Higgs Searches at the Tevatron

Gavin DaviesOn behalf of the CDF and DØ Collaborations

Higgs Searches at the Higgs Searches at the TevatronTevatron

Gavin Davies – SUSY08 2

OutlineOutline•Introduction

– Tevatron

•Standard Model (SM) Higgs– Introduction– Results

•Low & high mass•Combination

•Non-SM Higgs– Introduction– Results

•Prospects & Conclusions

(See parallel sessions for full details)

[ Thanks to all my Tevatron colleagues ]


Tevatron PerformanceTevatron Performance•Over 4fb-1 delivered per experiment. Over 0.2fb-1 recorded last

month.

•Expect ~6fb-1 by Apr 2009, and 8fb-1 by Oct 2010.

05/02 05/04 05/06 05/08

Average data-taking efficiency > 85%

Results presented here use up to 2.4fb-1

(3fb-1 results at ICHEP)

Many thanks to Accelerator Division


Standard Model HiggsStandard Model Higgs

•Higgs mechanism– Additional scalar field in SM Lagrangian mass to W,Z & fermions

– Predicts neutral, spin 0 boson• But not its mass

•Direct searches at LEP2– mH > 114.4 GeV

•Improved mt & mw tighten indirect

constraints: – mH < 160 GeV (EW fit)

– mH < 190 GeV if LEP2 limit included

A light (SUSY?) Higgs is favoured


SM Higgs Production & SM Higgs Production & DecayDecay

•Small production cross-sections– 0.1 -1 pb

•Branching ratio dictates search

•mH < 135 GeV– gg → H → bb overwhelmed by

multijet (QCD) background– Associated WH & ZH production

with H → bb decay– Main backgrounds: W+bb, Z+bb,

W/Z jj, top, di-boson, QCD

•mH > 135 GeV– gg → H → WW – Main background: WW

Exclu

ded

H bbH WW


SM Higgs TopologiesSM Higgs Topologies•Low mass:

•Intermediate mass:

•High mass:

bbWHqq

bbZHqq

bbZHqq

(*)WWWWHqq

(*)WWHgg

Leptonic decay of W / Z boson provides ‘handle’ for event.H bb reduce SM background

High PT opposite sign leptons.Missing ET


SM Higgs - snapshotSM Higgs - snapshot•Higgs searches at a hadron machine challenging

•Requires– Efficient triggering, lepton ID,

b-tagging, jet resolution..– Sizeable data-sets

•Well established– Analyses use multivariate techniques

• Neural net (NN), matrix element (ME)..

– Regular Tevatron combinations

•Approaching the SM expectation– Rapid improvement well beyond √L gain

•Even better to come


•Highest associated production cross-section

– Use electron and muon channels

•Selection– Isolated lepton, missing ET (MET), 2 jets

•Backgrounds– W+jets, QCD, top, di-boson

•Analyses– Separate channels: 1 &

2 b-tags, e or , central or forward – NN (and ME) based analyses using kinematic variables

, ,WH l bb l e


, ,WH l bb l e •Cross section limits derived from NN distributions

– Compare observed (obs) or expected (exp) limit to SM prediction

•Recent CDF analysis– Including isolated tracks– Increases acceptance by 25%

Lum Obs/SM Exp/SM

DØ 1.7 fb-1 10.9 8.9

CDF 1.9 fb-1 8.2 7.3

Exp/SM = 6.4 (14%

improvement)

mH=115GeV


, ( )ZH bb WH l bb •Large cross-section & acceptance but hard•Selection

– Two jets, MET (not aligned in with jets)

•Backgrounds– Physics: W/Z + jets, di-boson, top – from Monte Carlo– Instrumental: Mis-measured MET with QCD jets – from data

•Analyses– Optimised b-tagging, NN or decision tree (DT) discriminant

Exp/SM ~8 (both expts.)


And….And….•Not forgetting .. ZH → llbb

– 1 and 2 b-tags, NN event selection– Exp/SM ~ 16-20 at 115GeV (1fb-1)

•Sensitivity of existing channels rapidly improving

•Add additional channels– DØ

•

– Exp/SM ~ 45 at 120GeV– CDF

•

– Exp/SM ~ 40 at 120GeV•

– Exp/SM ~ 25 at 115GeV

– Adds 10% to CDF combination (≡ 20% more data)

bbqqHWZ /

H

H


(*)WWWWH •Intermediate mass range•Selection

– 2 like-sign high PT leptons (ee, e, )

•Backgrounds– WW,WZ, charge flips (from data)

•Analyses– DØ 1fb-1 and CDF 1.9fb-1

Exp /SM ~25 at 140GeV

Exp /SM ~20 at 160GeV


( ) , ,H WW l l l e

•Dominant for mH >135GeV

•Selection– 2 isolated, opposite sign leptons, MET

•Backgrounds– Drell-Yan, W+jets/QCD, tt, SM WW

•Analyses:– WW from spin 0 Higgs

• Leptons prefer to point in same direction

– Use NN, using kinematic variables and ME as inputs


( ) , ,H WW l l l e •Matrix element

– Use observed leptons & missing ET (xobs)

