Doubly heavy tetraquark states are the prime candidates for tightly bound exotic states. We present a systematic study of the mass spectra of the $S$-wave doubly heavy tetraquark states $QQ\overline{q}\overline{q}$ ($q=u$, $d$, $s$ and $Q=c$, $b$) with different quantum numbers $J^P=0^{+}$, $1^{+}$, and $2^{+}$ in the framework of the improved chromomagnetic interaction (ICMI) model. The parameters in the ICMI model are obtained by fitting the conventional hadron spectra and are used directly to predict the masses of the tetraquark states. For heavy quarks, the uncertainties of the parameters are obtained by comparing the masses of doubly and triply heavy baryons with those given by lattice QCD, QCD sum rules, and potential models. Several compact and stable bound states are found in both the doubly charmed and doubly bottomed tetraquark systems. The predicted mass of the $cc\overline{u}\overline{d}$ state is consistent with the recent measurement from the LHCb Collaboration.

The electromagnetic response of hot QCD matter to decaying external magnetic fields is investigated. We examine the validity of Ohm's law and find that the induced electric current increases from zero and relaxes towards the value from Ohm's law. The relaxation time is larger than the lifetime of the external magnetic field for the QCD matter in relativistic heavy-ion collisions. The lower-than-expected electric current significantly suppresses the induced magnetic field and makes the electromagnetic response incomplete. We demonstrate the incomplete electromagnetic response of hot QCD matter by calculations employing the parton transport model combined with the solution of Maxwell's equations. Our results show a strong suppression by two orders of magnitude in the magnetic field relative to calculations assuming the validity of Ohm's law. This may undermine experimental efforts to measure magnetic-field-related effects in heavy-ion collisions.

Heavy-ion collisions provide a unique opportunity to study the nature of X(3872) compared with electron-positron and proton-proton (antiproton) collisions. We investigate the centrality and momentum dependence of $X\left(3872\right)$ in heavy-ion collisions via the Langevin equation and instant coalescence model (LICM). When $X\left(3872\right)$ is treated as a compact tetraquark state, the tetraquarks are produced via the coalescence of heavy and light quarks near the quantum chromodynamic phase transition due to the restoration of the heavy quark potential at $T\to T_c$. In the molecular scenario, loosely bound $X\left(3872\right)$ is produced via the coalescence of $D^0\text{−}{\overline{D}}^{*0}$ mesons in a hadronic medium after kinetic freeze-out. We employ the LICM to explain both $D^0$ and $J/\psi$ production as a benchmark. Then we give predictions regarding $X\left(3872\right)$ production and the nuclear modification factor $R_{AA}^{X\left(3872\right)}$. We find that the total yield of tetraquark is several times larger than the molecular production in Pb-Pb collisions. Although the geometric size of the hadronic molecule is huge, the coalescence probability is small due to strict constraints on the relative momentum between $D^0$ and ${\overline{D}}^{*0}$ in the molecular Wigner function, which significantly suppresses the molecular yield.

Heavy-flavor hadrons produced in ultrarelativistic heavy-ion collisions are a sensitive probe for studying hadronization mechanisms of the quark-gluon-plasma. In this paper, we survey how different transport models for the simulation of heavy-quark diffusion through a quark-gluon plasma in heavy-ion collisions implement hadronization and how this affects final state observables. Utilizing the same input charm-quark distribution in all models at the hadronization transition, we find that the transverse-momentum dependence of the nuclear modification factor of various charm hadron species has significant sensitivity to the hadronization scheme. In addition, the charm-hadron elliptic flow exhibits a nontrivial dependence on the elliptic flow of the hadronizing partonic medium. Published by the American Physical Society 2024

The observation of the exotic states $X_{0,1}(2900)$ with four open flavors by the LHCb Collaboration has promoted great interest in searching for the open-charm and open-bottom tetraquark states. In this work, we perform a systematic study of the open-heavy tetraquark systems $qq\overline{q}\overline{Q}$ ($q=u$, $d$, $s$ and $Q=c$, $b$) within the improved chromomagnetic interaction model. The mass spectra and color-spin wave functions of $S$-wave open-heavy tetraquark states with quantum numbers $J^P=0^{+}$, $1^{+}$, and $2^{+}$ are calculated. A number of compact tetraquark states are predicted in both open-charm and open-bottom systems. The two-body decay properties of these tetraquark states are phenomenologically analyzed. Our results suggest that the exotic open-charm state $X_0(2900)$ observed by the LHCb Collaboration is likely a compact tetraquark state with quantum number $I{J}^P=00^{+}$ (isospin singlet). For the open-bottom systems, our results also suggest a candidate for the exotic state $X^{\pm }(5568)$ reported by the D0 Collaboration. The other tetraquark states predicted in this work may also be examined in future experiments.

We study the pair description of heavy tetraquark systems $|QQ\overline{Q}\overline{Q}⟩$ in the frame of a nonrelativistic potential model. By taking the two heavy quark pairs $(Q\overline{Q})$ as colored clusters, the four-quark Schrödinger equation is reduced to a two-pair equation, when the inner motion inside the pairs can be neglected. Taking into account all the Casimir scaling potentials between two quarks and using the lattice QCD simulated mixing angle between the two color-singlet states for the tetraquark system, we extracted a detailed pair potential between the two heavy quark pairs.