The formation of hot spots assumes the establishment of local equilibrium, which in its turn occurs if the thermal conductivity in the medium is sufficiently small.
In these reactions, with the increase of laboratory energy one observes that the transverse momenta of produced particles have a tail, which deviates from the single exponential Boltzmann spectrum, characteristic for global equilibrium.
The interest in this phenomenon was resurrected in the 1970s by the work of Weiner and Weström[8][9] who established the link between the hot spot model and the pre-equilibrium approach used in low-energy heavy-ion reactions.
[10][11] Experimentally the hot spot model in nuclear reactions was confirmed in a series of investigations[12][13][14][15] some of which of rather sophisticated nature including polarization measurements of protons[16] and gamma rays.
[20][21] With the advent of heavy ion accelerators experimental studies of hot spots in nuclear matter became a subject of current interest and a series of special meetings[22][23][24][25] was dedicated to the topic of local equilibrium in strong interactions.
The phenomena of hot spots, heat conduction and preequilibrium play also an important part in high-energy heavy ion reactions and in the search for the phase transition to quark matter.
Solitons are a solution of the hydrodynamic equations characterized by a stable localized high density region and small spatial volume.