Search for vectorlike $B$ quarks in events with one isolated lepton, missing transverse momentum, and jets at $\sqrt{s}=8\mathrm{TeV}$ with the ATLAS detector

A search has been performed for pair production of heavy vectorlike down-type ($B$) quarks. The analysis explores the lepton-plus-jets final state, characterized by events with one isolated charged lepton (electron or muon), significant missing transverse momentum, and multiple jets. One or more jets are required to be tagged as arising from $b$ quarks, and at least one pair of jets must be tagged as arising from the hadronic decay of an electroweak boson. The analysis uses the full data sample of $pp$ collisions recorded in 2012 by the ATLAS detector at the LHC, operating at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$. No significant excess of events is observed above the expected background. Limits are set on vectorlike $B$ production, as a function of the $B$ branching ratios, assuming the allowable decay modes are $B\to Wt/Zb/Hb$. In the chiral limit with a branching ratio of 100% for the decay $B\to Wt$, the observed (expected) 95% C.L. lower limit on the vectorlike $B$ mass is 810 GeV (760 GeV). In the case where the vectorlike $B$ quark has branching ratio values corresponding to those of an $SU(2)$ singlet state, the observed (expected) 95% C.L. lower limit on the vectorlike $B$ mass is 640 GeV (505 GeV). The same analysis, when used to investigate pair production of a colored, charge $5/3$ exotic fermion $T_{5/3}$, with subsequent decay $T_{5/3}\to Wt$, sets an observed (expected) 95% C.L. lower limit on the $T_{5/3}$ mass of 840 GeV (780 GeV).

Figure 1

Unit-normalized distributions of the 12 variables used in the BDT discriminant. The background contribution ($t\overline{t}$ combined with $W+\text{jets}$) is shown with a dark blue solid line, while signal distributions are shown for 700 and 800 GeV VLQ $B$ with $\mathrm{BR}(B\to Wt)=100\%$ with red dashed and light blue dotted lines, respectively. The following selection requirements, which define the signal region for the BDT analysis, are applied: $N_{\text{jets}}\ge 6$, $N_{\text{bjets}}\ge 1$, $N_{\mathrm{V}}\ge 1$, and $H_{\mathrm{T}}>500\mathrm{GeV}$.

Figure 2

Distributions of 9 of the 12 BDT input variables for data and for the background expectation, for control region TCR3. The lower panels show the bin-by-bin ratio of the data to the total background expectation. The data are shown with statistical error bars. The shaded bands show the total uncertainties on the background expectation, including both the statistical and systematic uncertainties.

Figure 3

Distributions of 3 of the 12 BDT input variables for data and for the background expectation, for control region TCR2. For the two rightmost plots, events with zero reconstructed hadronically decaying $W/Z$ boson candidates are included in the first bin, at zero. The lower panels show the bin-by-bin ratio of the data to the total background expectation. The data are shown with statistical error bars. The shaded bands show the total uncertainties on the background expectation, including both the statistical and systematic uncertainties.

Figure 4

Distributions of the BDT discriminant for data and for the background expectation, for the various control regions: WCR1 (top left), WCR2 (top right), TCR1 (middle left), TCR2 (middle right), and TCR3 (bottom). The lower panels show the bin-by-bin ratio of the data to the total background expectation. The data are shown with statistical error bars. The shaded bands show the total uncertainties on the background expectation, including both the statistical and systematic uncertainties.

