Identifying multiquark hadrons from heavy ion collisions

Modification of χ_{c1}(3872) and ψ(2S) Production in pPb Collisions at sqrt[s_{NN}]=8.16 TeV

The LHCb Collaboration measures production of the exotic hadron χ_{c1}(3872) in proton-nucleus collisions for the first time. Comparison with the charmonium state ψ(2S) suggests that the exotic χ_{c1}(3872) experiences different dynamics in the nuclear medium than conventional hadrons, and comparison with data from proton-proton collisions indicates that the presence of the nucleus may modify χ_{c1}(3872) production rates. This is the first measurement of the nuclear modification factor of an exotic hadron.

Hadron production from heavy ion collisions

A brief review of some topics in hadron production from heavy ion collisions is given. They include charged pion ratio as a probe of nuclear symmetry energy, in-medium effects on pion production, en- hanced Λc/D0 ratio, Λ local polarization, and X(3872) production.

Exotic Particles and heavy ion collisions

We discuss the structures of exotic candidates and why it is interesting to measure them in heavy ion collisions. We take the X(3872) and Tcc to illustrate our point.

NCQ scaling of f0(980) elliptic flow in 200 GeV Au+Au collisions by STAR and its constituent quark content

Searching for exotic state particles and studying their properties have furthered our understanding of quantum chromodynamics (QCD). The f0(980) resonance is an exotic state with relatively high production rate in relativistic heavy-ion collisions, decaying primarily into ππ. Currently the structure and quark content of the f0(980) are unknown with several predictions from theory being a qq¯ state, a qqq¯q¯ state, a KK¯ molecule state, or a gluonium state. We report the first f0(980) elliptic flow (v2) measurement from 200 GeV Au+Au collisions at STAR. The transverse momentum dependence of v2 is examined and compared to those of other hadrons (baryons and mesons). The empirical number of constituent quark (NCQ) scaling is used to investigate the constituent quark content of f0(980), which may potentially address an important question in QCD.

Evidence for X(3872) in PbPb collisions and studies of its prompt production at √SNN = 5.02 TeV with CMS

The first evidence for X(3872) production in relativistic heavy ion collisions, with a significance of 4.2 standard deviations, is reported. The X(3872) production is studied in lead-lead (PbPb) collisions at a center-of-mass energy of √SNN = 5.02 TeV per nucleon pair with the CMS detector. The measurement is performed in the rapidity and transverse momentum ranges |y| < 1.6 and 15 < pT < 50 GeV/c. The prompt X(3872) to ψ(2S) yield ratio is found to be ρPbPb = 1.08 ± 0.49(stat.) ± 0.52(syst.), to be compared with typical values of 0.1 for pp collisions. This provides new input to theoretical models of the X(3872) production mechanism, and of the nature of this exotic state.

Evidence for X(3872) in Pb-Pb Collisions and Studies of its Prompt Production at sqrt[s_{NN}]=5.02 TeV

The first evidence for X(3872) production in relativistic heavy ion collisions is reported. The X(3872) production is studied in lead-lead (Pb-Pb) collisions at a center-of-mass energy of sqrt[s_{NN}]=5.02  TeV per nucleon pair, using the decay chain X(3872)→J/ψπ^{+}π^{-}→μ^{+}μ^{-}π^{+}π^{-}. The data were recorded with the CMS detector in 2018 and correspond to an integrated luminosity of 1.7  nb^{-1}. The measurement is performed in the rapidity and transverse momentum ranges |y|<1.6 and 15<p_{T}<50  GeV/c. The significance of the inclusive X(3872) signal is 4.2 standard deviations. The prompt X(3872) to ψ2S yield ratio is found to be ρ^{Pb-Pb}=1.08±0.49(stat)±0.52(syst), to be compared with typical values of 0.1 for pp collisions. This result provides a unique experimental input to theoretical models of the X(3872) production mechanism, and of the nature of this exotic state.

Deciphering the Nature of X(3872) in Heavy Ion Collisions

Exploring the nature of exotic multiquark candidates such as the X(3872) plays a pivotal role in understanding quantum chromodynamics (QCD). Despite significant efforts, consensus on their internal structures is still lacking. As a prime example, it remains a pressing open question to decipher the X(3872) state between two popular exotic configurations: a loose hadronic molecule or a compact tetraquark. We demonstrate a novel approach to help address this problem by studying the X(3872) production in heavy ion collisions, where a hot fireball with ample light as well as charm (anti-)quarks is available for producing the exotics. Adopting a multiphase transport model (AMPT) for describing such collisions and implementing appropriate production mechanism of either molecule or tetraquark picture, we compute and compare a series of observables for X(3872) in Pb-Pb collisions at the Large Hadron Collider. We find the fireball volume plays a crucial role, leading to a 2-order-of-magnitude difference in the X(3872) yield and a markedly different centrality dependence between hadronic molecules and compact tetraquarks, thus offering a unique opportunity for distinguishing the two scenarios. We also make the first prediction of X(3872) elliptic flow coefficient to be tested by future experimental measurements.

Theoretical perspective on strangeness production

A brief review of some highlights and puzzles on strangeness production in heavy ion collisions is given. These include strangeness production and the nuclear equation of state; deeply subthreshold strangeness production; mean-field potentials on strange hadrons; phi meson in dense matter; anomalous strange hadron to pion ratios; density fluctuations on particle production; A hyperon polarization and the vorticity field, and exotic hadrons.

Strong binding and shrinkage of single and double (K) nuclear systems (K(-)pp, K(-)ppn, K(-)K(-)p and K(-)K(-)pp) predicted by Faddeev-Yakubovsky calculations

Non-relativistic Faddeev and Faddeev-Yakubovsky calculations were made for K(-)pp, K(-)ppn, K(-)K(-)p and K(-)K(-)pp kaonic nuclear clusters, where the quasi bound states were treated as bound states by employing real separable potential models for the K(-)-K(-) and the K(-)-nucleon interactions as well as for the nucleon-nucleon interaction. The binding energies and spatial shrinkages of these states, obtained for various values of the K interaction, were found to increase rapidly with the K interaction strength. Their behaviors are shown in a reference diagram, where possible changes by varying the K interaction in the dense nuclear medium are given. Using the Λ(1405) ansatz with a PDG mass of 1405 MeV/c(2) for K(-)p, the following ground-state binding energies together with the wave functions were obtained: 51.5 MeV (K(-)pp), 69 MeV (K(-)ppn), 30.4 MeV (K(-)K(-)p) and 93 MeV (K(-)K(-)pp), which are in good agreement with previous results of variational calculation based on the Akaishi-Yamazaki coupled-channel potential. The K(-)K(-)pp state has a significantly increased density where the two nucleons are located very close to each other, in spite of the inner NN repulsion. Relativistic corrections on the calculated non-relativistic results indicate substantial lowering of the bound-state masses, especially of K(-)K(-)pp, toward the kaon condensation regime. The fact that the recently observed binding energy of K(-)pp is much larger (by a factor of 2) than the originally predicted one may infer an enhancement of the K interaction in dense nuclei by about 25% possibly due to chiral symmetry restoration. In this respect some qualitative accounts are given based on "clearing QCD vacuum" model of Brown, Kubodera and Rho.