D0-France, October 14 th 2008 Gregorio Bernardi, LPNHE-Paris for the CDF and D Ø Collaboration

Gregorio Bernardi / DØ-France/ October 14-2008

D0-France, October 14th 2008 Gregorio Bernardi, LPNHE-Paris for the CDF and DØ Collaboration

WH, High Mass, Combined Higgs Searches and Prospects at the Tevatron

• Introduction

• Standard Model Higgs Searches

• Combination techniques

• Combination results:

• Prospects

2Gregorio Bernardi / DØ-France/ October 14-2008

SM Higgs boson production

• gg fusion– Dominates at hadron machines– Usefulness depends on the Higgs decay

channel

• WH, ZH associated production– Important at hadron colliders since can

trigger on 1 or 2 high-pT leptons, Jets and MET

• t t H (and bbH) associated production

– High-pT lepton, top reconstruction, b-tag

– Low rate at the Tevatron

• Vector Boson Fusion

– Two high-pT forward jets help to “tag” event

– Important at LHC, first results at Tev!

H


SM Higgs Production and Decays

Production cross section (mH 115-180) in the 0.8-0.2 pb range for gg H in the 0.2-0.03 pb range for WH associated vector boson production

Dominant Decays bb for MH < 135 GeV WW* for MH > 135 GeV

Production Decays


SM Higgs Low mass searches: datasets/methods

Missing ET 2 b-jets

Lepton Missing ET 2 b-jets

CDF DØ CDF DØ CDF DØ

Prel: 2.7 fb-1 1.7 fb-1 2.7 fb-1 2.1 fb-1 2.4 fb-1 2.3 fb-1

Pub: 1.0 fb-1 1.1 fb-1 1.0 fb-1 0.9 fb-1 1.0 fb-1 0.4 fb-1

Common to all analyses: b-tagging, Jet calibration & resolution, lepton-identification, Background cross-section

Differences: instrumental bckd, multivariate techniques

2 Leptons 2 b-jets

WH ZHbb ZHl l bb


Signal

u

dW*+

W+

H0

b

b

l+

l b

b

u

d

W+ l+

lLargebackground(also Wcc)

u

d

W+ l+

l

W+2 light jetwith one/two false b-tag

u

d

W*+

t

b

b

l+

l

W+

Single Top

b

b

u

d

W+ l+

l

Z0

“Diboson”

u

d

t

t

“ttbar”:Jets+leptonsfrom W decay

u

d

q

q

“Non-W from QCD”

and “Non-W from EW”,e.g. all Zbb processesIn which one lepton is lost

SM Higgs Searches @ Tevatron: WHlbb


Selecting WlbbSelect events using W decay signatures

Require one high-pT leptons: pT > 15 GeV)

Neutrinos missing transverse energy

WHlbb: MET > 20 GeVReconstruct vector boson mass

Use “OR'ing” of muon triggers: 100% efficiency +15% in sensitivity.

Single EM or EM+jets for e-channel (95% eff)

Select >=2 high-pT, central jets as a first step towards a Higgs signaturepT>20 GeV,|eta|<2.5

Reweight W+jets ALPGEN to data in eta, phi, delta-eta and delta-phi of jets, before b-tagging


Neural Network b-tagger

Secondary Vertex Tagger

Jet LIfetime Probability

Counting Signed Impact Parameter

NNbtag

LOOSE(eff=70%,

fake=4.5%)

All Higgs analyses uses Neural Network b-tagging algorithm,

Combining best aspects of secondary vertex algorithm

statistical impact parameters algo

Asymmetric tagging: Tight tagging for Single TagLoose tagging for Double Tag Large improvement compared

to the individual taggers:Loose 72% b-tagging eff.

