bare quarks (before hadronisation)

• Study of top- and antitop-quarks provide opportunities both to test the standard model and probe new physics beyond the standard model (BSM)

• Various properties of the top quark have been measured since its discovery at the Tevatron in 1995:• Most precise quantity measured is the top quark mass • Spin information can be deduced from the angular distributions of decay particles• Top-anti-top production charge asymmetry, an important test of QCD at high

energies

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45 top publications from Run-I since 2011

We will focus on latest results from Top14 from ATLAS

Frédéric Déliot 09.10.14

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Top production at the LHC LHC collides protons (pp) at √s = 7 TeV (2011) and √s = 8 TeV (2012), top quarks

predominantly produced in pairs via gluon gluon fusion Top decays almost entirely to a W boson and a b-jet ( ), so analyses are divided

into final state-dependent categories: single lepton (lepton+jets), dilepton and fully hadronic

Single leptonDilepton

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Gluon-gluon fusion

Typical event Selection: Single lepton:

At least four jets, two b-tagged Exactly one lepton Large missing transverse momentum

Dilepton At least two jets, two b-tagged 2 leptons Large missing transverse momentum

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Measurements of the Top-quark Mass

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Latest summary plots for mtop

ATLAS-CONF-2014-053

Sensitive to the top-quark mass since gluon radiation depends on the mass of the quarks

The mass is extracted from a measurement of the normalized differential cross section of top pair production in association with one jet, as a function of the inverse mass of the system, using:

Invariant mass of system

Reconstruction of the top-antitop system Unfold the cross section distribution (using SVD) Compare the distribution with the NLO+PS distribution (Powheg) at

different mass using a χ2

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Measurements of the Top-quark Mass

arXiv:1409.0832

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Template fit to the ratio of 3-jet mass (top candidates) to 2-jet mass (W candidates)

Top mass obtained from fit is less sensitive to uncertainty in the energy measurement of the jets

Binned likelihood top mass of Event selection

Hadronic Final State

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Measurements of the Top-quark Mass

arXiv:1406.5375

Mass extraction from the comparison between the theoretical and measured σtt

theoretical calculation using a well defined top mass scheme (pole mass)

almost no dependency on the top mass in the measured cross section:

almost no dependency on the mass definition in the MC

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Determination from top-antitop cross section

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Measurements of the Top-quark Mass

Fewer ambiguities in final state reconstruction and reduced combinatorial background if we only consider leptonic W decay, there is only one b-jet in the final state. Categorise events by a high-pT isolated lepton, missing transverse momentum and exactly two jets (b- and light)

Typical Q2 energy scale much lower than top-antitop pair production Less colour connection between top and protons Statistically independent sample

ATLAS-CONF-2014-055

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t-channel Associated Wt s-channel

Moti

vatio

n

MVA techniques separate signal from background (NN preprocesses input variables to enhance separation power)

Treat top pair production and single top as signal Distributions of m(lb) are constructed, since this is sensitive to top quark mass,

suing a range of discrete values for mtop

Met

hod

Systematics dominated by JES uncertainties

Value of mt in good agreement with other tt

measurements

Single top

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Measurements of the Spin Correlation between top-antitop quarks

Top-antitop pairs have been observed to be produced essentially unpolarized1 in pp collisions, however the correlation of the spin orientation of the top and anti-top can be studied, and is predicted to have a non-zero value

Some BSM models can alter both the production mechanism altering the spin correlation: E.g. heavy Higgs boson decay into a tt pair

… and decay of the tt pair, affecting how the spin information is accessed: E.g. supersymmetric models in which a top decays to a charged Higgs boson, subsequently

decaying to a lepton neutrino pair

Hence, measuring the spin correlation in tt events can simultaneously probe top production and decay effects due to new physics

