6 A GeV, where the purely hadronic scenario is realized. The data at the top SPS energy are also reproduced, which is achieved by a proper tune of the inter-fluid fric- tion in the quark-gluon phase. Quality of the reproduc- tion of above data is approximately the same as that with the hadronic EoS, as it is, e.g., seen from Figs. 2 and from Fig. 3. However, at top AGS and lower SPS energies (8 A -80 A GeV), where the mixed phase turns out to be really important, the 2-ph. EoS completely fails. The fact that the 2-ph.-EoS line perfectly hits 20 A -40 A - GeV experimental points in Fig. 3 is just a coincidence, shapes of the distributions are completely wrong, as seen from Figs. 2. This failure cannot be cured by variations of neither the friction nor the freeze-out criterion. However, the C$_{y}$ curvature energy dependence in the first-order-transition scenario manifests qualitatively the same “zig-zag” irregularity (Fig. 2), as that in the data fit, while the hadronic scenario produces purely monotonous behaviour. This “zig-zag” irregularity of the first-order-transition scenario is also reflected in the midrapidity values of the (net)proton rapidity spectrum (Fig. 3). As for the experimental data, it is still diffi- cult to judge if the “zig-zag” anomaly in the midrapid- ity values is statistically significant. In the conventional representation of the data (Fig. 4) without multiplying by y$_{cm}$ , the irregularity of the ( dN/dy )$_{cm}$ data is hardly visible. However, the conventional representation clearly demonstrates the overall trend of the data: the midra- pidity net-proton yield gradually decreases with the in- Notice that the proton rapidity distribution at 158 A GeV is well described within the color-glass-condensate framework based on small-coupling QCD [29]. This mechanism drastically differs from that of hadronic stop- ping. Therefore, it is not surprising that the 3FD model requires very different (from the hadronic one) phe- nomenological friction at the 158 A -GeV energy to repro- duce the data. The baryon stopping depends on a character of inter- actions (e.g., cross sections) of the matter constituents. If during the interpenetration stage of colliding nuclei a phase transformation 1 of the hadronic matter into quark- gluonic one happens, one can expect a change of the stopping power of the matter at this time span. This is a natural consequence of a change of the constituent content of the matter because hadron-hadron cross sec- tions differ from quark-quark, quark-gluon, etc. ones. This can naturally result in a non-monotonous behaviour of the shape of the (net)proton rapidity-spectrum at an incident energy, where onset of the phase transition oc- curs. Of course, the first-order transition does not hap- pen abruptly. Within the Gibbs construction the fraction of the quark-gluon phase is gradually increasing, as well as weights of the corresponding cross sections. Therefore, a non-monotonous behaviour will show up only if the dif- ference in cross sections in the hadronic and quark-gluon phases is large enough to override the above gradual in- crease of the fraction of the new phase. In fact, this is the case in the 3FD calculation with the phase transi- tion (2-ph. EoS). The friction in the quark-gluon phase was tuned to reproduce the data at the top SPS energy. Naturally, it does not continuously match the friction in the hadronic phase. In terms of parton-parton cross sec- tions, these cross sections in the quark-gluon phase turn out to be approximately twice as large as those in the hadronic phase $^{2}$. In the quark-gluon phase these cross sections are compatible with those used, e.g., in a multi- phase transport model [27] and a parton cascade model [28]. All above discussion concerns only central nuclear colli- sions. Experimental data on midcentral collisions is much less complete. The model calculations for midcentral col- lisions ( b ≈ 6 fm) reveal the same quantitative behaviour of the excitation functions of C$_{y}$ and ( dN/dy )$_{cm}$ both for hadr. EoS and 2-ph. EoS. cident energy, while the proton one stays approximately constant above the top SPS energy. Below the top SPS energy the proton and net-proton yields practically coin- cide. Model computations above the top SPS energy are at present not feasible because of high memory consump- tion required by the code (see discussion in Ref. [17]). 1 The term “phase transition” is deliberately avoided, since it usu- ally implies thermal equilibrium. 2 In the hadronic phase this parton cross section corresponds to the proton-proton one on the assumption of naive valence quark counting. FIG. 4: The same as in Fig. 3 but in conventional repre- sentation (without multiplying by y cm ) and in a wider energy range including RHIC data on Au+Au collisions at 5% cen- trality [26]. Proton data [7, 26] are also displayed. 4 1 10 100 s$^{1/2}$ [GeV] 0 20 40 60 80 ( d N /dy) cm hadr. EoS net-p 2-ph. EoS net-p E895,E917,E866 NA49 net-p NA49 p STAR net-p STAR p