Quantum interference between doubly and singly resonant top quark - PowerPoint PPT Presentation

Quantum interference between doubly and singly resonant top quark production Christian Herwig University of Pennsylvania APS DPF 2017

The ttbar and tWb processes have the same final state and thus there exists a quantum interference effect |A W W bb | 2 ∼ |A ( W tb ) | 2 + |A ( tt ) | 2 +2 R{A ( W tb ) A ( tt ) } The interference is largest when tWb "looks like" ttbar ATLAS separately generates ttbar and Wtb at NLO+PS with Powheg+Pythia 2 2 6 ¯ w+ b W − w- u~ u~ ¯ b ¯ u u b 5 ¯ t 5 t W + b t W − w+ t g g g g b b~ t~ W + 4 w- t b 3 u ¯ u 4 u b u ttbar tWb b~ b 1 1 6

The ttbar and tWb processes have the same final state and thus there exists a quantum interference effect |A W W bb | 2 ∼ |A ( W tb ) | 2 + |A ( tt ) | 2 +2 R{A ( W tb ) A ( tt ) } The interference is largest when tWb "looks like" ttbar ATLAS separately generates ttbar and Wtb at NLO+PS with Powheg+Pythia Interference effects are estimated by comparing two ad-hoc prescriptions: Diagram Removal (DR) and Diagram Subtraction (DS) Their difference is assigned as a systematic uncertainty

The ttbar and tWb processes have the same final state and thus there exists a quantum interference effect |A W W bb | 2 ∼ |A ( W tb ) | 2 + |A ( tt ) | 2 +2 R{A ( W tb ) A ( tt ) } The interference is largest when Wtb "looks like" ttbar Add’l details: Frixione et al. arXiv:0805.3067 Define Diagram Removal (DR) ATLAS separately generates ttbar and Wtb at NLO+PS with single top to take only the piece A ( W tb ) Powheg+Pythia6 Interference effects are estimated by comparing two ad-hoc prescriptions: Diagram Removal (DR) and Diagram Subtraction (DS) Their difference is assigned as a systematic uncertainty

The ttbar and tWb processes have the same final state and thus there exists a quantum interference effect |A W W bb | 2 ∼ |A ( W tb ) | 2 + |A ( tt ) | 2 +2 R{A ( W tb ) A ( tt ) } The interference is largest when Wtb "looks like" ttbar Add’l details: Frixione et al. arXiv:0805.3067 Define Diagram Removal (DR) ATLAS separately generates ttbar and Wtb at NLO+PS with single top to take only the piece A ( W tb ) Powheg+Pythia6 Define Diagram Subtraction (DS) single top Interference effects are estimated by comparing two ad-hoc as the entire expression, minus a gauge-invariant prescriptions: term that exactly cancels when A ( t ¯ t ) M 2 bW → m 2 Diagram Removal (DR) and Diagram Subtraction (DS) t Their difference is assigned as a systematic uncertainty

DS/DR disagreement large in extreme (search) phase space arXiv:0805.3067 magenta = 2L2b inclusive selection 6 Herwig (Pennsylvania)

DS/DR disagreement large in extreme (search) phase space arXiv:0805.3067 "Start worrying" magenta = 2L2b inclusive selection 7 Herwig (Pennsylvania)

Run 2 LHC has provided us with millions of Wt events Can we use this data to improve our understanding of tt-Wt interference? 8 Herwig (Pennsylvania)

e + p b b p µ − 9 Herwig (Pennsylvania)

e + p 2 ¯ w+ b b u~ b ¯ u ¯ t 5 t W − g g b t~ W + 4 w- t u u b~ b 1 6 p µ − 10 Herwig (Pennsylvania)

e + p 2 6 W − b w- u~ ¯ u 5 b W + b t w+ t g g b b b~ b 3 u ¯ 4 b u 1 p µ − 11 Herwig (Pennsylvania)

e + b p b p µ − 12 Herwig (Pennsylvania)

e + . m top ? b p b p . m top ? µ − 13 Herwig (Pennsylvania)

흂 Let m ij = m ( b i , ` j ), and define b 1 min-max m ( b , ` ) ) ≡ min { max ( m 11 , m 22 ) , max ( m 12 , m 21 ) } � 1 t t 흂 b 2 � 2 14 Herwig (Pennsylvania)

흂 Let m ij = m ( b i , ` j ), and define b 1 min-max m ( b , ` ) ) ≡ min { max ( m 11 , m 22 ) , max ( m 12 , m 21 ) } � 1 t wrong pairing: m bl usually large t correct pairing: m bl bounded by m top 흂 b 2 Important Properties: min-max m bl < m top for ttbar events not necessarily for Wt events! � 2 15 Herwig (Pennsylvania)

