Searches for the Standard Model Higgs at the Tevatron

CDF

Searches for the Standard Model Higgs at the

Tevatronpresented by

Per Jonsson

Imperial College London

On behalf of the CDF and DØ Collaborations

Moriond QCD 2006

La Thuile, Aosta Valley, Italy, March 18-25, 2006

CDF

Per Jonsson - Imperial College LondonMoriond QCD 2006 2Per Jonsson - Imperial College London

Outline• Introduction

• Higgs bb (low mass)

– ZH bb– WH lbb

• Higgs WW (high mass)

– WH WWW– H WW

• Limits:– combination

Main Injector & Recycler

Tevatron

Chicago

p source

Booster

p

p

p p 1.96 TeV

CDFDØ

36×36 bunches396 ns bunch crossing

Thanks to all colleagues at the Tevatron for their contributions to this talk

New results!

New result!

New result!

Only new results since last year’s Moriond are shown

CDF


Tevatron Performance

80-85% Average Efficiency

Currently in shutdown:• Silicon and Trigger upgrades

Run I

4-8 fb-1 expected by end of 2009

This talk:261-950pb-1

CDF


The DØ and CDF detectors

The Standard Model Higgs Boson is:• Key part of Electro-Weak symmetry breaking• A scalar with the Higgs mass as the only free parameter

DØ and CDF constrain the SM Higgs boson mass:• Indirectly: Top and W mass measurements• Directly: Search for Higgs boson production

CDF


EW constraints on HiggsmH constrained in the Standard Model

mH =89 +45 -30 GeV

Direct searches at LEP2:mH>114.4 GeV @95%CL

mH<175 GeV @95%CL(< 207 GeV if LEP2 limit incl.) A light Higgs is favored

68% CL

[GeV]

[GeV

]

LEPEWWG 18/03/06

CDF


SM Higgs Production• Production cross sections are small: 0.1-1

pb depending on mH:~1 in 1012 pp events is a Higgs

• MH< 135 GeV: decay to bb– gg H bb overwhelmed by QCD

background • searches can be performed in W/Z

associated production with lower background

• Best channels:– WHlbb, ZHbb

• MH > 135 GeV: decay to WW– gg H WW(*) l+l-– WH WWW(*) final states can be explored

We probably have a Higgs in our data already!

H bb

H WW(*)

Dominant decay modes

CDF


Backgrounds and Tools

• Lepton identification• Missing transverse energy

Measurements also rely on:• Jet reconstruction• B-tagging:

– Use track impact parameter (IP) measurements or secondary vertex reconstruction

IP

Both detectors are delivering the excellent performance needed

Understanding and modeling of background is critical:

• Especially so for advanced analysis techniques

• Electroweak backgrounds (W, Z, WZ, WW, Top):

– Monte Carlo distributions normalized to (N)NLO cross sections

• QCD and instrumental background taken from real data control samples

CDF

Per Jonsson - Imperial College LondonMoriond QCD 2006 8

Z/ ee + jets• Comparison of jet multiplicities and

distributions in data and MC:

– Pythia 6.319 vs. Sherpa 1.0.6 – Sherpa : Matrix Element + Parton

showers, using CKKW algorithm

• Event selection includes:– Electron pT > 25 GeV, – 70 GeV < Mee < 120 GeV

New result!

Sherpa agrees well with dataupto njet=4

L=~950 pb-1L=~950 pb-1

For details see Gavin’stalk yesterday.

CDF


ZH bb

• Event selection includes:– ≥ 1 tagged b-jets– Two jets with ET > 60/25 GeV– ET

miss > 70 GeV

• Backgrounds:– W/Z + heavy flavour jets– QCD– Di-bosons– Mis-tagged b-jets– Top pairs

• MH = 120 GeV, 80 < mjj < 120 GeV

– 6 events observed– 4.36 +/- 1.02 predicted

– pb

ET1

= 100.3 GeVET

2 = 54.7 GeVET

miss = 144.8 GeVMjj = 82.1 GeV

CDF


New result!

