1. INCLUSIVE STANDARD MODEL HIGGS SEARCHES HIGGS SEARCHES WITH ATLAS Francesco Polci LAL Orsay On behalf of the ATLAS collaboration. SUSY08 – Seoul (Korea)

2. THE HIGGS BOSON AT THE LHC PRODUCTION This talk is based on the results of the ATLAS CSC NOTE: update on the analysis techniques and discovery potentials. Almost ready for publication. DECAY gg fusion process is the more abundant, followed by the Vector Boson Fusion process. Francesco PolciSUSY08 – Seoul2

3. Important channel in the low mass region. It gives the best mass resolution thanks to excellent electromagnetic energy resolution H0H0   Francesco PolciSUSY08 – Seoul3 H→ 

4. ATLAS preliminary p T (Gev/c)  ATLAS preliminary p T (Gev/c)  Trigger efficiency for single photonsSELECTION Trigger: at least 2 isolated photons, with p T >20 GeV/c each   (respect to offline) = (93.6 ± 0.4)% Identification cut exploiting the shower shape. Fiducial cut: 0< |  | < 1.37 & 1.52 < |  | < 2.37. Isolation cut:  p T < 4 GeV/c, considering all tracks with p T >1GeV/c in a  R=0.3 cone around the electromagnetic cluster. Momentum cut: p T >25GeV/c and p T >40GeV/c for the two most energetic photons. Selection efficiency:  = 36.0 % ( 32.2% with pileup 10 33 cm -2 s -1 ) Within a mass window M H +/- 1.4GeV: Francesco PolciSUSY08 – Seoul4

5. The primary vertex identification is crucial for a precise measurement of M(  ). Tracker and calorimeter informations are combined: The calorimeter measure the  flight direction using longitudinal segmentation and pre-shower strips → vertex position accuracy is 19mm (17mm when using conversions). Combining with the tracker information we get ~0.1 mm Z 1 Z true O Z 2 Strips Middle Photon 1 Photon 2 PRIMARY VERTEX RECONSTRUCTION preliminary Calorimeter only Calorimeter and tracker z rec -z true (mm) Francesco PolciSUSY08 – Seoul5

6. Converted photons are an important issue: ~50% of the events have at least one converted photon! energy reconstruction is degraded: ad hoc calibration required. conversion vertex can be used to compute the direction Even conversions with only one reconstructed track are taken into account. CONVERSIONS preliminary ATLAS Francesco PolciSUSY08 – Seoul6 With pileup!

7. Background is evaluated with NLO simulations. It will be measured from data sidebands. Example: NLO  -jet processesBACKGROUNDS Within a mass window M H +/- 1.4GeV: Strategy for jet rejection: Longitudinal segmentation of the calorimeter. Fine segmentation of the first layer (  -strips) => good  0 rejection. Isolation of the electromagnetic cluster. Isolation based on tracks reconstructed by the inner detector. Francesco PolciSUSY08 – Seoul7

8. INVARIANT MASSES DISTRIBUTIONS From associated production, high levels of purity for signal achieved thanks to additional requirements. ATLAS preliminary WH → lv  ZH→ ll  H+1jet H+2jet Francesco PolciSUSY08 – Seoul8 ATLAS preliminary

9. Fit and likelihood ratio are used for setting discovery potential and handle systematics. Different fit based approaches for the evaluation of the significance: 1- fit only the mass distribution; 2- simultaneous fit to the mass, P T (  ) and cos  * distributions The two approaches are performed also in the Higgs mass floating configuration. Use of categories with different resolutions based on , jet multiplicity and presence of conversion improves the significance.SIGNIFICANCE Higgs   ** In Higgs rest frame P T (  )  it has softer spectrum and less pronounced rise at low values for background respect to the signal. cos  *  the photon decay angle in the H rest frame with respect to the H flight direction in the lab rest frame has an enhanced distribution for the background for collinear photons while is flat for the signal. Francesco PolciSUSY08 – Seoul9 ATLAS preliminary

10. H → ZZ(*) → 4l It is the “golden channel”! Observation of a clear peak on top of a smooth background! Clear signature Wide range of masses explored Francesco PolciSUSY08 – Seoul10

11. muons EVENTS SELECTION Trigger efficiency Trigger: - single isolated  (e) with p T >20 (25) GeV/c; - two  (e) with p T >10 (15) GeV/c. Kinematic: - 2 pairs of same flavor opposite charge lept. - calorimeter identification - |M ll1 -M Z | M 34 Fiducial cut: 0< |  | < 1.37 & 1.52 < |  | < 2.37. Isolation cut: - Calorimeter:  E T /p T < 0.23 - tracker:  p T /p T < 0.15 Vertexing cut on maximum lepton impact parameter: d 0 /  d0 <3.5 (6.0 ) for  (e) electrons ATLAS preliminary ATLAS preliminary Francesco PolciSUSY08 – Seoul11

