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Top Physics Results from the ATLAS Experiment

Marina CobalUniversity of Udine / INFN Udine

On behalf of the ATLAS Collaboration

PLHC2011Perugia 6-10 June 2011

First Results

with 2011 Data!!

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2010ATLAS recorded45 pb-1

2011

LHC status• 2010/2011 @ 7 TeV• Lpeak = 2.1x1032/1.3x1033 cm-2s-

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• ∫Ldt ~ 45 / ~736 pb-1

• Pile-up: <nvtx> = 2 / 6 <nvtx>

• Lumi uncertainty: 3.4%/4.5%

• Very high ATLAS data taking efficiency (93.7%) and fraction of good quality data from all sub-detectors

ATLAS recorded 736 pb-1

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Top quark pair production @ LHC• stt (7 Tev LHC) = 165+11

-16 pb – @ 172.5 GeV, Moch, Uwer,

Langenfeld (Phys. Rev. D78 (2008) 034003, arXiv:0907.2527) = 25 x stt (Tevatron)

• Few fb-1 expected by end of 2011 – 500k tt pairs (6 x Tevatron)

Dominant @ LHC: ≈80%M.Cacciari, S. Frixioneprivate communicationNason, Dawson, Ellis, 1988

LHC

Tevatron

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• https://twiki.cern.ch/twiki/bin/view/AtlasPublic/TopPublicResults• Top quark production cross section (35 pb-1, 2010 data)

– Single lepton pretag (ATLAS-CONF-2011-023)– Single lepton btag (ATLAS-CONF-2011-035)– Dilepton (ATLAS-CONF-2011-034)– Combination (ATLAS-CONF-2011-040)– All hadronic channel (ATLAS-CONF-2011-066)

• Top quark properties (35 pb-1, 2010 data) – Top quark mass (ATLAS-CONF-2011-033) – W helicity (ATLAS-CONF-2011-037)– tt + anomalous ET

miss (ATLAS-CONF-2011-036)– Search for FCNC top quark processes (ATLAS-CONF-2011-061)– Mass from the tt cross section (ATLAS-CONF-2011-054)– Single top (ATLAS-CONF-2011-027) – Single top t-channel (ATLAS-CONF-2011-088)– Search for tt resonances (ATLAS-CONF-2011-087)

Adam Roe, Thursday 4F

Results covered

R. Schwienhorst

V. Dao

D. Cinca, Friday SF

New

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5stt single lepton (no b-tagging)• Fit of projective likelihood based on uncorrelated variables• Three variables chosen:

• Binned maximum likelihood to 4 channels (3-jets, ≥4-jets; e,µ)

σtt = 171 ± 17(stat) +20-

17(syst) ± 6(lumi) pb

Syst. source Rel. unc. %Jet en. scale &Reconstruction -6.1 / +5.7

ISR/FSR -2.1 / +6.1QCD norm. 3.9QCD shape 3.4Parton shower& hadronisation 3.3

Total syst. -10.2 / +11.6

Lepton hm + jets

Lepton chargem + jets

exp(-8*aplanarity) m + jets

• Independent from b-tagging:– Avoids related systematics,

but worse S/B– Cross checked by cut&count

and 1 D-c2 likelihood

uncert. ~15%

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stt single lepton (b-tagging)• Multivariate method → split in 6 channels (=3,=4,≥5 jets;

e/µ)• Input variables: lepton η, exp(-8*aplanarity), exp(3*HT, 3p ),

b-tag weight• Profile likelihood fit extracts 16 parameters including σtt• Main systematics:

– W+jets HF content (7%)– Tagger calibration (7%)

σtt = 186 ± 10(stat) +21-20(syst) ± 6(lumi) pb

uncert. ~13%

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Single lepton analysis summary

Single lepton w/o btag Single lepton with btag

• Complementary and consistent results obtained from various analyses with and w/o b-tag info• Baseline and complementary cross sections are in agreement

within the uncertainties• Consistent with theoretical prediction (165+11

-16 pb)

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• Cut-based method– require 2 OS high PT leptons, 20 GeV– at least two energetic jets, 20 GeV– Fake rate and Z+jets dominating background

• ee/µµ: ETmiss > 40 GeV and |mll – mZ| > 10 GeV

• eµ: HT > 130 GeV• MC-assisted data-driven estimation of remaining Z+jets backgnd

– Uncertainty from ~100% to <50%• Fake lepton rate determined

in a data driven way – matrix methos)

