Search for $WH$ associated production in $p\overline{p}$ collisions at $\sqrt{s}\mathbf{=}1.96\mathrm{TeV}$

This report describes a search for associated production of $W$ and Higgs bosons based on data corresponding to an integrated luminosity of $\mathcal{L}\approx 5.3\mathrm{f}{\mathrm{b}}^{-1}$ collected with the D0 detector at the Fermilab Tevatron $p\overline{p}$ Collider. Events containing a $W\to \ell \nu$ candidate (with $\ell$ corresponding to $e$ or $\mu$) are selected in association with two or three reconstructed jets. One or two of the jets are required to be consistent with having evolved from a $b$ quark. A multivariate discriminant technique is used to improve the separation of signal and backgrounds. Expected and observed upper limits are obtained for the product of the $WH$ production cross section and branching ratios and reported in terms of ratios relative to the prediction of the standard model as a function of the mass of the Higgs boson ($M_H$). The observed and expected 95% C.L. upper limits obtained for an assumed $M_H=115\mathrm{GeV}$ are, respectively, factors of 4.5 and 4.8 larger than the value predicted by the standard model.

Figure 1

The probability for MJ background events to enter the final $e+2$ jet (post-2006 upgrade) sample. The solid, dashed, and dotted curves represent the result of fitted parameterizations in each interval.

Figure 2

The probability for MJ background events to enter the final $\mu +2$ jet (post-2006 upgrade) sample. The solid and dashed curves represent the result of the fitted parameterizations in each interval.

Figure 3

Comparison of simulated events, including data-determined MJ background, to the $W+2$ jet selected data (black points) for (a) isolated lepton $p_T$, (b) missing event transverse energy $E_T$, (c) transverse mass of the $(l,{\mathrm{E̸}}_T)$ system, (d) $p_T$ of $W$ boson candidates, (e) leading jet $p_T$, (f) $p_T$ of the second leading jet, (g) scalar sum of the $p_T$ of jets in the event ($H_T$), (h) transverse momentum of the dijet system, (i) separation $\Delta R$, and (j) azimuthal separation $\Delta \varphi$ for the two jets. The expectation for a $WH$ signal at $M_H=115\mathrm{GeV}$ has been scaled up by a factor of 300. The electron and muon selected samples are combined in the figures.

Figure 4

Dijet invariant mass distribution for the $W+2$ jet selected samples on linear and logarithmic scales for (a), (d) no $b$-tagging, (b), (e) events that contain two $b$-tagged jets, and (c), (f) events that fail the two-tagged requirement but contain a single NN $b$-tagged jet. The expectation for a $WH$ signal at $M_H=115\mathrm{GeV}$ has been scaled up by a factor of 10.

Figure 5

Dijet invariant mass distribution for the $W+3$ jet selected samples on linear and logarithmic scales for (a), (d) no $b$-tagging, (b), (e) events that contain two $b$-tagged jets, and (c), (f) events that fail the two-tagged requirement but contain a single NN $b$-tagged jet (the two leading jets in the $W+3$ jet samples are used to form the dijet invariant mass). The expectation for a $WH$ signal at $M_H=115\mathrm{GeV}$ has been scaled up by a factor of 10.

Figure 6

Comparison of the expected backgrounds to the two-$b$-tagged jet data sample in $W+2$ jet selected events. The expectation for a $WH$ signal at $M_H=115\mathrm{GeV}$ has been scaled up by a factor of 10.

Figure 7

Comparison of the expected backgrounds to the single-$b$-tagged jet data sample in $W+2$ jet selected events. The expectation for a $WH$ signal at $M_H=115\mathrm{GeV}$ has been scaled up by a factor of 10.

Figure 8

Comparison of the total backgrounds to data for the additional variables provided as inputs to the random forests. The distributions are compared after requiring two-$b$-tagged jets in $W+2$ jet events. Each variable is defined in Table 3. The expectation for a $WH$ signal at $M_H=115\mathrm{GeV}$ has been scaled up by a factor of 10.

Figure 9

Comparison of the total backgrounds to data for the additional variables provided as inputs to the random forests. The distributions are compared after requiring a single-$b$-tagged jet in $W+2$ jet events. Each variable is defined in Table 3. The expectation for a $WH$ signal at $M_H=115\mathrm{GeV}$ has been scaled up by a factor of 10.

Figure 10

Output RF distributions on linear and logarithmic scales for (a), (c) single (one-$b$-tag) and (b), (d) two-$b$-tagged $W+2$ jet events. The expectation for signal at $M_H=115\mathrm{GeV}$ (solid black line) is scaled by a factor of 10.

Figure 11

Output RF distributions on linear and logarithmic scales for (a), (c) single (one-$b$-tag) and (b), (d) two-$b$-tagged $W+3$ jet events. The expectation for signal at $M_H=115\mathrm{GeV}$ (solid black line) is scaled by a factor of 10.

Figure 12

The observed LLR as a function of hypothetical Higgs boson mass. Also shown are the medians of the resulting LLR distributions for the background-only hypothesis (dashed line), along with the $\pm 1\sigma$ and $\pm 2\sigma$ values (shaded bands), after generating multiple pseudoexperiments at each test mass point. The medians of the signal-plus-background hypothesis are shown by the dash-dotted line.

Figure 13

The RF discriminant output distribution minus the total background expectation for (a) $M_H=100\mathrm{GeV}$, (b) $M_H=115\mathrm{GeV}$, (c) $M_H=130\mathrm{GeV}$, and (d) $M_H=140\mathrm{GeV}$. The prefit uncertainties are shown by the shaded bands and the post-fit uncertainties are represented by the solid lines. The signal expectation is shown scaled to the obtained observed upper limit at each test mass point.

Figure 14

The expected and observed 95% C.L. upper limits from the likelihood fit, maximized over systematics, as a function of the hypothetical Higgs mass $M_H$. The limits are presented as ratios of $\sigma (p\overline{p}\to WH)\mathcal{B}\mathcal{R}(H\to b\overline{b})$ to the expected SM prediction.