Search for the Higgs boson produced in association with $Z\to {\ell }^{+}{\ell }^{-}$ using the matrix element method at CDF II

We present a search for associated production of the standard model Higgs boson and a $Z$ boson where the $Z$ boson decays to two leptons and the Higgs decays to a pair of $b$ quarks in $p\overline{p}$ collisions at the Fermilab Tevatron. We use event probabilities based on standard model matrix elements to construct a likelihood function of the Higgs content of the data sample. In a CDF data sample corresponding to an integrated luminosity of $2.7{\mathrm{fb}}^{-1}$ we see no evidence of a Higgs boson with a mass between $100\mathrm{GeV}/c^2$ and $150\mathrm{GeV}/c^2$. We set 95% confidence level upper limits on the cross section for $ZH$ production as a function of the Higgs boson mass $m_H$; the limit is 8.2 times the standard model prediction at $m_H=115\mathrm{GeV}/c^2$.

Figure 1

The distribution of ${tan}^{-1}\mathcal{D}$ where the discriminant $\mathcal{D}=(P_{ZH}-P_b)/L$ for expected backgrounds and data for events with one (left) and two (right) $b$ tags. The expected signal ($\times 10$) is overlaid. The ${\chi }^2$ is based on the data and total background expectation as shown, using the expected statistical uncertainties and the bin-to-bin uncorrelated systematic uncertainties on the background.

Figure 2

Confidence intervals in the measured signal fraction $S_{\mathrm{meas}}$ (along the $x$ axis) with 68% C.L., 95% C.L., and 99.7% C.L., for a range of true signal fraction $S_{\mathrm{true}}$ values (along the $y$ axis on the left) chosen in the simulated experiments. The signal cross section ratio equivalent to $S_{\mathrm{true}}$ is shown on the $y$ axis on the right. The intervals shown here are computed for a Higgs boson mass of $m_H=115\mathrm{GeV}/c^2$, and include statistical and systematic uncertainties. The vertical dashed line indicates the value of $S_{\mathrm{meas}}$ obtained from the data.