Revealing a deep connection between factorization and saturation: New insight into modeling high-energy proton-proton and nucleus-nucleus scattering in the EPOS4 framework

It is known that multiple partonic scatterings in high-energy proton-proton (pp) collisions must happen in parallel. However, a rigorous parallel scattering formalism, taking energy sharing properly into account, fails to reproduce factorization, which on the other hand is the basis of almost all pp event generators. In addition, binary scaling in nuclear scatterings is badly violated. These problems are usually "solved" by simply not considering strictly parallel scatterings, which is not a solution. I will report on new ideas (leading to EPOS4), which allow recovering perfectly factorization, and also binary scaling in AA collisions, in a rigorous unbiased parallel scattering formalism. In this new approach, dynamical saturation scales play a crucial role, and this seems to be the missing piece needed to reconcile parallel scattering with factorization. From a practical point of view, one can compute within the EPOS4 framework parton distribution functions (EPOS PDFs) and use them to compute inclusive pp cross sections. So, for the first time, one may compute inclusive jet production (for heavy or light flavors) at very high transverse momentum (pt) and at the same time in the same formalism study flow effects at low pt in high-multiplicity pp events, making EPOS4 a full-scale "general purpose event generator". I discuss applications, essentially multiplicity dependencies (of particle ratios, mean pt, charm production) which are very strongly affected by the saturation issues discussed in this paper.