Events / 20 GeV ATLAS Work in progress Total SM 흂 5 simulation 10 t t -1 s = 13 TeV, 36.1 fb b 1 Wt (DR) 4 Wt (DS) 10 3 10 2 10 10 � 1 1 t 1 − 10 0 50 100 150 200 250 300 350 400 450 500 reco min-max m [GeV] bl t Wt (DR) purity 1 0.5 흂 0 b 2 1 DS / DR 0.5 0 Analysis strategy: � 2 measure this spectrum! 16 Herwig (Pennsylvania)

Events / 20 GeV ATLAS Work in progress Total SM 흂 5 simulation 10 t t -1 s = 13 TeV, 36.1 fb b 1 Wt (DR) 4 Wt (DS) 10 3 10 2 10 10 � 1 1 t 1 − 10 0 50 100 150 200 250 300 350 400 450 500 reco min-max m [GeV] bl t Wt (DR) purity 1 0.5 All Wt here is NLO+PS 흂 Powheg+Pythia6 0 b 2 (DR and DS) 1 DS / DR 0.5 0 Analysis strategy: � 2 measure this spectrum! 17 Herwig (Pennsylvania)

Fiducial region: exactly 2 leptons, exactly 2 b-tagged jets m ll > 10 GeV and |m ll -m Z | > 5 GeV 18 Herwig (Pennsylvania)

Fiducial region: exactly 2 leptons, exactly 2 b-tagged jets m ll > 10 GeV and |m ll -m Z | > 5 GeV Single lepton triggers lepton p T > 28 GeV b-jet p T > 20 GeV tag at 60% efficiency WP , veto at 85% 19 Herwig (Pennsylvania)

ttbar and Wt are treated together as the signal process dominant backgrounds estimated from data using dedicated control regions (CRs) Events / 40 GeV ATLAS Work in Progress Total SM t t ATLAS Internal t t +hf Wt (DR) simulation 5 Z+jets t t +V -1 10 s = 13 TeV, 36.1 fb Diboson SR 4 10 3 10 2 10 450 0 50 100 150 200 250 300 350 400 450 [GeV] reco min-max m [GeV] bl 20 Herwig (Pennsylvania) (b) Logarithmic scale

Estimating tt with additional heavy flavor Events / 40 600 ATLAS Work in progress Data Total SM -1 s = 13 TeV, 36.1 fb t t t t +hf 1.8 Problem: × Wt (DR) Z+jets 1.2 × 500 t t +V if the identified b-jets 400 aren’t from top decays tt+b 300 ttbar can pass the Control kinematic endpoint! 200 Region 100 Solution: 0 0 50 100 150 200 250 300 350 400 450 normalize tt+b dilep_mbl_3b 3b m bl [GeV] 1.5 in a dedicated Data / SM 3 b-jet CR 1 0.5 21 Herwig (Pennsylvania)

m bl spectrum is unfolded to particle-level (with ttbar and Wt treated together) 450 0.9 [GeV] a.u. 400 0.8 350 0.7 bl reco min-max m 300 0.6 250 0.5 200 0.4 Response 150 0.3 Matrix 100 0.2 50 0.1 ATLAS Work in Progress 0 0 0 50 100 150 200 250 300 350 400 450 truth min-max m [GeV] bl 22 Herwig (Pennsylvania)

Results (blinded) 1 [1/GeV] ATLAS Work in Progress ATLAS Simulation Internal 1 − Bayesian Unfolding with 3 iterations 10 simulation bl Total uncertainty -1 σ d m s = 13 TeV, 36.1 fb d SR in tail is dominated DR pseudodata pseudo-data 1 2 − σ 10 stat unc. by data statistics stat syst ⊕ PhPy6 DR PhPy6 DS 3 − 10 Sensitive to DR/DS differences: 4 − 10 shape+normalization 5 − 10 1.5 1/Data Data compared to 1 state-of-the-art 0.5 generators 0 100 200 300 400 minimax m [GeV] bl 23 Herwig (Pennsylvania)

Conclusions We present the first measurement of the WWbb final state in a region of maximal tt-Wt interference Measurement is sensitive to the large differences between state-of-the-art generators We expect to reduce the systematic uncertainty associated with ATLAS’s treatment of the tt-Wt interference 24 Herwig (Pennsylvania)

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minus one. Black solid, red dashed, blue dotted, and green dot-dashed lines correspond to p ( veto ) = 10, 30, 50, and 70 GeV respectively. The magenta solid line with open boxes is T obtained without imposing any veto.