Top content in ATLAS ttH() measurements Jennet Dickinson for the - - PowerPoint PPT Presentation

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Top content in ATLAS ttH() measurements Jennet Dickinson for the - - PowerPoint PPT Presentation

Mar 21, 2023 628 likes •797 views

Top content in ATLAS ttH() measurements Jennet Dickinson for the ATLAS Collaboration Moriond EW March 17, 2019 ttH() analysis strategy 1 Fraction of Events Require 2 photons passing Cont. Bkg. 0.9 ATLAS Preliminary NTI

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SLIDE 1

Top content in ATLAS ttH(ɣɣ) measurements

Jennet Dickinson for the ATLAS Collaboration Moriond EW March 17, 2019

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SLIDE 2

BDT Output 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Fraction of Events 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 0.05 0.1 0.15 0.2 0.25 0.3 0.35
  • Cont. Bkg.

NTI Control Region H t t H Higgs t Non-t

ATLAS Preliminary

  • 1

= 13 TeV, 139 fb s Had region

  • Require 2 photons passing

tight ID and isolation criteria

  • Separate events by decay
  • f top quarks

(1) Hadronic region (4 categories) (2) Leptonic region (3 categories)

ttH(ɣɣ) analysis strategy

2 3/17/19 Jennet Dickinson

  • Boosted Decision Trees (BDTs) are trained on
  • bject-level variables to separate ttH MC signal

from background

– Background is modeled by data control sample failing photon tight ID or isolation (NTI)

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SLIDE 3

110 120 130 140 150 160 [GeV]

g g

m 5 10 15 20 25 30 Sum of Weights / 1.375 GeV

Data Continuum Background Total Background Signal + Background

Preliminary ATLAS

  • 1

= 13 TeV, 139 fb s

= 125.09 GeV

H

m All categories ln(1+S/B) weighted sum

ttH(ɣɣ) signal + background fit

  • Signal strength is extracted from a maximum

likelihood fit to all categories

  • Determination of continuum background is

completely data-driven (normalization and shape)

3/17/19 Jennet Dickinson 3

  • How top-like is this

background?

– Primarily composed of ttɣɣ and ɣɣ + jets – Small contribution from jets faking photons

  • Study this background using

reconstructed hadronic tops

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SLIDE 4

Reconstructing hadronic tops

  • Hadronic top decays correspond

to 3 quarks ~ 3 jets in ttH

  • Goal: identify these 3 jets

– Many possible combinations!

3/17/19 Jennet Dickinson 4

W t b q0 q

  • Train a dedicated BDT for top reconstruction

– Signal: jet triplets truth-matched to tops (ttH MC) – Background: other triplets (ttH MC) – Training variables: momenta & b-tag score of jets, angles between jets, mjjj

  • The jet triplet in each event with highest BDT score

is designated as the top candidate

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SLIDE 5

50 100 150 200 250 300 350 400 450 500 Top candidate mass [GeV]

20 40 60 80 100

Events

+ jets g g g g t t H t t Fitted total Data Preliminary ATLAS

  • 1

= 13 TeV, 139 fb s Two Tightest Had Categories

Template fit method

  • Exploit the shape difference in the top candidate

mass between samples with/without true tops

  • Construct templates from top mass distributions in

ttɣɣ, ɣɣ+jets and ttH Monte Carlo

3/17/19 Jennet Dickinson 5

  • Decompose the continuum

background by performing a template fit to data:

afttγγ(m) + bfγγ(m) + nttH

SM

ndata fttH(m)

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SLIDE 6

50 100 150 200 250 300 350 400 450 500 Top candidate mass [GeV]

100 200 300 400 500 600

Events

+ jets g g g g t t H t t Fitted total Data Preliminary ATLAS

  • 1

= 13 TeV, 139 fb s All Had Categories 50 100 150 200 250 300 350 400 450 500 Top candidate mass [GeV]

20 40 60 80 100

Events

+ jets g g g g t t H t t Fitted total Data Preliminary ATLAS

  • 1

= 13 TeV, 139 fb s Two Tightest Had Categories

  • Tighter ttH(ɣɣ) selection should give more top-like

background

Top fractions

in the hadronic region

3/17/19 Jennet Dickinson 6

ttɣɣ fraction (looser region)

  • These estimates of background passing ttH(ɣɣ)

selection direct further optimization efforts

ttɣɣ fraction (tighter region)

a = 0.21 ± 0.06 a = 0.31 ± 0.17

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SLIDE 7

7

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SLIDE 8

8

Thank you!

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SLIDE 9

Backup

9 3/17/19 Jennet Dickinson

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SLIDE 10

References

  • ATLAS publications

– ttH discovery: Phys. Lett. B 784 (2018) 173 – Latest ttH(ɣɣ): ANA-HIGG-2018-59-CONF

  • Other

– LHC HIGGS XS WG

10 3/17/19 Jennet Dickinson

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SLIDE 11

ttH production

in pp collisions

  • ttH production is a direct

probe of the Higgs-top Yukawa coupling

  • Measurements of this

process are challenging

– Low rate: at 13 TeV, SM σttH = 507 fb – Complex final states: decay products of 2 tops and Higgs

11 3/17/19 Jennet Dickinson

LHC HIGGS XS WG

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SLIDE 12

ttH(ɣɣ)

Analysis strategy

  • Events are pre-selected in two groups:

(1) leptonic (≥1 b-jet, ≥1 leptons) – 3 BDT categories (2) hadronic (≥1 b-jet, ≥3 jets, 0 leptons) – 4 BDT categories

  • Events are then further divided into

categories based on an XGBoost BDT discriminant

– Training uses energy and direction of photons, jets, leptons, jet b-tag flag, MET and MET_φ

12 3/17/19 Jennet Dickinson

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SLIDE 13
  • Define four hadronic ttH categories with different

S/B by slicing in BDT score

– Reject events with BDT score < 0.91

ttH(ɣɣ) category definition

in the hadronic channel

13

  • Tight BDT categories

have lower statistics, but higher ttH purity and better S/B ratio

– These are the most powerful categories

3/17/19 Jennet Dickinson

BDT Output 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Fraction of Events 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 0.05 0.1 0.15 0.2 0.25 0.3 0.35
  • Cont. Bkg.

NTI Control Region H t t H Higgs t Non-t

ATLAS Preliminary

  • 1

= 13 TeV, 139 fb s Had region

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SLIDE 14
  • Define three leptonic ttH categories with different

S/B by slicing in BDT score

– Reject events with BDT score < 0.70

ttH(ɣɣ) category definition

in the leptonic channel

14

  • Again, tightest BDT

category is the most powerful due to high S/B

  • Statistics in the leptonic

channel are lower

3/17/19 Jennet Dickinson

BDT Output 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Fraction of Events 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0.7 0.75 0.8 0.85 0.9 0.95 1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
  • Cont. Bkg.

NTI Control Region H t t H Higgs t Non-t

ATLAS Preliminary

  • 1

= 13 TeV, 139 fb s Lep region