Operation and performance of the Probe for Luminosity Measurement at LHCb

Abstract: The Probe for LUminosity MEasurement (PLUME) detector is a dedicated luminosity monitor at the LHCb interaction point. It is a hodoscope comprising 48 Hamamatsu R760 photomultiplier tubes that detect Cherenkov light produced by particles travelling in the direction opposite to the LHCb spectrometer acceptance. PLUME provides real-time, bunch-by-bunch luminosity measurements during LHCb data taking and serves as the primary detector for controlling the luminosity levelling process at LHCb. In addition, its data are used offline, together with other dedicated counters from LHCb sub-systems, to determine the integrated luminosity delivered to the experiment. This paper reports on the detector's operational performance during the 2024-2026 data-taking period of Run 3. The integrated luminosity recorded by the LHCb experiment in $pp$ collisions at $\sqrt{s} = 13.6$ TeV during this period, as measured by the PLUME online luminosity counters for detector performance monitoring, amounts to $\mathcal{L} = (26.71 \pm 1.07)$ fb$<sup>{-1}$. The luminosity values used in physics analyses are determined separately through dedicated offline calibrations based on van der Meer scans and Beam Gas Imaging techniques, and will be reported in a dedicated publication, as they are beyond the scope of this paper.