A search for a standard model Higgs boson produced in association with a
top-quark pair and decaying to bottom quarks is presented. Events with hadronic
jets and one or two oppositely charged leptons are selected from a data sample
corresponding to an integrated luminosity of 19.5 inverse femtobarns collected
by the CMS experiment at the LHC in pp collisions at a centre-of-mass energy of
8 TeV. In order to separate the signal from the larger t t-bar + jets
background, this analysis uses a matrix element method that assigns a
probability density value to each reconstructed event under signal or
background hypotheses. The ratio between the two values is used in a maximum
likelihood fit to extract the signal yield. The results are presented in terms
of the measured signal strength modifier, mu, relative to the standard model
prediction for a Higgs boson mass of 125 GeV. The observed (expected) exclusion
limit at a 95% confidence level is mu < 4.2 (3.3), corresponding to a best fit
value mu-hat = 1.2 +1.6 -1.5.

该研究使用了矩阵元方法来分离底夸克衰变产生的、与一对顶夸克产生的标准模型希格斯玻色子相关的信号和更大的背景，通过最大似然拟合来提取信号产生量的结果相对于 125 GeV 质量的希格斯玻色子标准模型预测的被观察（预计）的 95％置信度的排除极限为 μ <4.2（3.3），相应的最佳拟合值为 mu-hat = 1.2 +1.6 -1.5。