Wild Card ATLAS Haichen Wang University of California, Berkeley Lawrence Berkeley National Laboratory On behalf of the ATLAS Collaboration 54 th Rencontres de Moriond Electroweak Interactions and Unified Theories 17 March 2019
Measurement of ttH in Diphoton Final State using 139 fb -1 data collected by the ATLAS experiment during the LHC Run-2 Haichen Wang University of California, Berkeley Lawrence Berkeley National Laboratory On behalf of the ATLAS Collaboration 54 th Rencontres de Moriond Electroweak Interactions and Unified Theories 17 March 2019
Overview ● The associated production of the Higgs boson with top quarks provides a direct access to the top-Higgs coupling ● The diphoton channel is one of the leading channels in the ttH measurements ○ The diphoton resonant decay is an unambiguous signature for the Higgs boson ○ Robust background estimation from diphoton mass 3 sidebands
Photon performance in ATLAS ● Photons are selected using a cut-based multivariate discriminant based on shower shape variables in the EM calorimeter 1.1 ID ε ATLAS Preliminary 1 -1 s =13 TeV, 44 fb 0.9 Z → ll γ Data, µ < 40 0.8 Z ll MC, < 40 → γ µ Z → ll γ Data, µ > 40 0.7 Z → ll γ MC, µ > 40 0.6 <1.37, 1.52< <2.37, unconverted ⏐ η ⏐ ⏐ η ⏐ γ 0.5 FixedCutLoose isolation 1.06 10 20 30 40 50 60 70 80 90 100 MC 1.04 ε 1.02 / Data 1 0.98 ε 0.96 10 20 30 40 50 60 70 80 90 100 E [GeV] T ● A “tight” identification, typically used in analysis, has an efficiency > 90% for high p T photons, and a rejection at 10 3 - 10 4 4
B-tagging performance in ATLAS ● B-jets are tagged using a multivariate discriminant combining tracking, secondary vertex and decay chain information b-jet tagging efficiency -1 ATLAS Preliminary s = 13 TeV, 80.5 fb 0.9 MV2, = 70% Single Cut OP ∈ b 0.8 0.7 0.6 Data (stat. unc.) Data (total unc.) 0.5 t t MC 2 2 30 40 10 2 10 × Jet p [GeV] T ● The b-tagging is calibrated with a ttbar control sample ● In the ttH analysis, a b-tagger with a 77% efficiency is used, corresponding to a rejection of light jet at the level of a few hundreds 5
The diphoton sample in ATLAS Events / 1.0 GeV Select two energetic and well isolated 30000 Data ATLAS Preliminary 1 − Signal + background s = 13 TeV, 79.8 fb photons Continuum background m = 125.09 GeV H Diphoton Fiducial 20000 P T γ 1 /m γγ > 0.4, P T γ 2 /m γγ > 0.35 10000 | η | < 1.37 or 1.52 < | η | < 2.37 Quality requirement - Isolation and Data - Cont. Bkg identification criteria 500 0 ~ 1.5 million events with 105 GeV < 500 − 110 120 130 140 150 160 m γγ < 160 GeV at 139 fb -1 m [GeV] γ γ Assume the theoretical prediction, at 139 fb -1 , the LHC should have produced ● ~ 7,000,000 Higgs bosons ● ~ 70,000 via ttH production ● ~ 160 in the ttH γγ channel for the ATLAS experiment 6
Strategy Use photons to tag the Higgs Boson γ Use jets (b-jets), leptons, and E T miss to capture the characteristics of top quarks Directly use properties of the objects in the event to train a multivariate discriminant 7
Multivariate Training Training variables ● 4-momenta of photons, jets, leptons ● Whether or not a jet is b-tagged ● Missing transverse energy and its φ direction This discriminant is trained with ● Signal - Powheg Monte Carlo that models signal events ● Background - data control sample where the photon quality (isolation and/or identification) requirement is reversed ○ Mostly γγ + jets events, our main background before selection ○ See Jennet Dickinson’s talk for our understanding of the background composition 8
Training variables Some example training variables MET Photons Jets Leptons The training algorithm is a Boosted Decision Tree (BDT) 9
Multivariate ttH discriminant The BDT is trained for events with a lepton (leptonic) and events without a lepton (hadronic), separately 1 1 Fraction of Events Fraction of Events Cont. Bkg. Cont. Bkg. 0.9 0.9 ATLAS Preliminary ATLAS Preliminary NTI Control Region NTI Control Region -1 -1 s = 13 TeV, 139 fb s = 13 TeV, 139 fb t t H t t H 0.8 0.8 Non-t t H Higgs Non-t t H Higgs Had region Lep region 0.7 0.7 0.35 0.8 0.6 0.6 0.3 0.7 0.5 0.5 0.25 0.6 0.5 0.2 0.4 0.4 0.4 0.15 0.3 0.3 0.1 0.3 0.2 0.05 0.2 0.2 0.1 0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 0.7 0.75 0.8 0.85 0.9 0.95 1 0.1 0.1 0 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 BDT Output BDT Output Events with low BDT scores are removed The remaining events are classified into multiple categories with different signal-to-background-ratios (S/Bs) based on the BDT scores, to maximize the sensitivity 10
