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Searches for the SM Higgs Boson at the Tevatron Collider

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Moriond QCD – March 16 th , 2005. Searches for the SM Higgs Boson at the Tevatron Collider. Contents: Introduction to Run 2 at the Tevatron Higgs Sensitivity WG predictions H bb decay searches HWW decay searches Combined limits & conclusions. - PowerPoint PPT Presentation
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Searches for the SM Higgs Boson at the Tevatron Collider Tommaso Dorigo, University of Padova and INFN Representing the CDF and D0 Collaborations Contents: Introduction to Run 2 at the Tevatron Higgs Sensitivity WG predictions Hbb decay searches HWW decay searches Moriond QCD – March 16 th , 2005
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Page 1: Searches for the SM Higgs Boson at the Tevatron Collider

Searches for the SM Higgs Boson at the Tevatron Collider

Tommaso Dorigo, University of Padova and INFNRepresenting the CDF and D0 Collaborations

Contents:

Introduction to Run 2 at the Tevatron

Higgs Sensitivity WG predictions

Hbb decay searches

HWW decay searches

Combined limits & conclusions

Moriond QCD – March 16th, 2005

Page 2: Searches for the SM Higgs Boson at the Tevatron Collider

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Tevatron Run 2 – Quick OverviewTevatron is performing well – delivered 800 pb-1 so far. Lstart above 1032 now common. L has been following design curve!Upgrades continuing – electron cooling of antiprotons is critical. As L increases, CDF and D0 catching up by modifying trigger tables, improving DAQ Design curve means 8 fb-1 by 2009!

WE ARE HERE

Page 3: Searches for the SM Higgs Boson at the Tevatron Collider

SM Higgs: Production and Decay

mH (GeV/ c2)

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At the Tevatron, about five 120 GeV Higgs bosons are produced in a typical day of running (will be 15/day in two years).

Direct production occurs mostly via gluon-gluon fusion diagrams.Associated production through a virtual W or Z boson provides sensitivity in the region where LHC will have more trouble. At higher mass, the WW(*) final state becomes dominant.

e

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Even the WHWWW(*) process is promising despite the low yield, due to the striking signature of missing Et plus three leptons, two of which may be of the same charge but different flavor.

Page 4: Searches for the SM Higgs Boson at the Tevatron Collider

What we know about the Higgs

• Although they did not directly observe it, the LEP experiments have collected a wealth of information on the Higgs boson through comparisons of EW observables to EW theory + radiative corrections

• From theory we know its couplings, its decay modes, and how its mass impacts the W and top masses.

• If it exists, then we know its mass with about 60 GeV accuracy, and the direct search limit already cuts away a large part of the allowed mass region

• Latest LEP results: MH=126+73-48 GeV,

MH<280 GeV @ 95% CL (Winter ‘05).

Page 5: Searches for the SM Higgs Boson at the Tevatron Collider

Higgs Sensitivity WG PredictionsIn 2003 the Tevatron chances for Higgs discovery were re-evaluated

Idea: with available data and operating detectors, can better assess Tevatron reachSurprisingly, the new results meet or exceed 1998 Susy/Higgs WG ones.

CDF

BASEDESIGN Keys to success:

-mass resolution improvements;- optimized b-tagging;- shape information vs counting.

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Page 6: Searches for the SM Higgs Boson at the Tevatron Collider

Identification of High Pt LeptonsMost final states produced by Higgs decay involve high-Pt leptons. CDF and D0 have efficient lepton triggers and high purity ID selections

A host of electroweak measurements is being produced at the Tevatron.

Tau leptons are also starting to contribute appreciably to precision measurements (but no results for SM H searches with these yet).

