Speaker
Description
The upgraded LHCb experiment is pioneering the landscape of real-time data-processing techniques using an heterogeneous computing infrastructure, composed of both GPUs and FPGAs, aimed at boosting the performance of the HLT1 reconstruction. Amongst the novelties in the reconstruction infrastructure made for the Run 3, the introduction of a real-time VELO hit-finding FPGA-based architecture stands out. For the first time at any LHC experiment, the bidimensional clusters of active pixels on the silicon vertex detector are reconstructed before event-building, directly on the detector readout boards, at the full interaction rate of ~30MHz. In addition to saving HLT1 computing resources and reducing the DAQ bandwidth, the availability of well reconstructed particle hits at the readout level opens up the possibility of further processing in order to reconstruct even more complex quantities. Specifically, measuring hit rates at several positions on the detector sensors allows to measure and track the geometrical properties of the luminous region in real time. For such purpose, a set of programmable counters has been implemented in firmware. This set of counters uses minimal FPGA resources and it can be analysed to provide beam spot position, shape and inclination measurements. This is achieved via linearized computations based on principal component analysis (PCA), that are performed in real time on the LHCb slow control software. This method differs substantially from the usual techniques relying on track and vertex reconstruction, that are prone to misalignment biases and depend on the HLT running conditions. In this contribution, we describe the technical implementation of such a system for real-time beam-spot measurement, and report the results obtained with real data collected in 2024 and 2025.
Significance
This is the first time a particle physics experiment uses real-time hit reconstruction to characterise the luminous region. Luminosity and beam spot position are monitored by means of on-the-fly hit-statistics evaluation, transparently embedded in the readout in a complex detector at the full LHC average collision rate of 30 MHz. We present for the first time at a public conference the results obtained during the 2024 and 2025 data-taking periods.
References
This work is a spinoff of the following published work: G. Bassi et al., "A FPGA-Based Architecture for Real-Time Cluster Finding in the LHCb Silicon Pixel Detector", IEEE Trans. Nucl. Sci. 70 (2023) 1189, arXiv:2302.03972
The idea behind this endeavour, but not the results, has been presented last year at CHEP: arXiv:2503.10831
| Experiment context, if any | LHCb |
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