6% mistagTight 50% b-tagging eff. 0.3% mistag

B

Impact Parameter (dca)

Vertex Tagging(transverse plane)

Decay Lengh (Lxy)

Hard Scatter

(Signed) Track

Gain > 30% compared to our individual taggers (secondary vertex or impact parameter)


Dijet mass, GeV

WHl bb (l=e,): effect of b-tagging

Pre-tag Exclusive single tag (Tight)

Starting from a W+ 2 jet selection, apply NN_btagging:

Dijet mass, GeV


Dijet mass, GeV

WHl bb (l=e,): after b-tagging

Dijet mass, GeV

Exclusive single tag Double Loose tag

Higgs x10Backgrounds are measured one after the other (Wbb, Single

Top), WZ with Zbb remains the golden benchmark on which we can tune our analysis tools.

Starting from a W+ 2 jet selection, apply NN_btagging orthogonal samples

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WHl bb (l=e,): Neural Net and Limits

Use neural network to separate signal from backgroundFit the NN output

pT(j1)

pT(j2)

R(jj)

(jj)

pT(jj)

M(jj)

pT(l,MET)

NNwh

Future improvements (short term): include forward electrons and 3 jets sample. Improve NN with more backgd rejection and use Matrix Element approach

Higgs x10

11


Summary of WH published Results

PRL’05:S. Beauceron’s Ph.D thesis

PLB’07:L. Sonnenschein’s Habilitation

PRL’08:J. Lellouch’s Ph.D thesis

In the making:4 fb-1 result / N. Huske’s future thesis (’10)7 fb-1 result/ J. Brown’s future thesis (’11)

12


ICHEP Preliminary WHlbb

• WHlbb - signature: high pT lepton, MET and b jets– Backgrounds: W+bb, W+qq(mistagged), single top, Non

W(QCD)– Key issue: estimating W+bb background

• Shape from MC with normalization from data control regions

– Innovations: CDF: 20% acceptance from isolated tracks, NN jet corrections

TWO approaches Neural Net and Matrix Element+Boosted Decision Trees

Working on combining both for Tevatron combination.

Analysis Lum (fb-1)

Higgs Events

Exp. Limit

Obs. Limit

CDF NN 2.7 8.3 5.8 5.0

CDF ME+BDT 2.7 7.8 5.6 5.7

DØ NN 1.7 7.5 8.5 9.3

Results at mH = 115GeV: 95%CL Limits/SM

13


Other SM Higgs Searches

• CDF and DØ are performing searches in every viable

mode:

- CDF: VHqqbb: 4 Jet mode.

– CDF: H with 2jets• Simultaneous search for Higgs in

VH, VBF and ggH production modes

• Interesting benchmark for LHC

– DØ: H • Also model independent and

fermiophobic search

– DØ: WHbb, new mode• Dedicated search with hadronic

decays

– DØ: ttH, new mode

Analysis: Limits at 160 and

115GeV

Exp. Limi

t

Obs. Limi

t

DØ WHWWW 20 26

CDF VHqqbb 37 37

CDF H 25 31

DØ WHbb 42 35

DØ H 23 31

DØ ttH 45 64

14


Low mass (mH <~135 GeV): dominant decay: bbWHqq

bbZHqq

bbZHqq Use associated production modes to get better signal/background

High mass (mH>~135 GeV): dominant decay:

(*)WWH WWHgg

(*)WWWWHqq Intermediate mass:

bbH

VBF also contributes ~ 6%

SM Higgs Searches at Higg Mass at the Tevatron

15


Search for WH WWW* in di-lepton mode

Limits ~20 times higher than SM at 160 GeV useful in the intermediate region 125-140 GeV .. and also for fermiophobic Higgs searches

Data (Signal@ 160 GeV) 19 (0.10) 15 (0.21) 5 (0.11)

Topological Likelihood discriminant, built on 3 variables per channel (ET

miss, ’s

Same charge di-lepton from W’s (one from HWW, the other from prompt W)


High Mass: gg ➙H ➙WW*

➙ll(l=e,mu)

17


WW production W + jet/ production:

H WW* ll

Major backgrounds

Selection Strategy:• Presection: lepton ID, isolation,trigger, opposite charge leptons • Remove QCD and Zl+l-: ET > 20 GeV

• Higgs Mass Dependent Cuts: Invariant Mass (Ml+l-); Min. Transverse Mass Sum of lepton pT

l and ET (pT

l + ET)

• Anti tt(bar) cut: HT = PTjet < 100 GeV, or less than

2 jets• Spin correlation in WW pair: (l,l) < 2.0

•Then apply advanced analysis technique, Neural Net+Matrix element

Now measured at the Tevatron by both expts. in agreement with NLO calculation: ~13.5 pb


H WW* ll : Selection

Signal (x10)MT

min/GeV

Signal

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5 Matrix Elements discrim.

