Search for exotics with top at ATLAS

Diedi Hu Columbia University

On behalf of the ATLAS collaboration

EPSHEP2013, Stockholm July 18, 2013 Motivation Motivation

Due to its large mass, top offers a unique window to search for new physics:

mt ∼ ΛEWSB (246GeV): top may play a special role in the EWSB Many extensions of the SM explain the large top mass by allowing the top to participate in new dynamics.

Approaches of new physics search Measure properties: check with SM prediction. Directly search for new particles coupling to top tt resonances searches Search for anomalous single top production Vector-like searches (Covered in another talk ”Searches for fourth generation vector-like quarks with the ATLAS detector”) Search for exotics with tt and single top D.Hu, Columbia Univ. 2 / 16 tt resonance search with 14.3 fb−1 @ 8TeV tt resonance: benchmark models 2 specific models used as examples of sensitivity

Leptophobic Z 0 [2] Explains the top quark mass and EWSB through top quark condensate Z 0 couples strongly only to the first and third generation of quarks Narrow resonance: Γ/m 1.2% ∼ Kaluza-Klein gluons [3] Predicted by the Bulk Randall-Sundrum models with warped extra dimension Strongly coupled to the top quark Broad resonance: Γ/m 15% ∼

Search for exotics with tt and single top D.Hu, Columbia Univ. 3 / 16 tt resonance search with 14.3 fb−1 @ 8TeV tt resonance: lepton+jets(I) [4]

Backgrounds SM tt, Z+jets, single top, diboson and W+jets shape: directly from MC W+jets normalization, multijet: estimated from data

Event selection One top decays hadronically and The boosted selection the other semileptonically. helps improve efficiency Boosted selection in high signal mass 1 high pT R=1.0√ jet with region mjet > 100GeV, d12 > 40GeV;

1 R=0.4 jet close to lepton; 20 18 ATLAS Preliminary Simulation ≥ 1 R=0.4 b-jet(could overlap s=8 TeV 16 µ + jets, combined with the former 2 jets.) 14 µ + jets, boosted e + jets, combined miss 12

1 isolated lepton; E . [%] Efficiency e + jets, boosted T 10 Resolved selection 8 6 3 or 4 R=0.4 jets with ≥ 1 b-jet 4 miss 2 1 isolated lepton; ET . 0 (Same as boosted channel) 0 0.5 1 1.5 2 2.5 3 3.5 m [TeV] tt Search for exotics with tt and single top D.Hu, Columbia Univ. 4 / 16 tt resonance search with 14.3 fb−1 @ 8TeV tt resonance: lepton+jets(II)

tt invariant mass reconstruction miss pz (ν): from lepton+ET system with mW constraint

2 Resolved: χ algorithm using mt and Boosted: take R=1 jet as

mW , top pair pT balance as constraint hadronically decaying top; build the other top from ν, lepton and R=0.4 jet

0.3 ATLAS ATLAS 0.25 Preliminary Preliminary Simulation, s=8TeV 0.25 Simulation, s=8TeV Resolved Boosted 0.2 Z’ (0.5 TeV) Z’ (1.0 TeV) 0.2 Z’ (1.0 TeV) Z’ (1.5 TeV) 0.15 Z’ (1.5 TeV) Z’ (2.0 TeV) Z’ (2.0 TeV) 0.15 Z’ (3.0 TeV)

0.1 0.1 Fraction of events / 50 GeV Fraction of events / 100 GeV

0.05 0.05

0 0 0 0.5 1 1.5 2 2.5 3 3.5 0 0.5 1 1.5 2 2.5 3 3.5 mreco[TeV] mreco[TeV] tt tt The tt mass spectra are used for statistical analysis Search for exotics with tt and single top D.Hu, Columbia Univ. 5 / 16 tt resonance search with 14.3 fb−1 @ 8TeV tt resonance: lepton+jets(III)