– Integrate over LO theory predictions

for WW, ZZ, W+, W+jet, Higgs – Construct LR discriminant from probabilities

• Input ME information to NN– CDF – 4 ME’s and 6 kinematic variables– DØ – 1 ME and more kinematic variables

•Cross section limit derived from NN output distribution

LR (H WW)


( ) , ,H WW l l l e

Expected Limit / SM ~ 2.4Observed Limit / SM ~ 2.1

Expected Limit / SM ~ 2.5Observed Limit / SM ~ 1.6

Closing in on the SM


CombinationCombination• Compare background and signal

plus background hypotheses using

Poisson likelihoods• Systematics included in likelihoods

• Combine across channels within an experiment

• Combine across experiments

– Since 2005 sensitivity improved

by a factor of 1.7 beyond √L gain

Obs/SM Exp/SM

115GeV 3.7 3.3

160GeV 1.1 1.6


Non-SM HiggsNon-SM Higgs•MSSM

– 2 Higgs doublets, ratio of vev’s = tan– 2 charged (H) and 3 neutral Higgs particles: = h, H, A– At large tan

• Coupling to d-type fermions enhanced by tan→ α tan2• 2 A

– For low & intermediate masses• Br (→ bb) ~90%, Br (→ ) ~10%

→ and b→ bbborb→btopologies

bb


Neutral MSSM HiggsNeutral MSSM Higgs •Smaller BR but cleaner•Signal: (eτhad) (μτhad) (eμ) pairs + MET

•Background: →jet fakes

•Use visible mass:

– No significant excess in visible mass, so proceed to set limits• Initially on x Br then interpret in MSSM (FeynHiggs –

arXiv:0705.0746v2)

arXiv:0805.2491


• MSSM interpretation– Width included

– Limits very similar for <0

– Expect to reach tan~20 at low m A by end 2010

Neutral MSSM HiggsNeutral MSSM Higgs

Most powerful constraints on tan


Neutral MSSM Higgs Neutral MSSM Higgs bb + bb + b[b]b[b]

•Signal– At least 3 b-tagged jets– Look for peak in dijet mass

•Challenge - Large multijet background– Estimate from 2 b-tagged data and simulation– Composition: Sec. vertex mass (CDF) & fit to multiple b-tagging criteria

(DØ)

– No significant excess, so set limits, initially on x Br then interpret in MSSM

arXiv:0805.3556


•MSSM interpretation– Width included

Neutral MSSM Higgs Neutral MSSM Higgs bb + bb + b[b]b[b]


Going further…Going further…•Charged Higgs

–Separate t W+b & t H+b (H+ cs) via mass templates

•And beyond MSSM–Fermiophobic Higgs –Doubly charged Higgs

HR > 127 GeV

HL > 150 GeV

CDF also looked for e/ final states

arXiv:0803.1514arXiv:0805.15

34


•Rapid evolution

– Significantly better than √L– Increased trigger efficiency, lepton acceptance, improved b-tagging, NN discriminants

•For 2010 expect, beyond √L gain– x 1.4 improvement at high mass and x 2.0 improvement at low

mass

Prospects – SM HiggsProspects – SM Higgs

mH =160GeV mH =115GeV


Prospects – SM Higgs Prospects – SM Higgs (cont)(cont)

•End 2009– Very limited allowed range

•End 2010– Full exclusion

– 3 evidence possibleover almost entirerange

Assumes two

experiments


ConclusionsConclusions• Tevatron and CDF/ DØ experiments performing very well

– Over 4fb-1 delivered, with 8fb-1 expected by end of 2010

•New SM analyses keep coming in– Sensitivity almost improving linearly with luminosity– Well established, common effort across the Collaborations

•Closing in on the SM– 2008 - go beyond at 160GeV ?

•Have a clear roadmap to reach desired sensitivity

•Wide range of BSM searches, ready for discovery– Expect to reach tan ~20 at low mA

Exciting times ahead – will only get better!

ICHEP08?


