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. DPF Meeting 2004Amber Jenkins Imperial College London 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

3. DPF Meeting 2004Amber 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

4. DPF Meeting 2004Amber Jenkins Imperial College London 4 Higgs Searches at the Tevatron Search strategy: for a light Higgs (M H <135 GeV) - use associated WH or ZH production - dominant decay is H  bb for a heavy Higgs (M H >135 GeV) - use gg  H production - dominant decay is H  WW (*) - the WW leptonic signature is cleaner 135 GeV H  WW (*) SM Higgs decay For a heavy Higgs H  WW(*) offers a cleaner decay signal H  bb

5. DPF Meeting 2004Amber Jenkins Imperial College London 5 Cross-Section Enhancement From Extra Generations 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 4 th generation, m 4 =320 GeV E. Arik et al, SN-ATLAS-2001-006

6. DPF Meeting 2004Amber 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 p T leptons + missing E T Search for excess in the leptonic decay modes: ee, e  and  W+W+ e+e+ W-W- e-e- e nene (ll) Look for small opening angle between leptons  (ll)

7. DPF Meeting 2004Amber 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.

8. DPF Meeting 2004Amber Jenkins Imperial College London 8 Event Selection Event selection includes: Require two oppositely charged isolated leptons l = e,  – ee: p T (e 1 ) > 12 GeV, p T (e 2 ) > 8 GeV – e  p T (e) > 12 GeV, p T (  ) > 8 GeV –  p T (  1 ) > 20 GeV, p T (  2 ) > 10 GeV Missing E T greater than: 20 GeV (ee,e  ); 30 GeV (  Removal of mass resonances: –12 GeV < m(e,e) < 80 GeV –m  GeV and |  m(  )- m Z | > 15 GeV Azimuthal opening angle:  e,e)  e  Jet veto  rejects energetic jets: –ee, e  T1 < 90 GeV or E T1 < 50 GeV, E T2 < 30 GeV  : E T1 < 60 GeV, E T2 < 30 GeV Signal acceptance is ~ 0.02 – 0.2 depending on the Higgs mass/final state Cuts optimised for each final state  Beat down Z/  *, W+jets, WW & tt backgrounds

9. DPF Meeting 2004Amber Jenkins Imperial College London 9 Data vs. Monte Carlo: Opening Angle,  e  After final event selection After basic event preselection 160 GeV Higgs e  Final State

10. DPF Meeting 2004Amber Jenkins Imperial College London 10 Data vs. Monte Carlo: Dilepton Invariant Mass Distributions after basic event preselection 160 GeV Higgs ee Final State  Final State

11. DPF Meeting 2004Amber Jenkins Imperial College London 11 Data vs. Monte Carlo: Missing E T ee Final State  Final State 160 GeV Higgs Distributions after basic event preselection E T miss (GeV)

12. DPF Meeting 2004Amber Jenkins Imperial College London 12 Data vs. Monte Carlo Source ll’ = e + e - L = 177 pb -1  ll’ = e  L = 158 pb -1 ll’ =  L = 147 pb -1 tt  bl bl’ 0.06 ± 0.010.13 ± 0.010.03 ± 0.003 WZ  ll’ + X 0.04 ± 0.010.11 ± 0.01   e + e - 0.00 ± 0.01 WW  l l’ 1.17 ± 0.022.51 ± 0.051.28 ± 0.03 W+jets1.24 ± 0.070.34 ± 0.02 QCD/W+jets0.02 ± 0.02 QCD0.00 ± 0.500.00 ± 0.20 Z/    e + e - 0.10 ± 0.10 Z/        0.00 ± 0.053.90 ± 0.60 Z/        0.00 ± 0.10 0.00 ± 0.00 Total Background Sum2.70 ± 0.403.10 ± 0.305.30 ± 0.60 Expected Signal for SM Higgs (M H = 160GeV) 0.073 ± 0.0020.111 ± 0.0040.085 ± 0.001 Data225

13. DPF Meeting 2004Amber 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 M H all channels

14. DPF Meeting 2004Amber 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.

15. Back-up Slides

16. DPF Meeting 2004Amber Jenkins Imperial College London 16 SM Higgs Decay Modes 135 GeV H  bbH  WW (*)

17. DPF Meeting 2004Amber Jenkins Imperial College London 17 Suppressed Couplings to b,  Occurs beyond the SM in Top Color or Fermiophobic Higgs models L. Brucher, R. Santos, hep-ph/9907434 Increased H  WW (*) branching

18. DPF Meeting 2004Amber Jenkins Imperial College London 18 Higgs Sensitivity Reach

19. DPF Meeting 2004Amber Jenkins Imperial College London 19 Azimuthal Opening Angle:  ll An example: e + e - final state  C  ain background

20. DPF Meeting 2004Amber Jenkins Imperial College London 20 Missing E T An example: e + e - final state  C  ain background