Search for SM Higgs Boson Using Large Missing Transverse Energy and B-jets at DØ

Search for SM Higgs Boson Using Large Missing Transverse Energy and B-jets at

DØ

Tim ScanlonImperial College, London

on behalf of theDØ Collaboration

• Introduction• The DØ Detector• Previous Results• Analysis Method• Published Result• Future Analysis• Conclusion

Overview:

Tim Scanlon (Imperial College London) 2

Z Z

H

b

b

Introduction

• Motivation– ZHbb is a very sensitive way to search for the SM Higgs at

the Tevatron as we do not distinguish between the neutrino species

• (qqZH)xBr(Z, Hbb) = 0.015 pb @ mH=115 GeV

• (qqWH)xBr(Wl, Hbb) = 0.014 pb

• Characteristic Signal

– Large missing ET (ET)

– 2 b-tagged jets with high pT – The leading jets are boosted

and hence not back-to-back– Di-jet mass of b-jets– No isolated leptons

Jet1Jet2

ET


• Good calorimeter (ET) and tracking (b-tagging) essential

– Uranium/Liquid-Argon Calorimeter

• Central calorimeter provides coverage up to ||~1.1

• Two end calorimeters extend coverage up to ||~4.2

– Tracking• Silicon Microstrip Tracker

(SMT)– New Layer 0 for Run IIb

• Central Fibre Tracker (CFT)• Surrounded by 2T Solenoid

• DØ detector– Efficiency above 85%– Recorded 1.5 fb-1 of data

The DØ Detector


• 2005 Preliminary result– 95% C.L. limits on (ppZH) × Br(Hbb) = 8.5 ~ 12.2 pb @261pb-1

• Updated version of the analysis now accepted for publication:– Same dataset

• Improved:– Optimized event selection– Added “exclusive” single b-tag channel to double b-tag channel– Inclusion of WH limits in ET+jets sample, when the lepton from the W

is missed

Previous Preliminary Result


Analysis Issues

• The signal is well defined although it has significant backgrounds:– “Physics” backgrounds

• Well defined processes can be distinguished and accurately modeled– Some irreducible

• Dominant physics backgrounds– W+jets, Z+jets, top, ZZ, and WZ

– “Instrumental” backgrounds • Basically everything else• Mainly QCD multi-jet events with mismeasured jets

– back to back jets events where one jet is grossly mis-measured

» Large ET

– presence of fake jets, etc.• Generally low acceptance, but cross-section much larger

– Significant background• No easy way to estimate the magnitude and shape of this background

– Instrumental background forced us to apply more stringent selection cuts – Needed to devise a way of estimating and simulating its contribution


• ET > 50 GeV (basic Higgs signal)

• 2 or 3 jets with pT > 20 GeV, ||<2.5 (basic Higgs signal)

(dijet) < 165° (rejects QCD di-jet events)

• Isolated EM and muon veto (rejects top, W/Z+jets)

• HT < 240 GeV (rejects top)

• PTtrk > 20 GeV (rejects instrumental)

• -0.1 < A(ET,HT) < 0.2 (rejects instrumental)

• min (ET,jets) > 0.15 && ET > -40 * min (ET,jets) + 80

(ET, PTtrk) < 90° for “signal” region

> 90° for “sideband” region

Event Selection

Variables verified in W+jets sample.

(Used to measure instrumental background)

- HT = |pT(jets)|- PT

trk = |pT(tracks)|- A(ET,HT) = (ET-HT)/(ET+HT)

Determined by optimisation.


Estimating the Instrumental Background


• Fit of A(ET, HT)

– Estimate physics background from MC: Triple Gaussian function

– Instrumental background: 6th order polynomial function• Instrumental background = 696.1 ± 91.4 events (from fit)• Physics background = 2514.9 events (from MC)

– Normalise sideband region instrumental background in A(ET, HT) bins

• Model the instrumental background distributions in signal region

Instrumental Background

Sideband Region

Signal Region


b-tagging

(~P)

JLIP Performance in Data

• Identifying a b-jet– Track Impact Parameters (JLIP and

CSIP)– Secondary Vertex (SVT)– High pT Lepton– Neural Network Combination (more

later)

• Jet Lifetime Impact Parameter Tagger– JLIP identifies heavy flavour jets from large

impact parameter tracks– JLIP calculates a probability (P) that the jet is

a light-jet

• Analysis split into two different b-tagging channels– One JLIP tag ‘Exclusive’

• Ultra Tight JLIP (P < 0.001)– Two (or more) JLIP tags

• Loose JLIP (P < 0.01)• Extra Loose JLIP (P < 0.04)


Distributions

ET+jj (2 btags)Data : 25Exp : 27.0

ET+jj (1 btag)Data : 592Exp : 554.5

ET+jjData : 3210Exp : 3211


Background Composition and Acceptance

ET+bb (bj)

