LHC beam monitoring via real-time hit reconstruction in the LHCb VELO pixel detector

The increasing computing power and bandwidth of programmable digital devices opens new possibilities in the field of real-time processing of HEP data. The LHCb collaboration is exploiting these technology advancements in various ways to enhance its capability for complex data reconstruction in real time. Amongst them is the real-time reconstruction of hits in the VELO pixel detector, by means of real-time cluster-finding embedded in the readout board firmware. This reconstruction, in addition to saving data-acquisition bandwidth and high-level trigger computing resources, also enables further useful applications in precision monitoring and diagnostics of LHC beam conditions. In fact, clusters of pixels, being more reliable and robust indications of physical particle hits than raw pixel counts, are also exempt from the complications associated to the reconstruction of tracks, that involves alignment issues and is sensitive to multi-layer efficiency products. In this paper, we describe the design and implementation of a flexible system embedded in the readout firmware of the VELO detector, allowing real-time measurement of cluster density in several parts of the detector simultaneously, and separately for every bunch ID, for every single LHC collision, without any slowdown of data acquisition. Quantitative applications of this system to luminosity measurement and beam monitoring are demonstrated.