Dominant systematics on yields 8 10 ATLAS Preliminary Data 5 × Z’ (1.5 TeV) 7 10 •1 5 × g (2.0 TeV) ∫ L dt = 14.2 fb tt KK Systematics Resolved Boosted Note: R=1. jet systematics 106 Multi•jets W+jets tt 105 Other Backgrounds normalization 8.0% 9.0% also affect the resolved channel 4 s = 8 TeV Events / TeV Events / 10 JES of because only events failing the 103 R=0.4 jets 6.0% 0.70% 2 JES+JMS of 10 R=1. jets 0.30% 17% boosted selection will be 10 1 b-tag efficiency 4.0% 3.4% examined with the resolved 1.5 0 0.5 1 1.5 2 2.5 3 3.5 PDF 2.9% 6.0% 1 selection criteria. 0.5

Data/Bkg 0 0.5 1 1.5 2 2.5 3 3.5 mreco [TeV] Exclusion ranges @ 95% C.L. tt −1 14.3 fb @ 8TeV [4] s = 8 TeV Obs. 95% CL upper limit s = 8 TeV Obs. 95% CL upper limit 3 ) [pb] ) [pb] 3 t t 10 Exp. 95% CL upper limit Exp. 95% CL upper limit •1 10 L dt •1

L dt t t ∫ = 14.3 fb ∫ = 14.3 fb Exp. 1 σ uncertainty Exp. 1 σ uncertainty → Exclusion ranges Observed → σ Exp. 2 σ uncertainty 2 Exp. 2 uncertainty KK 0 10 2 Z 0.5-1.8TeV Leptophobic Z’ (LO x 1.3) 10 Kaluza•Klein gluon (LO)

BR(Z’ ATLAS ATLAS × Preliminary BR(g Preliminary KK gluon 0.5-2.0TeV 10 ×

Z’ 10 KK σ g σ −1 1 4.7 fb @ 7TeV [5] 1

10•1 10•1 Exclusion ranges Observed 0 10•2 Z 0.50-1.74TeV 10•2 0.5 1 1.5 2 2.5 3 0.5 1 1.5 2 2.5 KK gluon 0.70-2.07TeV g mass [TeV] Z’ mass [TeV] KK Search for exotics with tt and single top D.Hu, Columbia Univ. 6 / 16 tt resonance search with 14.3 fb−1 @ 8TeV tt resonance: full hadronic decays [6]

102

Two complementary 2 Obs. 95% CL upper limit Obs. 95% CL upper limit

) [pb] 10 ) [pb] t Exp. 95% CL upper limit t Exp. 95% CL upper limit t t Exp. 1 σ uncertainty Exp. 1 σ uncertainty techniques relying on jet → →

Exp. 2 σ uncertainty KK Exp. 2 σ uncertainty 10 substructure are used to 10 Leptophobic Z' (LOx1.3) KK gluon (LO) BR(Z' × BR(g ATLAS ATLAS × σ

identify top: HEPTopTagger σ HEPTopTagger 1 HEPTopTagger 1

Top Template Tagger 10-1 s = 7 TeV s = 7 TeV

-1 -1 ∫ L dt = 4.7 fb 10-1 ∫ L dt = 4.7 fb Exclusion ranges @ 95% C.L. 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0.8 1 1.2 1.4 1.6 1.8 2 −1 with 4.7 fb @ 7TeV g Mass [TeV] Z' Boson Mass [TeV] KK Use HepTopTagger 102 10 Obs. 95% CL upper limit Obs. 95% CL upper limit ) [pb] ) [pb] t Exp. 95% CL upper limit t Exp. 95% CL upper limit t t σ Exp. 1σ uncertainty Exp. 1 uncertainty Exclusion ranges Observed → → 10 σ Exp. 2σ uncertainty Exp. 2 uncertainty Z 0 0.70-1.00TeV KK Leptophobic Z' (LOx1.3) KK gluon (LO)