H

Neural Network Input Variables /Matrix Elements


Matrix elements discriminant

Matrix Elements + Neural Net Final Discriminant

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SM Higgs: HWW

• Most sensitive Higgs search channel at the Tevatron

Analysis Lumi (fb-1)

Higgs Events

Exp. Limit

Obs. Limit

CDF ME+NN 3.0 17.2 1.6 1.6

DØ NN 3.0 15.6 1.9 2.0

Results at mH = 165GeV : 95%CL Limits/SMBoth experiments

Approaching

SM sensitivity!

22


Systematics, including correlations, are taken into account when combining results.

Correlations of uncertainties of analyses inside one experiment (e.g. Jet calibration), or across experiments (e.g. PDF, theoretical cross-sections)

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 Flavor

Combining the results

23


Limit SettingLEP: low background, small systematics Tevatron: high background, large systematics (at low mass)

But SMALL signals in both cases Background only (b) and signal plus background (s+b)hypotheses are compared to data using Poisson likelihoods.

For the searches and to set limits, Tevatron experiments use generalized CLs method (modified frequentist, DØ) and Bayesian methods (CDF), and cross-check each other.

Systematic uncertainties are included in the likelihood, via Gaussian smearing of the expectation (‘profile likelihood’).

New compared to LEP: Background is constrained by maximising profile likelihood (‘sideband fitting’), useful in particular at low mass.

Limit setting approaches agree to better than 10%

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Constraining Systematic Uncertainties with Data

“Profiling”

AKA side band fitting Nuisance parameters introduced in the chi2 of the fit allow shifting of central value of the background estimation

Systematic uncertainty width gets also constrained

Shape of the systematic is also taken into account

Background prediction + uncertainty

25


Best fit for nuisance parameters

No smoking gun after all the checks, proceed to derive combined limit…

Centered close to good

sh

ift

in u

nit

s o

f syste

mati

c

un

cert

ain

tyDØ Higgs SM combination

Fitted Nuisance parameters for each systematic uncertainty

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Combining the Results

WHl bb 2.7 + 1.7 fb—1 / NN+NN e/, 1b/2b

ZHll bb 1.0 + 1.1 fb—1 / NN+NN e/, 1b/2b

ZH bb 2.7 + 2.1 fb—1 / NN+DT Z, Wł (2b)

H (gg,VBF,WH,ZH) 2.1 + __ fb-1 / NN . + 2 jets

H __ + 2.3 fb-1 / Di- mass

HWW* 2.4 + 2.3 fb—1 / NN&ME ee, e,

WHWWW* 1.7 + 1.1 fb—1 2D LHood

ee, e,

Total of 28 CDF + DØ channels combined

Channel Lumi /Technique Final state

27


Post-Moriond 2008, with up to 2.4 fb-1

Observed limit at mH= 160 Gev: 1.1 x SM (3.6 @ 115 GeV)Very close to excluding a 160 GeV SM Higgs. @ ICHEP: ~ 3 fb-1

3.3 SM at mH=115 GeV 1.6 SM at mH=160 GeV

arXiv:0804.3423

28


SM Higgs Limits with 3 fb-1

• Limits calculating and combination– Using Bayesian and CLs methodologies.– Incorporate systematic uncertainties using pseudo-

experiments (shape and rate included) (correlations taken into account between experiments)

