Production of cumulative pions and percolation of strings

Production of pions in high-energy collisions with nuclei in the kinematics prohibited for free nucleons ("cumulative pions") is studied in the fusing color string model.The model describes the so-called direct mechanism for cumulative production. The other, spectator mechanism dominates in production of cumulative protons but is suppressed for pions. In the model cumulative pions are generated by string fusion which raises the maximal energy of produced partons above the level of the free nucleon kinematics. Momentum and multiplicity sum rules are used to determine the spectra in the deep fragmentation region. Predicted spectra of cumulative pions exponentially fall with the scaling variable $x$ in the interval $1<x<3$ with a slope of the order 5$\div$5.6, which agrees well with the raw data obtained in the recent experiment at RHIC with Cu-Au collisioins. However the agreement is worse for the so-called unfolded data, presumably taking into account corrections due to the expermental set-up and having rather a power-like form.