1. Jessica Leonard Oct. 23, 2006 Physics 835
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
10/23/06
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%)
10/23/06
Jessica Leonard

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

5. CDF detector
=-ln(tan(/2))
10/23/06
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
10/23/06
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.
10/23/06
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
10/23/06
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
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Jessica Leonard

10. Monte Carlo Simulation
Event generation: PYTHIA, with CTEQ5L PDFs
Tau decays: TAUOLA package
Detector response: GEANT-based model
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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
10/23/06
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
10/23/06
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
10/23/06
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%
10/23/06
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%
10/23/06
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
10/23/06
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
10/23/06
Jessica Leonard

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

19. Exclusion limits
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Jessica Leonard

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

21. Excluded regions
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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
10/23/06
Jessica Leonard