Scientific context and motivation

Interactions between heavy ions at ultra-relativistic energies offers the possibility to explore the phase transition from hadronic(protons and neutrons) degrees of freedom to partonic degrees of freedom (quarks and gluons). This new state of matter, named quark gluon plasma (QGP), was created in the first microseconds after Big Bang. It is formed at several times the normal nuclear matter density or at temperature in excess of 170 MeV in nucleus-nucleus interactions.

In 1980 the hunt for QGP begun at Super Proton Synchrotron (SPS) -CERN, but by the year 2000 only indirect signals of QGP formation were presented. Starting with the four experiments from Relativistic Heavy Ion Collider(RHIC) -USA, BRAHMS, PHENIX, PHOBOS and STAR first experimental evidence of a liquid like state made from quarks and gluons begun to emerge from Au-Au interactions at 200 A GeV. Today, at Large Hadron Collider(LHC) from CERN two experiments have nucleus-nucleus interactions programs, ATLAS and ALICE(A Large Ion Collider Experiment), former being an heavy ion dedicated experiment using Pb-Pb interactions at a peak energy of 5.5 TeV. In the future at FAIR-GSI, Germany two experiments, PANDA and CBM will begun to study the QGP formation by increasing baryon densities in nucleus-nucleus collisions.

One of the most successful descriptions of QGP formation was made using hydrodynamic models building a Equation of State for the nuclear environment (EoS). Experimental evidence for this model came from the study of anisotropic flow of the participant region in nucleus-nucleus collisions. Anisotropic flow arises from the geometry of interaction from which an almond like shape of the fireball results. The study of direct and elliptic flow from the RHIC experiments gives us the present image of a liquid state with very low viscosity of QGP.

One of the main challenges in Heavy Ion Physics is to make fast analysis of high amount of experimental and simulated data. At LHC-CERN one p-p event is approximate 1 Mb in size. In the case of Pb-Pb this problem is much worse, one event having 100 Mb in size. In general, the amount of data generated at Large Hadron Collider (LHC) is estimated to reach 1 Peta Byte/year. The time taken to analyze the data and obtain fast results depends on high computational power. The classical approach is to build expensive clusters with large numbers of processors (CPU) in order to reduce the time needed to analyze the data.

Since 2006 a new technology has begun to emerge with the help of Graphics Processing Unit (GPU). NVIDIA launched that year an Application Programming Interface (API) named Compute Unified Device Architecture (CUDA) for NVIDIA graphical cards. After one year, in 2007, an open project begun to emerge, Open Computing Language (OpenCL) that can be used on different computing platforms of CPU and GPU (NVIDIA, ATI, IBM etc). The main advantage of using GPU programming over traditional CPU one is that graphical cards bring a lot of computing power at a very low price. Today a huge number of application (scientific, financial etc) begun to be ported or developed for GPU usage, including Monte Carlo tools or data analysis tools for High Energy Physics.

In this project we aim to study the behavior of anisotropic flow in nucleus-nucleus interactions from different experimental data with the help of new tools developed using GPU programming languages.