james howarth on behalf of the atlas collaboration the university of manchester

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Spin Correlations, Polarisation and

Charge Asymmetries in Top Quark Pairs

James Howarthon behalf of the ATLAS Collaboration

The University of Manchester

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Top Quark Properties

t

t

l+

ν

W+

W-

d

u

b

b

PropertiesMass, Mass difference,

Width, Charge, Spin, Polarisation,

Spin correlation, Asymmetries

ProductionCross section, generator studies, rare couplings

DecayW Polarisation, Colour Flow,

Branching Ratios, |Vtb|

The Top Quark

• Very heavy (Mtop =173 GeV)

• Narrow width short lifetime (10-25s)• Decays before hadronisation• “Bare” quark (almost)

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Dilepton Events

Dilepton Events:• 2 Leptons (opposite sign, isolated, trigger match)• At least 2 Jets• ET > 60 GeV (ee/μμ)

• HT > 130 GeV (eμ) HT = Scalar sum of all lepton & jet pT

• mll > 15 GeV and veto 81 GeV < mlll < 101 GeV (ee/μμ)

• Optional b-tagging (70% efficiency working point)

l+

ν

l-

νl

b

bt

t

W+

W-

miss

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Single Lepton

u

d

l-

νl

b

bt

t

W+

W-

Single Lepton Events:• 1 Lepton (isolated, trig match)• At least 4 Jets with at least 1 b-tag

(e + jets)• ET > 20 GeV • ET + mT > 30 GeV

(μ + jets)• ET > 35 (30) GeV • mT > 25 (30) GeV

mT =

miss miss

miss W W

W

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Spin Correlation and

Polarisation

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Introduction – Spin Variables

• Polarisation (P) and Spin Correlation (A) closely related.

Greg Mahlon, 3rd International Workshop on Top Quark Physics, Bruges, May 2010

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cos(θi/j)

• Angle between a decay particle from the top and some chosen spin quantisation axis in top rest frame.

• Top Helicity is a well known example.

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Spin Correlation

Spin Correlation

&

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• Looking at the double differential distribution requires full tt reconstruction.

• ΔΦ between two spin analysers in lab frame is also sensitive to spin correlations.

• Does not require tt reconstruction and sensitive to gluon gluon fusion.

• Precision test of the SM.

• Ideal variable for LHC!

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Polarisation

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lepton d u

αi/j (LO) 1.00 1.00 -0.31

αi/j (NLO) 1.00 0.93 -0.31

• Leptons and down type jets are the best analysers• Hard to distinguish u type jets from d type jets

F. Hubaut, E. Monnier, P. Pralavorio, K. Smolek, V. Simak, Eur.Phys.J. C44S2 (2005) 13-33

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Results

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Observation of Spin Correlation

AHELICITY : 0.40 ± 0.04 (stat) +0.08/-0.07 (syst)SM Prediction: 0.31

2.1 fb-1 7 TeV

• Perform likelihood fit with templates A=SM and A=0 and extract fSM.

• fSM = 1.30 ± 0.14 (stat.) +0.27/-0.22 (syst.)

• Extrapolate AHELICITY = fSM * PredHELICITY

• Exclude the A = 0 case with 5.1

First Observation of Spin Correlation in tt!

Consistent with hypothesis of spin ½

Phys. Rev. Lett: Volume 108 Issue 21(Prediciton) W. Bernreuther and Z. G. Si, Nucl. Phys. B 837, 90(2010)

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Top Quark Polarisation4.7 fb-1 7 TeV

αP : -0.060 ± 0.018 (stat) +0.046/-0.064 (syst)

• Template fit similar to Spin Correlation, hypotheses αP = +1 and αP = -1• Dominant Systematic comes from jet reconstruction, jet resolution and signal modeling.

• All results consistent with SM.

ATLAS-CONF-2012-133

Even

ts

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Charge Asymmetry

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Introduction – Charge Asymmetry

• |Δyl|=|yl+|-|yl-|: Doesn’t Require Reconstruction• |Δyt|=|yt+|-|yt-|: Easier to compare

AC

Asymmetry measurements at the LHC and Tevatron:

yAFB

• Small Asymmetry expected at Tevatron (~5%)• Large deviation seen (~15%)• LHC tt is pp collider and charge symmetric.• Asymmetry observable but expected to be very small

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Single Lepton Charge Asymmetry1.04 fb-1 7 TeV

Eur. Phys. J. C (2012) 72:2039: http://arxiv.org/abs/1203.4211

• Events reconstructed using Kinematic Likelihood Fitter (KL)• Rapidity distributions unfolded to parton level.

• Inclusive and differential measurements in mtt

MC@NLO (inclusive) Prediction AC = 0.006 ± 0.002

AC = -0.019 ± 0.028 (stat.) ± 0.024 (syst) inclusive

AC = -0.052 ± 0.070 (stat.) ± 0.054 (syst) mtt > 450 GeV

AC = -0.008 ± 0.035 (stat.) ± 0.032 (syst) mtt < 450 GeV

high

low

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Single Lepton Charge Asymmetry

Eur. Phys. J. C (2012) 72:2039: http://arxiv.org/abs/1203.4211

INCLUSIVE mtt > 450 GeVNote: Does not include most recent results from all colliders

AC = -0.019 ± 0.028 (stat.) ± 0.024 (syst) inclusive

AC = -0.052 ± 0.070 (stat.) ± 0.054 (syst) mtt > 450 GeV

AC = -0.008 ± 0.035 (stat.) ± 0.032 (syst) mtt < 450 GeV

high

lowD

0

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Dilepton Charge Asymmetry4.7 fb-1 7 TeV

• Asymmetry measured at parton level

• Background subtracted data corrected for detector acceptance.

