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/c2 to 200GeV/c2

• 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 compo

nents), two neutrino missing energy (2 quantities)

– Compared to the neural network method: discriminant more physics

• Likelihood Ratio Discriminator– Directly calculate the event probability: P(xobs;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 MH=160 Gev/c2 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/HW

W_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 PT (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 (gen

erator 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: