Search for Standard Model Higgs Boson Production in Association with a W Boson using Neural Networks with 7.5fb-1 of CDF data
Timo Aaltonen1, Barbara Alvarez2, Adrian Buzatu3, Giorgio Chiarelli4, Jake Connors5, Jay Dittmann6,
Martin Frank6, John Freeman7, Craig Group8, Richard Hughes5, Nazim Hussain3, Tom Junk7,
Azeddine Kasmi6 Benjamin Kilminster7, Shinhong Kim9, Mike Kirby7, Sandra Leone4, Jan Lueck10,
Thomas Muller10, Yoshikazu Nagai9, Yuri Oksuzian8, Tom Phillips11, Elisabetta Pianori12, Manfredi Ronzani4,
Alberto Ruiz13, Federico Sforza4, Rick Snider7, Marco Trovato4, Rocio Vilar13, Jesus Vizan13,
Andreas Warburton3, Jon Wilson5, Brian Winer5, Homer Wolfe5, Zhenbin Wu6, Weiming Yao14

1University of Helsinki, 2Michigan State University, 3McGill University, 4INFN Pisa, 5The Ohio State University, 6Baylor University, 7FNAL, 8University of Virginia,
9University of Tsukuba, 10Karlsruhe Institute of Technology, 11Duke, 12University of Pennsylvania, 13IFCA (CSIC-Univ. Cantabria & Univ. Oviedo), 14LBNL


 


- Abstract -
- Event Selection -
- NN b-jet energy correction -
- Bayesian Neural Network Inputs -
- Bayesian Neural Network Output -
- Systematics -
- Results -
- Public Note -


- Link to the webpage of the previous analysis presented on Summer 2010 -






Abstract:

We present a search for Standard Model Higgs boson decaying to two b quarks and produced in association with a W boson. This search uses data corresponding to an integrated luminosity of 7.5 fb-1. We select events with a high-pT lepton, a neutrino, and two jets. We require at least one of the jets to be identified as a b-quark jet (tagged) using three different tagging algorithms (SECVTX, JetProb and NN b-tagging). The discrimination between the Higgs signal and the large backgrounds in the W + 2 jets bin is increased through the use of a Bayesian neural network. We see no evidence for an excess of Higgs signal in the NN output distribution. We set a 95% confidence level upper limit on the WH cross section times the branching ratio of the Higgs to decay to a bbbar pair, expressed as a ratio to the SM cross section:
σ(pp -> WH)*BR(H->bb) < 3.64 x SM observed (2.78 expected) at M(H) = 115 GeV/c2
σ(pp -> WH)*BR(H->bb) < 1.34 (1.83) to 38.8 (23.4) x SM observed (expected) for M(H) = 100 to 150 GeV/c2

When this analysis is combined with an independent search that uses events with 3 jets we set the following limits:
σ(pp -> WH)*BR(H->bb) < 2.65 x SM observed (2.60 expected) at M(H) = 115 GeV/c2
σ(pp -> WH)*BR(H->bb) < 1.12 (1.79) to 34.4 (21.6) x SM observed (expected) for M(H) = 100 to 150 GeV/c2





Event Selection:

We analyze 7.5 fb-1 of events recorded by triggers for high pT electrons, muons, large missing transverse energy plus 2 jets (MET + 2 Jets triggers), and large missing transverse energy (MET trigger). We require events to have a high-pT lepton candidate, large missing transverse energy, and two jets with at least one b-tag with SECVTX. We classify our events into different lepton categories like central triggered leptons (electrons or muons); forward (plug) triggered electrons; or non-triggered muons, a category that is formed primary from the W &rarr &mu&nu decay where the muon failed the standard identification or entered into a detector gap region. A fourth category is formed by loose electron-like leptons that are primary from the decay of W &rarr e&nu or &tau&nu where the electron failed the standard identification or the &tau decays in a single charged hadron (one-prong). In addition to non-triggered electrons selected requiring an isolated track with significant deposits of energy in the calorimeter (loose isolated tracks), the acceptance of this category has been recently increased by selecting triggered electrons that fail the standard selection using a multivariate likelihood (loose electrons). Recently a cut to reject QCD events based on a multivariate technique that makes use of a Support Vector Machine was developed. This cut is applied to events with only one b-tagged jet (for plug electrons and loose electron-like leptons a cut based QCD veto is applied). Each lepton sample has distinct backgrounds and triggers requirements, and so has a different set of event selection cuts, which are summarized in the following tables:
Event Selection of Tight Leptons from Lepton Triggered Events
CategoryDouble SECVTXOne SECVTX + One JetProbOne SECVTX + One NN b-tagOne SECVTX
Missing Et > 10 GeV (Muons), 20 GeV(Central Electrons), > 25 GeV (Plug Electrons)
Two JetsET > 20 GeV, |&eta| < 2.0
b-tagging (one jet)tight SecVtx b-tag
b-tagging (another jet)tight SecVtx b-tagJetProb b-tagNN b-tagNo b-tag
QCD vetoPlug electron onlyQCD Veto


