An Examination of Backgrounds to Early-Run Minimum-Bias Events in ATLAS at the LHC

The initial parts of the Large Hadron Collider (LHC) run will be a source of critical information - about the ATLAS detector and about the physics of $pp$ collisions at $sqrt{s} = 14$ TeV, including parton distribution evolution and the cross-sections of $sigma_{pp}$. The accelerator itself will be the source of some detector interest, as we have a first look at what have so far been speculations on the quality of the vacuum in the experimental insertion, and the cleanliness of the beam from the accelerator. The shakedown period, with its low beam squeeze, low luminosity, and undemanding trigger menus, will be of great interest, avoiding the pileup and radiation levels that will arrive with higher luminosity -- making it an important opportunity to investigate minimum-bias events in relative isolation. For the short lifetime of the Minimum Bias Trigger Scintillators (MBTS), which are expected to fail within a few months of running, they will aid in discriminating the minimum bias signal of inelastic non-single-diffractive $pp$ collisions. Using single- or double-coincidence signatures, the MBTS system and other trigger and analysis strategies attempt to avoid triggering on otherwise empty bunch crossings and eliminate the effects of beam-gas collisions and beam-halo effects which would lead these spurious triggers that would reduce the general minimum-bias trigger efficiency. An examination of the effects of beam halo and beam -gas interactions on the minimum-bias trigger response is made. The signatures of the beam halo and beam gas are examined from the standard ATLAS tracking reconstruction.