1ATLAS, Phys. Rev. Lett. 111, 232002

arXiv:1407.4314v1

Measurements of spin correlation using full 7 and 8 TeV data sample of 4.6fb-1

Both single and dilepton final states explored

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Measurements of the Spin Correlation between top-antitop quarks

arXiv:1407.4314v1

1: Azimuthal angle Δφ: Both single and dilepton final states

2. “S-Ratio” of matrix elements for top production and decay from the fusion of like helicity gluons

Top and antitop have to be fully reconstructed

3 & 4. Double differential distribution

Top direction in tt rest frame is used as the spin quantization axis (“helicity” basis)

Event-by-event quantization axis that maximises spin correlation (“maximal” basis) used as the

top spin quantization axis

Four observables:

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Spin correlation strength

Correlation coefficient

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Results: arXiv:1407.4314v1

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Dilepton

Single lepton

Both in good agreement with SM predictions

stop between the top quark mass and 191 GeV are excluded at 95% CL

√s = 7 TeV

√s = 8 TeV

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Top-quark Charge Asymmetry• At LO in SM, quark pair production is symmetric under charge conjugation• However, at next-to-leading order (NLO) symmetry no longer valid due to

interferences between the Born and 1-loop diagram, and similarly for due to interferences between initial and final state radiation processes

• Results in a charge asymmetry, and consequently a forward-backward asymmetry in events

• Measurement of asymmetry in production allows search for unknown top quark production mechanisms which are invisible in the invariant mass spectrum

• Stringent test on QCD at very high energies

At the LHC..• Dominant production is gg fusion which is symmetric under charge conjugation• Difficult to define the quark direction since the two incoming beams are symmetricPerform measurement in the laboratory frame using the ATLAS detector and data collected at an integrated luminosiy of 4.6 fb-1 in the dilepton channel

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Top-quark Charge AsymmetryFinal state requirements:• exactly two charged leptons• ≥ 2 jets• Large missing transverse energy

Final state contains two neutrinos: challenging to reconstruct final state

Since charge asymmetry from is transmitted to leptons, can obtain a measurement of purely leptonic based asymmetry Doesn’t require full final state reconstruction Benefits from high precision lepton reconstruction LEPTONIC ASYMMETRY

Lepton pseudorapidities

After full reconstruction in the lab frame, the difference in the top(antitop) rapidities is computed and used to measure the asymmetry :

SM predictions at the LHC:(ATLAS)

(CMS)

Recent measurements from LHC (before Sep 2014)

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Top-quark Charge AsymmetryUpdate for 2014 dilepton measurement for 4.6 fb-1:• New analysis software framework• Latest MC simulation samples• Use a bin-by-bin correction method and unfolding (instead of

calibration) for leptonic symmetry measurement• Neutrino weighting technique to reconstruct kinematics

Samples and background estimation

Results:In agreement with

standard model prediction

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Top-quark Charge AsymmetryUpdate for 2014 dilepton measurement for 4.6 fb-1:• New analysis software framework• Latest MC simulation samples• Use a bin-by-bin correction method and unfolding (instead of

calibration) for leptonic symmetry measurement• Neutrino weighting technique to reconstruct kinematics

Samples and background estimation

Results:In agreement with

standard model prediction

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Outlook for future measurements:• Run-I data at √s = 8 TeV has L=20.3fb-1

• Cross section for top-antitop at 8TeV = 253+13-15pb

6 times more signal events for asymmetry measurement:• Will decrease stat. uncertainty• Will be feasible to perform differential

measurements of asymmetry as a function of rapidity, transverse momentum and invariant mass

• Differential measurements will increase sensitivity of asymmetry measurements to new BSM physics

• Differential measurements will enhance measurements of SM charge asymmetry during Run-II at √s = 14 TeV

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Thanks!

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Backup

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Backup Spin correlation

Electron channel

DILEPTON

arXiv:1407.4314v1

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Backup Spin correlation

Muon channel

DILEPTON

arXiv:1407.4314v1

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Backup Charge Asymmetry

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