Event Selection of Non-Triggered Muons from MET + 2 Jets (two different triggers) and MET triggered Events
CategoryDouble SECVTX tagOne SECVTX + One JetProbOne SECVTX + One NN b-tagOne SECVTX tag
Missing Et > 20 GeV
Two Jets MET + 2 Jets A: ET > 25 GeV, &Delta R>1.0; MET + 2 Jets B: ETj1> 40 GeV, ETj2> 25 GeV; All: ET > 20 GeV, |η| < 2.0
b-tagging (one jet)tight SecVtx b-tag
b-tagging (another jet)tight SecVtx b-tagJetProb b-tagNN b-tagNo b-tag
QCD Veto No QCD VetoQCD Veto


Event Selection of Loose Electron-Like Leptons from MET + 2 Jets, MET and high PT electron triggered Events
CategoryDouble SECVTX tagOne SECVTX + One JetProbOne SECVTX + One NN b-tagOne SECVTX tag
Missing Et > 20 GeV, not pointing to any jets in cone 0.4
Two Jets ET > 20 GeV, |&eta| < 2.0
b-tagging (one jet)tight SecVtx b-tag
b-tagging (another jet)tight SecVtx b-tagJetProb b-tagNN b-tagNo b-tag
QCD Veto QCD Veto

We estimate our expected background contribution to the sample in each lepton type and tag category. The following table shows our expected and observed number of background events for the different b-tagging categories combining all the lepton categories:



The expected and observed number of background events for the different b-tagging and lepton categories is shown in the following tables:

Lepton Type One Tag Two SECVTX Tags One SECVTX + One JETPROB tag One SECVTX + One NN b-tag
Central Triggered Leptons Event Yields Event Yields Event Yields Event Yields
Plug Triggered Electrons Event Yields Event Yields Event Yields Event Yields
Non-Triggered Muons Event Yields Event Yields Event Yields Event Yields
Loose Electron-Like Leptons Event Yields Event Yields Event Yields Event Yields




NN b-jet energy correction

The most sensitive variable for WH analysis is the dijet invariant mass. Improvement on dijet mass resolution directly results in improvement of the final sensitivity. To further improve the dijet mass resolution, b-jet energy corrections based on a multivariate technique are applied. More details can be found here




Bayesian Neural Network Inputs

We employ distinct Bayesian Neural Network discriminant functions which were optimized for three tagging categories: double SECVTX tag, one SECVTX tag + one JetProb/NN tag, and one SECVTX tag.
For each BNN function, we use the next input variables.
    double SECVTX tag
  • Dijet invariant mass: The invariant mass reconstructed from the two jets. For this variable, we apply NN b-jet energy correction.
  • PT Imbalance: The scalar sum of the lepton and jet transverse momenta minus the MET.
  • M max (lep + ν + jet): The invariant mass of the lepton, MET and one of the two jets, where the jet is chosen to give the maximum invariant mass.
  • Q x &etalep: The charge of the lepton times the &eta of the lepton.
  • Sum ET (loose jets): The scalar sum of the loose jet transverse energy.
  • PT(W): The transverse momentum of the reconstructed W.
  • HT: The scalar sum of the transverse energies of the jets, the lepton, and the MET.
    one SECVTX tag + one JetProb/NN tag
  • Dijet invariant mass: Same variable as double SECVTX input.
  • Sum ET (loose jets): Same variable as double SECVTX input.
  • Q x &etalep: Same variable as double SECVTX input.
  • M min (lep + ν + jet): The invariant mass of the lepton, MET and one of the two jets, where the jet is chosen to give the minimum invariant mass.
  • HT: Same variable as double SECVTX input.
  • PT(W): Same variable as double SECVTX input.
  • MET: Missing transverse energy.
    one SECVTX tag
  • Dijet invariant mass: Same variable as double SECVTX input.
  • KIT Flavor Separator: The neural network flavor separator to reduce no real b-quark jets.
  • Sum ET (loose jets): Same variable as double SECVTX input.
  • Q x &etalep: Same variable as double SECVTX input.
  • PT(W): Same variable as double SECVTX input.
  • HT: Same variable as double SECVTX input.
  • MET: Same variable as one SECVTX tag + one JetProb/NN tag input.
  • PT Imbalance: Same variable as double SECVTX input.
The following plots show the NN Inputs for pretag (control region) central leptons.





