Christian Ohm (Department of Physics, Stockholm University)
The ATLAS BPTX stations are comprised of electrostatic button pick-up detectors, located 175 m away along the beam pipe on both sides of ATLAS. The pick-ups are installed as a part of the LHC beam instrumentation and used by ATLAS for timing purposes. The usage of the BPTX signals in ATLAS is twofold; they are used both in the trigger system and for LHC beam monitoring. The ATLAS Trigger System is designed in three levels, each level sequentially reﬁning the selection of events to be saved for further oﬄine analysis. The BPTX signals are discriminated with a constant-fraction discriminator to provide a Level-1 trigger when a bunch passes through ATLAS. Furthermore, the BPTX detectors are used by a stand-alone monitoring system for the LHC bunches and timing signals. The BPTX monitoring software measures the phase between collisions and clock with high accuracy in order to guarantee a stable phase relationship for optimal signal sampling in the sub-detector front-end electronics. In addition to monitoring this phase, the properties of the individual bunches are measured and the structure of the beams is determined. On September 10th, 2008, the ﬁrst LHC beams reached the ATLAS experiment. During this period with single beam, the ATLAS BPTX system was used extensively to time in the read-out of the sub-detectors. In this paper, we present the performance of the BPTX system with focus on the monitoring system and its measurements of the ﬁrst LHC beams.