The $Z_{cs}(3985)$ structure, if a triangle/kinematic singularity, would disappear when heated in Heavy Ion Collisons

Abstract

Triangle and other kinematic singularities are very sensitive to the precise masses and widths of the intervening particles. Therefore, the effect that the heavy-ion collision medium can have on those masses and widths, as captured by finite-temperature field theory and reported in the literature, may erase the singularity from the spectrum if the effect is large enough and the loop completes before the hot gas freezes out. A very timely example is provided by the $Z_{cs}(3985)$ structure recently reported by BES-III in a $(D_s^-D^{*0}+D_s^{*-}D^0)$ spectrum recoiling against a $K^+$. If a new hadron, this would be a clear exotic $c\bar{c}s\bar{u}$ tetraquark candidate, the first of a charmonium-like family with strangeness: to accept this, alternative explanations first need to be tested and discarded. As shown in figure 1, the mass spectrum recoiling against the kaon is near the $m_{D_s^{*-}}+m_{D^0}$ threshold, whereas the production cross-section seems to peak around that for $\sqrt{s(e^-e^+)}=m_{D_{s2}^{*+}} +m_{D_s^{*-}} $. Therefore, this structure may well be caused by a triangle with the three charmed mesons $D_{s2}^{*+} / D_s^{*-} / D^0$ running in the loop, with the amplitude enhanced at the two thresholds. If so, the structure would be erased from the spectrum in Heavy Ion Collisions, whereas a hadron would continue to exist although with a mass decreased by a few percent as computed by other authors.