Luminosity determination in $pp$ collisions at $\sqrt{s}=13$ TeV using the ATLAS detector at the LHC

Abstract: The luminosity determination for the ATLAS detector at the LHC during Run 2 is presented, with $pp$ collisions at $\sqrt{s}=13$ TeV. The absolute luminosity scale is determined using van der Meer beam separation scans during dedicated running periods in each year, and extrapolated to the physics data-taking regime using complementary measurements from several luminosity-sensitive detectors. The total uncertainties in the integrated luminosities for each individual year of data-taking range from 0.9% to 1.1%, and are partially correlated between years. After standard data-quality selections, the full Run 2 $pp$ data sample corresponds to an integrated luminosity of $140.1\pm 1.2$ fb$^{-1}$, i.e. an uncertainty of 0.83%. A dedicated sample of low-pileup data recorded in 2017-18 for precision Standard Model physics measurements is analysed separately, and has an integrated luminosity of $338.1\pm 3.1$ pb$^{-1}$.

• The paper presents a refined luminosity calibration method using van der Meer scans to precisely map beam overlaps in 13 TeV pp collisions, achieving 0.83% precision.
• It leverages multiple detectors—including LUCID, track counting, and calorimeter systems—to provide robust cross-checks and enhance measurement confidence.
• The study thoroughly addresses systematic uncertainties such as beam dynamics and instrumental non-linearities to ensure accurate luminosity determination.

Overview of ATLAS Luminosity Determination in Proton-Proton Collisions

The ATLAS Collaboration has presented a meticulous study on determining the luminosity in proton-proton ($pp$) collisions at $\sqrt{s} = 13\,\TeV$ during the LHC's Run 2. This crucial measurement underpins both the precision of cross-section analyses and the sensitivity of new physics searches within the formidable data set accumulated between 2015 and 2018. In this overview, we will explore the methodologies employed, the calibrations of various luminosity detectors, and the comprehensive treatment of systematic uncertainties that the collaboration has tackled.

Luminosity Calibration via van der Meer Scans

The foundational calibration of the integrated luminosity in this study hinges on the van der Meer (vdM) scan technique. This method, pivotal since its original deployment at the ISR and further evolved through subsequent collider operations, entails incrementally scanning the transverse beam separation to map out the resulting luminosity curves. The measured luminosity as a function of beam separation provides a direct basis for calculating the overlap between colliding bunches, leading to an estimate of the absolute luminosity scale.

Sensitive challenges addressed in this paper include corrections for beam dynamics such as transverse deflections and optically induced distortions due to beam-beam interactions. These effects are meticulously quantified, ensuring precision in the determination of $\cap\capsigx\sigma_y$—the convolution of beam sizes key to the vdM methodology.

Deployment of Multiple Luminosity Detectors

Given the complex conditions of high-energy physics experiments, redundancy in measurement devices provides robustness against systematic uncertainties. The paper describes how the ATLAS detector leveraged multiple systems—such as LUCID, track counting in the inner detector, BCM diamond detectors, and signals from EMEC, FCal, and TileCal calorimeters—for comprehensive luminosity monitoring.

The LUCID Cherenkov detector, positioned near the beamline, served as a primary system during normal physics operation. Its real-time feedback capability was critical for LHC beam optimizations. However, in the intense pileup of Run 2, LUCID experienced non-linearities that required careful calibration against track-counting methods and cross-validation with other calorimeter readings.

Addressing Systematic Uncertainties

A detailed breakdown of systematic uncertainties marks a significant contribution of this paper. Not only are statistical errors analyzed, but comprehensive studies are conducted on possible shifts due to bunch population measurement inaccuracies, beam orbit drifts, jitter during vdM scans, and the effects of correlations in the calibration transfer process. Notably, pioneering work in understanding magnetic non-linearities in steering magnet settings introduces another layer of correction necessary for precise luminosity scale calibration.

Cross-comparisons between LUCID and reference measurements from track-counting data, and further comparisons with calorimeter-derived measures, serve as cross-checks. The study's robust treatment of these uncertainties results in an overall precision on the integrated luminosity of approximately $0.83\%$ for the combined Run 2 dataset.

Conclusion and Implications

The research detailed in this paper culminates in a refined luminosity determination protocol crucial for maintaining the integrity of the ATLAS physics program. The precision achieved in handling the various challenges of high-luminosity physics is not just a triumph for the operation at the LHC, but directly impacts the reliability with which researchers can probe the Standard Model and venture into new physics regimes.

Looking forward, techniques developed herein may well influence future endeavors at the HL-LHC, where precise luminosity measurements will continue to serve as a bedrock for high-energy physics discoveries. The ATLAS collaboration, through such meticulous methodological advancements, consolidates its role as a vanguard in the intricate quest of deciphering fundamental particle interactions.