First ALTAS data taking: background estimates for Higgs searches

1. First ALTAS data taking: background estimates for 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

2. Introduction Important issues before and during ATLAS startup • First 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 PDFs • Comparison 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 g/jet separation • Muon momentum scale and resolution (Z->mm, W->mn)

3. Introduction • Jet energy scale and resolution Z(mm)j samples (including forward jets very important for VBF channels) • Central jet veto: fake jets reconstruction rate • Missing Et calibration • b-tag efficiency and light-jet rejection • t-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

4. Background estimate for Higgs searches • Third step (analysis of first few fb-1): determine errors on expected background from the collected data using background enriched control samples • Higgs working group is planning to include in all the searches the strategy to get the bkg systematic errors from data control samples Personal Comment • We will work in a difficult situation: • detector performance and calibrations will not be “perfectly” understood • MC background predictions may (very likely) not agree wit observed data • competition with CMS will be very strong • We need to prepare clear searches strategies well in advanced to survive during the first data taking period

5. SM Higgs discovery potential Non-staged detector ATLAS Preliminary 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

6. SM Higgs Discovery channels Improvements w.r.t. TDR; some remarks: • VBF channels (H->tt and H->WW*) give large improvements to discovery potential • Some of the TDR channels are now in the process of being improved with more refined analyses: H-> gg (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 mass • Redundancy 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 5s can go from ~50% to less than 1% (inclusive H->gg)

8. Background estimate for Higgs searches • For some channels main tool to estimate bkg is the use of the mass side-bands (like inclusive H->gg 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 group • ttH H->bb: first study by Bonn group (J.Cammin) included in ATL-PHYS in preparation • H-ZZ*->4 leptons: study on reducible bkg (lepton isolation) started by Saclay group and E.Meoni, FC

9. Jet Jet Forward jets QCD f EW h Higgs Decay VFB analyses • Common features to VFB analyses: capability to distinguish between EW and QCD production processes • Central-jet veto • Forward-jet tag Zeppenfeld et al

10. 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 leptons • Missing Et • 2 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->tt 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 performance • Capability to distinguish EW from QCD Z production

11. ttH->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 background • Use ttbb events outside signal mass region to get bgk normalization • Work partially reported in ATL-PHYS in preparation • First estimate on bkg systematics derived with this method is of the order of 10% Bonn

12. H->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 leptons • Irreducible bkg. From mass side-bands (statistical error below mass peak trivial to compute) • Work almost finished Saclay

13. Conclusions • 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 tt 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