  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.
