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Higgs Bosons and b Quarks

Higgs Bosons and b Quarks. October, 2007 SLAC ATLAS Forum Sally Dawson. Laura Reina, Chris Jackson, Doreen Wackeroth. Plan:. Lightning review of SM Higgs physics Discussion of gg→bbh vs bg →bh Emphasize understanding of theoretical assumptions MSSM results for bg →bh

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Higgs Bosons and b Quarks

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  1. Higgs Bosons and b Quarks October, 2007 SLAC ATLAS Forum Sally Dawson Laura Reina, Chris Jackson, Doreen Wackeroth

  2. Plan: • Lightning review of SM Higgs physics • Discussion of gg→bbh vs bg →bh • Emphasize understanding of theoretical assumptions • MSSM results for bg→bh • Status of current (Summer, 2007) limits • Effects of squark/gluino loops on bg→bh • Why are these effects interesting?

  3. Precision measurements limit Higgs Mass • LEP EWWG (July, 2007): • Mt=170.9  1.8 GeV • Mh=76+36-24 GeV • Mh < 144 GeV (one-sided 95% cl) • Mh < 182 GeV (Precision measurements plus direct search limit) 2007 Best fit in region excluded from direct searches

  4. Producing the Higgs at the Tevatron NNLO or NLO rates Mh/2 <  < Mh/4

  5. SM Higgs Searches at Tevatron Tevatron Expected Tevatron Observed LP07

  6. Limits understood from Branching Ratios

  7. SM Production Mechanisms at LHC Bands show scale dependence All important channels calculated to NLO or NNLO Production with b’s very small in SM

  8. SM Higgs, CMS 2007 Includes radiative corrections Higgs + b’s aren’t discovery mode for SM Higgs

  9. pp  bbh • Why is bbh interesting? • Direct measurement of b quark Yukawa coupling (enhanced in MSSM at large tan ) • Higgs discovery mode in SUSY models at large tan  • Theoretical questions about b quark parton distribution functions (PDFs) • Why do NLO corrections? • Improved theoretical reliability • Often find large numerical results

  10. Which b mass? • (bbh)(mb/v)2 • Pole mass (from  decays): mb=4.62 GeV • MS bar mass: Makes a big numerical difference which b mass you use

  11. Use MS Renormalization • Compute the (s) corrections: • Define the running b mass • Large logarithms absorbed to 2-loops Lore: MS is best! h h h

  12. Scale and Scheme Dependence at NLO • NLO calculations improve scale dependence • Scale dependence enters in running of s() and PDFs, g(), as well as s3log() contributions • Formally, scale dependence is O(s4) but may be numerically large • Large remaining scale/scheme dependence between OS and MS at NLO • Effect  10-20% Scheme dependence Scale dependence

  13. What is the dominant process for Higgs + b Production? • Answer depends on whether you tag outgoing b’s • Is there double counting when including b initial state?

  14. The b quark as a parton Absorb collinear logs in b quark distributions • Altarelli-Parisi evolution of PDFs sums snlnn(2/mb2) • b quark PDF sln(2/mb2) relative to gluon PDF

  15. Two Schemes for PDFs: • 4 flavor number scheme (also called fixed flavor number scheme) • No b quarks in initial state • Lowest order process involving Higgs and b’s is ggbbh • 5 flavor number scheme (also called variable flavor number scheme) • Define b quark PDFs (absorbs large logarithms) • Higgs produced with no pT at lowest order (bb h) • Higgs pT generated at higher orders in expansion vs

  16. Counting Rules with b PDFs: Reordering of perturbation expansion (sln(Mh2/mb2))2.4 s2ln(Mh2/mb2).06 s2.01

  17. Re-ordering of Perturbation Theory • 0 b tag process in 5FNS: • LO: bb→h O(s2b2) • NLO: Virtual+real corrections O(s3b2) • NLO: bg →bh O(s2b) ,correction of O(1/ b) to tree level • NNLO: gg →bbh O(s2), correction of O(1/b2) to tree level • 1 b tag process in 5FNS: • LO process is bg→bh: Tree level, O(s2b) • NLO includes new subprocess: gg →bbh, O(1/ b) correction to LO b=log(Mh2/mb2)

  18. Inclusive Cross Section for bb h: 0 b tags bb h vs gg  bbh Almost no scale dependence at NNLO It really doesn’t matter which PDF scheme you use Harlander & Kilgore, hep-ph/0304035 Campbell et al, hep-ph/0405302

  19. What if only 1 b is tagged? 4FNS: gg→bbh pTb> 20 GeV || < 2 (Tevatron), 2.5 (LHC) R > 0.4

  20. What about distributions?Compare 4 and 5 Flavor Number PDF Schemes Higgs plus single b at LHC: NLO pTb> 20 GeV || < 2.5 R > 0.4

  21. Good Theoretical Understanding of Uncertainties

  22. Higgs in the MSSM • MSSM has 2 Higgs doublets: Hd and Hu • Physical CP-Even Higgs bosons • Pseudoscalar, A0, and two charged Higgs, H

  23. Higgs Couplings very different in MSSM Light Higgs Heavy Higgs Couplings to d, s, b enhanced at large tan  SM Couplings to u, c, t suppressed at large tan  Decoupling limit

  24. Large tan  Changes Relative Importance of Production Modes b, t h tan=1 tan=40 tan=7 tan ≥ 7, bb production mode dominates Kilgore

  25. Production of SUSY Higgs Bosons • For large tan , dominant production mechanism is with b’s • bbh can be 10x’s SM Higgs rate in SUSY for large tan  LHC tan=30 tan=3 Mh/H (GeV) SUSY Higgs are produced with b’s! *Maximal Mixing

  26. Enhancement in MSSM Note log scale! This is why the calculation is interesting! eff from FeynHiggs with MSUSY=Mg= =M2=1TeV, Ab=At=25 GeV Can observe heavy MSSM scalar Higgs boson

  27. Single b tag d/dh (fb/GeV) d/dH (pb/GeV) LHC Tevatron h H MSSM with Mh=MH=120 GeV, tan =40

  28. Single b tag NLO pTh (GeV) pTH (GeV) MSSM with Mh=MH=120 GeV, tan =40

  29. Higgs Decays also affected at large tan  • MSSM: At large tan , rates to bb and +- stay large • SM: Higgs branching rates to bb and +- turn off as rate to W+W- turns on (Mh > 160 GeV) Heavy H0 MSSM BRs Rate to bb and +- almost constant in MSSM A0 MSSM BRs SM

  30. MSSM limits from bg→bh (1 fb-1) LP, 2007 30 fb-1 CMS expects to get to tan  ~ 15 through bh; h→+, h→bb

  31. A Reliable Prediction • We have bg →bh at QCD NLO • PDF/scale/scheme uncertainties ~ 10-20% • Are squark/gluino contributions relevant? • Important for bb→H, A at LHC • For some parameters as large as -50% effects • Squark/gluino effects almost completely described by Improved Born Approximation Dittmaier et al, hep-ph/0611353

  32. Need SQCD for Reliable Predictions

  33. Squark/gluino loops important for large tan  and small MSUSY Note slow approach to decoupling limit for large tan

  34. gb→b Dawson, Jackson, hep-ph/0709.4519

  35. Can gb→b +jet be useful? • LHC More soon…..

  36. Conclusions • In the MSSM Higgs and b quarks go together at large tan  • Higgs production with b’s is dominant mechanism for tan  > 7 • Theoretical understanding of b PDFs: compatible answers in 4FNS and 5FNS for PDFs • SUSY QCD corrections can be the same size as QCD corrections

  37. Jan, 2007

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