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We present the first predictions for sub-threshold open charm and charmonium production in nuclear collisions. The production mechanism is driven by multi-step scatterings of nucleons and their resonance states, accumulating sufficient energy for the production of $J/Psi$, $Lambda_c + overline{D}$ and $D+overline{D}$. Our results are of particular importance for the CBM experiment at FAIR, as they indicate that already at the SIS100 accelerator one can expect a significant number of charmed hadrons to be produced. This opens new possibilities to explore charm dynamics and the formation of charmed nuclei.
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Recent measurements of various charm-hadron ratios in $pp$, $p$-Pb and Pb-Pb collisions at the LHC have posed challenges to the theoretical understanding of heavy-quark hadronization. The $Lambda_c/D^0$ ratio in $pp$ and $p$-Pb collisions shows larger values than those found in $e^+e^-$ and $ep$ collisions and predicted by Monte-Carlo event generators based on string fragmentation, at both low and intermediate transverse momenta ($p_T$). In AA collisions, the $D_s/D$ ratio is significantly enhanced over its values in $pp$, while the $Lambda_c/D^0$ data indicates a further enhancement at intermediate $p_T$. Here, we report on our recent developments for a comprehensive description of the charm hadrochemistry and transport in $pp$ and $AA$ collisions. For $pp$ collisions we find that the discrepancy between the $Lambda_c/D^0$ data and model predictions is much reduced by using a statistical hadronization model augmented by a large set of missing states in the charm-baryon spectrum, contributing to the $Lambda_c$ via decay feeddown. For $AA$ collisions, we develop a 4-momentum conserving resonance recombination model for charm-baryon formation implemented via event-by-event simulations that account for space-momentum correlations (SMCs) in transported charm- and thermal light-quark distributions. The SMCs, together with the augmented charm-baryon states, are found to play an important role in describing the baryon-to-meson enhancement at intermediate momenta. We emphasize the importance of satisfying the correct (relative) chemical equilibrium limit when computing the charm hadrochemistry and its momentum dependence with coalescence models.
Kaon production in pion-nucleon collisions in nuclear matter is studied in the resonance model. To evaluate the in-medium modification of the reaction amplitude as a function of the baryonic density we introduce relativistic, mean-field potentials for the initial, final and intermediate mesonic and baryonic states. These vector and scalar potentials were calculated using the quark-meson coupling (QMC) model. The in-medium kaon production cross sections in pion-nucleon interactions for reaction channels with $Lambda$ and $Sigma$ hyperons in the final state were calculated at the baryonic densities appropriate to relativistic heavy ion collisions. Contrary to earlier work which has not allowed for the change of the cross section in medium, we find that the data for kaon production are consistent with a repulsive $K^+$-nucleus potential.
By solving the rate equation in an expanding quark-gluon plasma, we study thermal production of charm quarks in central Pb+Pb collisions at the Future Circular Collider. With the charm quark production cross section taken from the perturbative QCD at the next-to-leading order, we find that charm quark production from the quark-gluon plasma can be appreciable compared to that due to initial hard scattering between colliding nucleons.
We present results for the $p p to p p omega$ reaction studied by considering two different scenarios: with and without the inclusion of nucleon resonance excitations. The recently measured angular distribution by the COSY-TOF Collaboration at an excess energy of $Q = 173$ MeV and the energy dependence of the total cross section data for $pi^- p to omega n$ are used to calibrate the model parameters. The inclusion of nucleon resonances improves the theoretical prediction for the energy dependence of the total cross section in $pp to ppomega$ at excess energies $Q < 31$ MeV. However, it still underestimates the data by about a factor of two, and remains a problem in understanding the reaction mechanism.
We study sub-threshold heavy quarkonium ($J/psi$ and $Upsilon$) photo-productions in $gamma A$ collisions as an independent test of the universality of the nucleon-nucleon short range correlation (SRC) in nuclear scattering processes. Just below the $gamma p$ threshold, the cross section is dominated by the mean field contribution of nucleons inside the nucleus. The SRC contributions start to dominate at lower photon energies, depending on the fraction of the SRC pairs in the target nucleus. We give an estimate of the cross sections in the sub-threshold region both for $J/psi$ and $Upsilon$. This may be helpful for future measurements at JLab as well as at the Electron-Ion Collider in the U.S., and especially in China.
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