Characterization of Interaction-Point Beam Parameters Using the pp Event-Vertex Distribution Reconstructed in the ATLAS Detector at the LHC

We present results from the measurement of the three-dimensional luminosity distribution with the ATLAS Inner Detector during early running. The spatial distribution of pp interactions is reconstructed by a dedicated algorithm in the High-Level Trigger that fits tracks and primary event vertices in real time, as well as by an offline reconstruction algorithm that takes full advantage of the high tracking efficiency and resolution of the Inner Detector through an unbinned maximum-likelihood fit. The number of these vertices provides online monitoring of the instantaneous luminosity, while luminous-centroid motion mirrors drifts in the interaction point (IP) orbit, and in the relative RF phase between the two rings. Similarly, the x, y and z luminous widths reflect the evolution of the transverse and longitudinal emittances. The length scales of the beam-position monitors and IP orbit bumps, which directly impact the accuracy of the transverse convolved beam sizes measured during beam-beam scans, are calibrated offline against the displacement of the luminous centroid measured, during these same scans, by the ATLAS trackers; this significantly improves the absolute accuracy of the luminosity calibration by beam-separation scans. Finally, the simultaneous determination, during such scans, of the transverse convolved beam sizes (from the luminosity variation) and of the corresponding luminous sizes can be used to disentangle the transverse IP sizes of the two beams.