Search for the Wh Production Using High-pT Isolated Like-Sign Dilepton Events in Run-II
CDF Collaboration

Authors
Toru Okusawa, Yoshi Seiya, Takayuki Wakisaka, and Kazuhiro Yamamoto
(Osaka City University)
Contact to authors and Higgs Conveners

• Blessed : April 11th 2008
• Data : CDF Run-II L = 1.9 fb^-1
• Documentation : CDF note 7307 V2.0 (public note)

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Contents

1. Abstract
2. Baseline Events Selection
3. Background Estimation
   1. Physics Backgrounds
   2. Residual Photon-Conversion Background
   3. Fake Lepton Background
4. Final Cut
5. Efficiency of Signal and Systematic Uncertainties
6. Cross Section Limit
7. Conclusion

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1. Abstract

We search for the neutral higgs production in association with a W boson using high-p_T isolated like-sign dilepton events in ppbar collisions at √s = 1.96 TeV. The data were collected with the CDF-II detector at the Fermilab Tevatron collider and correspond to an integrated luminosity of 1.9 fb^-1. We examine the dilepton events in the 2 dimensional grid of the 2nd lepton p_T (p_T2) versus the p_T of dilepton system (p_T12), and tune the final selection in order to maximize the sensitivity to higgs production. We select events with 2nd lepton p_T greater than 20 GeV/c and the p_T of dilepton system greater than 15 GeV/c. The expected number of signal events is 0.46 for the fermiophobic higgs of the mass 110 GeV/c² assuming the Standard Model cross section and 0.19 for the mass 160 GeV/c². The expected number of signal events for the Standard Model higgs is 0.02 and 0.18 for 110 GeV/c² and 160 GeV/c², respectively. The expected number of background events is 3.23 ± 0.69 events, while we observed 3 events in the data. From these results, we obtain the limits on the σ(ppbar→Wh)×Br(h →W⁺W^-) of 2.2 pb for the higgs of 110 GeV/c² and 1.4 pb for 160 GeV/c² at the 95% confidence level.

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2. Baseline Event Selection

We select at least one electron with E_T > 20 GeV and p_T > 10 GeV/c or one muon with p_T > 20 GeV/c, and at least one other electron with E_T > 6 GeV and p_T > 6 GeV/c or one muon with p_T > 6 GeV/c. Other lepton selection requirements include isolation, track quality, and the consistency of detector response with expectations for electrons or muons. Photon conversion resulting in electron candidates are rejected by identifying an oppositely charged track satisfying a conversion configuration. For the events containing exactly two leptons passing the lepton selection, we require that the two leptons originate from the same event vertex, and also apply a dilepton mass selection of (M_ll > 12 GeV/c²) and a Z-event veto. We finally require like-sign charge combinations. The final selection to improve the sensitivity for higgs events is not applied yet at this stage of the event selection.

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3. Background Estimation

1. Physics Backgrounds

   Backgrounds containing prompt real leptons (physics backgrounds) are estimated using Monte Calro Samples. The backgrounds can be classified into reducible and irreducible backgrounds Reducible Backgrounds : Drell-Yan, W+(heavy flavor hadrons), ttbar, and W+W-. Irreducible Backgrounds : WZ and ZZ.

   
   
2. Residual Photon-Conversion Background
   

   Residual photon-conversion events are one of the dominant backgrounds for the like-sign dilepton analysis. They arise from an electron originating from the photon conversion with an unobserved partner track with low momentum. We estimate the amount of this background by multiplying lepton + conversion events by the residual photon-conversion rate which is defined by Rres = (1−εcon)/ εcon, where εcon is the conversion detection efficiency. The efficiency is measured by comparing the conversions found in the real data with conversions from MC samples. The rate is obtained as a function of parent photon pT and shown in the right figure.
   	gif, eps
   	Residual photon-conversion rate as a function of photon pT.

