This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb$^{-1}$ of $p$+Pb data collected at $\sqrt{s_\mathrm{NN}}=8.16$ TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one $b$-tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be $\sigma_{t\bar{t}}= 58.1\pm 2.0\;\mathrm{(stat.)\;^{+4.8}_{-4.4} \;\mathrm{(syst.)}}\;\mathrm{nb}$, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be $R_{p\mathrm{A}} = 1.090\pm0.039\;(\mathrm{stat.})\;^{+0.094}_{-0.087}\;(\mathrm{syst.})$. The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions.
The figure shows the pre-fit distribution of events as a function of $H_{\mathrm{T}}^{\ell,j} = \sum_{\ell,j} p_{T}^{\ell,j}$, scalar sum of $p_T$ for all jets and leptons in the $\ell+$jets channel, in proton-lead (p+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 8.16$ TeV, with an integrated luminosity of 165 nb$^{-1}$. The data correspond to the $1\ell 1b$ $e$+jets channel in a pre-fit configuration. The stacked histograms represent different processes contributing to the event yield, including top quark pair production ($t\bar{t}$), single top, $W$ boson production with $b$, $c$, and light quarks, $Z$ boson production with $b$, $c$, and light quarks, diboson, and fake lepton backgrounds.
The figure shows the post-fit distribution of events as a function of $H_{\mathrm{T}}^{\ell,j} = \sum_{\ell,j} p_{T}^{\ell,j}$, scalar sum of $p_T$ for all jets and leptons in the $\ell+$jets channel, in proton-lead (p+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 8.16$ TeV, with an integrated luminosity of 165 nb$^{-1}$. The data correspond to the $1\ell 1b$ $e$+jets channel in a pre-fit configuration. The stacked histograms represent different processes contributing to the event yield, including top quark pair production ($t\bar{t}$), single top, $W$ boson production with $b$, $c$, and light quarks, $Z$ boson production with $b$, $c$, and light quarks, diboson, and fake lepton backgrounds.
The figure shows the pre-fit distribution of events as a function of $H_{\mathrm{T}}^{\ell,j} = \sum_{\ell,j} p_{T}^{\ell,j}$, scalar sum of $p_T$ for all jets and leptons in the $\ell+$jets channel, in proton-lead (p+Pb) collisions at a center-of-mass energy of $\sqrt{s_{\mathrm{NN}}} = 8.16$ TeV, with an integrated luminosity of 165 nb$^{-1}$. The data correspond to the $1\ell 1b$ $\mu$+jets channel in a pre-fit configuration. The stacked histograms represent different processes contributing to the event yield, including top quark pair production ($t\bar{t}$), single top, $W$ boson production with $b$, $c$, and light quarks, $Z$ boson production with $b$, $c$, and light quarks, diboson, and fake lepton backgrounds.
The transverse momentum spectra of D$^0$ mesons from b hadron decays are measured at midrapidity ($|y|$ $<$ $1$) in pp and PbPb collisions at a nucleon-nucleon center of mass energy of 5.02 TeV with the CMS detector at the LHC. The D$^0$ mesons from b hadron decays are distinguished from prompt D$^0$ mesons by their decay topologies. In PbPb collisions, the B $\to$ D$^0$ yield is found to be suppressed in the measured $p_\mathrm{T}$ range from 2 to 100 GeV$/c$ as compared to pp collisions. The suppression is weaker than that of prompt D$^0$ mesons and charged hadrons for $p_\mathrm{T}$ around 10 GeV$/c$. While theoretical calculations incorporating partonic energy loss in the quark-gluon plasma can successfully describe the measured B $\to$ D$^0$ suppression at higher $p_\mathrm{T}$, the data show an indication of larger suppression than the model predictions in the range of 2 $\lt$ $p_\mathrm{T}$ $\lt$ 5 GeV$/c$.
$ {{{\mathrm {B}}}\to {\mathrm {D^0}}} $ $ {p_{\mathrm {T}}} $ -differential cross section in pp collisions at ${\sqrt {\smash [b]{s}}} = $ 5.02 TeV.
$ {{{\mathrm {B}}}\to {\mathrm {D^0}}} $ $ {p_{\mathrm {T}}} $ -differential invariant yield in PbPb collisions normalized with $ {T_{\mathrm {AA}}} $ at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV.
The $\text {data}/\mathrm {FONLL}$ ratio for the $ {{{\mathrm {B}}}\to {\mathrm {D^0}}} $ $ {p_{\mathrm {T}}} $ spectra in pp collisions.
