Optical parameters Determination for absolute luminosity and total cross section measurements in ATLAS

ALFA (Absolute Luminosity For ATLAS) aims at measuring the absolute luminosity for the ATLAS experiment with an incertitude down to 2-3 \% and the total elastic cross section. The luminosity is related to the number of events, the highest the luminosity, the highest the number of events. This is, then, an important quantity for colliders like LHC (Large Hadron Collider). LHC is made of two beams circulating in two different beam pipes and colliding at four interaction points where the four physics experiments are located (ATLAS, CMS, ALICE, LHCb). ALFA detectors inserted into Roman Pots (RPs), have been placed around ATLAS at 240 m distance from the collision point (IP1) after six quadrupoles magnets and two dipoles defining the ALFA beam line which is part of the LHC ring.The detectors are made of scintillating optics fibers to catch elastic protons generated at IP1. These protons are tracked through the LHC magnets beam line which needs to be optimized in terms of optics parameters. We call high β optics, the optics used for special ALFA runs measurements. The high β optics parameters have been simulated to fulfill the ALFA requirements and have been tested on LHC in 2011 and 2012 during a certain number of LHC special runs. It has ended at the end of 2013. The parameters have been measured and compared with simulations. Some of them achieving a better value than expected. It allowed us to calculate systematic uncertainties and to evaluate the impact of some optics parameters on the total elastic cross section measurement.