SM Higgs Boson Searches at the CDF Experiment

1. SM Higgs Boson Searches at the CDF Experiment Luca Scodellaro Instituto de Fisica de Cantabria for the CDF Collaboration CORFU2005 Research Training Network Meeting 8th Hellenic Summer School on EPP 4th-11th September, Corfú (Greece)

2. Outline • Introduction What We Know and What We Can Do • Low Mass Higgs Searches WH→lvbb, ZH→vvbb, ZH→llbb • High Mass Higgs Searches H→WW, WH→WWW • Conclusions Summary and Perspectives _ _ _ _

3. What We Know about the Higgs • Direct searches at LEP: • Indirect limits from radiative corrections to W boson and top quark masses: MH≥114.4 GeV/c2 @ 95% CL MH =98 +52-36 GeV/c2 MH≤208 GeV/c2 @ 95% CL

4. Higgs Production at the Tevatron • Direct production via gluon fusion (main mechanism) • Associated production with a gauge boson (W/Z) Production Cross Section (pb)

5. Higgs Boson Decay Modes - 114.4<MH<135GeV/c2 H->bb dominating MH<114.4 GeV/c2 Excluded MH>135GeV/c2 H->W+W- dominating

6. What We Can Do at CDF • High Mass Higgs Searches - High background discrimination from leptonically decaying W’s - High cross section of Higgs direct production can be exploited • Low Mass Higgs Searches - H→bb decay hard to reconstruct over QCD jet production - Higgs associated production with a gauge boson provides the best experimental signature

7. Low Mass Higgs Searches _ • Based on H→bb decay reconstruction -High efficiency b quark tagging needed - Dijet mass most discriminating parameter • W/Z boson identification to reject background -Lepton identification and missing ET reconstruction needed to trigger on W±→l±ν, Z→νν and Z→l+l- decays _

8. b Quark Tagging • b quark hadronize producing long lived B mesons • B can travel before decaying Displaced tracks coming from a secondary vertex are looked for inside the jets • Efficiency for central jet ~42% False tag rate typically ~0.5%

9. _ W±H0→l±νbb Searches • Event selection: Isolated electron or muon (ET>20 GeV) Missing ET>20 GeV Vetos on Z0, g→e+e-, cosmics Two central jets (ET>15 GeV) At least one tagged jet • Sample composition is well understood 187 events observed in 319 pb-1 of data

10. _ W±H0→l±νbb Searches • Dijet mass resolution 18% • Fit to the dijet mass distribution allow to extract 95% CL limits on WH cross section • Limits range from 10 to 3 pb for MH from 110 to 150 GeV/c2 • Good agreement with a priori expectation

11. _ _ Z0H0→vvbb Searches • Event selection: Missing ET>70 GeV ΔΦ(ET, 2nd jet)>0.4 Veto on isolated leptons Two jets (ET>25 GeV) At least one tagged jet • Background estimation: QCD production from events failing ΔΦ cut Top and EWK processes from events with leptons

12. _ _ Z0H0→vvbb Searches • Good agreement data vs prediction in control regions: • Further background rejection provided from: • Leading jet and ET separation:ΔΦ(ET, 1st jet)>0.8 • HT significance: HT/HT>0.6 • ET of the leading jet: 1st jet ET>60 GeV • Cut on the dijet invariant mass (dependent on MH)

13. _ _ Z0H0→vvbb Searches • Final limits on ZH production cross section No Z→vv B.R. included

14. _ Z0H0→l+l-bb Searches • We did not look at data yet (blind analysis) • Event Selection: Z0→l+l- reconstructed 2 or 3 jets, ≥1 tagged Missing ET<50 GeV • Neural Network S/B improved from ~0.01 to ~0.1 • Extrapolated limit from RunI: 3.1pb (MH=120 GeV) ZH Z+2partons Z→μμ+2partons Zbb

15. High Mass Higgs Searches • Based on H0→W+W- decay - W±→l±v strong experimental signature • gg→H0→W+W- - high production cross section - opposite charged leptons and missing ET • W±H0→W±W+W- - smaller production cross section - same sign leptons→ very low background

16. gg→H0→W+W- Searches • Event selection: Two opposite charged leptons Missing ET>25 GeV Veto on jets to remove top • SM Higgs boson is a scalar (S=0): charged leptons preferentially aligned Mll cut to select Higgs production Mll<55-80 GeV for MH=140-180 GeV

17. gg→H0→W+W- Searches • 8 events observed in 200pb-1(8.9±0.1 expected) Mll<80GeV • From ΔΦlldistribution limits on H->WW production are derived • Limits ranges from 17.8 to 6.4 pb for Higgs boson mass from 140 to 180 GeV

18. W±H0→W±W*W*→l±l±X • Event selection begins requiring two same sign leptons (PT>20 and 6 GeV/c respectively) • Good agreement data-background expectations

19. W±H0→W±W*W*→l±l±X • A signal region is defined in the plane defined by the second lepton PT and the vector sum of the two lepton PT’s • 0 events are observed in the signal region while 0.95±0.8 are expected from background • Limits on WH production range from 12 to 8 pb for Higgs mass MH between 110 and 200 GeV

20. Summary of the Limits CDF and D0 Results SM Predictions

21. What Can We Do More? • Add more data (1fb-1 for winter 2006 are ready) • Understand background - more control samples - dijet mass fit instead of counting - neural networks to discriminate signal to background • Increase acceptances - forward electrons and taus not used yet • Improve b quark tagging - forward tagging still in progress - more algorithms: soft leptons and jet probability • Improve dijet mass resolution

22. Dijet Mass Resolution • We are still using RunI jet reconstruction • Work is in progress on - Associate tracks and calorimeter towers - b-specific corrections - Advanced multivariate techniques • Preliminary results: - σM/M~10% achievable

23. Perspectives • With all these tools we can get here: • We can reach 3σ evidence over significant preferred Higgs mass range Design Lum. Base Lum.

24. Back Up Slides

25. The Tevatron Collider

26. The CDF Detector

27. _ Z→bb Reconstruction

28. Background Cross Sections • W+jets: σmea = 2775±10(stat)±53(syst)±167(lum) pb σthe = 2687±54 pb • Z+jets: σmea = 254.9±3.3(stat)±4.6(syst)±15.2(lum) pb σthe = 251.3±5.0 pb • WW: σmea = 14.6+5.8-5.1(stat)+1.8-3.0(syst)±0.9(lum)pb σthe = 13.25±0.25 pb • ZZ: σmea (WZ+ZZ) < 15.2 pb σthe = 1.58±0.02 pb • WZ: σmea (WW+WZ)< 40 pb σthe = 3.06±0.06 pb

29. Integrated Luminosity at CDF