The differential yields of charged particles having pseudorapidity within $|\eta|<$ 1 are measured using xenon-xenon (XeXe) collisions at $\sqrt{s_\mathrm{NN}}$ = 5.44 TeV. The data, corresponding to an integrated luminosity of 3.42 $\mu$b$^{-1}$, were collected in 2017 by the CMS experiment at the LHC. The yields are reported as functions of collision centrality and transverse momentum, $p_\mathrm{T}$, from 0.5 to 100 GeV. A previously reported $p_\mathrm{T}$ spectrum from proton-proton collisions at $\sqrt{s}$ = 5.02 TeV is used for comparison after correcting for the difference in center-of-mass energy. The nuclear modification factors using this reference, $R_\mathrm{AA}^*$, are constructed and compared to previous measurements and theoretical predictions. In head-on collisions, the $R_\mathrm{AA}^*$ has a value of 0.17 in the $p_\mathrm{T}$ range of 6-8 GeV, but increases to approximately 0.7 at 100 GeV. Above $\approx$ 6 GeV, the XeXe data show a notably smaller suppression than previous results for lead-lead (PbPb) collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV when compared at the same centrality (i.e., the same fraction of total cross section). However, the XeXe suppression is slightly greater than that for PbPb in events having a similar number of participating nucleons.
The per-event differential invariant yield of charged particles having |eta|<1 in XeXe collisions at sqrt(s_NN)=5.44 TeV. The first systematic uncertainty describes uncertainties that are not fully correlated across points, while the second systematic uncertainty is a normalization uncertainty that is fully correlated across all points. Bins where no data point has been reported are denoted as 'empty'.
The per-event differential invariant yield of charged particles having |eta|<1 in pp collisions at sqrt(s)=5.44 TeV, after extrpolation from 5.02 TeV data. The first systematic uncertainty describes uncertainties that are not fully correlated across points, while the second systematic uncertainty is a normalization uncertainty that is fully correlated across all points. The data is measured and extrapolated as a differential cross section, and transformed into a differential yield using an inelastic cross-section of 70 mb.
The nuclear modification factor of charged particles having |eta|<1 in XeXe collisions at sqrt(s_NN)=5.44 TeV. The first systematic uncertainty describes uncertainties that are not fully correlated across points, while the second systematic uncertainty is a normalization uncertainty that is fully correlated across all points. Bins where no data point has been reported are denoted as 'empty'.
The transverse momentum ($p_\mathrm{t}$) spectrum of prompt D$^0$ mesons and their antiparticles has been measured via the hadronic decay channels D$^0 \to \mathrm{K}^- \pi^+$ and $\overline{\mathrm{D}}^0 \to \mathrm{K}^+ \pi^-$ in pp and PbPb collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair with the CMS detector at the LHC. The measurement is performed in the D$^0$ meson $p_\mathrm{t}$ range of 2-100 GeV and in the rapidity range of $|y| < $1. The pp (PbPb) dataset used for this analysis corresponds to an integrated luminosity of 27.4 pb$^{-1}$ (530 $\mu$b$^{-1}$). The measured D$^0$ meson $p_\mathrm{t}$ spectrum in pp collisions is well described by perturbative QCD calculations. The nuclear modification factor, comparing D$^0$ meson yields in PbPb and pp collisions, was extracted for both minimum-bias and the 10% most central PbPb interactions. For central events, the D$^0$ meson yield in the PbPb collisions is suppressed by a factor of 5-6 compared to the pp reference in the $p_\mathrm{t}$ range of 6-10 GeV. For D$^0$ mesons in the high-$p_\mathrm{t}$ range of 60-100 GeV, a significantly smaller suppression is observed. The results are also compared to theoretical calculations.
Nuclear modification factor of $(D^{0}+\overline{D}\hspace{.01cm}^{0})/2$ as a function of $p_{T}$ in the centrality range $0-100\%$ and $0-10\%$.
The $p_{T}$-differential production cross section of $(D^{0}+\overline{D}\hspace{.01cm}^{0})/2$ in pp collisions.
The $p_{T}$-differential production yields of $(D^{0}+\overline{D}\hspace{.01cm}^{0})/2$ divided by the nuclear overlap functions $T_{AA}$ for PbPb collisions in the $0-100\%$ and $0-10\%$ centrality ranges.
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