• Measured both lepton and top asymmetry.

• Results consistent with SM (predictions from MC@NLO).

COMB WITH L+JETS RESULT

ATLAS-CONF-2012-57

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Current ATLAS Asymmetry Results

• Current Status of the 7 TeV Asymmetry measurements at ATLAS• Everything consistent with SM

AC

-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08 0.1

ATLAS (combined)

ATLAS (dilep)

ATLAS (single lepton)

prediction

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tt + HF NEW!

• tt + HF (HF = bb, cc, b, c)

• Flavour coming from gluon splitting or proton• main irreducible background to ttH• Many BSM models also predict additional HF in tt events.

At least 3 b-tagged jets

At least 2 b-tagged jets

• Many experimental uncertainties cancel in ratio• Cross sections measured in fiducial region (with a factor to correct for detector acceptance)• Correction factor is 0.093 ± 0.004(stat.) in HF, 0.129 ± 0.001(stat.) in tt + j

Heavy Flavor Ratio

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tt + HF 4.7 fb-1 7 TeV NEW!

RHF = [7.1 ± 1.3 (stat.) +5.3/-2.0 (syst.)] %

RHF (LO) = [3.4 ± 1.1 (syst.)] %RHF (NLO) = [5.2 ± 1.7 (syst.)] %

• perform a binned maximum likelihood fit to extract σfid (tt + HF)

• σfid (tt + j) taken from observed yield in data.

• Use the displaced vertex mass in bins of b-tag purity (improves LF-c jet discrimination).

To be submitted to Physical Review D

• Dominant systematic is flavor composition

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Summary and Outlook

• LHC delivering precision measurements.

• First observation of non vanishing Spin correlations, measurements on Polarisation and Charge Asymmetry.

• 7 TeV Analyses for ATLAS Top Properties almost completed (one or two more to look forward to)

• Start to see 8 TeV properties results soon!

• Charge Asymmetry results so far statistics limited.

• Long Shutdown 1 will be a very exciting time for top properties and plenty to look forward to with 13 TeV!

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Backup

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References

“Spin Correlations: TEVATRON vs. LHC”, Greg Mahlon, 3rd International Workshop on Top Quark Physics, Bruges, May 2010https://agenda.irmp.ucl.ac.be/conferenceTimeTable.py?confId=538#20100530

“ttbar asymmetries in the SM”, Germán Rodrigo, LHC Workshop “From Charge Asymmetry to the Boosted Regime” 02-04 May 2012https://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=175916

“Top Charge Asymmetry at ATLAS”, Kerim Suruliz, LHC Workshop “From Charge Asymmetry to the Boosted Regime” 02-04 May 2012https://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=175916

Figures

Papers“ATLAS sensitivity to top quark and W boson polarization in tt events” ̄� , F. Hubaut, E. Monnier, P. Pralavorio, K. Smolek, V. Simak, Eur.Phys.J. C44S2 (2005) 13-33http://arxiv.org/abs/hep-ex/0508061

“Distributions and correlations for top quark pair production and decay at the Tevatron and LHC”W. Bernreuther and Z. G. Si, Nucl. Phys. B 837, 90(2010), and private communication.

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Systematics: Dilep Charge Asymm

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tt + HF – b tag purityHigh Purity Medium Purity

Low Purity

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Asymmetries Cartoon

http://cds.cern.ch/record/1493476/files/TOP-12-010-pas.pdfCMS Dilepton

CMS l+jets http://cds.cern.ch/record/1458955/files/plb.717.129.pdf

http://cds.cern.ch/record/1453785/files/ATLAS-CONF-2012-057.pdf

ATLAS Dilep + Combination

ATLAS l + jets http://link.springer.com/article/10.1140%2Fepjc%2Fs10052-012-2039-5

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lepton d u

αi/j (LO) 1.00 1.00 -0.31

αi/j (NLO) 1.00 0.97 -0.32


Double Differential DistributionP = 0, A = SM

[a]

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Physics Objects at ATLAS

JETS:• Anti-kT jets (R=0.4)• Corrected to EM Scale• |η| < 2.5, ET > 25 GeV• 75% tracks point to vertex• B-Tagged using Neural Net based tagger, eff 70%.

MUONS:• Muon Spectrometer track combined with Inner Detector track• Isolated from other objects•|η| < 2.5• PT > 20 GeV

ELECTRONS:• Central energy deposit combined with Inner Detector track• Isolated from other objects• |ηcluster| < 2.47, • 1.37 < |ηcluster | < 1.52• ET > 25 GeV

MISSING TRANSVERSE ENERGY:• Sum of transverse energies of all reconstructed physics objects.• Accounts for unassociated calorimeter cells. ?

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Event Model - Backgrounds

“lepton” Fakes (mismeasured leptons)• Data Driven• Matrix Method

Single Top:• MC@NLO + HERWIG & JIMMY• Dilepton Wt channel only.

Drell-Yan: ee, μμ, ττ• ALPGEN + HERWIG & JIMMY• Z + (0-5 jets inclusive)

W + jets:• ALPGEN + HERWIG & JIMMY• W + (0-5 jets inclusive)

DiBoson:• HERWIG (ALPGEN) & JIMMY• WW, WZ and ZZ

james howarth on behalf of the atlas collaboration the university of manchester

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