Counting states and the Hadron Resonance Gas: Does X(3872) count? Ortega, Pablo G. Entem, David R. Fernández, Francisco Ruiz Arriola, Enrique We analyze how the renowned X(3872), a weakly bound state right below the D¯D∗threshold, should effectively be included in a hadronic representation of the QCD partition function. This can be decided by analyzing the D¯D∗scattering phase-shifts in the JPC=1++channel and their contribution to the level density in the continuum from which the abundance in a hot medium can be determined. We show that in a purely molecular picture the bound state contribution cancels the continuum providing a vanishing occupation number density at finite temperature and the X (3872) does not count below the Quark-Gluon Plasma crossover happening at T∼150MeV. In contrast, within a coupled-channels approach, for a non vanishing c¯ccontent the cancellation does not occur due to the onset of the X (3940) which effectively counts as an elementary particle for temperatures above T _250MeV. Thus, a direct inclusion of the X (3872) in the Hadron Resonance Gas is not justified. We also estimate the role of this cancellation in X (3872) production in heavy-ion collision experiments in terms of the corresponding pT distribution due to a finite energy resolution. 2019-08-08T11:01:33Z 2019-08-08T11:01:33Z 2018-05-03 info:eu-repo/semantics/article P.G. Ortega et. al. Counting states and the Hadron Resonance Gas: Does X(3872) count?. Physics Letters B 781 (2018) 678–683 [https://doi.org/10.1016/j.physletb.2018.04.064] http://hdl.handle.net/10481/56611 10.1016/j.physletb.2018.04.064 eng http://creativecommons.org/licenses/by/3.0/es/ info:eu-repo/semantics/openAccess Atribución 3.0 España Elsevier B. V.