4 tainty are needed to see the evolution effect more dis- tinctly. Unfortunately, the calculation of correlation contri- bution Δ( t ) to the effective cross section σ exp eff is not yet possible in the framework of some kind of existing event generators. For instance, the double parton scat- terings are implemented in the Monte-Carlo generator PYTHIA [26, 27] taking into account some correlations which, however, are not quite adequate for the case of evolution effects under consideration (unlike the theoret- ical investigations in Ref. [11]). The implementation of the QCD evolution of two-parton distribution functions in some Monte-Carlo generator, as this was done for sin- gle distributions, is not a trivial task. However we can “predict” a functional form of p jet2 T -dependence of σ exp eff : In order to investigate the more delicate characteris- tics of double parton scatterings (distributions over var- ious kinematic variables with various kinematic cuts) it is also desirable to implement the QCD evolution of the two-parton distribution functions in some Monte Carlo event generator as this was done for the single distri- butions, for instance, within PYTHIA [26]. It is worth noticing once more that the evolution of the two-parton distribution functions has the same confidence status as the well-established evolution of the single distribution functions in the framework of the leading logarithm ap- proximation of perturbative QCD. Therefore the exper- imental direct or indirect observation of this evolution effect is significant to answer to many challenging ques- tions of yet poorly-understood aspects of QCD. σ exp eff = σ 0 $_{eff}$[1 + k ln( p jet2 T /p jet2 T 0 ] − 1 (11) inspired by the explicit expression for the correlation term [14] and the evolution variable t . The results from Eq. (11) are also shown in Fig. 1 at k = 0 . 1 (dashed line) and k = 0 . 5 (solid line) to illustrate the two possi- ble slopes of σ exp eff in the observation region: visually im- perceptible (practically constant) and noticeable falling slopes. The normalization point p jet2 T 0 = 22 . 5 GeV with σ 0 eff = 16 . 3 mb is fixed to reproduce the experimental value in the central p jet2 T bin. Discussions with D.V. Bandurin (especially), E.E. Boos, M.N. Dubinin, V.A. Ilyin, V.L. Ko- rotkikh, L.N. Lipatov, I.P. Lokhtin, S.V. Molodtsov, S.V. Petrushanko, A.S. Proskuryakov, V.I. Savrin, T. Sjostrand, P. Skands, D. Treleani, G.M. Zinovjev and N.P. Zotov are gratefully acknowledged. This work is partly supported by Russian Foundation for Basic Research (grants No 08-02-91001 and No 08-02-92496), Grants of President of Russian Federation for support of Leading Scientific Schools (No 1456.2008.2) and Russian Ministry for Education and Science (contract 02.740.11.0244). Acknowledgments IV. CONCLUSIONS We argue that the QCD dynamical correlations result effectively in the dependence of the experimentally ex- tracted σ exp eff on the resolution scale unlike naive accepted expectations. The measurements covering a larger range of the resolution scale variation with a smaller uncer- [1] AFS Collaboration, T. Akesson et al. , Z. Phys. C 34 , 163 (1987). [10] T. Sjostrand and P.Z. Skands, JHEP 0403 , 053 (2004). [11] T. Sjostrand and P.Z. Skands, Eur. Phys. J C 39 , 129 (2005). [12] E. Maina, JHEP 04 , 098 (2009); JHEP 09 , 081 (2009). [13] T.C. Rogers and M. Strikman, arXiv:0908.0251 [hep-ph]. [14] A.M. Snigirev, Phys. 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