Implicações fenomenológicas da alta densidade partônica na produção de diléptons para energias de raios cósmicos

Abstract

Particles coming from the space reach our atmosphere at all moment and could come with extremely high energy of orders of 1020 eV and because of this are denominated Ultra High Energy Cosmic Rays (UHECR). In the interaction of these cosmic rays with the atmosphere the center of mass energies can be larger than those obtained in the current laboratories, which allows to testing the strong interaction theory in an unexplored kinematical regime. The interaction of the UHECR with the atmosphere produce extended cosmic ray extensive air showers. This extensive air shower is a superposition of a hadronic cascade, which is situated in the central part, and an electromagnetic cascade, initiated by photons from decays neutral pions produced by the hadronic part, or through the Drell-Yan process, which considers the direct interaction of the primary particle with the nuclei of atoms in the atmosphere generating the dileptons. Our objective in this work is to estimate the high density effects in the Drell-Yan process and the consequent influence on the electromagnetic cascade induced by them. The dipole approach is used as the theoretical framework for cross sections calculation. In this formalism, the interaction between projectile and target is described by the dipole cross section σdip which is directly associated with the dynamics of Quantum Chromodynamics (QCD) at high energies. This interaction depends on the particles density in the target and any high density effect is included in this cross section. In this study we investigate the effects of the high parton density in the Drell-Yan process, in proton-proton (pp) and proton-nuclei (pA) collisions, considering mass and transverse momentum distributions of dileptons produced at LHC and UHECR energies. A comparison between the models of σdip proposed by Golec-Biernat and W¨usthoff (GBW) and Albacete et al. (rcBK), that describe the partonic saturation physics, in pp collisions for energies of RHIC and LHC accelerators and UHECR, are performed. For comparison, we calculated the cross section for production of dileptons using the first order expansion in the GBW, where high density effect are not present. The calculations presented in this work using the model rcBK are made and presented for the first time. For pA collisions we consider four different models, we performed a comparison between them considering the lead core to the LHC energy and nitrogen cores (air) to the energy of the UHECR. Our results demonstrate that the high density effects are very large, 102 to pp collisions and ≈ 103 to pA collisions, for small transverse momentum for the UHECR energies. These results show that the high density effects need be considered in the cross section calculation for the dileptons production, and consequently, in the simulations of UHECR interactions with the atmosphere.