L=261 pb-1L=261 pb-1

1 b-tag 2 b-tags

• Improved event selection includes:

– Two acoplanar jets with:

• ET > 20 GeV

– ETmiss > 50 GeV

– Sum of scalar jet ET < 240 GeV

• Separate analysis for single and double b-tagged events:

– Increased statistics

• mH = 115 GeV, 75 < mjj < 125 GeV:– 11 events observed– 9.4 +/- 1.8 predicted– pb

• Same analysis for WH lbb with missed lepton improves the WH limit

ZH bb

CDF


• Muon and electron channel combined:

– 1 or 2 tagged b-jets

– electron or muon with pT > 20 GeV

– ETmiss > 20 GeV

Per Jonsson - Imperial College London

• Main backgrounds:• W + jets• Top pairs• di-bosons (WW, WZ etc.)

e/

b jet

b jet

WHlbb , (l=e,

CDF


• New muon analysis• Re-optimized electron

analysis• Separate single and

double b-tag analysis

• Event selection includes:– Central isolated e/

• pT > 20 GeV– Missing ET > 25 GeV– ≥ two jets:

• ET > 20 GeV, |η| < 2.5– One or two tagged b-jets

• Limit from combined channels:– mH = 115 GeV, 85 < mjj < 140

GeV:• 6 (32 ST) events observed• 9.3 (45.1 ST) predicted• pb

New result!

WHlbb, (l=e,


CDF


• Event selection includes:– Two isolated like signed

leptons (ee,e– pT > 15 GeV

– ETmiss > 20 GeV

• mH=115 GeV)pb

WHWWW(*)llqq, (l=e,


Mainly di-bosons (WZ)

CDF 194 pb-1

L=363 pb-1

observed expected

ee 1 0.70 +/- 0.08

e 3 4.32 +/- 0.23

2 3.72 +/- 0.75

CDF


• Higgs mass reconstruction not possible due to two neutrinos

• Employ spin correlations to suppress the background– (ll) variable is particularly useful– Charged leptons from Higgs are collinear

• Event selection includes:– Two leptons (e/

• pT > 20/10 GeV– Missing ET greater than

• MH/4– Di-lepton invariant mass

• > 16 GeV• < MH/2-5 GeV

• ll distribution fitted to extract 95% CL limit

• SM cross section still far away, but 4th generation models getting closer

mH= 130 GeV

mH= 160 GeV

W+ e+

W- e-

HWW(*)l+l-, (l=e,

CDF


• L= 950 pb-1

• ee and echannels

• Event selection includes:– Isolated e/

• pT(e1, e2) > 15, 10 GeV• pT(e/1) > 15 GeV,• pT(e/2) > 10 GeV

– Missing ET greater than• 20 GeV (ee, e)

– Veto on• Z resonance• Energetic jets

• MH = 120 GeV:– 31 events observed– 32.7 +/- 2.3 (stat)

predicted– Bkg systematic

uncertainty: 15%– pb


New result!

ee

e

After cuts

160 GeV Higgs (x 10)

HWW(*)l+l- (l=e,

CDF


Currently a factor 15 away from mH =115 GeV

• With L= 2 fb-1, both experiments, NN b-tagging, NN analyses, track-cal jets, increased acceptance, new channels, full cross efficiency, and reduced systematics:– Cross section factor = 1.2

New result!

Combined limit• New combined limit from all SM Higgs

search channels!

• 14 orthogonal search channels (incl. single and double-tag analyses and

WH lbb w. missed lepton)

• Full account taken of systematic uncertainties

• High mass region benefits from HWW analyses

CDF


Limits

CDF


Summary

Many results already and more with increased stats are coming soon!

• The Tevatron and the experiments are performing well

• A wide range of Standard Model Higgs searches have been performed by both CDF and D0 with up to 1 fb-1 RunII data:– No deviation from the SM background

expectation observed

• Work underway to improve sensitivity:– First combination of all channels from D0

• Very exciting prospects for the future:

– Sensitivity to m H > 114 GeV starts at ~2 fb-1

– Exclusion up to 180 GeV possible with 8 fb-

1

CDF


Back-up slides

CDF


SM Higgs Sensitivity

• New Higgs sensitivity study from CDF + DØ in 2003:

Statistical power onlySystematics not included

Improved sensitivity from refined analysis and detailed simulation

No systematics

The SM Higgs is a challenge, understanding of bkgs critical!

CDF


Major Improvements

• 6x more lumi (2 fb-1 ): 2.4

• We need both CDF and D0: 1.4• Multivariate analysis: 1.7• NN b-tagging: 1.35• Di-jet mass resolution: 1.2• Additional search channels: 1.15

Documents

Searches for the Standard Model Higgs at the Tevatron