Post on 01-Jan-2016
description
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:
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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
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• 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
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• 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
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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
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• 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.
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Estimating the Instrumental Background
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• 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
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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)
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Distributions
ET+jj (2 btags)Data : 25Exp : 27.0
ET+jj (1 btag)Data : 592Exp : 554.5
ET+jjData : 3210Exp : 3211
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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)
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(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
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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
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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
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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
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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
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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