Status of ATLAS Higgs Search

Transcription

1 Canada s National Laboratory for Particle and Nuclear Physics Laboratoire national canadien pour la recherche en physique nucléaire et en physique des particules Status of ATLAS Higgs Search Reda Tafirout TRIUMF On behalf of the ATLAS Collaboration Winter Workshop on Nuclear Dynamics 2012, Puerto Rico Owned and operated as a joint venture by a consortium of Canadian universities via a contribution through the National Research Council Canada Propriété d'un consortium d'universités canadiennes, géré en co-entreprise à partir d'une contribution administrée par le Conseil national de recherches Canada

2 Outline Standard Model Higgs production & decay at the LHC ATLAS Detector and data taking at 7 TeV Higgs search strategies & specifics Combined results & exclusion limits 2

3 LHC (I) Fundamental ingredient of the Standard Model Pre-LHC era: MH<114.5 GeV (LEP), and 156< MH<177 GeV (Tevatron), excluded at 95% C.L. Several production mechanisms Primary goal of the LHC experimental program gluon fusion Vector boson fusion (VBF) gg fusion VV fusion tth WH ZH arxiv: , arxiv: Associated with W / Z Associated with top 3

4 LHC (II) Standard Model Higgs properties are well known Only unknown is MH: dictating different low / high mass search strategies arxiv: , arxiv:

5 LHC Superb LHC performance! Data quality requirements: fb-1 Well suited for Higgs searches with very good tracking and calorimetry (e, γ, μ, and jets) Data distributed and analyzed on a Computing Grid infrastructure (WLCG) 5

6 Higgs search strategies Twelve decay channels / topologies considered: ATLAS-CONF arxiv: Combination: profile likelihood ratio test statistic for signal strength μ=σ/σsm (Eur.Phys.J.C71:1554,2011) Full likelihood includes systematics and correlations Exclusion limits on μ based on CLs method (J. Phys. G 28 (2002) ) 6

7 (I) Selection: two isolated photons, ET>40 GeV (25 GeV) for leading (sub-leading) Backgrounds: γγ, γ+j, j+j with jets passing γ selection 9 event classifications with different S/B and mγγ resolutions: converted / non-converted, pseudo-rapidity regions, kinematics of diphoton system 7

8 (II) mγγ distribution used for the limit setting (resolution crucial). Combining event classifications Largest excess at GeV Significance: 2.8σ local, 1.5σ global (entire range) Observed exclusion: GeV, GeV 8

9 (I) Selection: two same flavor opposite sign lepton pairs 4e, 2e+2μ, 4μ (pt> 7 GeV with two leading pt> 20 GeV). Backgrounds: ZZ, Z+jets, ttbar Cuts on m12 and m34 (mh dependent) (control regions) 9

10 (II) m4ℓ distribution used for the limit setting (good resolution in low mass region / Higgs width small) Observed exclusion: GeV, GeV, GeV, GeV. Largest excess at mh=125, 244 and 500 GeV, with local significances of 2.1σ, 2.2σ and 2.1σ Comparable excess as observed in H γγ mode at ~125 10

11 Selection: two isolated e or μ with pt>20 GeV within mz window and missing transverse energy (ETmiss) Refined for different mh regions and data taking periods Background: ZZ, WZ, WW, top, W+jets, Z+jets mt distribution (from ℓℓ and ptmiss) for the limit setting No significant excess observed, exclude GeV 11

12 Selection: two isolated e or μ with pt>20 GeV and two jets with ET> 25 GeV within Z mass, and ETmiss <25 GeV. Refined for low/high Higgs mass and use of b-tagging Background: Z+jets, top, dibosons, QCD multijets mℓℓjj distribution used for the limit setting No significant excess observed, exclude GeV, GeV. 12

13 (I) Selection: two isolated ee, μμ, or eμ with pt>25 (15 GeV) for leading (sub-leading) with ETmiss and Z veto. Separate 0, 1 and 2 jets (VBF) Background: WW, top, Z+jets, W+jets with data driven normalizations using control samples Further topological cuts and b-jet veto for top rejection. 13

14 (II) mt distribution used for the limit setting No significant excess, GeV range excluded Expected exclusion range: GeV (if no SM signal) 14

15 Selection: isolated e or μ, two jets, and ETmiss Extra jets (in addition to above) are used to separate gluon fusion and VBF Backgrounds: W+jets, Z+jets, QCD multijets, top, dibosons mℓνjj used for the limit setting 15

16 (I) Selection: three event categories (leptonic and hadronic decays): Isolated leptons and τ hadronic candidate identified via narrow jets and low track multiplicity Separate 0, 1 and 2 jets (VBF) Background: Z+ jets (mainly from Z ττ), top, dibosons, fakes 16

17 (II) For the limit setting: mττ is used (calculated using different techniques depending on event topology) Observed (expected) upper limits: between 2.5 (3.2) and 11.9 (7.9) x SM prediction 17

18 (W/Z) (I) Three event topologies: W/Z selection with two btagged jets with ET>45 (25) GeV for leading (sub-leading) Exploit W and Z recoils against H, look at separate pt and ETmiss regions Background: W/Z + jets, top, QCD multijets, dibosons 18

19 (W/Z) (II) mbb distribution used for the limit setting no significant excess, observed (expected) upper limits at 95% CL are from 2.7 (2.6) to 5.3 (5.1) x σsm 19

20 Combined Results (I) Testing μ=1 (SM Hypothesis) Testing μ=1 (SM Hypothesis) Observed exclusion at 95%: , , GeV. Expected exclusion range: GeV. 20

21 Combined Results (II) Excess observed at mh=126 GeV from and Observed local significance 2.5σ (2.9σ expected) from all channels combined. Probability of background fluctuation Best fit of signal strength μ=σ/σsm Global probability of background fluctuation: 30% in GeV and 10% in GeV range. 21

22 Conclusion Exciting new era for Higgs hunting SM Higgs seriously cornered to a very small region at 95% C.L.: GeV or GeV Mass range excluded at 99% C.L.: GeV Excess more compatible with a SM Higgs in the low mass region around 126 GeV. Need more data to confirm or deny. LHC operations in 2011 have been truly remarkable with outstanding ATLAS data taking efficiency, processing and analysis. Thanks to a few kilo-people. Looking forward for 2012 LHC running. 8 TeV collisions already started. Higgs production x.s. increase of ~20% and expected integrated luminosity ~4 times more than

23 Thank You - Merci! Acknowledgments National Research Council (NRC) Canada Foundation for Innovation (CFI) British Columbia Knowledge Development Funds (BCKDF) National Science & Engineering Council (NSERC) 4004 Wesbrook Mall Vancouver, B.C. Canada V6T 2A3 Tel: Fax:

24 EXTRA MATERIAL 24

25 SM Higgs properties Higgs branching ratios in the low mass region and width as a function of its mass 25

26 Combined Results (III) Observed and expected (dashed) 95% CL limits for all the individiual channels used: 26

27 Combined Results (IV) Limits obtained with different combinations of channels: 27

28 Event Displays (I) Diphoton event with invariant mass of GeV. 28

29 Event Displays (II) 4 muons event with invariant mass of GeV. 29