Run II Standard Model Higgs Searches at DØ

1. Run II Standard Model Higgs Searches at DØ Gavin Davies Imperial College London On behalf of the DØ Collaboration SUSY2004 Epochal Tsukuba, Tsukuba, Japan

2. Chicago  p p 1.96 TeV Booster p CDF DØ Tevatron p p source Main Injector & Recycler Outline • Introduction • Tevatron • DØ detector • Higgs production / sensitivity • Results • Wbb / WH production • s(Z+b) / s(Z+j) ratio • HWW l+l-nn searches (Non-SM results – see A. Turcot) • Conclusion 36×36 bunches 396 ns bunch crossing Thanks to all my DØ colleagues Gavin Davies (Imperial)

3. Tevatron Performance - I 79% Average Efficiency 89% Current Efficiency Gavin Davies (Imperial)

4. Tevatron Performance - II FY04 integratedluminosity Measured Lumi 05/09/04 Gavin Davies (Imperial)

5. DØ detector “Build on existing strengths by improving the tracking and triggering” • New (tracking in B field) • Silicon tracker • Fibre tracker • Preshowers • Solenoid • Upgraded • Muon system • Calorimeter • DAQ/ trigger All needed for Higgs searches Gavin Davies (Imperial)

6. SM Higgs production • Cross sections small: 0.1 - 1 pb • MH 135 GeV: decay to bb • gg  H: QCD background too large • HW & HZ associated production • lower (but still large!) bkgd. • Best channels: WH→lνbb, ZH→ννbb, ZH→llbb • MH >135 GeV: decay to WW • gg  H WW(*)l+l- • BR small H  bb H  WW(*) Dominant decay modes Gavin Davies (Imperial)

7. No systematics SM Higgs sensitivity • New CDF+DØ in 2003 • More detailed simulation • More refined analysis • Improved sensitivity • Global fit – light Higgs • MH = 117 GeV • MH < 251 GeV at 95% C.L. +67 • LEP limit from direct searches • MH > 114.4 GeV at 95% C.L. -45 Gavin Davies (Imperial)

8. Cross Sections (fb) Higgs The Challenge • QCD & EWK cross sections much larger • Needle in the haystack… • Good understanding of detector, trigger.. • Good understanding of SM backgrounds • Use data & MC to estimate them • Useful: new enhanced LO MC’s (Alpgen) • Study background processes • Likelihood methods, NN’s require excellent modeling of backgrounds • Look at channels beyond SM (SUSY) with enhanced cross section (A. Turcot) Gavin Davies (Imperial)

9. b-tagging efficiency vs light quark mis-tag rate DØ RunII Preliminary b-jet tagging • Essential for Hbb searches Can b-tag up to |η| < 2.5 IP • Use track impact parameter (IP) measurements or secondary vertex reconstruction • Several algorithms available • Performance being improved (Also have on-line b-tagging) Gavin Davies (Imperial)

10. W(en)bb associated production - I • Background to WH search • Event selection • Central isolated e, pT > 20 GeV • Missing ET > 25 GeV • ≥ two jets: ET > 20 GeV, |h| < 2.5 • 2587 events (Lint=174 pb-1) • MC: Alpgen & Pythia • + detailed detector response • Cross sections normalized to MCFM NLO calculations Good understanding of data Gavin Davies (Imperial)

11. At least two b-tags W(en)bb associated production -II • Require jets to be b-tagged • Use various b-tag algorithms • All yield consistentresults At least one b-tag • Observe 8 events, expect 8.3±2.2 • Bkgd. dominated by top events Good agreement between data and MC Gavin Davies (Imperial)

12. 115 GeV W(en)bb associated production -III • Exactly 2 jets: suppress top production • Observe 2 events, expect 2.5±0.5 • Sample composition • Set limit on production of s(Wbb) < 20.3 pb(95%c.l.) s(WH) x B(Hbb) < 12.4 pbfor MH = 115 GeV(95%c.l.) • Systematics Gavin Davies (Imperial)

13. Motivation Background to ZH search Benchmark for SUSY Higgs production via gbbh Probes PDF of the b-quark Examples of ZQ (Zj) LO diagrams Measure cross section ratio s(Z+b)/s(Z+j) Many uncertainties cancel Z(ee/mm)bassociated production - I • Lumi of 184 (ee), 152 (mm) pb-1 • Event selection • Isolated e/m with pT > 15/20 GeV, |h| < 2.5/2.0 • Z peak for signal, side bands for bkgd. evaluation • Jet ET > 20 GeV, |h| < 2.5 • At least one b-tagged jet • MC: Pythia or Alpgen + detaileddetector response • Cross sections normalized to data • Relative b- and c-quark content from MCFM NLO calculations Gavin Davies (Imperial)

14. Z(ee/mm)bassociated production - II • Z+b/Z+j cross section ratio 0.024 ± 0.005 (stat) (syst) Theory: ~0.02 (hep-ph/0312024) • Systematics • Transverse energy spectrum of b-tagged jets • QCD and mistag bkgd. estimated from data • MC: Pythia Zb normalized to data + 0.005 – 0.004 Gavin Davies (Imperial)

15. H  WW(*)  l+l-nn (l = e, m) - I • Event selection • Isolated e/m • pT(e1) > 12 GeV, pT(e2) > 8 GeV • pT(e/m1) > 12 GeV, pT(e/m2) > 8 GeV • pT(m1) > 20 GeV, pT(m2) > 10 GeV • Missing ET> • 20 GeV (ee, em); 30 GeV (mm) • Veto on • Z resonance • Energetic jets • MC: Pythia + detailed detector response • Rates normalized to NLO cross section values • Lumi. of ~ 180 (ee), 160 (em) and 150 (mm) pb-1 Data vs MC after preselection Gavin Davies (Imperial)

16. e+ n W+ n W- e- H  WW(*)  l+l-nn (l = e, m) - II • Higgs mass reconstruction not possible due to two neutrinos • Spin correlations suppress bkgd. • Df(ll) variable is particularly useful Azimuthal angle between e and m (after event pre-selection) Higgs of 160 GeV • Charged leptons from Higgs collinear Good agreement between data and MC Gavin Davies (Imperial)

17. H  WW(*)  l+l-nn (l = e, m) - III • Signal acceptance is ~ 0.02 – 0.2 (Higgs mass/final state) • Number of events after cuts Excluded cross section times Branching Ratio • Dominant bkgd. in em sample DØ Run II Preliminary Higgs of 160 GeV Gavin Davies (Imperial)

18. Conclusions • SM Higgs searches at DØ underway • Good understanding of backgrounds • First Tevatron Run II Results • Limits set on Wbb, WH and HWW(*) production • s(Z+b) / s(Z+j) measured Many more results to come with improved statistics Gavin Davies (Imperial)