The first measurement of heavyflavour production by the LHCb experiment in its fixedtarget mode is presented. The production of $J/\psi$ and $D^0$ mesons is studied with beams of protons of different energies colliding with gaseous targets of helium and argon with nucleonnucleon centreofmass energies of $\sqrt{s_{NN}} = 86.6 $ and $ 110.4$ ${\rm GeV}$, respectively. The $J/\psi$ and $D^0$ production crosssections in $p{\rm He}$ collisions in the rapidity range $[2,4.6]$ are found to be $\sigma_{J/\psi} = 652 \pm 33$ (stat) $\pm 42$ (syst) nb$/$nucleon and $\sigma_{D^0} = 80.8 \pm 2.4$ (stat) $\pm 6.3$ (syst) $\mu$b$/$nucleon, where the first uncertainty is statistical and the second is systematic. No evidence for a substantial intrinsic charm content of the nucleon is observed in the large Bjorken$x$ region.
Mass distributions, fitted by an extended unbinned maximumlikelihood in $\sqrt{s_{\scriptscriptstyle\rm NN}} =86.6\mathrm{\,GeV} $ $ p\text{He}$ collisions; $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu} \rightarrow \mu ^ \mu ^+ $ (left); $ D ^0 \rightarrow K ^ \pi ^+ $ (right). The dashed blue line corresponds to the combinatorial background, the red line to the signal and the solid blue line to the sum of the two. 
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(left) $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ crosssection measurements as a function of the centreofmass energy; Experimental data, represented by black points, are taken from Ref. [38]. The band corresponds to a fit based on NLO NRQCD calculations [38]. (right) $c\overline{c}$ crosssection measurements as a function of the centreofmass energy; Experimental data, represented by black points, are taken from Ref. [39]; The yellow band corresponds to NLO pQCD calculations [40]. Red points correspond to the $ p\text{He}$ results from the present analysis. 
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Differential $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ production crosssections for (top) $ p\text{He}$ and differential $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ yields for (bottom) $ p\text{Ar}$ collisions, as a function of (left) centreofmass rapidity $y^*$ and (right) transverse momentum $ p_{\mathrm{T}} $. The data points mark the bin centres. The quadratic sum of statistical and uncorrelated systematic uncertainties are indicated by the vertical black lines. The correlated systematic uncertainties are indicated by the grey area. Theoretical predictions are described in the text. 
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Differential $ D ^0$ production crosssections for (top) $ p\text{He}$ and differential $ D ^0$ yields for (bottom) $ p\text{Ar}$ collisions, as a function of (left) centreofmass rapidity $y^*$ and (right) transverse momentum $ p_{\mathrm{T}} $. The data points mark the bin centres. The quadratic sum of statistical and uncorrelated systematic uncertainties are indicated by the vertical black lines. The correlated systematic uncertainties are indicated by the grey area. Theoretical predictions are described in the text. 
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Animated gif made out of all figures. 
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Systematic and statistical uncertainties on the $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ and $ D ^0$ yields in $\%$. Systematic uncertainties correlated between bins affect all measurements by the same relative amount. Ranges denote the minimum and the maximum values among the $y$ or $ p_{\mathrm{T}}$ bins. 
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Differential $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ production crosssections for $p$He and differential yields for $p$Ar collisions as a function of transverse momentum. The first uncertainty is bindependent (statistical and uncorrelated systematic uncertainties are quadratically added), and the second stands for the correlated systematic uncertainty. Yields are normalized to unity. 
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Differential $ { J \mskip 3mu/\mskip 2mu\psi \mskip 2mu}$ production crosssections for $p$He and differential yields for $p$Ar collisions as a function of rapidity. The first uncertainty is bindependent (statistical and uncorrelated systematic uncertainties are quadratically added), and the second stands for the correlated systematic uncertainty. Yields are normalized to unity. 
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Differential $ D ^0$ production crosssections for $p$He and differential yields for $p$Ar collisions as a function of transverse momentum. The first uncertainty is bindependent (statistical and uncorrelated systematic uncertainties are quadratically added), and the second stands for the correlated systematic uncertainty. Yields are normalized to unity. 
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Differential $ D ^0$ production crosssections for $p$He and differential yields for $p$Ar collisions as a function of rapidity. The first uncertainty is bindependent (statistical and uncorrelated systematic uncertainties are quadratically added), and the second stands for the correlated systematic uncertainty. Yields are normalized to unity. 
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The relative increase of the total efficiency (in $\%$), assuming $\lambda_\theta=0.2$ longitudinal polarisation rather than zero polarisation, in different bins of rapidity $y$ in the laboratory frame. 
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The relative increase of the total efficiency (in $\%$), assuming $\lambda_\theta=0.2$ longitudinal polarisation rather than zero polarisation in different bins of $ p_{\mathrm{T}} $. 
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Supplementary material full pdf 
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This ZIP file contains supplemetary material for the publication LHCbPAPER2018023. The files are: Supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps *.C : The figures in variuous formats 
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Created on 20 February 2019.Citation count from INSPIRE on 21 February 2019.