Evidence for a Particle Produced in Association with Weak Bosons and Decaying to a Bottom-Antibottom Quark Pair in Higgs Boson Searches at the Tevatron

We combine searches by the CDF and D0 Collaborations for the associated production of a Higgs boson with a $W$ or $Z$ boson and subsequent decay of the Higgs boson to a bottom-antibottom quark pair. The data, originating from Fermilab Tevatron $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$, correspond to integrated luminosities of up to $9.7{\mathrm{fb}}^{-1}$. The searches are conducted for a Higgs boson with mass in the range $100–150\mathrm{GeV}/c^2$. We observe an excess of events in the data compared with the background predictions, which is most significant in the mass range between 120 and $135\mathrm{GeV}/c^2$. The largest local significance is 3.3 standard deviations, corresponding to a global significance of 3.1 standard deviations. We interpret this as evidence for the presence of a new particle consistent with the standard model Higgs boson, which is produced in association with a weak vector boson and decays to a bottom-antibottom quark pair.

Figure 1

Background-subtracted distribution of the reconstructed dijet mass $m_{jj}$, summed over all input channels. The $VZ$ signal and the background contributions are fit to the data, and the fitted background is subtracted. The fitted $VZ$ and expected SM Higgs ($m_H=125\mathrm{GeV}/c^2$) contributions are shown with filled histograms.

Figure 2

Background-subtracted distribution for the discriminant histograms, summed for bins with similar signal-to-background ratio ($s/b$), for the $H\to b\overline{b}$ ($m_H=125\mathrm{GeV}/c^2$) search. The solid histogram shows the uncertainty on the background after the fit to the data as discussed in the text. The signal model, scaled to the SM expectation, is shown with a filled histogram. Uncertainties on the data points correspond to the square root of the sum of the expected signal and background yields in each bin.

Figure 3

The log-likelihood ratio $LLR$ as a function of Higgs boson mass. The dark and light-shaded bands correspond to the regions encompassing 1 s.d. and 2 s.d. fluctuations of the background, respectively. The dot-dashed line shows the median expected $LLR$ assuming the SM Higgs boson is present at $m_H=125\mathrm{GeV}/c^2$.

Figure 4

The observed 95% credibility level upper limits on SM Higgs boson production ($R_{95}$) as a function of Higgs boson mass. The dashed line indicates the median expected value in the absence of a signal, and the shaded bands indicate the 1 s.d. and 2 s.d. ranges in which $R_{95}$ is expected to fluctuate. The dot-dashed line shows the median expected limit if the SM Higgs boson is present at $m_H=125\mathrm{GeV}/c^2$.

Figure 5

The best-fit cross section times branching ratio $({\sigma }_{WH}+{\sigma }_{ZH})\times \mathcal{B}(H\to b\overline{b})$ as a function of $m_H$. The dark and light-shaded regions indicate the 1 s.d. and 2 s.d. measurement uncertainties, and the SM prediction is shown as the smooth, falling curve with a narrow band indicating the theoretical uncertainty. The expected cross section fit values assuming the SM Higgs boson is present at $m_H=125\mathrm{GeV}/c^2$ are shown with dot-dashed lines for the cases of the expected SM rate (dark blue) and the best fitted rate from data (light magenta).

Figure 6

The $p$ value as a function of $m_H$ under the background-only hypothesis. Also shown are the median expected values assuming a SM signal is present, evaluated separately at each $m_H$. The associated dark and light-shaded bands indicate the 1 s.d. and 2 s.d. fluctuations of possible experimental outcomes.