BR(Z' 1

1.28-1.32TeV × BR(g

1 × σ

KK gluon 0.70-1.48TeV σ

-1 Use Top Template Tagger 10 s = 7 TeV s = 7 TeV

-1 -1 ∫ L dt = 4.7 fb 10-1 ∫ L dt = 4.7 fb Exclusion ranges Observed 10-2 ATLAS Top Template Tagger ATLAS Top Template Tagger

KK gluon 1.02-1.62TeV 1 1.2 1.4 1.6 1.8 2 1 1.2 1.4 1.6 1.8 2 g Mass [TeV] Z' Boson Mass [TeV] KK Search for exotics with tt and single top D.Hu, Columbia Univ. 7 / 16 W 0 → tb search with 14.3 fb−1 @ 8TeV

W 0 tb: theory [7] →

W 0 carries new charged interaction as a result of enlarged symmetry group. Example of phenomenological models Extra dimensions: Kaluza-Klein excitations of SM W Little Higgs: predicts new particles including W 0 Model-independent search Use an effective model describing the coupling of the W 0 to the fermions. 0 Vij 0 5 0 5 0µ L = √ f¯i γµg (1 + γ ) + g (1 γ )W fj + h.c. 2 2 Rij Lij −

Search for W 0 in t+b decay channel Explore models potentially inaccessible to leptonic search (W 0 → lν). 0 WR can not decay to a lepton and νR if mνR > mW 0 Leptophobic W 0 W 0 may couple more strongly to the third generation quarks Can exploit the distinct signature of top quark.

Search for exotics with tt and single top D.Hu, Columbia Univ. 8 / 16 W 0 → tb search with 14.3 fb−1 @ 8TeV

W 0 tb: analysis strategy →

Background tt, Z+jets, single top, diboson, W+jets shape: from MC W+jets normalization, multijet: estimated from data Event selection miss Preselection: 1 lepton, ET , 2 or 3 jets with ≥ 1 b-jet Signal region: ≥ 2 b-jets, MW 0 ≥ 270GeV Control region:

≥ 2 b-jets, mW 0 < 270GeV: Derive W+jets normalization; verify the variable modeling =1 b-jet: check the modeling of kinematic variables tb invariant mass reconstruction miss pz (ν): from lepton+ET system with mW constraint b from top: the b-jet/jet giving the mWb closest to mt 0 b from W : the other b-jet or highest pT jet

Search for exotics with tt and single top D.Hu, Columbia Univ. 9 / 16 W 0 → tb search with 14.3 fb−1 @ 8TeV

W 0 tb: TMVA → Use Boosted Decision Tree discriminators (BDT) 1 BDT for 2-jet events: 14 variables used for training Most discriminating variables: mtb, pT (t), ∆φ(l,top−jet) 1 BDT for 3-jet events: 13 variables used for training Most discriminating variables: mtb, pT (t), sphericity

Search for exotics with tt and single top D.Hu, Columbia Univ. 10 / 16 W 0 → tb search with 14.3 fb−1 @ 8TeV

W 0 tb: result → Dominant systematics tt MC generator 10% b-tag efficiency 15% − 25% for events with ≥ 2b The BDT output distributions are used for statistical analysis Exclusion ranges @ 95% C.L. with 14.3 fb−1 @ 8TeV

Exclusion ranges Expected Observed 0 WL 0.50 1.56TeV 0.50 1.74TeV W 0 0.50 − 1.72TeV 0.50 − 1.84TeV R − −

Search for exotics with tt and single top D.Hu, Columbia Univ. 11 / 16 b∗ → Wt search with 4.7 fb−1 @ 7TeV b∗ Wt: theory [8] → Phenomenological models Randall-Sundrum models; . Consider b∗ produced singly through its coupling to a b and gluon

gs ¯ µν b b L = 2Λ Gµνbσ (kL PL + kR PR )b∗ + h.c. Exploit the weak coupling of excited-quarks to the third generation quarks L = √gs W +t¯γµ(g P + g P )b + h.c. 2 µ L L R R ∗ b∗ can also decay to bZ and bH cross-section and branching ratio of b∗ → Wt depend on the couplings and b∗ mass general search for Wt resonances under 3 b∗ coupling scenarios

b t Left-handed: k = gR = 0 R g b∗ b Right-handed: kL = gL = 0 b W

b b Vector-like: kL = gL, kR = gR Search for exotics with tt and single top D.Hu, Columbia Univ. 12 / 16 b∗ → Wt search with 4.7 fb−1 @ 7TeV b∗ Wt: dilepton channel →

