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1. Charmonium, B _c and X(3872) Transport at the LHC

   https://doi.org/10.1051/epjconf/202327602001  

   Wu, Biaogang; Tang, Zhanduo; He, Min; Rapp, Ralf (January 2023, EPJ Web of Conferences)

   Kim, Y.; Moon, D.H. (Ed.)

   We deploy a kinetic-rate equation to evaluate the transport of J /ψ, ψ(2 S ), B c and X (3872) in ultrarelativistic heavy-ion collisions and compare their production yields to experimental data from the Large Hadron Collider. The rate equation has two main transport parameters: the equilibrium limit and reaction rate for each state. The temperature-dependent equilibrium limits include charm- and bottom-quark fugacities based on their initial production. The reaction rates for charmonia, bottomonia and B c rely on charm- and bottomquark masses and binding energies from a thermodynamic T -matrix approach. For the X (3872) particle, internal structure information is encoded in reaction rates and initial conditions in the hadronic phase via two different scenarios: a loosely bound hadronic molecule vs. a compact tetraquark. 

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2. Recombination of $B_c$ mesons in ultrarelativistic heavy-ion collisions

   https://doi.org/10.1103/PhysRevC.109.014906  

   Wu, Biaogang; Tang, Zhanduo; He, Min; Rapp, Ralf (January 2024, Physical Review C)

   High-energy heavy-ion collisions have been suggested as a favorable environment for the production of Bc mesons, due to a much larger abundance of charm and bottom quarks compared to elementary reactions. Motivatedby recent CMS data for Bc production in Pb-Pb(5.02 TeV) collisions at the Large Hadron Collider (LHC),we deploy a previously developed transport approach for charmonia and bottomonia to evaluate the kinetics of Bc mesons throughout the fireball formed in these reactions. The main inputs to our approach are two transport parameters: the Bc’s reaction rate and equilibrium limit. Both quantities are determined by previous calculations via a combination of charm and bottom sectors. In-medium binding energies of Bc mesons are calculated from a thermodynamic T matrix with a lattice-QCD constrained potential, and figure in their inelastic reaction rates. Temperature-dependent equilibrium limits include charm- and bottom-quark fugacities based on their initial production. We compute the centrality dependence of inclusive Bc production and transverse-momentum (pT) spectra using two different recombination models: instantaneous coalescence and resonance recombination. The main uncertainty in the resulting nuclear modification factors, RAA, is currently associated with the Bc cross section in elementary pp collisions, caused by the uncertainty in the branching ratio for the Bc → J/ψμν decay. Our results indicate a large enhancement of the RAA at low pT , with significant regeneration contributions up to pT ~ 20 GeV. Comparisons to CMS data are carried out but firm conclusions will require a more accurate value of the branching ratio, or alternative channels to measure the Bc production in pp collisions. 

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3. Charm and bottom hadrons in hot hadronic matter

   https://doi.org/10.1016/j.physrep.2025.05.002  

   Das, Santosh K; Torres-Rincon, J; Rapp, R (May 2025, Physics reports)

   Heavy quarks, and the hadrons containing them, are excellent probes of the QCD medium formed in high-energy heavy-ion collisions, as they provide essential information on the transport properties of the medium and how quarks color-neutralize into hadrons. Large theoretical and phenomenological efforts have been dedicated thus far to assess the diffusion of charm and bottom quarks in the quark-gluon plasma and their subsequent hadronization into heavy-flavor (HF) hadrons. However, the fireball formed in heavy-ion collisions also features an extended hadronic phase, and therefore any quantitative analysis of experimental observables needs to account for the rescattering of charm and bottom hadrons. This is further reinforced by the presence of a QCD cross-over transition and the notion that the interaction strength is maximal in the vicinity of the pseudo-critical temperature. We review existing approaches for evaluating the interactions of open HF hadrons in a hadronic heat bath and the pertinent results for scattering amplitudes, spectral functions and transport coefficients. While most of the work to date has focused on 𝐷-mesons, we also discuss excited states as well as HF baryons and the bottom sector. Both the HF hadro-chemistry and bottom observables will play a key role in future experimental measurements. We also conduct a survey of transport calculations in heavy-ion collisions that have included effects of hadronic HF diffusion and assess its impact on various observables. 

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4. Evidence of Mass Ordering of Charm and Bottom Quark Energy Loss in Au+Au Collisions at RHIC

   https://doi.org/10.1140/epjc/s10052-022-11003-7  

   Abdallah, M. S.; Aboona, B. E.; Adam, J.; Adamczyk, L.; Adams, J. R.; Adkins, J. K.; Aggarwal, I.; Aggarwal, M. M.; Ahammed, Z.; Anderson, D. M.; et al (December 2022, The European Physical Journal C)

   Abstract Partons traversing the strongly interacting medium produced in heavy-ion collisions are expected to lose energy depending on their color charge and mass. We measure the nuclear modification factors for charm- and bottom-decay electrons, defined as the ratio of yields, divided by the number of binary nucleon–nucleon collisions, in $$\sqrt{s_{\textrm{NN}}}=200$$ s NN = 200 GeV Au+Au collisions to p + p collisions ( $$R_{\textrm{AA}}$$ R AA ), or in central to peripheral Au+Au collisions ( $$R_{\textrm{CP}}$$ R CP ). We find the bottom-decay electron $$R_{\textrm{AA}}$$ R AA and $$R_{\textrm{CP}}$$ R CP to be significantly higher than those of charm-decay electrons. Model calculations including mass-dependent parton energy loss in a strongly coupled medium are consistent with the measured data. These observations provide evidence of mass ordering of charm and bottom quark energy loss when traversing through the strongly coupled medium created in heavy-ion collisions. 

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5. Hydrodynamic description of D meson production in high-energy heavy-ion collisions *

   https://doi.org/10.1088/1674-1137/abf645  

   Ding, Chi; Ke, Wei-Yao; Pang, Long-Gang; Wang, Xin-Nian (July 2021, Chinese Physics C)

   Abstract The large values and constituent-quark-number scaling of the elliptic flow of low- D mesons imply that charm quarks, initially produced through hard processes, might be partially thermalized through strong interactions with quark-gluon plasma (QGP) in high-energy heavy-ion collisions. To quantify the degree of thermalization of low- charm quarks, we compare the meson spectra and elliptic flow from a hydrodynamic model to experimental data as well as transport model simulations. We use an effective charm chemical potential at the freeze-out temperature to account for the initial charm quark production from hard processes and assume that they are thermalized in the local comoving frame of the medium before freeze-out. mesons are sampled statistically from the freeze-out hyper-surface of the expanding QGP as described by the event-by-event (3+1)D viscous hydrodynamic model CLVisc. Both the hydrodynamic and transport models can describe the elliptic flow of mesons at GeV/ c as measured in Au+Au collisions at GeV. Though the experimental data on spectra are consistent with the hydrodynamic result at small GeV/ c , they deviate from the hydrodynamic model at high transverse momentum, GeV/ c . The diffusion and parton energy loss mechanisms in the transport model can describe the measured spectra reasonably well within the theoretical uncertainty. Our comparative study indicates that charm quarks only approach local thermal equilibrium at small , even though they acquire sizable elliptic flow comparable to light-quark hadrons at both small and intermediate . 

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