(105 GeV)

ET+bb (bj)

(115 GeV)

ET+bb (bj)

(125 GeV)

ET+bb (bj)

(135 GeV)

# Data 10 (29) 11 (33) 10 (37) 9 (44)

# Predicted BKG

8.9 ± 1.7 (32.2 ± 5.9)

9.4 ± 1.8 (34.0 ± 6.1)

9.8 ± 1.8 (35.2 ± 6.0)

10.5 ± 2.0 (37.3 ± 6.6)

# ZH (Hbb)Acceptance (%)

0.25 (0.24)0.86 ± 0.16

0.21 (0.20)1.04 ± 0.20

0.15 (0.14)1.18 ± 0.22

0.091 (0.087)1.34 ± 0.24

# WH (Hbb)Acceptance (%)

0.18 (0.18)0.36 ± 0.07

0.15 (0.14)0.43 ± 0.08

0.098 (0.096)0.47 ± 0.09

0.062 (0.061)0.55 ± 0.10

Composition

SingleTag (%)

Double Tag (%)

Zjj 8 3

Zbb 5 16

Wjj 38 16

Wbb 5 12

Top 16 33

WZ/ZZ 1 7

Instrumental

26 13

• Large contribution from WH decays• Single Tag Channel

– Main background is Wj– Instrumental background is 26%

• Double Tag Channel– Main background top decay– Instrumental background reduced to 13%

• On same level as the W+jj and Z+bb• Systematics: Signal 19%, Background 19%

– b-tagging (~14%) and Jet energy scale (~8%)

Double (Single) Tagged Channel (within ±1.5 mass window)


(ppZH)xBr(Hbb) Limit

• Significant progress since preliminary result– New limits are more than 2 times better– Limits set from combined double and exclusive single tag channels

• Also measured limits for WH with escaped lepton• Results combined with other DØ and CDF result

Expected/Observed Limits

105 Gev 115 Gev 125 Gev 135 Gev

ZH Limits (pb) 3.1/3.4 2.7/3.2 2.4/2.9 2.1/2.5

WH Limits (pb) 7.6/8.3 6.3/7.5 6.0/7.4 5.0/6.3


Future Analysis

• Significant progress made on next generation of analysis

• Several improvements expected to significantly improve the limit 1 fb-1 of data– Full calibration of calorimeter– Lower systematic errors– New NN b-tagging– NN event selection

• New preliminary limit expected by early 2007


New NN b-tagging (released summer 2006)

• A new b-tagging tool – Combines various variables from

the track based b-tagging tools in a Neural Network

– Substantial improvement in performance over constituent input b-taggers

– Trained on Monte Carlo

– Certified on data

– Performance measured on data

– Increase of 1/3 in efficiency for a fixed fake rate of 0.5 %

• Significantly increase Higgs sensitivity

Fake-jets with a very loose

tag

Data

MC


Conclusions

• Search for the Higgs boson with large missing transverse energy– Very important channel in search for the SM Higgs

• Search for 2 b-tagged jets and large missing ET

• Main difficulty predicting the instrumental background

– 0.3 fb-1 analysis accepted for publication in PRL• Results were two times better than preliminary result• Also measured WH with escaped lepton

– 1 fb-1 preliminary result expected early next year• Numerous improvements• Expect significantly improved limit

– Current and future results• Vital role in combined SM Higgs search

Backup Slides


Systematic Errors

Signal Errors Single Tag (%)

Double Tag (%)

Trigger Efficiency 6 6

Jet Identification 7 7

Jet Energy Scale 9 7

Jet Resolution 5 5

Taggability Scale Factor 1 1

b-Tag 3 14

Total 0.14 0.19

• Each systematic source varied by ±1– Analysis repeated

• Uncertainties dominated by Jet Energy Scale and b-tagging

Background Errors Single Tag (%)

Double Tag (%)

Trigger Efficiency 6 6

Jet Identification 6 6

Jet Energy Scale 8 11

Jet Resolution 2 2

Taggability Scale Factor 1 1

b-Tag 5 12

Instrumental b-Tag 8 9

Instrumental Prediction 3 2

Cross-section 5 5

Total 0.18 0.19


Sensitivity Prospects

Ingredient Equivalent Luminosity Gain (@

115 GeV)

1 fb-1 3

NN b-tagging 2.0

NN Event Selection 1.7

Di-jet Mass Resolution

1.5

Increased Acceptance

1.2

Reduced Systematics

1.2

Total 22

• Largest improvement in sensitivity from– Increased luminosity – NN b-tagging

Documents

Search for SM Higgs Boson Using Large Missing Transverse Energy and B-jets at DØ