Figure 5

Distribution of four BDT input variables for data in the BDT signal region, with $N_{\text{jets}}\ge 6$, $N_{\text{bjets}}\ge 1$, $N_{\mathrm{V}}\ge 1$, and $H_{\mathrm{T}}>500\mathrm{GeV}$. Superimposed is the expectation for the total background. The variables shown in the upper row are the two variables with the highest sensitivity in the BDT training, namely $H_{\mathrm{T}}$ (left), and $\mathrm{\Delta }R(\ell ,\text{bjet}1)$, the angular separation between the lepton and the leading $b$-tagged jet (right). The lower row shows two of the multiplicity-related variables, namely the number of jets (left) and the number of hadronic $W/Z$ candidates (right). Also shown are the expected signal contributions for VLQ $B$ masses of 600 and 700 GeV, assuming $\mathrm{BR}(B\to Wt)=1$. The data are shown with statistical error bars, and the shaded band shows the total uncertainty on the background expectation. The lower panels show the bin-by-bin ratio of the data to the background expectation.

Figure 6

Distribution of the BDT discriminant for data in the signal region of the BDT analysis, with $N_{\text{jets}}\ge 6$, $N_{\text{bjets}}\ge 1$, $N_{\mathrm{V}}\ge 1$, and $H_{\mathrm{T}}>500\mathrm{GeV}$. Superimposed is the expectation for the total background, as well as signal contributions that would be expected for VLQ $B$ masses of 600 and 700 GeV, in both cases for a branching ratio of unity for the decay $B\to Wt$. The data are shown with statistical error bars, and the shaded band shows the total uncertainty on the background expectation. Uniform binning is used in the left plot, and the right one shows the same data in the nonuniform binning used to determine the final exclusion limits, with the background-dominated region of BDT values from $-1$ to $+0.95$ combined in a single bin. The lower panels show the bin-by-bin ratio of the data to the background expectation.

Figure 7

$H_{\mathrm{T}}$ distributions for events in the signal region for the cuts-based analysis, with $N_{\text{jets}}\ge 6$, $N_{\text{bjets}}\ge 1$, $N_{\mathrm{V}}\ge 1$, and $H_{\mathrm{T}}>800\mathrm{GeV}$. The figure shows events with (left) $N_{\mathrm{V}}=1$ and (right) $N_{\mathrm{V}}\ge 2$. Superimposed is the expectation for the total background, as well as signal contributions that would be expected for VLQ $B$ masses of 600 and 700 GeV, in both cases for a branching ratio of unity for the decay $B\to Wt$. The data are shown with statistical error bars, and the shaded band shows the total uncertainty on the background expectation. The lower panels show the bin-by-bin ratio of the data to the background expectation.

Figure 8

Predicted pair production cross section as a function of the VLQ $B$ mass and 95% C.L. observed and expected upper limits for an $SU(2)$ singlet VLQ $B$, as obtained with the BDT analysis. The uncertainty on the theoretical cross section is indicated by the width of the theory band.

Figure 9

For a variety of VLQ $B$ mass values, the expected and observed 95% C.L. exclusion limits of the BDT analysis. The results are shown in terms of the decay branching ratios, with $\mathrm{BR}(B\to Hb)$ plotted on the vertical axis, $\mathrm{BR}(B\to Wt)$ on the horizontal axis, and the branching ratios of the three decay modes considered, namely $B\to Wt/Hb/Zb$, assumed to sum to unity. Shown with a closed star (circle) are the branching ratio values for the particular case of an $SU(2)$ doublet (singlet) implementation of the VLQ $B$ model.

Figure 10

Expected (left) and observed (right) 95% C.L. lower limits on the mass of the VLQ $B$. The results are shown in terms of the decay branching ratios, with $\mathrm{BR}(B\to Hb)$ plotted on the vertical axis, $\mathrm{BR}(B\to Wt)$ on the horizontal axis, and the branching ratios of the three decay modes considered, namely $B\to Wt/Hb/Zb$, assumed to sum to unity.

Figure 11

Observed and expected 95% C.L. upper limits, obtained with the BDT analysis, on the cross section for $T_{5/3}$ pair production, as a function of $T_{5/3}$ mass. The colored, charge $5/3$ exotic fermion is assumed to decay into a same-sign $W$ boson and top quark, via $T_{5/3}\to Wt$. The uncertainty on the theoretical cross section is indicated by the width of the theory band.