Background tt, Wt, diboson, Drell-Yan shape: from MC Drell-Yan normalization, and fake dileptons: estimated from data

Event selection ATLAS Data b* (800 GeV) miss 10 •1 2 leptons with opposite charges, ET , L dt = 4.7 fb Wt ∫ tt

1 jet Events / GeV s = 7 TeV Diboson 1 Z(e+e•/µ+µ•)+jets ee, µµ channels: Z(τ+τ•)+jets Fake dileptons m < 81GeV or m > 101GeV •1 (ll) (ll) 10 Dilepton channel Veto to suppress Zττ : 10•2 ∆φ miss + ∆φ miss > 2.5 (l1,ET ) (l2,ET ) 10•3 Discriminating variable: 0 500 1000 1500

HT the scalar sum of ET of leptons, jet and HT [GeV] miss ET

Search for exotics with tt and single top D.Hu, Columbia Univ. 13 / 16 b∗ → Wt search with 4.7 fb−1 @ 7TeV b∗ Wt: lepton+jets channel →

Background tt, Z+jets, single top, diboson, W+jets shape: from MC W+jets normalization, multijet: estimated from data

ATLAS Event selection Data miss 102 Lepton + jets channel b’ (800 GeV) 1 leptons, ET , 3 jets with 1b-jet Wt •1 ≥ L dt = 4.7 fb Other top Reject multijet events Events / GeV ∫ W+jets HF miss 10 s = 7 TeV W+light jets µ channel: ET > 25GeV, Z+jets miss W Diboson ET + ET > 60GeV Multijet miss W 1 e channel: ET > 30GeV, ET > 30GeV Discriminating variable: •1 miss 10 Reconstruct mb∗ from lepton, ET , and 0 400 800 1200 1600 2000 jets assuming pz (ν)=0 reconstructed mass [GeV]

Search for exotics with tt and single top D.Hu, Columbia Univ. 14 / 16 b∗ → Wt search with 4.7 fb−1 @ 7TeV

b∗ Wt: result →

Combine 2 channels for statistical analysis. 102 ATLAS Expected limit [pb] s = 7 TeV Expected ± 1σ

Wt pp→ b* → Wt Expected ± 2σ

Dilepton channel: use the HT distribution →

b* Observed limit ∗ → 10 b* (κb=g =1;κb =g =0) Lepton+jets channel: use the reconstructed b mass spectra L L R R pp

σ Theory uncertainty Cross-section limit @ 95% C.L. as a function of the b∗ L 1 b b mass(kL = gL = 1, kR = gR = 0) •1 Expected limit: > 910GeV; Observed limit: > 870GeV. 10 •1 ∫ L dt = 4.7 fb Coupling limits at each mass in 3 scenarios 400 600 800 1000 1200 mb* [GeV] Left-handed; right-handed; vector-like. b L 1 b R 1 1 κ b L/R κ m [GeV] m [GeV] mb* [GeV] b* b* κ 0.9 300 0.9 300 0.9 300 400 0.8 400 0.8 400 0.8 500 500 500 600 0.7 0.7 600 0.7 600 700 700 800 0.6 700 0.6 800 0.6 900 0.5 800 0.5 900 0.5 1000 ATLAS ATLAS ATLAS 0.4 0.4 0.4 95% CL limit 95% CL limit 95% CL limit 0.3 0.3 0.3 s = 7 TeV; L dt = 4.7 fb•1 s = 7 TeV; L dt = 4.7 fb•1 s = 7 TeV; L dt = 4.7 fb•1 0.2 ∫ 0.2 ∫ 0.2 ∫ pp→ b* → Wt; Left•handed b*•quarks pp→ b* → Wt; Right•handed b*•quarks pp→ b* → Wt; Vector•like b*•quarks 0.1 0.1 0.1 0 0 0 •0.2 0 0.2 0.4 0.6 0.8 1 •0.2 0 0.2 0.4 0.6 0.8 1 •0.2 0 0.2 0.4 0.6 0.8 1 g g g L R L/R Search for exotics with tt and single top D.Hu, Columbia Univ. 15 / 16 Summary Summary

