ИСТИНА

In the talk we discuss some general features of the novel perspective diagnostic tool for the mixed-phase generation at the new facility NICA planned in JINR. It is planned that the mixed phase produced at the heavy-ion collider NICA will be characterized by rather high density and temperature and also by some admixture of quark-gluon bubbles to hadronic phase. So, the whole program of physical experiments at such conditions will depend upon the exact identification of the state of hot and dense matter emerged from nucleus-nucleus collisions. One can expect that in such collisions a chiral symmetry of QCD (broken at normal conditions) will be partially restored. One of the most distinguished features of such symmetry restoration should be dropping the lightest scalar meson (sigma, or f0(500)) mass and thus narrowing its width due to renormalization of the sigma-meson propagator in dense nuclear (or hadronic) medium. On the other hand, some QCD-based models predict chiral symmetry restoration even in a single strongly excited hadron (due to decoupling of valence quarks from the \bar{q}q condensate). Recently we considered generation of dibaryon resonances in NN collisions at intermediate energies. Within the above dibaryon model, the dibaryon generation is accompanied with intensive sigma-meson production as well. Due to partial chiral symmetry restoration inside the excited dibaryon, we predict some dropping of the sigma-meson mass and width already in a single NN collision. It has been proven both experimentally and theoretically that numerous pion pairs (in the form of sigma-mesons) are generated also in high-energy pp collisions, at 10-200 GeV, at least. So, while propagating through dense and hot hadronic matter, these sigma-mesons will undergo further chiral symmetry restoration, and thus their mass and width will drop additionally due to renormalization. The leading decay mode for the sigma-meson is a scalar-isoscalar pion pair. But there is also some decay channel sigma -> gamma gamma, the branching ratio of which (related to dipion channel) will depend crucially upon the sigma mass and width, which, in turn, depend upon the temperature and average density of matter inside the fireball. Thus one can extract from characteristics of the gamma-gamma signal (yield and energy width) very useful information about the basic parameters of the mixed phase emerged in heavy-ion collisions. The first experiments registered signals presumably from sigma-meson decay performed at JINR Nuclotron and previously at CELSIUS have given rather encouraging results and clearly shown the possibility of studying signals for digamma diagnostics of hot and dense hadronic matter. In the talk we discuss some details of the novel diagnostic tool and some characteristics of the gamma-gamma signal in view of previous experimental data on the gamma-gamma yield in hadronic collisions.