Real-time monitoring of LHCb beam spot properties based on FPGA hit reconstruction

Abstract

We present a real-time trackless method to monitor the centre of the LHCb luminous region and the relative alignment of the two halves of the LHCb Vertex Locator, using cluster counts accumulated in selected regions of the detector and processed in firmware. A Principal Component Analysis performed on simulated data yields a set of weight vectors that reduce the position estimators to scalar products of the counter rates, executable with negligible computational overhead. With pp collision data from 2024, the transverse position of the luminous region is reconstructed with a statistical precision of approximately 4 micrometre per coordinate in a 90 millisecond accumulation window. A measurement of the longitudinal centre is demonstrated with a statistical precision of a few millimetres in simulated studies. The introduction of run-time configurable firmware counters has been studied in simulation to improve the transverse and longitudinal resolutions by a factor of three with respect to the fixed-geometry baseline.