Combined Search for the Standard Model Higgs Boson Decaying to $b\overline{b}$ Using the D0 Run II Data Set

We present the results of the combination of searches for the standard model Higgs boson produced in association with a $W$ or $Z$ boson and decaying into $b\overline{b}$ using the data sample collected with the D0 detector in $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ at the Fermilab Tevatron Collider. We derive 95% C.L. upper limits on the Higgs boson cross section relative to the standard model prediction in the mass range $100\mathrm{GeV}\le M_H\le 150\mathrm{GeV}$, and we exclude Higgs bosons with masses smaller than 102 GeV at the 95% C.L. In the mass range $120\mathrm{GeV}\le M_H\le 145\mathrm{GeV}$, the data exhibit an excess above the background prediction with a global significance of 1.5 standard deviations, consistent with the expectation in the presence of a standard model Higgs boson.

Figure 1

Background-subtracted data distributions of ${log}_{10}(s/b)$ in (a) the $VZ$ analysis after a fit of the $VZ$ and background contributions to the data and (b) the $VH,H\to b\overline{b}$ search for $M_H=125\mathrm{GeV}$ after a fit of the backgrounds to the data. The background-subtracted data are shown as points, and the signal is shown as the red histogram in each plot. The blue lines indicate the posterior uncertainty on the background prediction.

Figure 2

(a) The 95% C.L. cross section upper limit ratios versus $M_H$, and (b) LLR distribution versus $M_H$, for the combined $VH,H\to b\overline{b}$ analyses. The solid lines represent the observed values in the data. The short-dashed black (red) lines represent the median expected values under the background-only (signal-plus-background) hypothesis at each mass. The long-dashed blue lines show the expected outcome from injecting a SM Higgs boson signal with $M_H=125\mathrm{GeV}$. The green and yellow shaded bands correspond to the regions enclosing 1 and 2 s.d. variations about the median expected values under the background-only hypothesis, respectively.

Figure 3

The best-fit value for $\sigma (VH)\times \mathrm{BR}(H\to b\overline{b})$ as a function of $M_H$. The green shaded band corresponds to the 1 s.d. uncertainty around the best-fit cross section. Also shown is the SM prediction including the theoretical uncertainties.