Search for vectorlike light-flavor quark partners in proton-proton collisions at $\sqrt{s}=8\mathrm{TeV}$

A search is presented for heavy vectorlike quarks (VLQs) that couple only to light quarks in proton-proton collisions at $\sqrt{s}=8\mathrm{TeV}$ at the LHC. The data were collected by the CMS experiment during 2012 and correspond to an integrated luminosity of $19.7{\mathrm{fb}}^{-1}$. Both single and pair production of VLQs are considered. The single-production search is performed for down-type VLQs (electric charge of magnitude $1/3$), while the pair-production search is sensitive to up-type (charge of magnitude $2/3$) and down-type VLQs. Final states with at least one muon or one electron are considered. No significant excess over standard model expectations is observed, and lower limits on the mass of VLQs are derived. The lower limits range from 400 to 1800 GeV, depending on the single-production cross section and the VLQ branching fractions $\mathcal{B}$ to $W$, $Z$, and Higgs bosons. When considering pair production alone, VLQs with masses below 845 GeV are excluded for $\mathcal{B}(W)=1.0$, and below 685 GeV for $\mathcal{B}(W)=0.5$, $\mathcal{B}(Z)=\mathcal{B}(H)=0.25$. The results are more stringent than those previously obtained for single and pair production of VLQs coupled to light quarks.

Figure 1

Vectorlike quarks (denoted $Q$) can be produced in proton-proton collisions either singly through electroweak interactions (the $t$ channel mode (upper) is shown as an example), or in pairs via the strong interaction (lower). For single production we consider in the present work only vectorlike quarks with electric charge of magnitude $1/3$ (denoted D).

Figure 2

The reconstructed mass of the VLQ candidate in the $W^{+}qq$ event category (upper) and the $W^{-}qq$ event category (lower), in the muon channel (left) and the electron channel (right). The contributions of simulated events where the $W$ boson is produced in association with light-flavor (LF) jets and heavy-flavor (HF) jets are shown separately. The distribution for a heavy VLQ signal (indicated as $\mathrm{D}q$ representing a down-type VLQ produced in association with a SM quark) of mass 1100 GeV and ${\tilde{\kappa }}_W=1$ (for ${\mathcal{B}}_W=0.5$ and ${\mathcal{B}}_Z={\mathcal{B}}_H=0.25$) is scaled up by a factor of 15 for visibility. The enhanced $D$ quark signal contribution in the $W^{-}qq$ event category in comparison to the $W^{+}qq$ event category is clearly shown. The hatched bands represent the combined statistical and systematic uncertainties, and the highest bin contains the overflow.

Figure 3

The reconstructed VLQ candidate mass in the $W^{-}qq$ category for the muon channel (left) and the electron channel (right), for the background prediction and the data. The solid bold (blue) line is the background distribution estimated from data, with a final shape correction that accounts for the difference between the $W+\text{jets}$ simulation in the control region and the $W^{-}qq$ signal region. The dashed (blue) line is the same, but without the shape correction. The dotted (grey) line represents the SM prediction from simulation. The lower panel shows the ratio of the data to the data-driven background distribution with shape corrections. For bins from 1000 GeV onwards, a wider bin width is chosen to reduce statistical uncertainties in the background estimation from the data control region. The horizontal error bars on the data points indicate the bin width.

Figure 4

The reconstructed mass of the VLQ candidate in the Zqq event category, in the muon channel (upper) and the electron channel (lower). The contributions of simulated events where the $Z$ boson is produced in association with light-flavor (LF) jets and heavy-flavor (HF) jets are shown separately. The distribution for a heavy VLQ signal (indicated as $\mathrm{D}q$ representing a down-type VLQ produced in association with a SM quark) of mass 1100 GeV and ${\tilde{\kappa }}_W=1$ (for ${\mathcal{B}}_W=0.5$ and ${\mathcal{B}}_Z={\mathcal{B}}_H=0.25$) is scaled by a factor of 10 for better visibility. The hatched bands represent the combined statistical and systematic uncertainties.

