Search for Doubly-Charged Higgs at CDF

 

Introduction
  Doubly-charged Higgs particles are predicted by left-right (LR) symmetric 
models, and SUSY LR models predict low-mass H++ (~100 GeV to 1 TeV).  These 
particles decay to leptons, and we have searched ~240 pb-1 of CDF Run 2 
data (Mar, 2002-Nov, 2003) for evidence of their production.  We searched in  
same-sign electron mass windows of +-10% of a given H++ mass (~3 sigma of
the detector resolution).  Our search is sensitive to any doubly-charged 
particle decaying to dielectrons, whether produced singly or in pairs.  

Dielectron Background Prediction
  In the 80-100 GeV mass range, the dielectron search has little 
sensitivity due to the overwhelming background from Z production.  The
background occurs when the Z decays to opposite-sign electrons and one
of the electrons radiates a photon, which subsequently converts.  When the
wrong sign conversion track is associated with the electron cluster, the 
event is reconstructed with 2 same-sign electrons.

  We estimate the Z background using the data in the 80-100 GeV mass 
range and search the region above 100 GeV.  The mass region of 100-130
GeV is dominated by this background; above this mass, QCD and Z processes 
are expected to contribute equally to the background.  



Dimuon Background Prediction
  The Standard Model background to the same-sign dimuon sample is the smallest
of the three.  The QCD and W+jet backgrounds contribute most at low masses and the WZ
background contributes more at high masses.



Electron-muon Background Prediction
  The QCD and W+jet background to the electron-muon sample is significant if we 
do not require a muon stub attached to a track for the muon.  This reduces the 
acceptance relative to the dielectron and dimuon channels, but also reduces the 
background to less than an event for a given doubly-charged Higgs mass bin.  



Background Prediction vs Mass

  As a function of doubly-charged Higgs mass, the background rises as the mass 
increases away from the low-mass cut-off due to the increasing search window size. 
At high masses, the background falls steadily.




Total Acceptance
  We estimate the total acceptance for the pair-production of doubly-charged
Higgs particles using the PYTHIA event generator and a GEANT-based detector
simulation.  



Results
  We use the low mass region (<80 GeV) as a test of our background prediction.  We predict 1.1
dielectron, 0.9 dimuon, and 0.6 electron-muon events in this region and we 
observe 1 dielectron event.  In our search region, we observe 0 events.  These 
results provide a 95% confidence level cross section limit for pair-production 
of doubly-charged particles.



  To provide some context, we show the current world limits in the doubly-charged Higgs
mass versus lepton coupling plane: