Search for $ZH\to {\ell }^{\mathbf{+}}{\ell }^{\mathbf{-}}b\overline{b}$ production in $9.7{\mathrm{fb}}^{\mathbf{-}1}$ of $p\overline{p}$ collisions with the D0 detector

We present a search for the standard model Higgs boson produced in association with a $Z$ boson in $9.7{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions collected with the D0 detector at the Fermilab Tevatron Collider at $\sqrt{s}=1.96\mathrm{TeV}$. Selected events contain one reconstructed $Z\to e^{+}{e}^{-}$ or $Z\to {\mu }^{+}{\mu }^{-}$ candidate and at least two jets, including at least one jet likely to contain a $b$ quark. To validate the search procedure, we also measure the cross section for $ZZ$ production and find that it is consistent with the standard model expectation. We set upper limits at the 95% C.L. on the product of the $ZH$ production cross section and branching ratio $\mathcal{B}(H\to b\overline{b})$ for Higgs boson masses $90\le M_H\le 150\mathrm{GeV}$. The observed (expected) limit for $M_H=125\mathrm{GeV}$ is a factor of 7.1 (5.1) larger than the standard model prediction.

Figure 1

Trigger correction factors for the $\mu \mu$ channel in Run 2b3 as a function of (a) jet multiplicity, (b) ${\mathrm{E̸}}_{\mathrm{T}}$, (c) ${\eta }_{\det }$ of the triggered muon, and (d) ${\eta }_{\det }$ of the other muon. The correction applied to the single muon trigger is the product of all four components.

Figure 2

Jet multiplicity distribution in the inclusive sample, summed over all lepton channels, along with the background expectation. The signal distribution for $M_H=125\mathrm{GeV}$ is scaled by a factor of 500.

Figure 3

The dilepton mass spectra, along with the background expectation, for the (a) $\mu \mu$, (b) $\mu {\mu }_{\mathrm{trk}}$, (c) $ee$ and (d) $e{e}_{\mathrm{ICR}}$ channels in the pretag sample. The signal distributions for $M_H=125\mathrm{GeV}$ are scaled by a factor of 500. The mass resolution in the $ee$ channel is superior to that in the $\mu \mu$ channel due to the better energy resolution of the electromagnetic calorimeter compared to the momentum resolution of the tracking system in the relevant $p_{\mathrm{T}}$ range.

Figure 4

The $p_T$ spectra of the (a) leading and (b) subleading jets, along with the background expectations, summed over all lepton channels in the pretag sample. The signal distributions, for $M_H=125\mathrm{GeV}$, are scaled by a factor of 500.

Figure 5

Distribution of the dijet invariant mass before the kinematic fit, along with the background expectation, summed over all lepton channels in the pretag sample. The signal distribution, for $M_H=125\mathrm{GeV}$, is scaled by a factor of 500.

Figure 6

The dijet invariant mass for the simulated $ZH$ signal, at $M_H=125\mathrm{GeV}$, summed over all lepton channels in the pretag sample, shown before and after the kinematic fit.

Figure 7

Distributions in ST events of (a) the dijet invariant mass corrected by the kinematic fit, (b) the $p_T$ of the leading jet from the Higgs boson candidate, (c) the $p_T$ of the subleading jet from the Higgs boson candidate, (d) the $p_T$ of the dijet system divided by the scalar sum of the transverse momenta of the two jets, (e) the collinearity of the two leptons, and (f) $\cos {\theta }^{*}$ [43]. The signal distributions for $M_H=125\mathrm{GeV}$ are scaled by a factor of 100.

Figure 8

Distributions in DT events of (a) the dijet invariant mass corrected by the kinematic fit, (b) the $p_T$ of the leading jet from the Higgs boson candidate, (c) the $p_T$ of the subleading jet from the Higgs boson candidate, (d) the $p_T$ of the dijet system divided by the scalar sum of the transverse momenta of the two jets, (e) the collinearity of the two leptons, and (f) $\cos {\theta }^{*}$. The signal distributions for $M_H=125\mathrm{GeV}$ are scaled by a factor of 50.

Figure 9

The $t\overline{t}$ RF output ($M_H=125\mathrm{GeV}$) for all lepton channels combined in the pretag sample (a) trained for ST events and (b) trained for DT events. The arrows indicate the $t\overline{t}$ RF selection requirement used to define the $t\overline{t}$-enriched and -depleted subsamples. The signal distributions for $M_H=125\mathrm{GeV}$ are scaled by a factor of 500.

Figure 10

Global RF output ($M_H=125\mathrm{GeV}$) for all lepton channels combined for (a) pretag events evaluated with the ST-trained RF and (b) pretag events evaluated with the DT-trained RF. The signal distributions for $M_H=125\mathrm{GeV}$ are scaled by a factor of 500.

Figure 11

The $t\overline{t}$ RF output for all lepton channels combined in ST and DT events for $M_H=115\mathrm{GeV}$ (a),(b), for $M_H=125\mathrm{GeV}$ (c),(d), and for $M_H=135\mathrm{GeV}$ (e),(f). The signal distributions correspond to the $M_H$ used for the RF training and are scaled by a factor of 40. The arrows indicate the $t\overline{t}$ RF selection requirement used to define the $t\overline{t}$-enriched and -depleted subsamples.

Figure 12

Global RF distributions for ST and DT events in the $t\overline{t}$-depleted region for $M_H=115\mathrm{GeV}$ (a),(b), for $M_H=125\mathrm{GeV}$ (c),(d), and for $M_H=135\mathrm{GeV}$ (e),(f). The signal distributions correspond to the $M_H$ used for the RF training and are scaled by a factor of 40.

Figure 13

Global RF distributions for ST and DT events in the $t\overline{t}$-enriched region for $M_H=115\mathrm{GeV}$ (a),(b), for $M_H=125\mathrm{GeV}$ (c),(d), and for $M_H=135\mathrm{GeV}$ (e),(f). The signal distributions correspond to the $M_H$ used for the RF training and are scaled by a factor of 400.

Figure 14

LLR distributions obtained from $B$ and $S+B$ pseudoexperiments, using the global RF output as the final variable, for the $VZ$ search. The vertical line indicates the LLR obtained from the data.

Figure 15

Distribution of $VZ$ cross sections obtained from $B$ and $S+B$ pseudoexperiments. The observed cross section from the data and the SM cross section are also shown.

Figure 16

Global RF output distributions for the $VZ$ search after the fit to data in the $S+B$ hypothesis in (a) ST events, (c) DT events, and (e) ST and DT events combined. Distributions are summed over all $Z\to \ell \ell$ channels. The $VZ$ signal distribution, scaled to the measured ${\sigma }_{VZ}$, is compared to the data after subtracting the fitted background in (b) ST events, (d) DT events, and (e) ST and DT events combined. Data points are shown with Poisson statistical errors. Also shown is the uncertainty on the background after the fit.

Figure 17

Dijet invariant mass distributions for the $VZ$ search after the kinematic fit and after the fit to the data in the $S+B$ hypothesis in (a) ST events, (c) DT events, and (e) ST and DT events combined. Distributions are summed over all $Z\to \ell \ell$ channels. The $VZ$ signal distribution, scaled to the measured ${\sigma }_{VZ}$, is compared to the data after subtracting the fitted background in (b) ST events, (d) DT events, and (e) ST and DT events combined. Also shown is the uncertainty on the background after the $S+B$ fit.

Figure 18

Global RF output distributions in the $t\overline{t}$-depleted region, assuming $M_H=125\mathrm{GeV}$, after the fit to the data in the $B$ hypothesis for (a) ST events, (c) DT events, and (e) ST and DT events combined. Background-subtracted distributions for (a), (c), and (e) are shown in (b), (d), and (f), respectively. Signal distributions for $M_H=125\mathrm{GeV}$ are shown with the SM cross section scaled by a factor of 5 in (b), (d), and (f).

Figure 19

Global RF output distributions in the $t\overline{t}$-enriched region, assuming $M_H=125\mathrm{GeV}$, after the fit to the data in the $B$ hypothesis for (a) ST events, (c) DT events, and (e) ST and DT events combined. Background-subtracted distributions for (a),(c), and (e) are shown in (b),(d), and (f), respectively. Signal distributions for $M_H=125\mathrm{GeV}$ are shown with the SM cross section scaled by a factor of 50 in (b),(d) and (f).

Figure 20

Observed and expected LLR values for the $S+B$ and $B$ hypotheses, along with the $\pm 1$ and $\pm 2$ s.d. bands for the $B$ hypotheses, as well as observed and expected cross section upper limits (along with the $\pm 1$ and $\pm 2$ s.d. bands for the expected limit) relative to the SM cross section, (a),(b) for the $ee$ channel, (c),(d) for the $\mu \mu$ channel, (e),(f) for the $e{e}_{\mathrm{ICR}}$ channel, and (g),(h) for the $\mu {\mu }_{\mathrm{trk}}$ channel.

Figure 21

(a) Observed and expected LLR values as a function of $M_H$ for the $S+B$ and $B$ hypotheses, along with the $\pm 1$ and $\pm 2$ s.d. bands for the $B$ hypotheses, for all lepton channels combined. (b) Expected and observed cross section upper limits at the 95% C.L. for $ZH\to {\ell }^{+}{\ell }^{-}b\overline{b}$ production, relative to the SM cross section.