Figure 5

Reconstructed mass distributions for $WqWq$ (uppermost), $WqZq$ (middle), and $WqHq$ (lowest) reconstruction from a kinematic fit. Plots on the left are for the $\mu +\text{jets}$ channel and on the right, for the $e+\text{jets}$ channel. The distribution for pair-produced VLQs of mass 700 GeV for ${\mathcal{B}}_W=1.0$ (uppermost), ${\mathcal{B}}_W={\mathcal{B}}_Z=0.5$ (middle) and ${\mathcal{B}}_W={\mathcal{B}}_H=0.5$ (lowest) are scaled up for visibility by a factor of 5, 10 and 5, respectively. The hatched bands represent the combined statistical and systematic uncertainties.

Figure 6

Mass distributions for the $WqWq$ (upper left), $WqZq$ (upper right), and $WqHq$ (lower) reconstructions from a kinematic fit for the combination of the $\mu +\text{jets}$ and $e+\text{jets}$ channel, for events with $S_{\mathrm{T}}>1240\mathrm{GeV}$. The distribution for pair-produced VLQs of mass 800 GeV for ${\mathcal{B}}_W=1.0$ (upper left), ${\mathcal{B}}_W={\mathcal{B}}_Z=0.5$ (upper right) and ${\mathcal{B}}_W={\mathcal{B}}_H=0.5$ (lower) is scaled up for visibility by a factor of 5, 10 and 15, respectively. The hatched bands represent the combined statistical and systematic uncertainties. The horizontal error bars on the data points only indicate the bin width.

Figure 7

Mass distribution for the $WqHq$ reconstruction from a kinematic fit, for combined $\mu +\text{jets}$ and $e+\text{jets}$ channels and for events with $S_{\mathrm{T}}>1240\mathrm{GeV}$. Events appearing also in the $WqWq$ sample have been removed. The distribution for pair-produced VLQs of mass 800 GeV for ${\mathcal{B}}_W={\mathcal{B}}_H=0.5$ is scaled up by a factor of 15 for visibility. The hatched band represent the combined statistical and systematic uncertainties. The horizontal error bars on the data points only indicate the bin width.

Figure 8

The reconstructed mass of the VLQ candidate in the $ZqHq$ event category, in the muon channel (upper) and the electron channel (lower). The contributions of simulated events where the $Z$ boson is produced in association with light-flavor (LF) jets and heavy-flavor (HF) jets are shown separately. The distribution for a heavy VLQ signal of mass 600 GeV and ${\tilde{\kappa }}_W=0.1$ (for ${\mathcal{B}}_W=0.5$ and ${\mathcal{B}}_Z={\mathcal{B}}_H=0.25$) is scaled up by a factor of 10 for visibility. The hatched bands represent the combined statistical and systematic uncertainties.

Figure 9

The reconstructed mass of the VLQ candidate in the dileptonic $VqZq$ event category, in the muon channel (upper) and the electron channel (lower). The contributions of simulated events where the $Z$ boson is produced in association with light-flavor (LF) jets and heavy-flavor (HF) jets are shown separately. The distribution for a heavy VLQ signal of mass 600 GeV and ${\tilde{\kappa }}_W=0.1$ (for ${\mathcal{B}}_W=0.5$ and ${\mathcal{B}}_Z={\mathcal{B}}_H=0.25$) is scaled up by a factor of 10 for visibility. The hatched bands represent the combined statistical and systematic uncertainties.

Figure 10

The $S_T$ variable in the dileptonic $WqWq$ event category, in the muon channel (upper) and in the electron channel (lower). The contributions of simulated events where the $Z$ boson is produced in association with light-flavor (LF) jets and heavy-flavor (HF) jets are shown separately. The distribution for a heavy VLQ signal of mass 600 GeV and ${\tilde{\kappa }}_W=0.1$ (for ${\mathcal{B}}_W=0.5$ and ${\mathcal{B}}_Z={\mathcal{B}}_H=0.25$) is scaled up by a factor of 10 for visibility. The hatched bands represent the combined statistical and systematic uncertainties.