More NN Input Plots
Lepton Type Pretag One Tag Two SECVTX Tags One SECVTX + One JETPROB tag One SECVTX + One NN b-tag
Central Triggered Leptons Plots Plots Plots Plots Plots
Plug Triggered Electrons Plots Plots Plots Plots Plots
Non-Triggered Muons Plots Plots Plots Plots Plots
Loose Electron-Like Leptons Plots Plots Plots Plots Plots





Bayesian Neural Network Output (Control region)

The Bayesian neural network output is a value between 0 and 1. Values close to 1 correspond to "more signal-like", values close to zero correspond to "less signal-like". We train a separate neural network for each Higgs signal mass. The following plots show the BNN output before applying b-tagging (control region). The plots are shown for central leptons.


Bayesian Neural Network outputs before b-tagging, for both linear scale (Left) and log scale (Right).
From top to bottom, Central leptons, Plug electrons, Non-triggered muons, and Loose electron-like leptons, respectively.





Bayesian Neural Network Output (Signal region)

The following plots show the BNN output for all the b-tagging categories (signal region) combining all the considered lepton categories. Left plots show the normalized BNN output, middle (right) plots show the BNN output with linear (log) scale.


All Leptons, Double SECVTX tags (Normalized to a unit area) All Leptons, Double SECVTX tags (Linear scale) All Leptons, Double SECVTX tags (Log scale)
All Leptons, One SECVTX + One JetProb tags (Normalized to a unit area) All Leptons, One SECVTX + One JetProb tags (Linear scale) All Leptons, One SECVTX + One JetProb tags (Log scale)
All Leptons, One SECVTX + One NN tags (Normalized to a unit area) All Leptons, One SECVTX + One NN tags (Linear scale) All Leptons, One SECVTX + One NN tags (Log scale)
All Leptons, One SECVTX tags (Normalized to a unit area) All Leptons, One SECVTX tags (Linear scale) All Leptons, One SECVTX tags (Log scale)



The same kind of plots are shown below only for central leptons combining all the categories with 2 b-tagged jets (ST+ST, ST+JP and ST+NN):


More NN Output Plots
Central Triggered Leptons Plots
Plug Triggered Electrons Plots
Non-Triggered Muons Plots
Loose Electron-Like Leptons Plots





Systematics:

We address systematic uncertainty on the signal acceptance from several different sources:
  • Lepton reconstruction
  • Trigger
  • Initial and final state radiation, and Parton distribution functions
  • Jet energy scale
  • b-tagging scale factor
  • Luminosity
The following link shows the systematic uncertainty for each lepton/b-tag categories due to each effect on the signal acceptance.

Systematics


The uncertainty in the shape of the BNN discriminant due to the JES is also taken into account (jet-energy corrections that describe ± 1 &sigma variations to the default correction factor are applied). Also a shape systematic is considered for the uncertainty in the renormalization scale used to generate the W + jets MC samples by halving and doubling the default value.




Results:

We perform a binned likelihood fit of the Bayesian neural network distribution where we constrain the backgrounds to their estimated rates within uncertainties. For optimal sensitivity, we perform a separate simultaneous search in each tag channel and lepton category.
The left plot and table show the combined result using all the lepton and tag categories. The right plot and table show the result when the presented analysis is combined with an independent WH search using events with 3 jets using 5.6 fb-1 of data.


Limits for the presented analysis using events with 2 jets.
Rates relative to Standard Model Expectation.


Mass (GeV/c2) Observed Expected
100
1.34
1.83
105
2.10
2.08
110
3.42
2.26
115
3.64
2.78
120
4.68
3.22
125
5.84
4.01
130
8.65
5.13
135
10.2
7.02
140
16.4
9.39
145
24.7
15.3
150
38.8
23.4

Limits combining the presented analysis using events with 2 jets with
and independent search that uses events with 3 jets.
Rates relative to Standard Model Expectation.

Mass (GeV/c2) Observed Expected
100
1.12
1.79
105
2.06
1.98
110
2.78
2.17
115
2.65
2.60
120
3.40
3.06
125
4.36
3.69
130
6.09
4.80
135
7.71
6.40
140
12.3
8.84
145
18.9
14.2
150
34.4
21.6




The evolution of the improvements in the Higgs boson sensitivity of this analysis over time is summarized on the following plot:







Public Note:

More details about the analysis you can be find in this public note.
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