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First ALTAS data taking: background estimates First ALTAS data taking: background estimates for Higgs searchesfor Higgs searches

Introduction: important issues before and during data taking startup

SM and MSSM Higgs “discovery” channels, how well do we need to control bkg. systematics

Background studies with the data: a few examples

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IntroductionIntroduction

Important issues before and during ATLAS startupImportant issues before and during ATLAS startupFirst step (before data taking starts): good understanding of the MC used to estimate the background and optimize analyses:

NLO MC (Higgs wg very active in this field)LO+PS shower matching as alternative method to control gluon rad.Understanding of PDFsComparison with TEVATRON data

Second step (beginning of data taking): calibration of relevant experimental quantities (mainly sub-detector working groups):

Electromagnetic energy scale and resolution e/Jet and /jet separation Muon momentum scale and resolution (Z->, W->)

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IntroductionIntroduction

Jet energy scale and resolution Z()j samples (including forward jets very important for VBF channels)Central jet veto: fake jets reconstruction rateMissing Et calibration b-tag efficiency and light-jet rejection -tag efficiency and jet rejection

Very important that Higgs working group members are involved in calibration studies and are aware of up-to-date calibrations and of SM measurements (B-Physics, top, EW, QCD working groups): W, Z, tt, jets, direct photons, heavy flavor jets

Higgs searches must foresee control samples on which claimed calibration is double-checked

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Background estimate for Higgs Background estimate for Higgs searchessearches

Third step (analysis of first few fb-1): determine errors on expected background from the collected data using background enriched control samplesHiggs working group is planning to include in all the searches the strategy to get the bkg systematic errors from data control samples

Personal CommentPersonal CommentWe will work in a difficult situation:

detector performance and calibrations will not be “perfectly” understoodMC background predictions may (very likely) not agree wit observed datacompetition with CMS will be very strong

We need to prepare clear searches strategies well in advanced to survive during the first data taking period

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SM Higgs discovery potentialSM Higgs discovery potential

Provocative statement:

With non-staged detector, “ATLFAST like” detector performance, small errors on Bkg normalization and shapes the SM Higgs can be discovered with less than 10 fb-1

Provocative statement:

With non-staged detector, “ATLFAST like” detector performance, small errors on Bkg normalization and shapes the SM Higgs can be discovered with less than 10 fb-1

Non-staged detector

ATLASATLAS

PreliminaryPreliminary

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SM Higgs Discovery channelsSM Higgs Discovery channels

Improvements w.r.t. TDR; some remarks:VBF channels (H->and H->WW*) give large improvements to discovery potentialSome of the TDR channels are now in the process of being improved with more refined analyses: H->(high Pt and jet tagging), H->ZZ*->4 leptons (NN and likelihood based analyses), ttH with H->bb (likelihood based jet pairing and selection), …… Typically ~3 channels expected to give “evidence” for each massRedundancy very important for:

Robustness of the discovery against “hidden systematics” (often uncorrelated between channels)Higgs coupling measurements

S/B ratio quite small in some channels: maximum systematics on bkg to reach 5 can go from ~50% to less than 1% (inclusive H->)

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ChannelChannel Main Main backgroundbackground

S/BS/B Bkg.Bkg.

sys for 5sys for 5Proposed Proposed

technique/commentstechnique/comments

H-> Irreduc. Reducible j

2-3% 0.4% Side-bands stat Err ~0.5% for 30-100 fb-1

ttH H->bb ttjj 30% 6% Mass side-bandsAnti b-tagged ttjj ev. Under study J.Cammin

H->ZZ*-> 4 lep ZZ->4l and llReducible tt, Zbb

300-600% 60% Mass side-bandsStat Err <30% 30fb-1

H->WW*->ll WW*, tW 30-50% 6% No mass peakBkg enriched region ?Study to be performed

VBF channelsIn general

Rejection QCD/EW Study forward jet tag and central jet veto

Use EW ZZ and WW leptonic

Study to be performed

VFB H->WW tt, WW, Wt 50-200% 10% Study Z,W,WW and tt plus jets

Work started Wisconsin

VBF H-> Zjj, tt 50-400% 10% Missing Et calibrationStudy to be performed

MSSM (bb)H/A->

Z->, Wj 25% tg=15

MA=300

5% Mass side-bandsStat Err ~5% 30fb-1

MSSM(bb)H/A ->

Z/*-> 12% tg=15

MA=150

~2% Mass side-bandsStat Err ~2% 30fb-1

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Background estimate for Higgs Background estimate for Higgs searchessearches

For some channels main tool to estimate bkg is the use of the mass side-bands (like inclusive H->and H->ZZ*->4 leptons); it is important to have under control reducible bkg and bkg mass shapes

In other channels like VBF H->WW or ttH H->bb the bkg shape doesn’t allow the use of this technique: it is very important to get the bkg estimate from the data

It is our general strategy to address this problem in any analyses

As first exercise some studies launched related to:VBF H->WW*: under study by the Wisconsin groupttH H->bb: first study by Bonn group (J.Cammin) included in ATL-PHYS in preparationH-ZZ*->4 leptons: study on reducible bkg (lepton isolation) started by Saclay group and E.Meoni, FC

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QCD

EW

VFB analysesVFB analysesCommon features to VFB analyses: capability to distinguish between EW and QCD production processes

Central-jet veto

Forward-jet tag

Jet

Jet

Forward jets

Higgs Decay

Zeppenfeld et al

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VFB H->WW* VFB H->WW* Working plans to get control samples on bkg and check performance on main selection variables as forward jet tag and central jet veto

Main bkg is tt+jets: select a control sample with the following cuts

2 large Pt leptonsMissing Et2 tagging jets in opposite hemispheres 1 b-jet

Look into this inclusive channel and look for extra-jets (rapidity and Pt distribution of extra-jets)Other important bkg’s are WWjj (EW) and Zjj (Z->QCD)Select Z inclusive sample with Z-> 2 leptons and two tagged forward jets to study (classified by number of extra-jets):

Central jet veto performanceCapability to distinguish EW from QCD Z production

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ttH->H->bbttH->H->bb

ttH H->bb Main ideas:Use ttjj (with anti-btagged jets) to check MC capability to reproduce extra jets production and understand mass shape of the backgroundUse ttbb events outside signal mass region to get bgk normalizationWork partially reported in ATL-PHYS in preparationFirst estimate on bkg systematics derived with this method is of the order of 10%

Bonn

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H->ZZ*->4 leptH->ZZ*->4 lept

H->ZZ*->4 leptons Main ideas:Reducible bkg can be studied on tt events with one isolated lepton 2 j with Mjj=MW and at least one b-jet: study lepton isolation variables on other leptonsIrreducible bkg. From mass side-bands (statistical error below mass peak trivial to compute)Work almost finished

Saclay

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ConclusionsConclusions

The main effort of the Higgs wg group during the first data taking period will be the understanding of the detector performance by participating to the work of the calibration and sub-detector working groups

More refined analysis techniques are being developed and this is pushing us towards more exclusive and difficult channels: we are planning to include in every analysis a clear strategy on how to determine bkg systematics from the (first) collected data

VBF H->WW and very challenging channels: bkg studies started by the Wisconsin group and results should be ready by the September workshop

Probably studies on other channels like ttH with H->bb and reducible bkg in H->ZZ*-> 4 leptons will be also ready