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Low-mass Higgs Searches at the Tevatron

Low-mass Higgs Searches at the Tevatron. 4 New results in H->bb channels : ZH-> bb D0 - 0.3 fb -1 CDF - Update from 0.3 to 1 fb -1 WH-> l bb D0 - 0.4 fb -1 CDF - Update from 0.8 to 1 fb -1 ZH->llbb D0 - New Channel ! 0.4 fb -1 CDF - New Channel ! 1 fb -1.

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Low-mass Higgs Searches at the Tevatron

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  1. Low-mass Higgs Searches at the Tevatron 4 New results in H->bb channels : ZH-> bb D0 - 0.3 fb-1 CDF - Updatefrom 0.3 to 1 fb-1 WH-> lbb D0 - 0.4 fb-1 CDF - Update from 0.8 to 1 fb-1 ZH->llbb D0 - New Channel !0.4 fb-1 CDF - New Channel ! 1 fb-1 L=1 fb-1 L=1 fb-1 Ben Kilminster Ohio State University/CDF for CDF/D0 L=1 fb-1 ICHEP 2006: Tev Low-mass Higgs

  2. Standard Model mass generation via Higgs tR tR <H0> <H0> tL • Mass ~ Inertia : how hard it is to move free quark or lepton • Mass caused by transition between left-handed fermion to right-handed particle via Higgs field, H0 • For instance, top quark mass, Mt: Mt = tR<H0>tL ICHEP 2006: Tev Low-mass Higgs

  3. What we know about Higgs Expected Higgs Mass • Required Higgs boson not yet discovered !! “Standard Model” (SM) • Simplest Higgs mechanism possible • Higgs is 1 particle • H • spin 0 • electrically neutral • interacts with all SM particles • couples more strongly with higher mass particles • LEP Direct : • MH > 114 GeV @ 95% • New CDF/D0 top mass (174.1  2.1 GeV) & new LEP W mass (80.392  0.029 GeV) • MH = 85 +39-28 GeV • MH < 166 GeV @ 95 % CL LEP EWWG Low mass Higgs Favored !! SM not wrong yet ! ICHEP 2006: Tev Low-mass Higgs

  4. What we know about Higgs Decay by mass [GeV] Production (pp @ 1.96 TeV c.o.m.) Decay Excluded Low mass region: MH < 135 GeV H  bb dominates WH & ZH - easier to identify than gg -> H 95% CL Most likely MH 68% CL ICHEP 2006: Tev Low-mass Higgs

  5. Fermilab’s Tevatron • World’s highest-energy particle collisions • ~4 miles circumference protons-antiprotons • 2 multi-purpose detectors: D and CDF • Run I (1992-1996) • s = 1.8 TeV • Integrated luminosity 120 pb-1 • Run II (2001-present) • s = 1.96 TeV • Integrated luminosity by July ‘06: • Delivered > 1.6 fb-1 • Higgs analyses use up to 1 fb-1 • Design goal of 8 fb-1 by 2008 Good slope after shutdown! 1 fb-1 delivered May 2005 July, 2006 ICHEP 2006: Tev Low-mass Higgs

  6. Review of low mass Higgs channels ZH l+l- bb WH lbb 2 b jets ~ 1/2 MH each 2 leptons ~ 50 GeV each Z mass constraint Cleanest signal ZH    bb 2 b jets ~ 1/2 MH each 1 lepton ~ 50 GeV each Missing ET ~ 50 GeV Highest production X-sec 2 b jets ~ 1/2 MH each 0 leptons Missing ET ~ 100 GeV Largest expected signal ICHEP 2006: Tev Low-mass Higgs

  7. B-Tagging Techniques • All channels have 2 jets originating from b quarks • Require one or both to be “b-tagged” Algorithm exploits long b lifetime and large mass to look for displaced vertices or tracks with impact parameter “Mistags” of tagged light-quark jets can be understood from “negative tags” Negative tag (wrong side) Positive tag (right side) Interaction point primary vertex 2nd vertex Interaction point 2nd vertex Lxy < 0 Lxy > 0 Charm-jets and mistagged jets can be controlled by strictness of cut on LXY / XY

  8. B-Tagging Techniques at CDF B-Tag Efficiency Light quark mistag rate (Positive Tag) (Negative Tag) Can improve purity with a Neural Network trained to discriminate b from c and light jets ICHEP 2006: Tev Low-mass Higgs

  9. Identifying bb resonances : D0 • Z-> bb • H->bb benchmark • Can be used to determine b-jet energy scale • New D0 analysis finds evidence for Z->bb in dijet data • Background derived from data • 1168 Events in peak (300 pb-1) • MZ = 81.0  2.2 GeV measured • 83  2 GeV expected (from MC) ICHEP 2006: Tev Low-mass Higgs

  10. ZH ->  bb ZH    bb 2nd jet 180o 2 b jets ~ 50 GeV each 0 leptons Missing ET ~ 90 GeV Most expected signal Fake Missing ET 1st jet Tev’s most sensitive Channel Most difficult background: Di-jet QCD ICHEP 2006: Tev Low-mass Higgs