• Main systematics– Jet energy scale (5%)– Parton shower model (5%)– Fakes (4%)

stt dileptonCan also simultaneously extract:– σttbar/σZ, lumi uncertainty cancels in ratio– σttbar, σWW, σZ→ττ from fit ET

miss vs Njets – σttbar and b-tagging efficiency

Z masslow ET

miss

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9stt dilepton• Cross-section extraction

– Profile likelihood method to combine channels

– Events in SR are compatible with top quarksHT: sum of lepton and jet momentaDistribution after requiring 1 b-tag

105 events selected, 101 ± 9 expectedData well modeled by MC+DD methods

Stransverse mass: generalizationof mT concept with two neutrinos

σtt = 173 ± 22(stat) +18-16 (syst) +8

-7(lumi) pb

Additionally requiringb-tag improves S/B

uncert. ~17%

σtt = 171 ± 22(stat) +21-16 (syst) +7

-6(lumi) pb

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• Combined dilepton and single lepton channels

– Single lepton with b-tag, dilepton w/o b-tag– Statistical uncertainty ~4%– Systematic uncertainty ~8%– Luminosity uncertainty ~3%

stt combination

stat

stat+sys

σtt = 180 ± 9(stat) ± 15(syst) ± 6 (lumi) pb

δσ = 10%

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11stt summary

• stt in full hadronic channel• Challenging measurement:

Measured < 261 pb @ 95% CLExpected < 314 pb @ 95% CL

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12Top quark mass• Single lepton selection

– Main aim: reduce JES systematic• Template fit to R32 = mjjb(t) / mjj(W)• Systematics: (b-)JES , ISR/FSR• Cross-checked by

– kinematic fit templates – 2D templates with Jet Scaling Factor

µ+jets

mtop = 169.3 ± 4.0(stat) ± 4.9(syst) GeV

µ+jets

µ+jets

13mtop from stt

• Use most accurate measurement– Multivariate analysis with b-tag

• Exploit dependence of stt from mtop– Assume mtop

MC = mtoppole

• mtop from combined uncorrelated th. and exp. likelihood: max determines the extracted mtop

pole – fit performed for 3 theoretical calculations

• 13% uncertainty on stt ~ 5 GeV uncertainty on mtop

• Default analysis → NNLO Langenfeld– (Phys. Rev. D80 (2009) 054009)

mtop = 166.4+7.8-7.3 GeV

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Single top: t-channel

σt = 76+41-21 pb

Observed (Expected)

Significance: 6.2 (5.7) s

Statistical uncertainty (17%)B-tagging (+12

-9 %), JES((+23-2 %)

Signal Modelling (+36-20 %)

• Single lepton NN analysis (check with cut&count)

• Background: QCD multijet, W+jets

• 22 variables– most important: Mlnb by M(jet1, jet2)

New Result!!156 pb -1

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Single top: Wt-channel

σWt < 158 pb at 95%

• Wt-channel: cut-based analysis– single lepton+dilepton– SM predicts sWt = 15 pb– Combine dilepton and l+jets channel

• Include systematic uncertainty correlations– Dominant: JES, b-tag modeling, ISR/FSR, MC

stat• Expected (observed) limit on sWt

corresponds to 6.3 (10.5) * sWtSM

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W helicity• Wtb structure probed →

set limits on new physics• V-A coupling predicts:

• F0 ≈ 0.7 (long. polarization)• FR ≈ 0 (RH polarization) • FL≈ 0.3 (LH polarization)

• Use e+jets and µ+jets channels• Can extract directly from cos θ* or unfold to

parton level and calculate asymmetry

Templatemethod

Asymmetrymethod

FL 0.41 ± 0.12

0.36 ± 0.10

F0 0.59 ± 0.12

0.65 ± 0.15

FR Fixed 0 -0.01 ± 0.07

b

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W helicity• V-A coupling predicts:

• F0≈0.7 (long. Polarization)• FR≈ 0 (RH polarization) • FL≈ 0.3 (LH polarization)

• Wtb structure probed → set limits on new physics

• Use e+jets and µ+jets channels• Can extract directly from cos θ* or unfold to

parton level and calculate asymmetry

FL

F0 FR

Templatemethod

Asymmetrymethod

FL 0.41 ± 0.12

0.36 ± 0.10

F0 0.59 ± 0.12

0.65 ± 0.15

FR Fixed 0 -0.01 ± 0.07

b

• Search for anomalous ETmiss in ttbar (lepton+jets) events

– benchmark: TT pair, TtA0• A0 dark matter candidate• Enhanced cross-section due to spin states

– Signal region: • ETmiss >80 GeV, mT>120 GeV; dilepton veto: pT > 15 GeV, tracks, loose

electrons

tt + anomalous ETmiss

Observed: 17 evtsExpected 17.2 ± 2.6

Observed 17

Alwall, Feng, Kumar et al. (2010)Berger, Cao (2009)

