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Periodic Report Summary 1 - QCDENSE (Precision tools for high-energy QCD scattering at the LHC and Auger)

The main goal of this project is to develop theoretical and phenomenological tools in the framework of Quantum Chromodynamics (QCD) at high gluon densities oriented to the understanding on data from the Heavy Ion program at the LHC and from the Pierre Auger Observatory on cosmic rays. One main goal of this proposal is to provide a precise characterisation of the initial state of heavy ion collisions in order to allow a precise extraction of the transport parameters of the produced medium, presumably a Quark Gluon Plasma. This implies achieving a precise description of the full phase space and collision geometry dependence of the initial quark and gluon distributions and their correlations. The theoretical work of this project focuses in the calculation of higher order corrections to the Color Glass Condensate (CGC) effective theory for QCD high-energy scattering, both at the level of the non-linear evolution equations and of production processes. The theoretical results shall be used to build a Monte Carlo simulation tool for the characterisation of the initial stages of proton and nuclear collisions. The small Bjorken-x component of the nuclear wave function, a key ingredient for the calculation of any production processes studied in this project, shall be constructed under the Gaussian approximation and empirically constrained through global fits to data from several collision systems (e+p, p+p, p+A and A+A). Other objective of this project is to translate the advances in our understanding of high-energy QCD scattering gained by the detailed study of LHC data into the simulation tools used in the analyses of Ultra High Energy Cosmic Rays. This will allow for theoretically well constrained extrapolations over more than two orders of magnitude in the collision energy of several of the hadronic observables of great relevance for the development of the air-showers and, hence, for the interpretation of the experimental results.
The QCDense team project is composed by the project leader Javier L. Albacete, a Ramón y Cajal researcher at Universidad de Granada, and two Ph.D. students, Pablo Guerrero and Alba Soto.
During the first two years of the project we have studied accelerator data on proton-proton and proton-lead collisions collected at the Large Hadron Collider (LHC, CERN, Switzerland). The focus of our research is to understand the scattering between hadrons, particles composed of quarks and gluons, at ultra-high energies. In particular, we have studied the scattering amplitude extracted from data which, at LHC energies, exhibit a new, intriguing feature: the hollowness of the inelasticity profile. We have provided a first dynamical explanation of the hollowness effect based on an effective description of the proton as composed of hot spots, or domains of high gluonic densities confined within a radius much smaller than the proton radius. Further, the positions of hot spots inside the proton are strongly correlated and multiple, simultaneous scattering between hot spots within the projectile and target proton are allowed and described in terms of the Glauber model. In another article we studied the production of particles —neutral pions— produced at very forward rapidities, i.e. of produced pions flying out very close to the beam pipe. This kinematic region is interesting because it provides access to the region of the wave function of the target —a proton or a lead nucleus— at very small-x, where it is dominated by high gluon densities. Indeed, we found that the Color Glass Condensate effective theory, which correctly copes with high-gluonic density effects allows a very good description of such data, thus providing another suggestive indication for the importance of saturation effects - characteristic of high gluon density systems in available data. Finally, we have also explored the importance of taking into account non-linear saturation effects in the calculation of the neutrino-nucleon cross section. In another article we have shown that neglecting this important physical effect in the analysis of data from neutrino observatories may led to underestimate the ultra high-energy neutrino fluxes impinging on Earth from outer space.
The studies completed in the first two years of the project provide the baseline for the research work to be carried out in the next two years: extending the techniques devised and knowledge acquired in the analysis of proton data from the LHC data to the case of heavy ion collisions where multiple nucleons collide simultaneously— and also to the energy domain characteristic of ultra-high energy cosmic rays.

Javier López Albacete
Investigador Ramón y Cajal
Dpto. de Física Teórica y del Cosmos
Campus de Fuentenueva s/n. E-18071
Tel: (+34) 958 241726

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