Search for ZHllbb with 1.1/fb using NN

Andy Haas and Shaohua Fu

Higgs Results for Lepton Photon 2007

Special ADM

August 8, 2007

Data / MC samples

All p17 pass3 data, L = 1.07 fb-1

2EMhighpt, 2MUhighpt skims

MC

ZH  eebb

Z+nlp (n = 0,1,2,3,4,5, HF skimmed) K-factor = 1.23

Z+bb, Z+cc K-factor = 1.35 (30% error for 2tag, 50% error for 1tag)

ZH  bb

Z+nlp: normalized to Z + 2jets data

Z+bb K-factor = 1.5 (this is higher than ee channel b/c lack of Z+cc)

Applied Z pT re-weighting, Z mass bug correction, and object ID corrections

Other MC: ttbar, WZ, ZZ

Analysis

Event selection

ZH  eebb: 2 Loose_trk electrons pT > 15 GeV at CC/EC

ZH  bb: 2 loose muons pT > 10 GeV

2 jets pT > 15 GeV with || < 2.5 (Rej > 0.5)

70 < Mee < 110 GeV, all triggers

QCD background: reversed EM likelihood or muon isolation data samples

B-tagging

1T sample: single VERYTIGHT tagged but not at least double L4 tagged

2L sample: at least double L4 tagged

NN selection

Andy’s NN scripts

ROOT TMultiLayerPerceptron

6 (8) hidden neurons, 150 training epochs

10 variables: Mbb (Mjj), pTjet1, pTjet2, R(l1,l2), (j1,j2), (j1,j2), R(Z,j1), Z, MET, SET

Mjj mass cut: within signal’s mean  3

All backgrounds included (QCD from data)

Training weights: #events × b-tagging probabilities TRF

Train on ee and  channels separately

Train on double-tagged and single-tagged samples separately

NN output

Systematic uncertainties

Luminosity: 6.5% uncertainty (or normalized to Z peak in mu)

Lepton-ID: 4% uncertainty

Other jet errors (resolution, jet ID): <1.5% and negligible

Cross-section uncertainties (see below)

QCD: 50% normalization error – very small contribution

NN output shape uncertainties – being added...

Limits

Combine 2L and 1T samples, combine ee and  channels

Calculate confidence levels using profile likelihood fitting

Provide a BEST GUESS for the central values of background expectations and uncertainties, then find the BEST FIT to data within the uncertainties

Using Wade’s limit codes (package collie, CLFit2)

SM factor limit goes from 13 -> 16 for ZH115 using CLFit->CLFit2

Summary

Combined limits:
factor of 16 away from SM for ZH115, ~4 for ZZ

CDF, also 16 (including MET corrected di-jet resolution!)

Updated analysis notes for both ZH  eebb and ZH  bb

Wish to send combined ZH  llbb conf. note to Lepton Photon

Plans

Combine with Matrix Element method (which will include MET di-jet mass corrections) for 2 fb-1 analyses, publish!

Other missing pieces may go into 2 fb-1 analyses: di-jet mass resolution improvement, new JES, new Alpgen Z+HF MC, improved b-tagging, …

Back-up

A. Haas & S. Fu

PowerPoint Presentation

shfu

2007-03-14T12:53:56