Search for single production of vector-like quarks decaying to a top quark and a W boson in proton-proton collisions at s = 13 TeV

Abstract

A search is presented for the single production of vector-like quarks in proton-proton collisions at $\sqrt{s}=13\text{TeV}$ . The data, corresponding to an integrated luminosity of 35.9 ${\text{fb}}^{-1}$ , were recorded with the CMS experiment at the LHC. The analysis focuses on the vector-like quark decay into a top quark and a $W$ boson, with one muon or electron in the final state. The mass of the vector-like quark candidate is reconstructed from hadronic jets, the lepton, and the missing transverse momentum. Methods for the identification of $b$ quarks and of highly Lorentz boosted hadronically decaying top quarks and $W$ bosons are exploited in this search. No significant deviation from the standard model background expectation is observed. Exclusion limits at 95% confidence level are set on the product of the production cross section and branching fraction as a function of the vector-like quark mass, which range from 0.3 to 0.03 $\text{pb}$ for vector-like quark masses of 700 to 2000 $\text{GeV}$ . Mass exclusion limits up to 1660 $\text{GeV}$ are obtained, depending on the vector-like quark type, coupling, and decay width. These represent the most stringent exclusion limits for the single production of vector-like quarks in this channel.

Fig. 1

Leading order Feynman diagrams for the production of a single vector-like $\mathrm{B}$ or $X_{5/3}$ quark in association with a $\mathrm{b}$ (left) or $\mathrm{t}$ (right) and a light-flavour quark, and the subsequent decay of the VLQ to $\mathrm{t}\mathrm{W}$

Fig. 2

Distributions of $m_{\mathrm{reco}}$ for the $\mathrm{B}$ +$\mathrm{b}$ production mode, obtained for simulated events with a muon in the final state, reconstructed with a $\mathrm{t}$ tag (top) and with the ${\chi }^2$ method (bottom) for right-handed VLQ couplings and various VLQ masses $m_{\mathrm{B}}$. Signal events are shown assuming a production cross section of 1$\text{pb}$ and a relative VLQ decay width of 1%

Fig. 3

Distributions of $m_{\mathrm{reco}}$ in data and simulation in the control region for the muon (upper) and electron (lower) channels for events reconstructed with a $\mathrm{t}$ tag (left) and with the ${\chi }^2$ method (right). The VLQ signal is shown for the $\mathrm{B}$ +$\mathrm{b}$ production mode and right-handed VLQ couplings. The vertical bars illustrate the statistical uncertainties on the data, while the shaded area shows the total uncertainties for the background simulation. The lower panels show the ratio of data to the background prediction. The dark and light gray bands correspond to the statistical and total uncertainties, respectively

Fig. 4

Distributions of $m_{\mathrm{reco}}$ in the validation region of the two most sensitive categories in the muon channel (upper) and electron channel (lower). The lower panels show the difference of data and background expectations in units of the total (stat. and sys.) uncertainty on the background estimate

Fig. 5

Distributions of $m_{\mathrm{reco}}$ measured in the signal region for events with a jet in the forward direction with $|\eta |>2.4$ in the muon channel. Shown are the sensitive categories: $\mathrm{t}$ tag (upper left), $\mathrm{W}$ tag (upper right), $\ge 2$ $\mathrm{b}$ tag (middle left), 1 $\mathrm{b}$ tag (middle right) and 0 $\mathrm{b}$ tag (lower). The background prediction is obtained from control regions as detailed in the main text. The distributions from two example signal samples for the $\mathrm{B}$ +$\mathrm{b}$ production mode with right-handed VLQ couplings with a cross section of 1$\text{pb}$ and a relative width of 1% are shown for illustration

Fig. 6

Distributions of $m_{\mathrm{reco}}$ measured in the signal region for events with a jet in the forward direction with $|\eta |>2.4$ in the electron channel. Shown are the sensitive categories: $\mathrm{t}$ tag(upper left), $\mathrm{W}$ tag(upper right), $\ge 2$ $\mathrm{b}$ tag (middle left), 1 $\mathrm{b}$ tag (middle right) and 0 $\mathrm{b}$ tag (lower). The background prediction is obtained from control regions as detailed in the main text. The distributions from two example signal samples for the $\mathrm{B}$ +$\mathrm{b}$ production mode with right-handed VLQ couplings with a cross section of 1$\text{pb}$ and a relative VLQ width of 1% are shown for illustration

Fig. 7

Upper limits at 95% $\text{CL}$ on the product of the VLQ production cross section and branching fraction for the $\mathrm{B}$ +$\mathrm{b}$ production mode for a relative VLQ width of 1% and left- and right-handed VLQ couplings (upper left and right), for 10% relative VLQ width and left-handed VLQ couplings (lower left), and a comparison of the observed exclusion limits for relative VLQ widths of 10, 20, and 30% for left-handed couplings (lower right). The dashed lines show the theoretical predictions

Fig. 8

Upper limits at 95% $\text{CL}$ on the product of the VLQ production cross section and branching fraction for the $\mathrm{B}$ +$\mathrm{t}$ and $X_{5/3}$+$\mathrm{t}$ production modes for right-handed VLQ couplings assuming a relative VLQ width of 1% (upper left and right), for the $X_{5/3}$+$\mathrm{t}$ production mode with left-handed VLQ couplings and a 10% relative width (lower left) and a comparison of the observed exclusion limits for left-handed couplings for relative widths of 10, 20, and 30% (lower right). The dashed lines show the theoretical predictions