Beam backgrounds in the ATLAS detector during LHC loss map tests at beta*=40cm and beta*=80cm at Ebeam=6.5 TeV

In this note the beam-background measurements with the ATLAS detector during lossmap tests of the LHC are described. Loss maps taken at beta*=40 cm and the normal 2015 setting of beta*=80 cm are compared. In the first case several collimator settings were explored, resulting in significant changes of beam backgrounds in ATLAS. Besides the studies of the dependence of background on collimation, which are important for optimisation of the LHC performance, these tests provide a clean environment to study the relative importance of beam halo losses on the experiment. The results show that the halo-related component of beam background in ATLAS decreases exponentially with increasing aperture of the tertiary collimators, the slope in terms of nominal sigma being about -0.5. From the data it is also shown that in normal operation conditions of LHC run 2 the beam halo losses contribute at most at the percent level to the total background, the dominant part coming from beam-gas interactions. The data are also used to demonstrate that the event-byevent correlation of backgrounds seen by the Beam Conditions Monitor and fake jets seen in the calorimeters in halo-related background events is very weak. The data are compared with dedicated Monte Carlo simulations and excellent agreement is demonstrated for several observables, in particular background rates per unit of beam intensity loss and pT and azimuthal distributions of fake jets.