Exclude MT< 275 (300) GeVFor MA0< 50 (10) GeV

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19FCNC in decay• Look for tZq vertex (q =u,c):

leptonic W/Z decays to suppress QCD background

• Cut & Count:– 3 leptons (> 25 GeV, > 20 GeV, > 15

GeV), two same flavor and OS– 2 jets ( >30 GeV, >20 GeV), ETmiss>20

GeV

Expected BR(tqZ) < 17%Observed BR(tqZ) < 12‐3

+4 %

@ 95% CL

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FCNC in production• look for gqt vertex

– anomalous single top production• Single lepton Events

– exactly 1 b‐tagged jet– exactly 1 lepton (e/μ)

• Neural Network with 13 input variables+ binned likelihood ratio

• No excess observed: limit on σqg*BR(tWb)

• Systematics: ISR, JES, HF content in W+jets

Observed σqgt*BR(tWb) < 17.3 pbExpected σqgt*Br(tWb) < 17.4‐

5.4+8.2 pb

21Search for high mtt• In many new physics models

top plays a special role – Narrow resonance: leptophobic

topcolor Z' as benchmark model– Wide resonance: a Kaluza-Klein

gluon gKK, which appears in Randall-Sundrum models

• mtt from 3 or 4 jets, e/m, and n– DRmin variant (4 highest pT jets

considered, jet removed if too far from other objects and has too high mass)

• Normalization uncertainties – W (35%), diboson (5%), QCD in e

(30%) and m (50%) channels• Shape uncertainties

– b-tag (11%), JES (9%), ISR/FSR(7%)

New Result!!200 pb -1

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For a narrow Z’, observed 95% C.L. limits range from ≈38 pb to 3.2 pb for masses from MZ’= 500 GeV to 1300 GeV.

In Randall-Sundrum models, KK gluons with masses below 650 GeV excluded at 95% C.L.

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Conclusions • Top physics in ATLAS just started. With only 35 pb-1 of data:

– Cross section measurement already competitive ~10% uncertainty– Large number of measurements already performed

• New results presented on single top and Mttbar with 2011 data

• Statistic limited analysis will soon become very competitive Already ≈0.7 fb-1 recorded . 1 fb-1 coming soon!

• Focus to reduce impact of systematics– Improve even further detector understanding– Use advanced analysis techniques

• 2011 year of precision → more top measurements and maybe discoveries!– Cross sections in various channels, differential cross sections and top

mass– Charge asymmetry, spin correlation, top charge– t’t’ , same sign top, W’ search in single top

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• Single lepton cut based (check with likelihood)

• 1 lepton, 1 b-jet, 1 light jet, ET mi ss >25 GeV

• Background: QCD multijet, W+jets• Final cut M t o p ∈ [130, 210] GeV, h | (u-jet)|

> 2.5• Likelihood ratio using : cosDf (lep,Etmiss),

DR(bjet,lep), h (untagged jet), reconstructed Mtop

Single top: t-channel

σt = 53+27-24(stat) +38

-27(syst) = 53 +46-36 pb

To be updated

• Significance: 1.6

25Search for high Mttbar • Search for massive neutral bosons into a ttbar pair

– leptophobic topcolor Z' as benchmark model (hepph: 9911288)• Search for anomalous high mass tt + X production →

– Quantum Black Holes as benchmark model (JHEP 0805(2008)003)

• M reconstruction using 3 different methods. – DRmin variant (4 highest pT jets considered, jet removed if too far

from other objects and has too high mass). M build from lepton, n and ¾ remaining jets

• Main systematics: JES, JER, btagging• Bayesian approach using binned likelihood to the Mttbar • No evidence for resonance found

To be updated

Observed 95% CL limit from 55(2.2)pb at M=500(2000) GeV

Exclude MQBH < 2.35 TeV @ 95% CL

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The Top quark: from theory…• Weak isospin partner of b-quark• Spin ½ fermion, charge 2/3• Produced in pairs via qq annihilation or

gg fusion• Lifetime ~ 5 . 10-25 s • Decays (BR ~ 100%) in Wb

Most precise predictions of production cross section by approximate NNLO calculations, precision of 6% to 9% : For Mtop = 172.5 GeV/c2, tt ~xxx

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…to experiment

• Mass known with a precision < 1% • Top properties (charge, width, …) deeply investigated • Single top identified• Top samples now land for searches for new physics

Discovery at the Tevatron 1995 (67 pb-1) 2011 ( > 5 fb-1

analyzed), 10 fb-1 by the end of RunII)

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Top decay