Measurements of the charge asymmetry in top-quark pair production in the dilepton final state at $\sqrt{s}=8\mathrm{TeV}$ with the ATLAS detector

Measurements of the top-antitop quark pair production charge asymmetry in the dilepton channel, characterized by two high-$p_{\mathrm{T}}$ leptons (electrons or muons), are presented using data corresponding to an integrated luminosity of $20.3{\mathrm{fb}}^{-1}$ from $pp$ collisions at a center-of-mass energy $\sqrt{s}=8\mathrm{TeV}$ collected with the ATLAS detector at the Large Hadron Collider at CERN. Inclusive and differential measurements as a function of the invariant mass, transverse momentum, and longitudinal boost of the $t\overline{t}$ system are performed both in the full phase space and in a fiducial phase space closely matching the detector acceptance. Two observables are studied: $A_{\mathrm{C}}^{\ell \ell }$ based on the selected leptons and $A_{\mathrm{C}}^{t\overline{t}}$ based on the reconstructed $t\overline{t}$ final state. The inclusive asymmetries are measured in the full phase space to be $A_{\mathrm{C}}^{\ell \ell }=0.008\pm 0.006$ and $A_{\mathrm{C}}^{t\overline{t}}=0.021\pm 0.016$, which are in agreement with the Standard Model predictions of $A_{\mathrm{C}}^{\ell \ell }=0.0064\pm 0.0003$ and $A_{\mathrm{C}}^{t\overline{t}}=0.0111\pm 0.0004$.

Figure 1

Distributions of the jet multiplicity, lepton $p_T$, and lepton $\eta$ for data (points) and predictions (histograms) for all channels combined after event selection. The data/expected ratio is also shown. The shaded area corresponds to the detector systematic uncertainty, the signal modeling systematic uncertainty, and the normalization uncertainty in signal and background. In the lepton $p_{\mathrm{T}}$ distribution, the last bin includes the overflow.

Figure 2

Distributions of $p_{\mathrm{T},t\overline{t}}$, $m_{t\overline{t}}$, and $|{\beta }_{z,t\overline{t}}|$ for data (points) and predictions (histograms) after kinematic reconstruction. The data/expected ratio is also shown. The shaded area corresponds to the detector systematic uncertainty, the signal modeling systematic uncertainty, and the normalization uncertainty in signal and background. In the $p_{\mathrm{T},t\overline{t}}$ and $m_{t\overline{t}}$ distributions, the last bin includes the overflow.

Figure 3

Rectangular migration matrix for the $\mathrm{\Delta }|y|$ observable in the fiducial volume. The first four columns correspond to the $ee$ channel, followed by $\mu \mu$ and $e\mu$. The numbers are normalized by row for each channel.

Figure 4

Input distributions for the inclusive $t\overline{t}$ (top) and leptonic (bottom) asymmetry measurements. The first 4 $\mathrm{\Delta }|y|$ and 16 $\mathrm{\Delta }|\eta |$ bins correspond to the $ee$ channel, followed by $\mu \mu$ and $e\mu$. The bin boundaries are symmetric around zero and are defined as [0.0, 0.75, 5.0] and [0.0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.7, 1.9, 5.0] for $\mathrm{\Delta }|y|$ and $\mathrm{\Delta }|\eta |$, respectively. The data/expected ratio is also shown.

Figure 5

Summary of all the measurements in this paper for the leptonic asymmetry in the fiducial volume (top) and full phase space (bottom). The predictions shown in blue are obtained using Powheg-hvq $+$ Pythia6 at NLO where the uncertainties are statistical, and the corresponding theoretical uncertainties are small compared to the experimental precision. The inclusive measurement in the full phase space is compared to a $\mathrm{NLO}+\mathrm{EW}$ prediction [23].

Figure 6

Summary of all the measurements in this paper for the $t\overline{t}$ asymmetry in the fiducial volume (top) and full phase space (bottom). The predictions shown in blue are obtained using Powheg-hvq $+$ Pythia6 at NLO where the uncertainties are statistical, and the corresponding theoretical uncertainties are small compared to the experimental precision. The inclusive measurement in the full phase space is compared to a $\mathrm{NLO}+\mathrm{EW}$ prediction [23].

Figure 7

Data distribution after the unfolding procedure compared with the Powheg-hvq $+$ Pythia6 prediction at NLO for the inclusive $\mathrm{\Delta }|\eta |$ (left) and $\mathrm{\Delta }|y|$ (right) observables in the fiducial volume. The data/expected ratio is also shown.

Figure 8

Comparison of the inclusive $A_{\mathrm{C}}^{\ell \ell }$ and $A_{\mathrm{C}}^{t\overline{t}}$ measurement values in the full phase space to the SM NLO $\mathrm{QCD}+\mathrm{EW}$ prediction [23] and to two benchmark BSM models [77], one with a light octet with mass below the $t\overline{t}$ production threshold (left) and one with a heavy octet with mass beyond the reach of the LHC (right), for various couplings as described in the legend. Ellipses corresponding to $1\sigma$ and $2\sigma$ combined statistical and systematic uncertainties of the measurement, including the correlation between $A_{\mathrm{C}}^{\ell \ell }$ and $A_{\mathrm{C}}^{t\overline{t}}$, are also shown.