Observing the Λc baryon is difficult because of its very short life time (cτ ~ 60 microns). This measurement was only possible because of the excellent pointing resolution of the STAR Heavy Flavor Tracker (HFT). As Fig. 1 shows, this enabled a clear observation of the Λc mass peak containing 215 ± 21 events
Two mechanisms for charmed baryon formation have been proposed. In the first, Λc baryons are produced from the fragmentation of energetic charm quark, via string fragmentation. This mechanism is embodied in the PYTHIA event generator. In the second mechanism, Λc baryons are produced through the coalescence of nearby (in phase space) charm quark and light quarks. In relativistic heavy-ion collisions, the coalescence mechanism had been expected to be dominant due to the extreme high parton density environment created in these collisions.
Figure 2 shows the ratio of Λc yield to D0 meson yield as a function of particle transverse momentum (pT) compared to various model calculations. The data show a factor of ~5 enhancement compared to the p+p collision baseline from the PYTHIA model with charm fragmentation (green solid line) constrained by previous ee, ep and pp collisions. Other colored lines in the figure depict model calculations incorporating the coalescence hadronization mechanism for charm quarks in the hot and dense medium. These calculations show comparable magnitude and pT dependence as seen in the data while they differ in the medium and coalescence parameters. Our new result suggests that the coalescence mechanism plays an important role for charm quark hadronization in heavy-ion collisions.
The analysis was led by NSD scientists from the RNC group together with collaborators from Czech Technical University, University of Illinois Chicago, University of Science and Technology of China and other STAR institutions.
Reference
[1] J. Adam et al. [STAR Collaboration], “Observation of enhancement of charmed baryon-to-meson ratio in Au+Au collisions at √sNN = 200 GeV, Phys. Rev. Lett. 124, 172301 (2020).