The search for exotic physics in tt pair and single top productions has been presented: tt resonances, W 0, b∗. No evidence of new physics has been observed. Limits @ 95% C.L. have been placed.

Signal Limit 0 Z mZ 0 > 1.80TeV

KK gluon mgKK > 2.00TeV 0 W m 0 > 1.74TeV L WL 0 W m 0 > 1.84TeV R WR ∗ b m ∗ > 0.87TeV L bL Updates with full 2012 dataset will come soon

Search for exotics with tt and single top D.Hu, Columbia Univ. 16 / 16 ReferencesI

[1] ATLAS Exotics Public Results

[2] Harris, Robert M. et al Cross Sections for Leptophobic Topcolor Z 0 decaying to tt Eur.Phys.J.C72 (2012) 2072

[3] K. Agashe, A. Belyaev, T. Krupovnickas, G. Perez and J. Virzi LHC Signals from Warped Extra Dimensions Phys. Rev. D 77, 015003 (2008)

[4] ATLAS Collaboration −1 A search for tt resonances√ in lepton plus jets events with ATLAS using 14 fb of proton-proton collisions at s = 8 TeV ATLAS-CONF-2013-052 [5] ATLAS Collaboration A search for tt resonances√ in the lepton plus jets final state with ATLAS using 4.7 fb−1 of pp collisions at s = 7 TeV arXiv:1305.2756

Search for exotics with tt and single top D.Hu, Columbia Univ. 17 / 16 ReferencesII

[6] ATLAS Collaboration Search for resonances decaying√ into top-quark pairs using fully hadronic decays in pp collisions with ATLAS at s = 7 TeV JHEP 1301 (2013) 116

[7] ATLAS Collaboration √ Search for W 0 → tb in proton-proton collisions at a centre-of-mass energy of s = 8 TeV with the ATLAS detector ATLAS-CONF-2013-050

[8] ATLAS Collaboration √ Search for single b∗-quark production with the ATLAS detector at s = 7 TeV PLB 721, 171 (2013)

Search for exotics with tt and single top D.Hu, Columbia Univ. 18 / 16 BACKUP

Search for exotics with tt and single top D.Hu, Columbia Univ. 19 / 16 tt resonance (lepton+jets): χ2 algorithm used in event reconstruction

2 If one of the jets has mjet > 60GeV, the χ is modified:

Search for exotics with tt and single top D.Hu, Columbia Univ. 20 / 16 tt resonance (lepton+jets): The reconstruction tt invariant mass

Search for exotics with tt and single top D.Hu, Columbia Univ. 21 / 16 tt resonance (lepton+jets): The tt invariant mass spectra

Search for exotics with tt and single top D.Hu, Columbia Univ. 22 / 16 tt resonance (lepton+jets): Average impact of the dominant systematic uncertainties

Search for exotics with tt and single top D.Hu, Columbia Univ. 23 / 16 tt resonance (lepton+jets): Data and expected background event yields

Search for exotics with tt and single top D.Hu, Columbia Univ. 24 / 16 tt resonance (lepton+jets): Upper 95% C.L. cross section limits times branching ratio on Z 0 decaying to tt