Performance of Categorization Event yields in a narrow mass window around 125 GeV ● The S/B goes beyond 1 in the best “hadronic” and “leptonic” categories ● Contamination of non-ttH Higgs signals are strongly suppressed; ttH purity reaches 90% level in the best categories ● Best categories also correspond to categories with best diphoton resolutions 11
Diphoton mass distributions (hadronic) Events / 1.375 GeV Events / 1.375 GeV 18 30 Data ATLAS Preliminary Data ATLAS Preliminary -1 -1 s = 13 TeV, 139 fb s = 13 TeV, 139 fb 16 Continuum Background Continuum Background 25 Total Background m = 125.09 GeV Total Background m = 125.09 GeV H H 14 Had 1 category Had 2 category Signal + Background Signal + Background 12 20 10 15 8 6 10 4 5 2 110 120 130 140 150 160 110 120 130 140 150 160 m [GeV] m [GeV] γ γ γ γ Events / 1.375 GeV Events / 1.375 GeV 100 80 ATLAS Preliminary ATLAS Preliminary Data Data -1 -1 s = 13 TeV, 139 fb s = 13 TeV, 139 fb Continuum Background Continuum Background Total Background m = 125.09 GeV 80 Total Background m = 125.09 GeV H H 60 Signal + Background Had 3 category Signal + Background Had 4 category 60 40 40 20 20 110 120 130 140 150 160 110 120 130 140 150 160 m [GeV] m [GeV] γ γ γ γ 12
Diphoton mass distributions (hadronic) Events / 1.375 GeV Events / 1.375 GeV 18 30 Data ATLAS Preliminary Data ATLAS Preliminary -1 -1 s = 13 TeV, 139 fb s = 13 TeV, 139 fb 16 Continuum Background Continuum Background 25 Total Background m = 125.09 GeV Total Background m = 125.09 GeV H H 14 Had 1 category Had 2 category Signal + Background Signal + Background Events / 1.375 GeV 12 20 18 ATLAS Preliminary Data 10 -1 15 s = 13 TeV, 139 fb Continuum Background 16 8 Total Background m = 125.09 GeV 6 10 H 14 4 Signal + Background Had 1 category 5 2 12 110 120 130 140 150 160 110 120 130 140 150 160 10 m [GeV] m [GeV] γ γ γ γ 8 Events / 1.375 GeV Events / 1.375 GeV 100 80 ATLAS Preliminary ATLAS Preliminary Data Data -1 -1 s = 13 TeV, 139 fb s = 13 TeV, 139 fb Continuum Background 6 Continuum Background Total Background m = 125.09 GeV 80 Total Background m = 125.09 GeV H H 60 Signal + Background Had 3 category Signal + Background Had 4 category 4 60 2 40 40 110 120 130 140 150 160 20 20 m [GeV] γ γ 110 120 130 140 150 160 110 120 130 140 150 160 m [GeV] m [GeV] γ γ γ γ 13
Diphoton mass distributions (hadronic) Events / 1.375 GeV Events / 1.375 GeV 18 30 Data ATLAS Preliminary Data ATLAS Preliminary -1 -1 s = 13 TeV, 139 fb s = 13 TeV, 139 fb 16 Continuum Background Continuum Background / 0.5 GeV 25 0.16 H , m = 125 GeV ATLAS Simulation Preliminary Total Background m = 125.09 GeV Total Background → γ γ m = 125.09 GeV H H 14 H -1 s = 13 TeV, 139 fb Had 1 category Had 2 category Signal + Background Signal + Background 0.14 Events / 1.375 GeV 12 20 18 ATLAS Preliminary Data γ 0.12 γ Had 1 10 m MC 1/N dN/d -1 15 s = 13 TeV, 139 fb 0.1 Continuum Background 16 Signal Model 8 0.08 Lep 3 Total Background m = 125.09 GeV 6 10 MC H 14 Signal Model 0.06 4 Signal + Background Had 1 category 5 0.04 2 12 0.02 110 120 130 140 150 160 110 120 130 140 150 160 10 0 m [GeV] m [GeV] 115 120 125 130 135 140 γ γ γ γ m [GeV] γ γ 8 Events / 1.375 GeV Events / 1.375 GeV 100 80 ATLAS Preliminary ATLAS Preliminary Data Data -1 -1 s = 13 TeV, 139 fb s = 13 TeV, 139 fb Continuum Background 6 Continuum Background Total Background m = 125.09 GeV 80 Total Background m = 125.09 GeV H H 60 Signal + Background Had 3 category Signal + Background Had 4 category 4 60 2 40 40 110 120 130 140 150 160 20 20 m [GeV] γ γ 110 120 130 140 150 160 110 120 130 140 150 160 m [GeV] m [GeV] γ γ γ γ 14
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