Search for neutral MSSM Higgs bosons decaying to  pairs in pp collisions at 1.96 TeV (CDF)

1. Search for neutral MSSM Higgs bosons decaying to  pairs in pp collisions at 1.96 TeV (CDF) Jessica Leonard Oct. 23, 2006 Physics 835

2. MSSM Higgs • How do particles have mass? • SM Higgs mechanism • Problems with SM Higgs may be solved in MSSM • Five Higgs in MSSM: 2 charged, 3 neutral • This paper deals with neutrals (): • 1 CP-odd: A • 2 CP-even: h (lighter), H (heavier) • MSSM parameters: • mA • tan  = vu/vd, where vu, vd are vev’s of neutral Higgs that couple to up- and down-type fermions Jessica Leonard

3. MSSM Higgs • For large tan b, one CP-even Higgs has similar mass, couplings to A • Dominant  production mechanisms: • Gluon fusion • bb fusion • Dominant decay modes for A, mass-degenerate CP-even state: •  -> bb (~90%) •  ->  (~10%) Jessica Leonard

4. The search at CDF • Tevatron: CM energy 1.96 TeV • Data collected at CDF II between 2002-04 • Integrated luminosity 310 pb-1 •  ->  for 90 < mA < 250 GeV/c2 • One  -> lepton + neutrinos (e, ) • Other  -> hadrons + neutrino (had) Jessica Leonard

5. CDF detector =-ln(tan(/2)) Jessica Leonard

6. Event selection • For the  ->  search, each event must contain a lepton candidate (e or ) and hadrons consistent with a  decay. • Pre-select with a “lepton plus track” condition • Lepton and track must both point into central cal. • Must have azimuthal separation of > 10o • e/ determination: • Electrons only deposit energy in EM calorimeter • Muons seen in muon chambers outside cal. • Momentum of neutrinos: missing transverse energy Jessica Leonard

7. Event selection • Hadron reconstruction • Energy from  decays are deposited in a small, contiguous region • Four-momentum of this jet determined from tracks of charged particles and energies of 0’s within “signal cone” • Particles in a ring outside the signal cone must have very low transverse energy (< 1 GeV total). This isolation cuts out quark and gluon jets. Jessica Leonard

8. Event selection • Hadron reconstruction • The jets must have 1 or 3 tracks (charged particles), and sum of charges 1 • Invariant mass must be < 1.8 GeV/c2 ( = m) • Require (Ecl/Psigtrk)(0.95-f)>0.1 to reject electrons • Ecl: value of energy deposit in the calorimeter • f: ratio of electromagnetic to hadronic energy • Psigtrk: scalar sum of track momenta in signal cone • Require ETcl > 15 GeV to reject muons Jessica Leonard

9. Event selection • Hadron reconstruction efficiency: • 38% at pThad = 15 GeV/c • 46% at pThad > 25 GeV/c • Probability of misidentifying quark or gluon jet • 1.5% at ETjet = 20 GeV • 0.1% at ETjet = 100 GeV Jessica Leonard

10. Monte Carlo Simulation • Event generation: PYTHIA, with CTEQ5L PDFs • Tau decays: TAUOLA package • Detector response: GEANT-based model Jessica Leonard

11. Backgrounds • Dominant background: Z/* ->  • Second-highest: “fake” ’s (misidentified quark or gluon jets) from processes like di-jet, multi-jet, W+jets, +jets • Third: Z/* -> ll, WW, WZ, ZZ, tt • Background contributions determined by Monte Carlo Jessica Leonard

12. Eliminating backgrounds • Cut out low-energy multi-jet backgrounds • Each e/ candidate must pass pT threshold of 10 GeV/c • Each had candidate must pass pT threshold of 15 GeV/c, must also have opposite charge • Require |pTl|+|pThad|+|ETmiss| > 50 GeV Jessica Leonard

13. Eliminating backgrounds • Cut out W+jets backgrounds • Define: • pvis = (pl + phad)   • pETmiss = ETmiss  • Require pETmiss > 0.6pvis - 10 GeV/c • Removes ~85% W+jets events, keeps ~95% of signal • Cut out Z -> ll decays with misidentified lepton: reject events with lepton + 1-prong had invariant mass within 10 GeV/c2 of Z mass Jessica Leonard

14. Signal Acceptance • For mA = 90 GeV/c2, combined acceptance for ehad and had channels is 0.8% • For mA = 250 GeV/c2, combined acceptance is 2.0% Jessica Leonard

15. Uncertainties • Systematic uncertainties for particle ID efficiency: had 3.5%, e 1.3%,  4.6% • Uncertainties in trigger efficiency: ehad 2.1%, had 1.4% • Jet ->  misidentification 20% (3% bg. overall) • Uncertainty in signal acceptance • Due to event-level cuts < 2% • Due to PDFs: 5.7% Jessica Leonard

16. Expected events from SM • Number of expected standard model events passing all cuts: 4965(stat)28(sys)25(lumi) for a range of 438 to 554 events • Breakdown: • 405 from Z/* ->  • 75 from jet ->  misidentification • 16 from all remaining background sources Jessica Leonard

17. Data • 487 observed events • Reconstructed the mass (mvis) for each event: invariant mass of visible tau decay products and ETmiss • NOTE: This is incorrect! • Binned the data to compare with background and hypothetical Higgs data • No signal evidence observed for mA = 90 through 250 GeV/c2 Jessica Leonard

18. Example plot: mA = 140 GeV/c2 Jessica Leonard

19. Exclusion limits Jessica Leonard

20. Excluded regions • Plotted excluded regions in parameter space for four situations • mhmax vs. no-mixing •  > 0 and  < 0 Jessica Leonard

21. Excluded regions Jessica Leonard

22. Conclusions • LEP excluded mA < 93 GeV/c2, higher mA for small tan  • This search also excluded regions of higher tan  • Results similar to those of D0 Jessica Leonard