– Backgrounds can be constrained in the fit

• Low mass combination difficult due to ~70 channels, not updated yet

but Expected sensitivity of CDF/DØ combined: <3.0xSM @ 115GeV

Combination of CDF and D0 done at high Mass > 150 GeV

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SM Higgs Combination at High Mass

High mass only

Expected: 1.2 @ 165, 1.4 @ 170 GeV

Observed: 1.0 @ 170 GeV

30


SM Higgs Combination at High Mass

• Result verified using two independent methods(Bayesian/CLs)

M Higgs(GeV) 160 165 170 175

Method 1: Exp 1.3 1.2 1.4 1.7

Method 1: Obs

1.4 1.2 1.0 1.3

Method 2: Exp 1.2 1.1 1.3 1.7

Method 2: Obs

1.3 1.1 0.95 1.2

95%CL Limits/SM

SM Higgs Excluded: mH = 170 GeV

We exclude at 95% C.L. the production of a SM Higgs boson of 170 GeV

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• Since 2005, our high Higgs mass experimental sensitivity has improved by a factor of 1.7 (i.e. taking out gain due to luminosity)– NN discriminants– Lepton acceptance

• For 2010, we estimate an additional improvement in analysis sensitivity by a factor of 1.4– increased lepton efficiency (10% per lepton)– multivariate analyses (~30% in sensitivity)

• Potential improvements not included in estimate– add channels– …

Projection assumptions: High mass Higgs (Dec. 07)

32


• Since 2005, our analysis sensitivity has improved by a factor of 1.7 well beyond improvement expected from sqrt(luminosity)– Acceptance/kin. phase space/Trigger efficiency– Asymmetric tagging for double b-tags – b-tagging improvements (NN b-tagging)– improved statistical techniques/event NN discriminant for channel with largest effort applied (WH) factor was 2.1

• For 2010, we estimate that we will gain an additional factor of 2.0 beyond improvement expected from sqrt(luminosity)– b-tagging improvements

• DØ: Layer 0 (~8% per tag efficiency increase)• add semileptonic b-tags (~5% per tag efficiency

increase)– Di-jet mass resolution (18% to 15% in σ(m)/m)– increased lepton efficiency (10% per lepton)– improved/additional multivariate techniques (~20% in

sensitivity)

Sensitivity and Projections – MH = 115 GeV

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Expected Higgs sensitivity in 2009/2010

Assumes two experiments

2010 2009

Projection: DØ X 2

By the time LHC produces Physics (2009), precision EW meas. + Tevatron might allow SM Higgs only with mass between 118 and 145 GeV, definitely only a light Higgs boson, which will take some time to be found at LHC (> 1 fb-1) LHC/Tevatron complementarity H vs H bb

2009

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Higgs sensitivity, 3-σ evidence

Assumes two

experiments2010 2009

Projection: DØ X 2

• With data accumulated by the end of 2010, we will be able to explore much of the SM Higgs mass region allowed by the constraints from precision measurements and LEP direct exclusion– Expected 95% CL exclusion over whole allowed range, (except

possibly around 130 GeV) - assuming the Higgs does not exist at these masses

– Three-sigma evidence for a Higgs possible over almost entire range, and probable for the low end and high end.

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ConclusionNew Higgs analyses available, large common effort by both CDF and

Dzero collaborations. Sensitivity constantly progressing ~linearly with Luminosity.

The Tevatron experiments have achieved sensitivity to the SM Higgs boson production cross section at High Mass, still working on low Mass

• 2009 will also teach us how well we perform at low mass, with the golden WZ/ZZ (Zbb) benchmark

• The Higgs boson search is entering its most exciting era, since it is for the first time within reach in the very near future (< 3 years).

• We exclude at 95% C.L. the production of a SM Higgs boson of 170 GeV– We expect large exclusion, or

evidence, with full Tevatron data set and improvements

Report for conseil scientifique In2p3, july 2000

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Expected

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Systematics: ZH-llbb / CDF vs DØ

Documents

D0-France, October 14 th 2008 Gregorio Bernardi, LPNHE-Paris for the CDF and D Ø Collaboration