Investigation of exotic state X(3872) in pp collisions at $$\sqrt{s}=7$$, 13 TeV

We have used the dynamically constrained phase space coalescence model to study the production of the exotic state X(3872) based on the hadronic final states generated by the parton and hadron cascade model (PACIAE) with $$|y|<1$$ | y | < 1 and $$p_T < 15.5$$ p T < 15.5 GeV/c in pp collisions at $$\sqrt{s}=7$$ s = 7 and 13 TeV, respectively. Here the X(3872) is assumed to consist of bound state $$D\bar{D^*}$$ D D ∗ ¯ , which can form three possible structures for the tetraquark state, the nucleus-like state, and the molecular state. The yields of three different structures X(3872) were predicted. The transverse momentum distribution and the rapidity distribution of three different structures X(3872) are also presented. Sizable difference can be found in the transverse momentum and rapidity distributions for the three different X(3872) structures.

The nature of X(3872) from high-multiplicity pp collisions

The structure of exotic resonances that do not trivially fit the usual quark model expectations has been a matter of intense scientific debate during the last two decades. A possible way of estimating the size of these states is to study their behavior when immersed in QCD matter. Recently, LHCb has measured the relative abundance of the exotic $$X(3872)$$ X ( 3872 ) over the ordinary $$\psi (2S)$$ ψ ( 2 S ) . We use the comover interaction model to study the yield of a compact $$X(3872)$$ X ( 3872 ) . To confirm the reliability of the model in high-multiplicity pp collisions, we describe the suppression of excited over ground $$\Upsilon $$ Υ states. With this at hand, we show that the size of the compact $$X(3872)$$ X ( 3872 ) would be slightly larger than that of the $$\psi (2S)$$ ψ ( 2 S ) . If the $$X(3872)$$ X ( 3872 ) is instead assumed to be a meson molecule of large size, we argue that its evolution in QCD matter should be described via a coalescence model, as suggested by data on deuteron production. We show that the predictions of this model for the $$X(3872)$$ X ( 3872 ) are in contrast with data.

On nuclear coalescence in small interacting systems

The formation of light nuclei can be described as the coalescence of clusters of nucleons into nuclei. In the case of small interacting systems, such as dark matter and $$e^+e^-$$ e + e - annihilations or pp collisions, the coalescence condition is often imposed only in momentum space and hence the size of the interaction region is neglected. On the other hand, in most coalescence models used for heavy ion collisions, the coalescence probability is controlled mainly by the size of the interaction region, while two-nucleon momentum correlations are either neglected or treated as collective flow. Recent experimental data from pp collisions at LHC have been interpreted as evidence for such collective behaviour, even in small interacting systems. We argue that these data are naturally explained in the framework of conventional QCD inspired event generators when both two-nucleon momentum correlations and the size of the hadronic emission volume are taken into account. To include both effects, we employ a per-event coalescence model based on the Wigner function representation of the produced nuclei states. This model reproduces well the source size for baryon emission and the coalescence factor $$B_2$$ B 2 measured recently by the ALICE collaboration in pp collisions.

The study of exotic state $$Z_c^{\pm }(3900)$$ decaying to $$J/\psi \pi ^{\pm }$$ in the pp collisions at $$\sqrt{s}$$ = 1.96, 7, and 13 TeV

A dynamically constrained phase-space coalescence model and PACIAE model are used to predict the exotic resonant state $$Z_c^{\pm }(3900)$$ Z c ± ( 3900 ) yield in pp collisions at $$\sqrt{s} = 1.96, 7$$ s = 1.96 , 7 and 13 TeV, respectively, which are estimated to be around $$10^{-6}$$ 10 - 6 to $$10^{-5}$$ 10 - 5 based on the $$J/\psi \pi ^{\pm }$$ J / ψ π ± bound state in the decay chain of b hadrons. The energy dependence of the transverse momentum distributions and rapidity distributions with $$|y|<6$$ | y | < 6 and $$p_{T}<10$$ p T < 10 GeV/c are also calculated for $${Z_c^{+}(3900)}$$ Z c + ( 3900 ) and $${Z_c^{-}(3900)}$$ Z c - ( 3900 ) . The production of $${Z_c^{+}(3900)}$$ Z c + ( 3900 ) and its anti-particle $${Z_c^{-}(3900)}$$ Z c - ( 3900 ) are found to be quite similar to each other.

Understanding the energy dependence of $$B_2$$ in heavy ion collisions: interplay of volume and space-momentum correlations

The deuteron coalescence parameter $$B_2$$ B 2 in proton+proton and nucleus+nucleus collisions in the energy range of $$\sqrt{s_{NN}}=$$ s NN = 900–7000 GeV for proton + proton and $$\sqrt{s_{NN}}=$$ s NN = 2–2760 GeV for nucleus + nucleus collisions is analyzed with the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport model, supplemented by an event-by-event phase space coalescence model for deuteron and anti-deuteron production. The results are compared to data by the E866, E877, PHENIX, STAR and ALICE experiments. The $$B_2$$ B 2 values are calculated from the final spectra of protons and deuterons. At lower energies, $$\sqrt{s_{NN}}\le 20$$ s NN ≤ 20 GeV, $$B_2$$ B 2 drops drastically with increasing energy. The calculations confirm that this is due to the increasing freeze-out volume reflected in $$B_2\sim 1/V$$ B 2 ∼ 1 / V . At higher energies, $$\sqrt{s_{NN}}\ge 20$$ s NN ≥ 20 GeV, $$B_2$$ B 2 saturates at a constant level. This qualitative change and the vanishing of the volume suppression is shown to be due to the development of strong radial flow with increasing energy. The flow leads to strong space-momentum correlations which counteract the volume effect.

Production of light nuclei at colliders – coalescence vs. thermal model

The production of light nuclei in relativistic heavy-ion collisions is well described by both the thermal model, where light nuclei are in equilibrium with hadrons of all species present in a fireball, and by the coalescence model, where light nuclei are formed due to final-state interactions after the fireball decays. We present and critically discuss the two models and further on we consider two proposals to falsify one of the models. The first proposal is to measure a yield of exotic nuclide 4Li and compare it to that of 4He. The ratio of yields of the nuclides is quite different in the thermal and coalescence models. The second proposal is to measure a hadron-deuteron correlation function which carries information whether a deuteron is emitted from a fireball together with all other hadrons, as assumed in the thermal model, or a deuteron is formed only after nucleons are emitted, as in the coalescence model. The p − 3He correlation function is of interest in context of both proposals: it is needed to obtain the yield of 4Li which decays into p and 3He, but the correlation function can also tell us about an origin of 3He.