Measurement of the $t\overline{t}Z$ and $t\overline{t}W$ cross sections in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS detector

A measurement of the associated production of a top-quark pair ($t\overline{t}$) with a vector boson ($W$, $Z$) in proton-proton collisions at a center-of-mass energy of 13 TeV is presented, using $36.1{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. Events are selected in channels with two same- or opposite-sign leptons (electrons or muons), three leptons or four leptons, and each channel is further divided into multiple regions to maximize the sensitivity of the measurement. The $t\overline{t}Z$ and $t\overline{t}W$ production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are ${\sigma }_{t\overline{t}Z}=0.95\pm 0.08_{\mathrm{stat}}\pm 0.10_{\mathrm{syst}}\mathrm{pb}$ and ${\sigma }_{t\overline{t}W}=0.87\pm 0.13_{\mathrm{stat}}\pm 0.14_{\mathrm{syst}}\mathrm{pb}$ in agreement with the Standard Model predictions. The measurement of the $t\overline{t}Z$ cross section is used to set constraints on effective field theory operators which modify the $t\overline{t}Z$ vertex.

Figure 1

The BDT distributions for the $t\overline{t}$ control regions (a) $2\ell$-Z-6j1b, (b) $2\ell$-Z-5j2b, (c) $2\ell$-Z-6j2b. The shaded band represents the total uncertainty. The “Other” background contains SM processes with small cross sections producing two opposite-sign prompt leptons, including the $t\overline{t}Z$ process, whose contribution is negligible.

Figure 2

Distributions in the $2e$-SS-1b-VR validation region: (a) the invariant mass $m_{\ell \ell }$ of the lepton pair and (b) leading lepton transverse momentum $p_{\mathrm{T}}$. The shaded band represents the total uncertainty. The “Other” background contains SM processes with small cross sections producing two same-sign prompt leptons. The last bin in each of the distributions includes the overflow.

Figure 3

Distributions in fake-lepton control regions: (a) scalar sum of transverse momenta of leptons and jets, $H_{\mathrm{T}}$, in the $2e$-SS-$2b$-CR region, (b) missing transverse momentum, $E_{\mathrm{T}}^{\mathrm{miss}}$, in the $e\mu$-SS-$1b$-CR region and (c) subleading lepton transverse momentum, $p_{\mathrm{T}}$, in the $2\mu$-SS-$1b$-CR region. The shaded band represents the total uncertainty. The “Other” background contains SM processes with small cross sections producing two same-sign prompt leptons. The last bin in each of the distributions includes the overflow.

Figure 4

Distributions in the $2\mu$-SS, $e\mu$-SS and $2e$-SS regions of (a),(c),(e) the missing transverse momentum $E_{\mathrm{T}}^{\mathrm{miss}}$ and (b),(d),(f) the jet multiplicity. The shaded band represents the total uncertainty. The “Other” background contains SM processes with small cross sections producing two same-sign prompt leptons. The last bin in each of the distributions includes the overflow.

Figure 5

Distributions of (a) the leading lepton transverse momentum $p_{\mathrm{T}}$ and (b) the leading jet $p_{\mathrm{T}}$ in the $3\ell$-WZ-CR control region before the fit. The “Other” background contains SM processes with small cross sections producing three prompt leptons. The shaded band represents the total uncertainty. The last bin in each of the distributions includes the overflow.

Figure 6

Distributions of (a) the subleading lepton transverse momentum $p_{\mathrm{T}}$ and (b) missing transverse momentum $E_{\mathrm{T}}^{\mathrm{miss}}$ for events belonging to any of the four trilepton regions targeting the $t\overline{t}W$ process. The distributions are shown before the fit. The “Other” background contains SM processes with small cross sections producing three prompt leptons. The shaded band represents the total uncertainty. The last bin in each of the distributions includes the overflow.

Figure 7

Distributions of (a) the number of jets, (b) the transverse momentum $p_{\mathrm{T}}(\ell \ell )$ and (c) the mass $m_{\ell \ell }$ of the reconstructed $Z$ boson candidate for events in $3\ell$-Z-2b4j. The distributions are shown before the fit. The “Other” background contains SM processes with small cross sections producing three prompt leptons. The shaded band represents the total uncertainty. The last bin in each of the distributions includes the overflow.

Figure 8

Distribution of (a) the leading lepton transverse momentum $p_{\mathrm{T}}$ and (b) jet multiplicity in the $4\ell$-ZZ-CR control region. The distributions are shown before the fit. The shaded band represents the total uncertainty. The last bin in each of the distributions includes the overflow.

Figure 9

Distributions, for all tetralepton signal regions combined, of (a) the number of jets, (b) the invariant mass of the OSSF lepton pair closest to the $Z$ boson mass, $m_{Z_1}$, (c) the pseudorapidity separation $\mathrm{\Delta }\eta$ for that pair of leptons and (d) the azimuthal angle $\mathrm{\Delta }\varphi$ between the remaining two leptons. The “Other” background contains SM processes with small cross sections producing four prompt leptons. The distributions are shown before the fit. The shaded band represents the total uncertainty. The first and last bin include the underflow and overflow, respectively.

Figure 10

The BDT distributions for the OS dilepton signal regions, (a) $2\ell$-Z-6j1b, (b) $2\ell$-Z-5j2b, (c) $2\ell$-Z-6j2b. The distributions are shown after the fit. The “Other” background contains SM processes with small cross sections producing two opposite-sign prompt leptons. The shaded band represents the total uncertainty. The last bin of each distribution contains the overflow.

Figure 11

Event yields in data compared with the results of the fit that extracts ${\sigma }_{t\overline{t}Z}$ and ${\sigma }_{t\overline{t}W}$ simultaneously in the (a) trilepton and (b) tetralepton signal regions targeting the $t\overline{t}Z$ process. Yields for the control regions used to extract the normalization of the $WZ$ and $ZZ$ backgrounds are also shown. The “Other” background summarizes all small SM backgrounds described in Sec. 3. The shaded band represents the total uncertainty.

Figure 12

Event yields in data compared with the results of the fit that extracts ${\sigma }_{t\overline{t}Z}$ and ${\sigma }_{t\overline{t}W}$ simultaneously in the regions targeting the $t\overline{t}W$ process. The “Other” background summarizes all small SM backgrounds described in Sec. 3. The shaded band represents the total uncertainty.

Figure 13

The result of the simultaneous fit to the $t\overline{t}Z$ and $t\overline{t}W$ cross sections along with the 68% and 95% confidence level (C.L.) contours. The cross shows the SM calculations and their uncertainties, including renormalization and factorization scale uncertainties as well as uncertainties including ${\alpha }_{\mathrm{S}}$ variations.

Figure 14

The value of the profile-likelihood test statistic as a function of $c/{\mathrm{\Lambda }}^2$, for (a) ${\mathcal{C}}_{\varphi Q}^{(3)}$, (b) ${\mathcal{C}}_{\varphi t}$, (c) ${\mathcal{C}}_{tB}$, and (d) ${\mathcal{C}}_{tW}$. In the ${\mathcal{C}}_{\varphi Q}^{(3)}$ and ${\mathcal{C}}_{\varphi t}$ distributions, another, deeper minimum exists for large negative values of $C_i$, which is excluded by indirect measurements. There, the vertical axis is chosen such that the value of the likelihood at the minimum near $C_i=0$ is zero.