  11. ZH MET+bb at CDF • Mjj in EWK control region: • one lepton • met away from second jet # Leptons • MjjSignal region: • no leptons • met away from second jet • MET in QCD control region: • no leptons • met close to second jet (MET, J2) • Improvements : (S/√B)2=6.3 gain in Lum. • Includes WH -> lbb ( lepton not detected) • Improved EWK lepton veto • Dijet mass fit separately 1-tag, 2-tags • ZH / SM = 14 for MH : 115 GeV L=1 fb-1

  12. ZH->  bb D0 • Instrumental background (from energy mismeasurement) in signal region understood by parameterization of Met Result: Dijet mass fit in 1 b-tag & 2 b-tags L = 261 pb-1 ZH < 3.4 pb for MH : 115 GeV ICHEP 2006: Tev Low-mass Higgs

  13. WH -> l  bb WH lbb Most difficult background: W+bb jet production 2 b jets ~ 50 GeV each 1 lepton ~ 40 GeV each Missing ET ~ 40 GeV WH Highest production X-sec ICHEP 2006: Tev Low-mass Higgs

  14. WH->l bb CDF • Variety of b-jet identification scenarios • optimized to find the best a priori limit • BEST : Separate 1-tag + NN-tag • and 2-tag scenario L=1 fb-1 Result: Dijet mass fit WH < 3.4 pb for MH : 115 GeV ICHEP 2006: Tev Low-mass Higgs

  15. WH->lbb D0 Result: Dijet mass fit in 1 b-tag & 2 b-tags L = 378 pb-1 ZH < 2.4 pb for MH : 115 GeV ICHEP 2006: Tev Low-mass Higgs

  16. ZH -> l+l- bb ZH l+l- bb 2 b jets ~ 50 GeV each 2 leptons~ 40 GeV each Z mass constraint Cleanest signal ICHEP 2006: Tev Low-mass Higgs

  17. ZH->llbb D0 • New analysis with 389 pb-1 (Z->ee), 320 pb-1 (Z->+-) Dijet mass after 2 b-tags Dijet mass before b-tagging ZHM=115 = 0.1 evts Result: Dijet mass fit ZH < 7.9 pb (Z->ee) ZH < 11 pb (Z-> ) for MH = 115 GeV Total BKG : 13 evts ICHEP 2006: Tev Low-mass Higgs

  18. ZH->llbb CDF Method 2D Neural Net Discriminant (1,1) TT Fakes Z+jets ZH ZZ, ZW (1,0) (0,0) Training on : TT,ZH,Z+jets Allow other shapes to fall in place: Fakes, ZZ, ZW • 2D Neural Network trained to separate Signal from Background • Z+jets vs. ZH “x” axis (85% BKG) • ZH vs. ttbar “y” -axis (8% BKG) • Optimized design with 9 inputs ICHEP 2006: Tev Low-mass Higgs

  19. ZH->llbb CDF results Results in Data : ee,  combined (1,1) TT ZH vs TTBAR axis Fakes ZH Z+j ZZ, ZW (1,0) (0,0) Expected : 103 +- 17 Observed : 104 events Result: Entire 2D distribution fit Brand new result : 1 fb-1 ZH < 2.2 pb @ 95% CL for MH : 115 GeV L=1 fb-1 Note: ZH * 5 ! ICHEP 2006: Tev Low-mass Higgs

  20. Summary of Observed limits ICHEP 2006: Tev Low-mass Higgs

  21. Summary Accelerator Division, CDF, and D0 working together against the clock ! • CDF/D0 fully exploring all Low Mass Higgs • ZH -> l+ l- bb channel added by both CDF and D0 • CDF has updated WH-> l bb, ZH -> llbb with 1 fb-1 • Experimental techniques providing factors of equivalent luminosity Limits will improve with luminosity and smarts ! 4 - 8 fb-1 can find us a light Higgs Projected Luminosity 8 4 0 400 fb-1 2009 Now

  22. BACKUPS ICHEP 2006: Tev Low-mass Higgs

  23. Higgs: ZHllbb ICHEP 2006: Tev Low-mass Higgs

  24. Summary CDF & D0 Preliminary

  25. SM / MSSM Compatibility If MSSM is theory, is it worth looking for SM Higgs ? • For MA > 200 GeV • light MSSM Higgs h behaves like SM Higgs • Wh and Zh couplings same as WH and ZH • H branching ratios same as h • SM searches valid • If only one Higgs accessible at Tevatron/LHC, LC may be required to distinguish SM from MSSM(Carena, Haber, Logan, Mrenna Phys.Rev.D65 055005, 2002) • MA < 200 GeV • For large tan (> 3), SM-like Higgs is suppressed • Discovery potential mainly in MSSM • Small tan, SM searches valid ICHEP 2006: Tev Low-mass Higgs

  26. CDF sees Zbb decays in Run 2 Double b-tagged events with no extra jets and a back-to-back topology are the signal-enriched sample: Et3<10 GeV, DF12>3 Among 85,784 selected events CDF finds 3400±500 Zbb decays - signal size ok - resolution as expected - jet energy scale ok! This is a proof that we are in business with small S/N jet resonances! CDF expects to stringently constrain the b-jet energy scale with this dataset ICHEP 2006: Tev Low-mass Higgs

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