First measurements of beam-beam effects in beam-separation, luminosity-calibration scans at the LHC

At the CERN Large Hadron Collider (LHC), absolute luminosity calibrations obtained by the van der Meer ( vdM ) method are affected by the mutual electromagnetic interaction of the two beams. In the last few years, increasingly stringent, physics-driven requirements on the accuracy of the absolute luminosity scale, combined with the unparalleled precision of the luminosity measurements achieved by the LHC experiments, motivated comprehensive simulation studies to accurately model beam–beam effects and provide simulation-based corrections to vdM -based luminosity calibrations. This paper reports the first attempt at an experimental validation of that model, in dedicated accelerator measurements under controlled conditions, of the predicted impact of beam-beam effects on the luminosity measured at the LHC under luminosity-calibration conditions. This is the first measurement of beam-beam interaction effects on luminosity at the LHC. The results show good agreement with the predictions of multiparticle simulations, and justify the recently proposed strategy for correcting beam-beam biases on absolute luminosity calibrations at hadron colliders. This marks a critical step forward in precise luminosity calibration for past, current, and future datasets, including the High-Luminosity Large Hadron Collider era.