And…And…

Thank you for your attention

Please see the parallel sessions for full details

Search for the SM Higgs Boson at DØ - Suyong ChoiSearch for Charged Higgs Bosons at DØ - Yvonne PetersSearch for Charged Higgs in Top Quark Decays at CDF -

Geumbong YuSearch for BSM Higgs Bosons at DØ - Ingo Torchiani


Backup slides


CDF and DCDF and DØ experimentsØ experiments• Both detectors extensively upgraded for Run IIa

– New silicon vertex detector– New tracking system– Upgraded muon chambers

• CDF: New plug calorimeter & ToF

•DØ– New solenoid & preshowers– Run IIb: New inner tracking layer

& L1 trigger


DDØØ data taking data taking


ZH → llbb

ZH → llbb

WH → lbb

H →WW → lvlv

Total


B-taggingB-tagging• Critical for low mass H bb

– Improves S/B by > 10

• Use lifetime information– Correct for MC / data differences

• Measured at given operating points

• Analyse separately (“tight”) single & (“loose”) double tags

CDF: Secondary vertex reconstruction

• Neural Net - improves purity• Inputs: track multiplicity, pT, vertex

decay length, mass, fit

• Loose = 50% eff, 1.5 % mistag• Tight = 40% eff, 0.5 % mistag

DØ: Neural Net tagger

Secondary vertex & dca based inputs, derived from basic taggers High efficiency, purity

Loose = 70% eff, 4.5% mistag Tight = 50% eff, 0.3% mistag


DDØ B-taggingØ B-taggingSeveral mature algorithms used:

• 3 main categories:

- Soft-lepton tagging

- Impact Parameter based

- Secondary Vertex reconstruction

Several mature algorithms used:

• 3 main categories:

- Soft-lepton tagging

- Impact Parameter based

- Secondary Vertex reconstruction


B-tagging – (DB-tagging – (DØ) Ø) CertificationCertification

•Have MC / data differences – particularly at a hadron machine

– Measure performance on data

• Tag Rate Function (TRF) Parameterized efficiency & fake-rate as function of pT and η

– Use to correct MC b-tagging rate

•b and c-efficiencies– Measured using a b-

enriched data sample

•Fake-rate– Measured using QCD data


Tau IDTau ID• CDF: Isolation based

• Require 1 or 3 tracks, pT > 1GeV in the isolation cone

– For 3 tracks total charge must be ±1

– pThad > 15 (20) GeV for 1 (3)

prongs– Mhad < 1.8 (2.2) GeV

• Reject electrons via E/p cut• Validated via W/Z measurements• Performance

– Efficiency ~ 40-50%– Jet to tau fake rate ~0.001-0.005

• DØ: 3 NN’s for each tau type

• Validated via Z’s

Efficiency (NN>0.9): ~0.65, jets ~0.02


, ,ZH llbb l e •Cleanest channel, but low cross

section•Selection:

– Loose lepton ID, 2 jets

•Backgrounds:– Z+jets, top, WZ, ZZ, QCD

•Analyses – Constrain dijet mass– NN event selection

Exp/SM ~ 16


Limit SettingLimit SettingLEP: Low background, small systematics Tevatron/LHC: High background, large systematics

• Systematics, including correlations, taken into account• Main systematics (depending on channel):

- Luminosity and normalisation- QCD background estimates- Input background cross-sections- Jet energy scale and b-tagging- Lepton identification- K-factors on W/Z + heavy flavour

• Systematic uncertainties included in likelihood via gaussian smearing of expectation (‘profile likelihood’)

• Background constrained by maximising profile likelihood (‘sideband fitting’)

• Tevatron experiments use LEP CLs (modified frequentist) and Bayesian methods

• Limit setting approaches agree to within 10%


Spring 08 SM Spring 08 SM CombinationCombination

WH→lbb 1.9 + 1.7 fb—1 / NN+NN

e/, 1b/2b

ZH→ll bb 1.0 + 1.1 fb—1 / NN+NN

e/, 1b/2b

ZH→ bb 1.7 + 2.1 fb—1 / NN+DT Z→, W→ł (1b/2b)

H→ (gg,VBF,WH,ZH)

2.1 + ….. fb-1 / NN .

+ 2 jets

H→ …... + 2.3 fb-1 / Di- mass

H→WW* 2.4 + 2.3 fb—1 / NN&ME

ee, e,

WH→WWW* ….. + 1.1 fb—1 /2D LHood

ee, e,

Total of 28 CDF + DØ channels combined

Channel Lumi /Technique Final state



ChannelCDF

Obs (exp)DØ

Obs (exp

WH→lbb 8.2 (7.3) 10.9 (8.9)

ZH→ll bb 16 (16) 17.8 (20.4)

ZH→ bb 8.0 (8.3) 7.5 (8.4)

H→ 30.5 (24.8) -

H→ - ~45

H→WW* 1.6 (2.5) 2.1 (2.4)

WH→WWW* - 24 (18)


MSSM benchmarksMSSM benchmarks•Five additional parameters due to radiative correction

– MSUSY (parameterizes squark, gaugino masses)

– Xt (related to the trilinear coupling At → stop mixing)

– M2 (gaugino mass term) (Higgs mass parameter)

– Mgluino (comes in via loops)

•Two common benchmarks– Max-mixing - Higgs boson mass

mh close to max possible value

for a given tan– No-mixing - vanishing mixing in

stop sector → small mass for h


Neutral MSSM HiggsNeutral MSSM Higgs Limits: x Br ( ) Evolution:Full 2010 dataset (2 expts.)

Width:

Higgs Searches at the Tevatron

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