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Low-mass Higgs Searches at the Tevatron

Presenting the latest findings from the Tevatron particle collider regarding the search for low-mass Higgs boson in H->bb decay channels. Explore the importance of these results in understanding mass generation, the properties of the Higgs boson, and the implications for the Standard Model. Gain insights into the Fermilab's Tevatron, b-tagging techniques, and strategies to identify bb resonances and critical channels like ZH->ννbb and WH->lνbb. Dive into the complexities of identifying signals amidst challenging backgrounds and the advances in techniques to improve detection accuracy.

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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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