A Search for Higgs Decaying to WW (*) at DØ

1. A Search for Higgs Decaying to WW(*)at DØ presented by Amber Jenkins Imperial College London on behalf of the D Collaboration Meeting of the Division of Particles and Fields University of California, Riverside, August 30th 2004

2. Overview • The DØ experiment in Run II • Why HWW(*)? • Event signature • Selection criteria • Comparison of Monte Carlo and data • The  x BR(HWW(*)) limit • Conclusions Amber Jenkins Imperial College London

3. The Upgraded DØ Detector • Completely new tracking system, inside 2T magnetic field • Inner Si vertex detector (SMT) provides b-tagging capability • Excellent Run I calorimetry exploited in Run II • Upgraded 3-tier trigger and data acquisition system Amber Jenkins Imperial College London

4. Higgs Searches at the Tevatron SM Higgs decay • Search strategy: • for a light Higgs (MH <135 GeV) - use associated WH or ZH production - dominant decay is Hbb • for a heavy Higgs (MH >135 GeV) - use ggH production - dominant decay is HWW(*)-the WW leptonic signature is cleaner 135 GeV Hbb HWW(*) For a heavy Higgs HWW(*) offers a cleaner decay signal Amber Jenkins Imperial College London

5. Cross-Section Enhancement From Extra Generations E. Arik et al, SN-ATLAS-2001-006 • Extra quark generations could increase production cross section significantly • Enhancement factor depends on Higgs and quark masses • A factor of 8.5 is expected for a 4th generation, m4=320 GeV Amber Jenkins Imperial College London

6. HWW(*): Experimental Signature • Higgs mass reconstruction not possible due to two neutrinos • Spin correlations suppress background:  WW comes from spin 0 Higgs • leptons prefer to point in the same direction • Event signature: 2 high pT leptons + missing ET • Search for excess in the leptonic decay modes: ee, e and  e+ ne W+ Look for small opening angle between leptons (ll) ne W- e- Amber Jenkins Imperial College London

7. Data and Monte Carlo Samples • Analysed data collected between April 2002 and September 2003  Final integrated luminosities of 177 pb-1 (ee), 158 pb-1 (e) and 147 pb-1 () for each final state • Removed runs with hardware failures and incomplete luminosity information • Monte Carlo events were generated using Pythia or ALPGEN followed by a full detector simulation • rates normalized to NLO cross section values; • events overlaid with an average of 0.8 minimum bias events. Amber Jenkins Imperial College London

8. Cuts optimised for each final state Event Selection Event selection includes: • Require two oppositely charged isolated leptons l = e, m • ee: pT(e1) > 12 GeV, pT(e2) > 8 GeV • em: pT(e) > 12 GeV, pT(m) > 8 GeV • mm: pT(m1) > 20 GeV, pT(m2) > 10 GeV • Missing ET greater than: 20 GeV (ee,em); 30 GeV (mm) • Removal of mass resonances: • 12 GeV < m(e,e) < 80 GeV • m(m,m) > 20 GeV and |m(m,m)- mZ| > 15 GeV • Azimuthal opening angle: Df(e,e) < 1.5; Df(e/m,m) < 2.0 • Jet veto  rejects energetic jets: • ee, em : ET1 < 90 GeV or ET1 < 50 GeV, ET2 < 30 GeV - mm: ET1 < 60 GeV, ET2 < 30 GeV  Beat down Z/*, W+jets, WW & tt backgrounds Signal acceptance is ~ 0.02 – 0.2 depending on the Higgs mass/final state Amber Jenkins Imperial College London

9. Data vs. Monte Carlo:Opening Angle, e After basic event preselection After final event selection e Final State 160 GeV Higgs Amber Jenkins Imperial College London

10. Data vs. Monte Carlo:Dilepton Invariant Mass Distributions after basic event preselection ee Final State  Final State 160 GeV Higgs Amber Jenkins Imperial College London

11. Data vs. Monte Carlo: Missing ET Distributions after basic event preselection ee Final State  Final State ETmiss (GeV) 160 GeV Higgs Amber Jenkins Imperial College London

12. Data vs. Monte Carlo Amber Jenkins Imperial College London

13. The HWW(*) Cross-Section Limit • Cross-section limits have been calculated at 95% C.L. in each leptonic channel by counting events • Combine likelihood functions from each channel to obtain overall result • Upper limit on  x BR(HWW(*)) set: • Agrees well with predictions from NLO calculations  x BR(HWW) lies between 6.6 and 40.1 pb, depending on MH all channels Amber Jenkins Imperial College London

14. Conclusions • D has performed a search for HWW(*) in leptonic final states using 147-177 pb-1 of Run II data • Comparison between data and Monte Carlo looks good • The number of events observed is consistent with expectations from Standard Model backgrounds • An upper limit on  x BR(HWW(*)) of between 6 and 40 pb has been set • result is highly consistent between all three channels • We look forward to improving the measurement with more data and higher statistics. Amber Jenkins Imperial College London

15. Back-up Slides

16. SM Higgs Decay Modes 135 GeV Hbb HWW(*) Amber Jenkins Imperial College London

17. Suppressed Couplings to b,t L. Brucher, R. Santos, hep-ph/9907434 • Occurs beyond the SM in Top Color or Fermiophobic Higgs models Increased HWW(*) branching Amber Jenkins Imperial College London

18. Higgs Sensitivity Reach Amber Jenkins Imperial College London

19. Azimuthal Opening Angle: Dfll An example: e+e- final state MC Main background Amber Jenkins Imperial College London

20. Missing ET An example: e+e- final state MC Main background Amber Jenkins Imperial College London