Mark NeubauerUniversity of Illinois at Urbana-Champaign(for the ATLAS Collaboration)

DPF 2011 MeetingBrown University

August, 10th 2011

Search for Diboson Production in the Lepton + MET + Jets Final

State

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• Diboson production: gauge boson self couplings Couplings intimately related to gauge structure

of the SM and electroweak symmetry breaking Sensitive probe of new phenomena

Higgs, SUSY, LED, technicolor, Higgsless, … Important background to many searches

Higgs, SUSY, LED, technicolor, Higgsless, … Demonstrate sensitivity to small cross section

processes in the SM

Dibosons @ Hadron Colliders

Mark Neubauer DPF 2011 @ Brown University / Aug 10, 2011

σ(pb)

O(1-10) pb {

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• Dibosons play a central role in Higgs searches for mH > 135 GeV

WW is the dominant decay channel

ZZ channels very clean

Dibosons in Higgs Search

Mark Neubauer DPF 2011 @ Brown University / Aug 10, 2011

This talk focuses on lepton + MET + 2 or 3 jets

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• Signal rates and theoretical uncertainties (mf, mr, PDF and αs)from arXiv:1101.0593 [hep-ph]

Higgs Boson Production @ LHC

Mark Neubauer DPF 2011 @ Brown University / Aug 10, 2011

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• Data collected in 2011 (pp collisions at √s = 7 TeV) Peak luminosity Linst = 2.11 × 1033 cm-2 s-1

Mean number of interactions per bunch crossing μ=5.7 ΔL / L = ± 3.7% (2011, preliminary)

LHC Delivers, ATLAS Collects

Mark Neubauer DPF 2011 @ Brown University / Aug 10, 2011

2011 to-date: ε≈ 95%

1.04 pb-1 used for results in this talk

DPF 2011 @ Brown University / Aug 9, 2011

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• Single boson and diboson cross section measurements in the leptonic decay channels

ATLAS Diboson Measurements

Mark Neubauer

DPF 2011 @ Brown University / Aug 9, 2011

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Lepton + MET + Jets Motivation

Mark Neubauer

• WW dominant Higgs decay channel for MH>135 GeV lvlv dominates our Higgs sensitivity

for 135 < MH< 200 GeV lvqq dominates WW sensitivity for 240 < MH< 600 GeV due to:

Larger W branching fractionDominant background (W+jets) falls

off rapidly with increasing M(lvjj)Full Higgs mass reconstruction

lvqq complements ZZ channels with comparable sensitivity

Weak Boson Fusion (WBF) in lvqq channel a stepping stone to Weak Boson Scattering (WBS) for deep study of EWSB

DPF 2011 @ Brown University / Aug 9, 2011

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• Select events with one high-pT e/μ, 2 or 3 jets, and large MET

• M(lvjj) to discriminate signal and background: M(lv) = M(W)

Objection selectionLepton: pT > 30 GeV, |η| < 2.5Jet: ET > 25 GeV, |η| < 4.5“Neutrino”: MET > 30 GeV

Event selection / categorization71 < M(jj) < 91 GeVRequire real pν,z solutionSeparate categorization for H+0j and H+1j

Data-driven background Signal modeled by POWHEG + PYTHIA

Analysis Overview

Mark Neubauer

DPF 2011 @ Brown University / Aug 9, 2011

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Data-Driven QCD Estimates

Mark Neubauer

H + 1 j

H + 0 j

e±

e±

μ±

μ±

DPF 2011 @ Brown University / Aug 9, 2011

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• Good agreement with expectations

Yields after MET Fits

Mark Neubauer

Process Events in 1.04 fb-1

W/Z + jets 38080 ± 1170

QCD Multi-jets 2027 ± 43

Top 1330 ± 270

Dibosons 1180 ± 240

Total Expected Background 42600 ± 1200

Observed 41687

Expected Signal (mH=400 GeV)

58 ± 15

• Expected and observed yields after MET fit Real M(lvjj) solution requirement not applied

DPF 2011 @ Brown University / Aug 9, 2011

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Some Kinematic Distributions

Mark Neubauer

H + 1 j

H + 0 j

DPF 2011 @ Brown University / Aug 9, 2011

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Dijet Invariant Mass

Mark Neubauer

Signal selection: [71, 91] GeVSideband selections: [50, 60] GeV (low sideband) and [100,110] (high sideband)

DPF 2011 @ Brown University / Aug 9, 2011

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• Double-exponential model, well-motivated by MC and validated in M(jj) sidebands of the W boson

Background Model

Mark Neubauer

High masssideband

Low masssideband

DPF 2011 @ Brown University / Aug 9, 2011

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• No evidence for a resonance in the M(lvjj) spectrum

M(lvjj) Distributions

Mark Neubauer

DPF 2011 @ Brown University / Aug 9, 2011

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Cross Section Limits

Mark Neubauer

95% C.L. limits @ 400 GeV:

σ < 3.1 pb (2.7 × SM)

DPF 2011 @ Brown University / Aug 9, 2011

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M(lvjj) Distribution

Mark Neubauer

DPF 2011 @ Brown University / Aug 9, 2011

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Background-Subtracted M(lvjj)

Mark Neubauer

DPF 2011 @ Brown University / Aug 9, 2011

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• Diboson production provides a sensitive probe of the Higgs and new physics

• Channels involving a W->qq’ decay play an important role in the searches, especially at large diboson invariant mass

• We searched for Higgs production in the lepton + MET + 2 or 3 jet final state and place limits on the production

• Many improvements anticipated Better pileup suppression Weak boson fusion Charm tagging Multivariate techniques More data!

Summary

Mark Neubauer