Search for exotics with tt and single top D.Hu, Columbia Univ. 25 / 16 tt resonance (lepton+jets): Upper 95% C.L. cross section limits times branching ratio on KK gluon decaying to tt

Search for exotics with tt and single top D.Hu, Columbia Univ. 26 / 16 tt resonance (full hadronic): HEPTopTagger top-quark candidate mass and reconstructed tt mass

180 1050 ATLAS Data 2011 ATLAS Data 2011 178 1000 -1 -1 ∫ L dt = 4.7 fb tt ∫ L dt = 4.7 fb tt 176 950 s = 7 TeV Mass [GeV] s = 7 TeV t 174 t 900 172 850 170 800 168 750 Top-Quark Candidate Mass [GeV] Top-Quark 166

164 700 4 6 8 10 12 14 4 6 8 10 12 14 <µ> <µ>

Search for exotics with tt and single top D.Hu, Columbia Univ. 27 / 16 tt resonance (full hadronic): the reconstructed tt mass for Z 0 and KK gluon signals in HEPTopTagger analysis

40 60 ATLAS Simulation ATLAS Simulation 35 Z' (0.8 TeV) KK gluon (0.8 TeV) 50 30 s = 7 TeV Z' (1.3 TeV) s = 7 TeV KK gluon (1.3 TeV) Z' (2.0 TeV) KK gluon (2.0 TeV) 40 25 Events / 100 GeV Events / 100 GeV 20 30 15 20 10

10 5

0 0 500 1000 1500 2000 500 1000 1500 2000 tt Mass [GeV] tt Mass [GeV]

Search for exotics with tt and single top D.Hu, Columbia Univ. 28 / 16 tt resonance (full hadronic): OV3 distributions of Top Template Tagger for the leading jets

ATLAS ∫ L dt = 4.7 fb-1 plead > 500 GeV s = 7 TeV T precoil > 450 GeV Arbitrary Units T 10-1 Data 2011 Multijet

Z'->tt(2.0 TeV)

10-2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Leading Jet OV 3 Search for exotics with tt and single top D.Hu, Columbia Univ. 29 / 16 tt resonance (full hadronic): tt selection efficiency of HepTopTagger and Top Template Tagger

Search for exotics with tt and single top D.Hu, Columbia Univ. 30 / 16 tt resonance (full hadronic): distributions of tt mass.

350 35 Data 2011 Data 2011 σ Z' (1 TeV) = 1.3 pb 30 g (1.6 TeV) σ = 0.35 pb 300 KK tt tt 250 25 Multijet Multijet

Events / 100 GeV 200 ATLAS Events / 100 GeV 20 ATLAS

-1 -1 150 ∫ L dt = 4.7 fb 15 ∫ L dt = 4.7 fb s = 7 TeV s = 7 TeV 100 10 HEPTopTagger Top Template Tagger 50 5

0 500 1000 1500 2000 2500 3000 1000 1500 2000 2500 3000 tt Mass [GeV] tt Mass [GeV]

Search for exotics with tt and single top D.Hu, Columbia Univ. 31 / 16 W 0 tb: Expected BDT output distributions →

Search for exotics with tt and single top D.Hu, Columbia Univ. 32 / 16 W 0 tb: Disitributions of the reconstructioned mW 0 and → pT (top)

Search for exotics with tt and single top D.Hu, Columbia Univ. 33 / 16 W 0 tb: Data and expected background event yields →

Search for exotics with tt and single top D.Hu, Columbia Univ. 34 / 16 W 0 tb: Upper 95% C.L. cross section limits times → branching ratio on WL0 and WR0

Search for exotics with tt and single top D.Hu, Columbia Univ. 35 / 16 W 0 tb: Observed and expected 95% C.L. on g /g as a → 0 function of mW 0 for WL0 and WR0

Search for exotics with tt and single top D.Hu, Columbia Univ. 36 / 16 b∗ Wt: Data and expected background event yields →

Search for exotics with tt and single top D.Hu, Columbia Univ. 37 / 16