S. Chakrabarti(K. Tschann-Grimm, Y. Hu, P. Grannis)

SUNY @ Stony Brook

Search for the SM Higgs boson in jj final state

NSF Site Visit Nov. 19, 2009

Outline

•Motivation

•Preselection

•Multivariate Method

•Results

•Summary

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Analysis presented here is for 3.9 fb-1 from Run 2b(combine with similar search in Run IIa with 1.0 fb-1

Motivation

• The channels involving tau decays of either Higgs or W/Z have about half the XS *BR as the W(lv)H(bb) or Z(vv)H(bb), so add measurably to Higgs sensitivity

• Simultaneous search for associated VH production and Vector Boson Fusion (VBF)/ gluon gluon fusion (GGF) signals

• First Tevatron search for SM Higgs in tau final states

• Mainly sensitive at low mass

• Discuss here mainly the Run 2b analysis (3.9 fb-1); then combine with published Run 2a result (1.0 fb-1)

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low high

Higgs in jj final states - Chakrabarti

Higgs production

Higgs decays

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L= 3.9 fb-1

Higgs in jj final states - Chakrabarti

Signal channels

Z() H(bb)

H() Z(qq)

H() W(qq’)

VBF: qqq’q’WWq’q’H()

GGF: ggH()+2jets

VH/VBF→jj

Event Preselection Only one isolated muon, pT>15 GeV(from

decay) One hadronic tau candidate, pT>15 GeV

At least two jets pT>20 GeV and ||<3.4 Opposite sign mu-tau pair requirement Veto on electrons spatial separation of jets, , No b tagging (allow W/Z qq’)

At preselection

Multivariate Analysis

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Higgs in jj final states - Chakrabarti

Comparison of data and MC for 3 of BDT input variables (at preselection level):

No single variable allows good separation of (very small) signal and background. Select 17 well-modeled variables to train Boosted Decision Trees (BDT)* to differentiate signal/background.

ST= |pT| for ,, jets, MET C= HT/HE where HE(HT) is scalar sum of total energy (transverse energy) for all jets

* DTs are iteratively trained learning networks in which events are sorted into ‘signal-like’ and ‘background-like’ nodes using successive selections on some input variable. Boosting is a technique of weighting those events that are misclassified more heavily in the next iteration.

BDTs for ττjj selection

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32 BDTs trained: one for each of 4 signals (VBF and GGF done together) with respect to each of 4 bkgds (ttbar, W+jets, Z+jets, Multijet). Train for low (<135 GeV) and high (>135 GeV) Higgs mass.

Representative BDT outputs for HW signal vs tt, W+jets, multijet background:

HW vs tt HW vs W+jets HW vs MJ

Signal (dotted histogram) tends to high BDT; bknd being trained against (stacked colored histogams) tend to low BDT.

Max BDT

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Max BDT ttbar Max BDT W+jets

Higgs in jj final states - Chakrabarti

For each background j (j = tt, W+jets, MJ), use the Maximum BDT output BDTj = maxi BDT(i,j) over signals (i = ZH, HZ, HW, VBF).

Final selection sample is obtained after cuts on BDTj (>-0.2, >-0.2, >0). Z+jets background is not well discriminated since its event topology is similar to the signals.

Max BDT MJ

Signal tends toward high MaxBDT; backgrounds tend to small MaxBDT

Weighted Avg BDT for Zjets

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After the MaxBDT cuts, form a final variable for limit setting from the Z+jets BDTs averaged over signals:

BDTZjets = i*BDT(i, Zjets)/ i (i = XS*BR*Acceptance for signal i)

Low MH

Use BDTZjets distribution for final limit setting. Use modified frequentist method: Form log likelihood ratios (LLR) from the BDTZjet

distributions to conform to Bknd only or Signal+bknd over many pseudo experiments. Obtain CLb (CLs+b) for B (S+B) to be less likely than observed. Scale up signal until CLs=CLs+b/CLb reaches 5% to obtain signal 95% CL limits.) The pseudoexpts allow fluctuations within statistical and systematic uncertainties (correlations included). Major systematics: energy scale (4.5%) Lumi (6.1%),

MJ bknd (15%) Cross sections (~10%), jet energy scacle(7.5%)

Yields after MaxBDT cuts:

Limits

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Run 2b results: LLR (more negative is more signal-like). Black dotted curve is LLR expected if Bknd only (green/yellow for ±1, ±2bands) and black solid line is observed in our data.

Corresponding expected and observed 95% CL XS limits/(SM expectation) (Run 2b only)

Combine with Run 2a result (PRL 102, 251801 (2009)) to get limits for 4.9 fb-1. For MH=115 GeV, expected/observed limit ratio to SM is 16/27.

Run 2b

Summary

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Higgs in jj final states - Chakrabarti

First results for Higgs search with tau final states

This analysis included in the combined Tevatron limits (3/09)

below.

Sensitive at low mass; 95% CL limit currently 27x SM prediction

Update and improve with >6 fb-1 for Moriond 2010 and

publication with echannel