   
   
3. Fake Lepton Background
   

   Fake lepton events are also a dominant background for the like-sign dilepton analysis. The fake electron backgrounds result from π±, overlap of π0 + track and residual photon-conversions. The fake muon backgrounds result from hadrons that punch-through the calorimeter, and π± and K± which decay in flight to muons. We also consider leptons from semileptonic decays of heavy-flavor hadrons as one of fake lepton backgrounds. We estimate the backgrounds by multiplying lepton + isolated track events by the fake-lepton rate derived from inclusive jet samples. The fake-lepton rate (Rfake) is defined as the rate of fake leptons relative to isolated tracks. We subtract the residual photon-conversion component from the fake electron rate, because we estimate the amount of residual photon-conversion events separately.
   	gif, eps
   	Fake-lepton rate as a function of isolated track pT.

• Background expectation and observed number of events for the like-sign dilepton events for each dilepton channel before the final selection.
  We distinguish eμ and μe according to the lepton trigger and the lepton type with the leading p_T.

  gif

• Comparison of kinematic distributions (1st lepton p_T,2nd lepton p_T, and dilepton system p_T) between the data and the background prediction for the like-sign dilepton events.

  	ee	eμ	μe	μμ
  pT1
  	gif, eps	gif, eps	gif, eps	gif, eps
  pT2
  	gif, eps	gif, eps	gif, eps	gif, eps
  pT12
  	gif, eps	gif, eps	gif, eps	gif, eps

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4. Final Selection

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We examine the baseline like-sign dilepton events on the 2 dimensional plane of the 2nd lepton pT (pT2) versus the dilepton system pT (pT12). By scanning this 2D plane of pT2 and pT12, we find no significant discrepancies between the data and the background expectations for all the cut values. We then determine the optimized selection criteria for signal events as pT2 ≥ 20 GeV/c and pT12 ≥ 15 GeV/c by minimizing the expected upper limit on the cross section times branching fraction. Using these selection criteria, we define 4 regions and background predictions and observed data for each region: Region1 : pT2 ≥ 20 GeV/c and pT12 < 15 GeV/c Region2 : pT2 < 20 GeV/c and pT12 < 15 GeV/c Region3 : pT2 < 20 GeV/c and pT12 ≥ 15 GeV/c Region4 : pT2 ≥ 20 GeV/c and pT12 ≥ 15 GeV/c
	gif, eps
	Expected upper limit on the σ(ppbar→Wh)×Br(h →W+W-) at the 95% confidence level.

• Background expectation and observed number of events for the like-sign dilepton events for each region defined by using the final selection criteria.
  

  gif

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5. Efficiency of Signal and Systematic Uncertainties

Efficiencies for higgs events shown for each higgs mass. The uncertainties shown here include Monte Carlo statistics and those associated with Monte Carlo scale factors.	Systematic Uncertainties on the signal efficiency.
gif	gif

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6. Cross Section Limit

We set the 95% confidence level upper limit on the cross section times branching fraction σ(ppbar→Wh)× Br(h→W⁺W^-) using a Bayesian approach.

Cross section upper limit at the 95% confidence level	Ratio of the limit to the Standard Model prediction.	Ratio of the limit to the fermiophobic higgs production.
gif	gif	gif

Cross section upper limit at the 95% confidence level	Ratio of the limit to the Standard Model prediction.	Ratio of the limit to the prediction for the fermiophobic higgs production.
gif, eps	gif, eps	gif, eps

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7. Conclusion

We have searched for the neutral higgs boson produced in associated with a W boson using high-p_T like-sign dilepton events with the data corresponding to an integrated luminosity of 1.9 fb^-1. We do not see any significant disagreements between background expectations and data. Given this, we applied an optimized selection on the 2nd lepton p_T and the dilepton system p_T in order to obtain the limits on Higgs production cross section. We found that a 2nd lepton p_T cut of 20 GeV/c and the dilepton system p_T cut of 15 GeV/c was an optimal choice. With this final selection, we found the expected number of fermiophobic higgs events for the mass 110 GeV/c² to be 0.46 assuming the Standard Model production cross section and 0.19 for the 160 GeV/c² higgs. The expected number of background events in the final sample was 3.23 ± 0.69 events, while the number of observed events is 3. We obtained the upper limits on the production cross-section times the branching fraction for the higgs with masses in the region from 110 GeV/c² to 200 GeV/c², 2.2 pb for the 110 GeV/c² higgs and 1.4 pb for 160 GeV/c².

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Takayuki Wakisaka