12. Backgrounds are (|  | 5 GeV ): qq,gg  ZZ (  )  4l (l=e,m,t) => 56 fb (NLO); qq  Zbb  4l => 810 fb (NLO ); qq  Zbb  3l => 1.3 pb (NLO ); qq,gg  tt => 833 pb (NLO); qq,gg  WZ  3l => 386 fb (NLO); Z  2l+X => 4030 pb (NNLO) Z  ee mass peak is affected electron bremsstrahlung.BACKGROUNDS Background will be estimated in sidebands  low systematic uncertainties Mass resolution @ M Z  1.8 GeV Look to the Z with first data to understand lepton reconstruction and detectors response. Francesco PolciSUSY08 – Seoul12 ATLAS preliminary

13. ISOLATION  IMPACT PARAMETER Lepton from b-quark decay do not point towards primary vertex Reducible backgrounds have activity around leptons from b-decay Transverse impact parameter significance in signal and reducible background events. Normalized calorimetric and track isolation (  R=0.2) for the signal (m H = 130) and the Zbb and tt backgrounds in the 4  channel. ATLAS preliminary Francesco PolciSUSY08 – Seoul13 ATLAS preliminary Calorimetric isolationTrack isolation H4H4 H  4e

14. Mass resolution as a function of the Higgs mass. Open circles denote the resolution Z mass constraint improves resolution. Selection efficiency as a function of the Higgs mass, for each of the three decay channels. EFFICIENCY & RESOLUTION Francesco PolciSUSY08 – Seoul14 ATLAS preliminary H4H4 H  4e Z on shellZ off shell

15. INVARIANT MASS DISTRIBUTIONS Francesco PolciSUSY08 – Seoul15 ATLAS preliminary

16. SIGNIFICANCE Preliminary ATLAS preliminary Poisson probability Significance estimations with number counting and performing a full range fit are consistent. Other approaches (background only sideband fit, two dimensional fit on M ll and M Z* with Higgs mass floated) are also explored. Francesco PolciSUSY08 – Seoul16 ATLAS preliminary

17. Interesting for 2M W <M H <2M Z where all other decay modes are suppressed. Signature is 2 , 2e, e  + E T miss. Three channels: Measure spin and CP properties possible for heavy H → WW → lvqq H → WW  - H→WW → e v  v (H+0jet) - H → WW → e v  v - H → WW → l v q q (only for M H =300 GeV) VBF (H+2jet) Francesco PolciSUSY08 – Seoul17

18. High backgrounds: WW, Wt, ttbar, Z  2l, bb,cc,QCD multijet SELECTION AND BACKGROUNDS No mass peak  use transverse mass. Reconstruction: –Trigger : single or double lepton selection 1  20i or 1e25i; –Offline: select events with exactly two isolated (tracking and calorimeter) opposite sign primary leptons and E T miss. –Specific reconstructions for different channels Francesco PolciSUSY08 – Seoul18 The challenge: precise knowledge of the backgrounds.

19. Fit the transverse mass and the transverse momentum of the candidates in two bins of the dilepton opening angle  in the transverse plane; Account for the ratio of the background in the two regions Extract the signal and background mixture in the signal region. EVENTS SELECTION: H+ 0jets Signal regionControl region preliminary The transverse mass distribution in the region  20 GeV. Francesco PolciSUSY08 – Seoul19 preliminary ATLAS preliminary

20. Two distinct signatures: 1.Two forward “tag” jets (large  separation with high-p T ) with large M jj 2.No jet activity in the central region (between the two tag jets): jet veto Typical cuts require: Tag jets are assumed to be the highest E T jets in opposite hemispheres, with E T >~40 GeV,  jj >~4, M jj >500-1000 GeV. Higgs decay products between tag jets in  No additional jet activity in the event: central jet veto. EVENTS SELECTION FOR VBF Francesco PolciSUSY08 – Seoul20

21. Expected significance from the H→WW+0j, WW→e  channel for an integrated luminosity L= 10 fb -1. Expected significance from the H→WW+2j, WW→e  channel for an integrated luminosity L= 10 fb -1.SIGNIFICANCE preliminary Preliminary Francesco PolciSUSY08 – Seoul21 preliminary

22. The median p-value for the SM Higgs Boson exclusion as a function of the Higgs mass for the various channels and for their combination. The expected luminosity for the exclusion of a Higgs Boson with mass m H @ 95% CL COMBINATION OF THE CHANNELS Zooming… ATLAS IS WELL READY TO CATCH THE HIGGS! Francesco PolciSUSY08 – Seoul22 ATLAS preliminary