Speaker
Description
In proton-nucleus collisions, the top quark is a novel and theoretically precise probe of the nuclear gluon density at high virtualities $Q^2\approx m_{\mathrm{top}}$ and in the less explored high Bjorken-$x$ region. The first observation of the inclusive $\mathrm{t\bar{t}}$ production has been performed using 174$\pm$6 $\mathrm{nb}^{-1}$ of data in pPb collisions at $\sqrt{\smash [b]{s_{_{\mathrm {NN}}}}}=8.16$ TeV. However, the total uncertainty of about 17% is not sufficient for imposing constraints on current nPDF parameterizations, the dominant source of uncertainty in the theoretical prediction of $\sigma(\mathrm{pPb}\rightarrow\mathrm{t\bar{t}} + X)$. The prospects of measuring $\sigma$ differentially have recently been examined and a feasibility study of the measurement with the CMS detector at the High-Luminosity LHC (HL-LHC) era is therefore carried out as a function of the reconstructed lepton $p_{\mathrm{T}}$ and rapidity. The relative statistical uncertainty in both variables is found to be at the level of 4$-$5% in each bin, and it is expected to be the dominant uncertainty at $\sqrt{\smash [b]{s_{_{\mathrm {NN}}}}}=8.16$ TeV for an integrated luminosity scenario of 2 $\mathrm{pb}^{-1}$. The motivations for measurements of top quarks in nucleus-nucleus collisions are multifold and are discussed: the top quark decay products are sensitive to the energy loss of heavy quarks, and a probe of the space-time structure of the QGP at HL-LHC.
