First Measurement of Charm Production in its Fixed-Target Configuration at the LHC

The first measurement of heavy-flavor production by the LHCb experiment in its fixed-target 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 nucleon-nucleon center-of-mass energies of $\sqrt{s_{\mathrm{NN}}}=86.6$ and 110.4 GeV, respectively. The $J/\psi$ and $D^0$ production cross sections in $p\mathrm{He}$ collisions in the rapidity range [2, 4.6] are found to be ${\sigma }_{J/\psi }=652\pm 33(\text{stat})\pm 42(\text{syst})\mathrm{nb}/\text{nucleon}$ and ${\sigma }_{D^0}=80.8\pm 2.4(\text{stat})\pm 6.3(\text{syst})\mu \mathrm{b}/\text{nucleon}$, 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.

Figure 1

Mass distributions, fitted by an extended unbinned maximum-likelihood in $\sqrt{s_{\mathrm{NN}}}=86.6\mathrm{GeV}$ $p\mathrm{He}$ collisions; $J/\psi \to {\mu }^{-}{\mu }^{+}$ (left); $D^0\to K^{-}{\pi }^{+}$ (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.

Figure 2

Left: $J/\psi$ cross-section measurements as a function of the center-of-mass energy. Experimental data, represented by black points, are taken from Ref. [42]. The band corresponds to a fit based on NLO NRQCD calculations [42]. Right: $c\overline{c}$ cross-section measurements as a function of the center-of-mass energy. Experimental data, represented by black points, are taken from Ref. [43]. The yellow band corresponds to NLO pQCD calculations [44]. Red points correspond to the $p\mathrm{He}$ results from the present analysis.

Figure 3

Differential $J/\psi$ production cross sections for (top) $p\mathrm{He}$ and differential $J/\psi$ yields for (bottom) $p\mathrm{Ar}$ collisions, as a function of (left) center-of-mass rapidity $y^{*}$ and (right) transverse momentum $p_T$. The data points mark the bin centers. 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. The lower panel of each plot shows the ratio of data to HELAC-ONIA $pp$ predictions.

Figure 4

Differential $D^0$ production cross sections for (top) $p\mathrm{He}$ and differential $D^0$ yields for (bottom) $p\mathrm{Ar}$ collisions, as a function of (left) center-of-mass rapidity $y^{*}$ and (right) transverse momentum $p_T$. The data points mark the bin centers. The quadratic sum of statistical and uncorrelated systematic uncertainties are indicated by the vertical black lines. The correlated systematic uncertainties are indicated by the gray area. Theoretical predictions are described in the text. The lower panel of each plot shows the ratio of data to HELAC-ONIA $pp$ predictions.