b-tagging and tth, h → bb analysis on fully simulated events in the atlas experiment

September 27, 2005 FAKT 2005, Vienna Slide 1

B-Tagging and ttH, H → bb Analysis

on Fully Simulated Events

in the

ATLAS Experiment

A.H. WildauerUniversität Innsbruck

CERN ATLAS Computing Group


Overview

• Introduction

• The ttH, H→bb Channel

topology, cross section, backgrounds

• B-Tagging Algorithms

impact parameter, weight, performance on ttH

• Analysis of ttH

reconstruction, event selection, bkg rejection

comparison with fast simulation


Introduction

• I am a PhD student in the

Austrian Doctoral Student Program

at CERN.

Main Working Areas

• start-up: work on e/gamma trigger efficiencies for the High Level Trigger TDR

• work on Atlas reconstruction software (Athena) with focus on Inner Detector• development of vertex software and its Event Data Model

• development and performance of b-tagging software and its EDM• analysis of the ttH, Hbb channel on AOD level


• promising discovery channel for a light Standard Model Higgs Boson

ttHjjb lb bb

efficient b-tagging very important for signal reconstruction

channel has to be fully reconstructed to reduce combinatorics

complex final state

• 6 jets where 4 are b-jets (εb4!!)

• 1 W has to decay leptonicaly (trigger!)• 1 neutrino: missing energy!

W

W


Signal and Background

• fully simulated events with “initial” detector layout (2 pixel layers)

Signal:

• ttH(120) → lb jjb bb (0.52 pb, H(120)→bb 70%)

• mH chosen to be 120 Gev/c2

60k events from private production on the Grid

Background:

• ttjj background (474 pb) – 250k events

• ttbb (QCD) (gg: 8.1 pb, qq: 0.5 pb) – 50k events

• ttbb (EW) none produced large background! good rejection needed:

efficient b-tagging very important to reduce background


• b-tagging: identify jets which come from a b-quark

• How? By using the properties of B-hadrons:

• longer lifetime

• reconstructable 2nd vertex

• semileptonic decay modes

“Dependencies”:- Tracking- Vertex reconstruction- Jet finding

B-Tagging

BB a0 < 0

a0 > 0Secondary Vertex

Primary Vertex

Jet-Axis

Lepton


• most common way to tag b-jets: the signed IP significance distribution

• better than IP alone: give higher weight to well measured tracks!

Impact Parameter Tagging

• knowledge of primary vertex important to calculate IP!

)σ(a

a)S(a

0

00

z Signed Impact Significance rφ Signed Impact Significance


Likelihood/Weight

• Significance distributions are used as input pdfs to calculate a jet weight

• or a normalized b-tag likelihood.

• typical likelihood/weight plot for combined tagging in z and rphi:

tracks Bkg(S)

Sig(S)lnW


B-Tagging Performance

• B-Tagging performance is given in 2 connected quantities:

• light jet rejection Ru at a given b-jet selection efficiency εb: u1uR

Ru\εb 70% 60% 50%

1D 5 13 34

2D 24 62 180

CB 31 81 220

• numbers are without 2nd vertex tagger

• !performance depends heavily on truth matching and jet cleaning!

• more important: performance in an actual analysis (e.g. )BS


ttH Event Reconstruction

• 2 jets out of 4 b-jets out of 6 reco jets need to be assigned to the Higgs …

full reconstruction necessary to reconstruct Higgs Boson

Event Selectionb

b

bb

ℓ

jj

H W

W

t

t

→ 1 (e) with pt > 20(25) GeV, |η|<2.5

→ 6 jets with pt > 20 GeV, |η|<5.

→ 4 jets tagged as b-jets (cut defined at εb = 60%)

→ 2 reconstructed tops with |mtop|<20 GeV

→ this leaves 2 b-jets for the reco of the Higgs


Cut Flow Signal

ttH(120) AOD TDR J. Cammin(improved analysis)

All Events 100 % 100 % 100 %

1l 6j 50.8 46.2

4 bjets 4.15 (8) 3.8 (8) 3.8

2 tops reco 2.0 (48) 2.3 (60) 3.7

Higgs reco 0.7 (35) 0.8 (35) 1.5

• comparison of my analysis (AOD) with fully simulated events to 2 analyses based on fast simulation and older detector layout (3 pixel layers)

• numbers in () are relative to previous cut

• problem with top reconstruction? (might be at W→l reco)


Cut Flow ttjj Background

ttjj AOD TDR J. Cammin(improved analysis)

All Events 100 % 100 % 100 %

1l 6j 17.7 15.4

4 bjets 0.035 (0.2) 0.01 (0.1) 0.01

2 tops reco 0.013 (37) 0.0047 (47) 0.01 (92.3)

Higgs reco 0.0007 (5.4) 0.0001 (2.1) 0.0013 (13)

• selection efficiency and background rejection comparable• ttbb (QCD) background also comparable with earlier analyses

• cut flow in the background with largest cross section: ttjj


Reconstructed Masses in Signal

m =173.3 GeV

= 9.3 GeV

m =172.2 GeV

= 10.1 GeV

t→jjb: TDR: 174 ± 11.7 GeV, J.Cammin: 174.7 ± 7.7 GeV

t→lb: TDR: 174 ± 8.8 GeV, J.Cammin: 174.6 ± 8.6 GeV

• tail in the Higgs mass spectrum due to mismatched b quarks

GeV GeV GeV


Number of expected Events at 30 fb-1

• 30 fb-1 is the anticipated integrated luminosity after 3 years of low lumi run

• tt is always forced to decay to lb ljj with BR ~ 29%

ttH(120) AOD TDR J. Cammin(improved analysis)

All Events 3166 3166 3166

1l 6j 1609 1462

4 bjets 128 117 120

2 tops reco 61 70 117

Higgs reco 22 25 47

ttjj AOD TDR J. Cammin(improved analysis)

All Events 4.1M 4.1M 4.1M

1l 6j 730k 631k

4 bjets 1435 410 410

2 tops reco 533 192 377

Higgs reco 29 5 53

simulated events after cuts

Signal: ~300 events left

ttjj background: ~10 events left

→ no detailed analysis possible


Conclusion and Outlook

• first look at ttH channel with fully simulated events and initial detector layout

• “realistic” b-tagging performance looks OK on ttH channel

(no SV tagger in use for this analysis so far)

• cut flow on sig and bkg in agreement with earlier studies

• small discrepancies in the Wl reconstruction under study

• lack of simulated events

a lot more are needed (factor 10)

• might need to use fast simulation for more background …

b-tagging and tth, h → bb analysis on fully simulated events in the atlas experiment

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