Investigation on Higgs physics Group Ye Li Graduate Student UW - Madison
Search for a SM Higgs Boson decaying to W pairs • Matrix Element and Likelihood Ratio Method • pp-bar Collisions at 1.96 TeV • Integrated Luminosity: 1.1 fb -1 • Standard Model Higgs Boson • 9 Higgs Mass Hypotheses – ranging from 110GeV/c 2 to 200GeV/c 2 • Channel: two leptons + missing energy
• Matrix Element Method: – Make maximum use of the data – Probabilities used as discriminants – Calculated using the LO theoretical prediction for the fully-differential cross-section – 8 variables: two lepton momentums (6 components), two neutrino missing energy (2 quantities) – Compared to the neural network method: discriminant more physics
• Likelihood Ratio Discriminator – Directly calculate the event probability: P ( x obs ; α ) – Use PDF, detector acceptance and efficiency function and detector resolution function – Missing information of two neutrinos • Accurate calculation requires the missing info • Smearing due to it is larger than the detector level smearing – Discriminator:
• Result (take M H =160 Gev/c 2 as the example): – 323 candidates: 286.1 +- 23.3 bg and 3.9 +- 0.3 signal at NNLL calculation – The observed 95% C.L. limit is 1.3pb which is 3.4 times the Standard Model prediction while the median of expected limit is 4.8+2.2-1.4 (systematics included) • See plots at: – http://www- cdf.fnal.gov/physics/new/hdg/HWW_ME/plots. htm
Search for the SM Higgs boson in the ZH->llbb Channel • Small background of mostly real Z + jets events due to the requirement of two leptons and a Z mass constraint • Loosen the tight and loose lepton cuts from standard high P T (top) analyses to improve statistics • 2 Artificial Neural Networks: – One to correct two candidate Higgs jets – The other to maintain signal efficiency and improve signal discrimination
• Background: – Bg w/ 2 genuine leptons and 1 b- or c-jet (generator modeling shape) – Bg w/ a fake lepton (use lepton fake rate) – Bg w/ a mistaged light jet (use mistag matrix) • Event Selection:
• Result: – The data agrees well with the background – Limit:
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