CDF

D0

CDF

Page 7: Searches for the SM Higgs Boson at the Tevatron Collider

Tagging b-jetsIdentifying b-jets is of paramount importance for low-mass Higgs boson searches.Three methods are well-tested and used:– Soft lepton tagging– Secondary vertex tagging– Jet Probability tagging

For double tag searches, efficiency factors get squared! To retain signal, both CDF/D0 have loose and tight tagging optionsEfficiency drops at low jet Et and high rapidity but is 45-50% for central b-jets from Higgs decayMistag rates are kept typically at 0.5%

I.P.B

SV tagging: tracks with significant IP are used in a iterative fit to identify the secondary vertex inside the jet

D0

CDF

Tight/loose SV tag eff.

Page 8: Searches for the SM Higgs Boson at the Tevatron Collider

Can we see dijet resonances if they are there?

The S/N is not higher than 1/5 at the most in the signal region

– good testing ground for H!– can use to test/improve dijet mass resolution with

advanced algorithms

We barely saw it in Run 1…

Can we use it in Run 2 ??

A low mass Higgs search entails believing that we can:- appropriately reconstruct hadronically-decaying objects

- accurately understand our background shapes

All of that can be proven if we see the Zbb decay in our data.

Page 9: Searches for the SM Higgs Boson at the Tevatron Collider

CDF sees Zbb decays in Run 2!

Double b-tagged events with no extra jets and a back-to-back topology are the signal-enriched sample: Et

3<10 GeV, 12>3

Among 85,784 selected events CDF finds 3400±500 Zbb decays

- signal size ok- resolution as expected- jet energy scale ok!

This is a proof that we are in business with small S/N jet resonances!

CDF expects to stringently constrain the b-jet energy scale with this dataset

Page 10: Searches for the SM Higgs Boson at the Tevatron Collider

Low Mass H SearchesThe only chance to see Hbb at the Tevatron is through associated production with bosons

ZHllbb is the cleanest signature, but it yields too few eventsW/ZHjjbb has the lowest S/N but the high BR helps at larger Higgs massWHlbb is next-to-best, but CDF was “unlucky” in Run I The best channel is ZHbb

CDF has a new combination of Run 1 results with ZHllbb, bb channels. They search events with two jets with <2.6, missing Et>40 GeV, no isolated track with Pt>10 GeV. The limit is obtained by a fit to the mass distribution of b-tagged events.

The Run 1 CDF limit is now at 7.2 to 6.6 pb for MH=110 to 130 GeV.

Page 11: Searches for the SM Higgs Boson at the Tevatron Collider

New search for WH in Run 2To search for WHlbb eventsa detailed understanding of the composition of the W+jets sample is mandatory. In the 2-jet bin CDF finds 62 events with a b-tag,where 66.5±9.0 are expected, mostly from Wbb production and mistags.

A fit to the dijet mass distribution allows to extract a 95% CL limit of 5 pb to SM WH production.The obtained limitis consistent bothwith a priori predictions and with expectations based on HSWG results.

Page 12: Searches for the SM Higgs Boson at the Tevatron Collider

WH Search in D0D0 also study their W+2jet bin with b-tagging in 174 pb-1 of high-Pt leptons from Run 2 data. They find 76 events with one b-tag (exp. 72.6±20.0), 6 with two b-tags (exp. 4.4±1.2).

The dijet mass distributions show no anomaly with 1 b-tag. The 2-tag distribution is divided in search windows to set limits to Higgs production. 95%CL limits on

WH*B(Hbb) are set at 9-12 pb for MH=115-135 GeV

By-product: a 95% CL limit is set to Wbb production (R>0.75, Pt>20 GeV) at 6.6 pb.

Page 13: Searches for the SM Higgs Boson at the Tevatron Collider

High Mass Searches: HWW(*)The SM production of WW pairs has been measured by CDF in Run 1 and by both CDF and D0 in Run 2: excellent agreement with NLO.

To search for Higgs boson decays, events with two high-Pt leptons (e,) and large missing Et are selected; the tt background is rejected with a jet veto.

Then both experiments use the helicity-preferred alignment of charged leptons in to discriminate known backgrounds.

W+ e+

W- e-

Page 14: Searches for the SM Higgs Boson at the Tevatron Collider

CDF results on HWW

8 events are observed in 184 pb-1 of Run 2 data with the Mll <80 GeV cut, with an expected background of 8.9±1.0.

A likelihood fit to the ll distribution is performed to extract a limit on the HWW cross section as a function of its mass.

The result is WW*B(WWll)>5.6 pb for MH=160 GeV.

CDF searches for HWW events by selecting two tight leptons (ee,e) with Et

e(Pt)>20 GeV and

missing Et>25 GeV (50 GeV if ll<20°). A strict jet veto (Et<15 GeV if ||<2.5) rejects top candidates.Finally, a small dilepton mass is required (Mll<55-80 GeV for MH=140-180 GeV).

Page 15: Searches for the SM Higgs Boson at the Tevatron Collider

D0 Results on HWWD0 also searches for HWW decays by selecting events with two oppositely charged leptons (ee,e: Pt>12,8 GeV; : Pt>20,10 GeV), missing Et>20 GeV (30 for ), and imposing a loose jet veto (Et<90 GeV, or Et

1,Et2 <50,30 GeV).

Combining the three channels they find 9 events, when 11.2±3.2 are expected from background sources in 177 pb-1 of Run 2 data.

They can thus exclude *B>5.7 pbat 95% C.L. for MH=160 GeV.

The azimuthal angle ll between the two leptons is then required to be less than 1.5 for electron pairs (2.0 for the e, combinations).

Page 16: Searches for the SM Higgs Boson at the Tevatron Collider

WHWWW(*) SearchCDF also searches for the striking signature of three W bosons in 193.5 pb-1 of Run 2 data.First, the dataset with a lepton with Pt>20 GeV and a second with Pt>6 GeV of same charge is analyzed and found in agreement with expectations.

Then, optimized cuts are applied to the second lepton (e.g. Pt>18 GeV for MH>160 GeV) and on the vector sum of leptons transverse momenta (Pt

ll>35 GeV).Zero events are observed, when 0.95±0.61± 0.18 are expected from known sources.95% CL limits are thus set at 12 (8) pb for MH=110 (160) GeV.

Page 17: Searches for the SM Higgs Boson at the Tevatron Collider

Putting it all together…

Page 18: Searches for the SM Higgs Boson at the Tevatron Collider

Summary and OutlookThe Higgs boson is being hunted at the Tevatron in all advantageous search channels. D0 and CDF are competing – that’s good! – but will soon start to also combine their results.

No surprises with the analyzed 200 pb-1 samples, but we have already three times more data on tape to look at!

We are on track to supersede the LEP2 lower limit on MH in time for Moriond QCD 2007!

By the end of 2009, the Tevatron might be able to see a MH=115 GeV Higgs at 5, or exclude it all the way to 180 GeV.

…but that will require both cunning and the Tevatron delivering according to the design plan!

What I feel I can promise at 95% CL: exclusion up to 135 GeV, 3evidence at 115 GeV.

Page 19: Searches for the SM Higgs Boson at the Tevatron Collider

CDF and D0 in Run 2

Page 20: Searches for the SM Higgs Boson at the Tevatron Collider

The CDF DetectorCDF significantly upgraded from Run 1:

• New L00+SVX+ISL silicon detector• New central tracker • Extended muon coverage to ||<1.5• New end-plug calorimeters• SVT measures IP to 45 m at Level 2!

The challenge is now a smooth operation for many years of running…

Page 21: Searches for the SM Higgs Boson at the Tevatron Collider

The D0 DetectorMassively upgraded from Run 1 to include:• 77,000 ch scintillating fiber tracking • 2.0 Tesla solenoid• 800,000 channel silicon detector (4 barrel layers, 2-sided disks)• Extended muon coverage (MDT)Tracker working well despite lowvolume (R=1/3 RCDF)High performance b-tag to ||<2.0

Page 22: Searches for the SM Higgs Boson at the Tevatron Collider
Page 23: Searches for the SM Higgs Boson at the Tevatron Collider
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Page 25: Searches for the SM Higgs Boson at the Tevatron Collider

CDF: aWWee candidate

CDF: a WWecandidate


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