Observation of Top Quark Production in Proton-Nucleus Collisions

The first observation of top quark production in proton-nucleus collisions is reported using proton-lead data collected by the CMS experiment at the CERN LHC at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{NN}}=8.16\mathrm{TeV}$. The measurement is performed using events with exactly one isolated electron or muon candidate and at least four jets. The data sample corresponds to an integrated luminosity of $174{\mathrm{nb}}^{-1}$. The significance of the $t\overline{t}$ signal against the background-only hypothesis is above 5 standard deviations. The measured cross section is ${\sigma }_{t\overline{t}}=45\pm 8\mathrm{nb}$, consistent with predictions from perturbative quantum chromodynamics.

Figure 1

Invariant mass distributions of the $W$ candidate, $m_{j{j}^{\prime }}$, in the 0 (left), 1 (center), and 2 (right) $b$-tagged jet categories after all selections. The red and orange areas correspond to the signal simulation (correct and wrong assignments, respectively), while the blue one corresponds to the estimated nontop background contributions. The error bars indicate the statistical uncertainties.

Figure 2

Invariant mass distributions of the $t\to j{j}^{\prime }b$ candidates, $m_{\mathrm{top}}$, in the 0 (left), 1 (center), and 2 (right) $b$-tagged jet categories after all selections. All signal and background parameters are kept fixed to the outcome of the $m_{j{j}^{\prime }}$ fit. Symbols and patterns are the same as in Fig. 1.

Figure 3

Total $t\overline{t}$ cross sections measured in the $e+\mathrm{jets}$, $\mu +\mathrm{jets}$, and combined $\ell +\mathrm{jets}$ channels in $p\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=8.16\mathrm{TeV}$, compared to theoretical $\mathrm{NNLO}+\mathrm{NNLL}$ predictions, and to scaled $\sqrt{s}=8\mathrm{TeV}$ $pp$ results [38, 39]. The total experimental error bars (theoretical error bands) include statistical and systematic (PDF and scale) uncertainties added in quadrature.