Origin of structures in the PLUME collision-clock phase measurement

Determine the origin of the dips and structures observed in the PLUME TELL40 measurements of the collision phase relative to the LHC clock, which remain unexplained after preliminary studies found no correlation with proton–proton interaction-vertex positions, the SMOG2 operating regime, or temperature variations in the LHCb experimental area.

Background

PLUME measures the arrival time of particles produced in proton–proton collisions relative to the LHC clock, while the BPTX system measures the passage time of the individual LHC beams at a fixed longitudinal position. Although the two systems generally agree, their time evolution can differ by up to approximately 200 ps because they measure different physical quantities.

The paper reports unexplained dips and structures in the PLUME TELL40 collision-phase measurements. Preliminary investigations excluded correlations with the proton–proton interaction-vertex position, the SMOG2 operating regime, and temperature variations in the LHCb experimental area, and further studies involving LHC accelerator experts were ongoing.

References

The origin of the dips and structures observed in the PLUME TELL40 measurements is not yet understood and is currently under investigation. Preliminary studies suggest that these features are not correlated with the position of the \proton\proton interaction vertices, the SMOG2 operating regime, or temperature variations in the LHCb experimental area.

— Operation and performance of the Probe for Luminosity Measurement at LHCb  (2609.10288 - Alessio et al., 9 Sep 2026) in Section 6.4, subsection “Measurement of the collision phase relative to the LHC clock”