Scientists study how Bc mesons form to gain more information from ultra-relativistic heavy-ion collisions

The Science

High-energy collisions of atomic nuclei provide a unique opportunity to recreate the quark-gluon plasma (QGP) in the laboratory, for a brief moment. The QGP is a fundamental, extremely hot form of nuclear matter in which protons and neutrons dissolve into quarks and gluons. It filled the early universe in the first few microseconds after the Big Bang. Scientists use collisions of heavy ions (particles with an electrical charge) to produce large numbers of the heavy charm and bottom quarks. These quarks are excellent probes of QGP formation. Specifically, the recombination of freely moving charm and bottom quarks facilitates the production of B_c mesons—particles made of an equal number of quarks and antiquarks—when the QGP decays.

The Impact

A QGP formed in high-energy heavy-ion collisions only lasts for a short time before disintegrating into thousands of particles that can be observed in detectors. These detectors track signatures—the signals produced by specific types of particles. The discovery and study of QGP formation in heavy-ion experiments requires signatures that do not occur in other types of collisions, such as proton-proton collisions. In this study, researchers carried out theoretical simulations of charm and bottom quarks diffusing through the QGP. They found that the recombination of these quarks enhances the production of B_c mesons. This mechanism does not occur in proton-proton collisions and thus can serve as a clean signature of QGP formation.

Summary

Researchers from the HEFTY Topical Collaboration investigated the recombination of charm and bottom quarks into B_c mesons in the QGP. They have developed a transport model that simulates the kinetics of heavy-quark bound states through the expanding QGP fireball formed in high-energy heavy-ion collisions. Previous research has successfully used this model to describe the production of charm-anticharm and bottom-antibottom bound states, and thus can provide predictions for B_c particles (charm-antibottom bound states). The researchers used realistic spectra of charm and bottom quarks, computed from their diffusion through the QGP, to evaluate their recombination processes. The results show a large enhancement of the B_c yield in collisions of lead (Pb) nuclei, relative to that in proton collisions. The largest effect is predicted for slow-moving B_c mesons in “head-on” collisions of the Pb nuclei, where a large QGP fireball with appreciable numbers of charm and bottom quarks is formed.

The theoretical calculations agree with pioneering data of the CMS collaboration at the Large Hadron Collider (LHC). However, the data are not yet sensitive to slow-moving B_c mesons; future data will therefore provide a critical test of this QGP signature.



Funding

This work was supported by the Department of Energy Office of Science, Office of Nuclear Physics through the Topical Collaboration in Nuclear Theory on Heavy-Flavor Theory (HEFTY) for QCD Matter. This research was also funded by the National Science Foundation and the National Natural Science Foundation of China.