The factorization bias in the Van der Meer method: Run 2 experiences at the CMS experiment

TL;DR

This paper investigates the bias in the Van der Meer method used for luminosity calibration at CMS, caused by non-factorizable proton densities, and compares multiple estimation techniques from Run 2 data.

Contribution

It presents various methods to estimate and analyze the factorization bias in the Van der Meer luminosity calibration at CMS during Run 2.

Findings

Different methods provide consistent estimates of the bias.

The beam-imaging method effectively reconstructs transverse proton densities.

Analysis of scan data reveals the magnitude of the factorization bias.

Abstract

The luminosity measurement of the CMS experiment at the CERN LHC is calibrated with Van der Meer (VdM) scans. A bias occurs in the VdM method due to the assumption of transversely factorizable proton densities of the LHC beams. Here, the different methods applied in Run 2 to estimate the size of the factorization bias are reported. The beam-imaging method reconstructs the transverse proton densities from beam-imaging scans. Additional methods exploit offset scans, analyze the evolution of the measured luminous region, and evaluate diagonal scans.