Commissioning of the Absolute Luminosity For ATLAS detector at the LHC

The startup of the LHC (Large Hadron Collider) has initialized a new era in particle physics. The standard model of particle physics has for the last 40 years with tremendous success described all measurements with phenomenal precision. The experiments at the LHC are testing the standard model in a new energy regime. To normalize the measurements and understand the potential discoveries of the LHC experiments it is often crucial to know the interaction rate - the absolute luminosity. The ATLAS (A Toroidal LHC ApparatuS) detector will measure luminosity by numerous methods. But for most of the methods only the relative luminosity is measured with good precision. The absolute scale has to be provided from elsewhere. ATLAS is like the other LHC experiments mainly relying of absolute luminosity calibration from van der Meer scans (beam separation scans). To cross check and maybe even improve the precision; ATLAS has built a sub-detector to measure the flux of protons scattered under very small angles as this flux relates directly and with good precision to the absolute luminosity. This sub-detector is called ALFA (Absolute Luminosity For ATLAS) and is mainly use in very special beam condition of LHC, as the measurement of the flux at very small angles requires very high β* beam optics. To reach sufficiently small angles the ALFA detectors are positioned about 240 m from the interaction point. To approach close enough to the beam (few millimeters or even less) the detectors moves inside aperture of the LHC beam pipe. To not pollute the ultra high vacuum of LHC the ALFA detectors are contained in Roman Pots. The detector itself is build up from 500 µm squared scintillating fibers. To improve resolution the fibers are staggered. The fibers are readout by MAPMTs (MultiAnode PhotoMultiplier Tubes). To eliminate background two stations containing an upper and a lower detector are positioned at each side of the interaction point. The combined tracks for 4 stations will give the flux and angle of the scattered particles, from which the luminosity can be calculated with good precision. Before the installation of the ALFA detector system several tests were made to optimize and evaluate the performance of the detectors. This culminated in a test beam campaign in 2010 in the North Area of CERN. At the test beam a full ALFA station was tested inclusive final electronics, Detector Control System and readout system. The EUDET beam telescope was used for as high precision particle track reference. Several beams were supplied including a thoroughly set up pion beam that matched the rate and size of the EUDET telescope sensors and a large size muon beam to test the full surface of the ALFA detectors. The main result from the test beam was internal alignment with high precision of the ALFA detector parts. After installation of the ALFA detector system in the LHC winter shutdown 2010-2011 it was commissioned and integrated into ATLAS. Already In 2011 this allowed for a measurement of the total cross section at center of mass energy 7 TeV using special high β* = 90 m beam optics. A major conceptual upgrade of the ALFA trigger system was made in the winter shutdown 2011-2012 which potentially doubled the data taking time useful for physics in all runs after. In 2012 the total cross section measurement was repeated with similar optics but for a center of mass energy 8 TeV and in addition a moderate high intensity fill made it possible for ATLAS using the sub-detector ALFA to have a dedicated data taking for diffractive physics where the very forward particles passing the ALFA detectors were measured and used for triggering. In 2012 the improved beam optics of β* = 1000 m and a very aggressive positioning of the ALFA detectors near the beam also made it possible to take data in a position that potentially would allow for absolute luminosity determination. In 2013 the ALFA detector was used in almost all lead-proton fill to measure outgoing protons. Beam induced heating (impedance heating) was observed on the ALFA detectors and tests revealed that the detectors were in danger of overheating with massive degradation as a consequence. A short term solution was found and implemented and the implementation of a long term solution by minimization of the impedance losses is ongoing