Using pile-up collisions as an abundant source of low-energy hadronic physics processes in ATLAS and an extraction of the jet energy resolution

TL;DR

This paper demonstrates how pile-up collisions in ATLAS can be used as a rich source of low-energy hadronic physics processes and shows how to extract the jet energy resolution from this dataset, improving measurements at low transverse momentum.

Contribution

It introduces a novel method to utilize pile-up collisions as an unbiased data source for low-energy physics and jet energy resolution extraction in ATLAS.

Findings

Pile-up data provides up to 50 times more dijet events than traditional methods.

The method reduces statistical uncertainty in jet energy resolution below 65 GeV.

Validation confirms the utility of pile-up data for physics measurements.

Abstract

During the 2015-2018 data-taking period, the Large Hadron Collider delivered proton-proton bunch crossings at a centre-of-mass energy of 13 TeV to the ATLAS experiment at a rate of roughly 30 MHz, where each bunch crossing contained an average of 34 independent inelastic proton-proton collisions. The ATLAS trigger system selected roughly 1 kHz of these bunch crossings to be recorded to disk. Offline algorithms then identify one of the recorded collisions as the collision of interest for subsequent data analysis, and the remaining collisions are referred to as pile-up. Pile-up collisions represent a trigger-unbiased dataset, which is evaluated to have an integrated luminosity of 1.33 pb$^{-1}$ in 2015-2018. This is small compared with the normal trigger-based ATLAS dataset, but when combined with vertex-by-vertex jet reconstruction it provides up to 50 times more dijet events than the conventional single-jet-trigger-based approach, and does so without adding any additional cost or requirements on the trigger system, readout, or storage. The pile-up dataset is validated through comparisons with a special trigger-unbiased dataset recorded by ATLAS, and its utility is demonstrated by means of a measurement of the jet energy resolution in dijet events, where the statistical uncertainty is significantly reduced for jet transverse momenta below 65 GeV.