Figure 11

The 95% C.L. exclusion limits on the product of the production cross section and the branching fraction, considering only single production of down-type VLQs, and assuming a neutral current coupling of zero (upper) or a charged current coupling of zero (lower). The median expected and observed exclusion limits are indicated with a dashed and a solid line, respectively. The inner (green) band and the outer (yellow) band indicate the regions containing 68% and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The corresponding LO theory predictions are superimposed. The predictions are represented by a solid black line centered within a blue band, which shows the uncertainty of the calculation. The uncertainties are determined based on the choice of PDF set along with the renormalization and factorization scales.

Figure 12

The median expected (upper) and observed (lower) combined lower mass limits represented in a triangular form, where each point of the triangle corresponds to a given set of branching fractions for the decay of a VLQ into a boson and a first-generation quark. The limit contours are determined assuming that ${\tilde{\kappa }}_W=1.0$, which means that the signal is dominated by electroweak single production. The black shaded band near ${\mathcal{B}}_W=0$ shows a region where the results cannot be reliably interpreted because ${\tilde{\kappa }}_Z$ diverges, as explained in the text.

Figure 13

The median expected (upper) and observed (lower) combined lower mass limits represented in a triangular form, where each point of the triangle corresponds to a given set of branching fractions for a VLQ decaying into a boson and a first-generation quark. The limit contours are determined assuming ${\tilde{\kappa }}_W=0.7$, which means that the signal will be dominated by electroweak single production for most of the parameter space represented by the triangles. The black shaded band near ${\mathcal{B}}_W=0$ represents a region where results cannot be reliably interpreted because ${\tilde{\kappa }}_Z$ diverges, as explained in the text.

Figure 14

The median expected (upper) and observed (lower) combined lower mass limits represented in a triangular form, where each point of the triangle corresponds to a given set of branching fractions for a VLQ decaying into a boson and a first-generation quark. The limit contours are determined assuming ${\tilde{\kappa }}_W=0.4$, which means that the signal is dominated by electroweak single production in most of the parameter space represented by the triangles, but in which the relative importance of the pair-produced signal has increased. The black shaded band near ${\mathcal{B}}_W=0$ represents a region where results cannot be reliably interpreted because ${\tilde{\kappa }}_Z$ diverges, as explained in the text.

Figure 15

The median expected (upper) and observed (lower) combined lower mass limits represented in a triangular form, where each point of the triangle corresponds to a given set of branching fractions for a VLQ decaying into a boson and a first-generation quark. The limit contours are determined assuming ${\tilde{\kappa }}_W=0.1$, which means that the signal is dominated by strong pair production for most of the parameter space represented by the triangles. The black shaded band near ${\mathcal{B}}_W=0$ indicates a region where results cannot be reliably interpreted because ${\tilde{\kappa }}_Z$ diverges, as explained in the text.

Figure 16

The median expected (upper) and observed (lower) combined lower mass limits represented in a triangular form, where each point of the triangle corresponds to a given set of branching fractions for a VLQ decaying into a boson and a first-generation quark. The limit contours are determined assuming that ${\tilde{\kappa }}_W$ and ${\tilde{\kappa }}_Z$ are so small that the single-production modes can be neglected, and therefore that the heavy quarks can only be produced in pairs via strong interaction. The white area in the triangle with expected limits indicates mass limits below 400 GeV.

Figure 17

The 95% C.L. exclusion limits on the production cross section determined assuming different sets of model parameters (${\mathcal{B}}_W=0.5$, ${\mathcal{B}}_Z=0.25$ (upper), and ${\mathcal{B}}_W=1$ (lower)) as a function of the hypothetical VLQ mass, and for the scenario where only strong pair production of the VLQs is considered. The median expected and observed exclusion limits are indicated with a dashed and a solid line, respectively. The inner (green) band and the outer (yellow) band indicate the regions containing 68% and 95%, respectively, of the distribution of limits expected under the background-only hypothesis. The cross section from a full NNLO calculation [25], including uncertainties in the PDF description and the renormalization and factorization scales, is shown by the magenta band.