[Paper Review] Calibration of the light-flavour jet mistagging efficiency of the $b$-tagging algorithms with $Z$+jets events using 139 $\mathrm{fb}^{-1}$ of ATLAS proton-proton collision data at $\sqrt{s} = 13$ TeV

Aad, Georges, Abbott, Braden Keim+2970 more|arXiv (Cornell University)|Jan 1, 2023

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a data-driven calibration of the light-flavour jet mistagging efficiency in ATLAS $b$-tagging algorithms using 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s} = 13$ TeV. By employing modified 'flip tagger' algorithms and fitting the jet-flavour-sensitive secondary-vertex mass in $Z$+jets events, the method simultaneously corrects both $b$-jet and light-flavour mistagging efficiencies, reducing uncertainties from heavy-flavour jet modeling and yielding scale factors consistent with unity within uncertainties.

ABSTRACT

The identification of $b$-jets, referred to as $b$-tagging, is an important part of many physics analyses in the ATLAS experiment at the Large Hadron Collider and an accurate calibration of its performance is essential for high-quality physics results. This publication describes the calibration of the light-flavour jet mistagging efficiency in a data sample of proton-proton collision events at $\sqrt{s}=13$ TeV corresponding to an integrated luminosity of 139 fb$^{-1}$. The calibration is performed in a sample of $Z$ bosons produced in association with jets. Due to the low mistagging efficiency for light-flavour jets, a method which uses modified versions of the $b$-tagging algorithms referred to as flip taggers is used in this work. A fit to the jet-flavour-sensitive secondary-vertex mass is performed to extract the scale factor from data, while simultaneously correcting the $b$-jet efficiency. With this procedure the heavy-flavour uncertainties are considerably lower than in previous calibrations of the mistagging scale factors, where they were dominant. The scale factors obtained in this calibration are consistent with unity within uncertainties.

Motivation & Objective

• To improve the accuracy of light-flavour jet mistagging efficiency calibration in $b$-tagging algorithms used in ATLAS Run 2 physics analyses.
• To reduce systematic uncertainties dominated by heavy-flavour jet modeling in previous calibration methods.
• To simultaneously calibrate both $b$-jet efficiency and light-flavour mistagging efficiency using data from $Z$+jets events.
• To validate the method using a data-driven extrapolation technique between nominal and modified tagger responses.

Proposed method

• Uses $Z$+jets events from 139 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV to enrich the sample in mistagged light-flavour jets.
• Employs modified 'flip tagger' algorithms that reduce $b$- and $c$-jet tagging efficiencies while preserving light-flavour mistagging efficiency.
• Performs a fit to the jet-flavour-sensitive secondary-vertex mass distribution to extract scale factors for $b$-jet and light-flavour mistagging efficiencies.
• Simultaneously corrects both $\varepsilon_b$ and $\varepsilon_{\text{light}}$ in simulation using data-driven scale factors to reduce modeling uncertainties.
• Applies the scale factor definition $\text{SF}_f = \varepsilon_f^{\text{data}} / \varepsilon_f^{\text{MC}}$ to correct simulation-based tagging efficiencies.
• Validates the extrapolation between nominal and flip tagger performance using an alternative data-driven method based on low-level algorithm corrections.

Experimental results

Research questions

• RQ1 How can the light-flavour jet mistagging efficiency be accurately calibrated when data samples are dominated by $b$-jets?
• RQ2 To what extent do uncertainties from heavy-flavour jet modeling affect previous mistagging efficiency calibrations?
• RQ3 Can simultaneous calibration of $b$-jet and light-flavour mistagging efficiencies improve precision and reduce systematic uncertainties?
• RQ4 How well do the flip tagger response and nominal tagger response extrapolate to each other in the context of mistagging efficiency calibration?

Key findings

• The light-flavour mistagging scale factor ($\text{SF}_{\text{light}}$) is measured to be consistent with unity within uncertainties, indicating good agreement between data and simulation after correction.
• The $b$-jet efficiency scale factor ($\text{SF}_b$) is also extracted from data, reducing reliance on simulation-based estimates and lowering associated uncertainties.
• Systematic uncertainties from heavy-flavour jet modeling are significantly reduced compared to previous calibration methods, which previously dominated the total uncertainty.
• The calibration method using $Z$+jets events enables the use of unprescaled lepton triggers, improving data statistics and precision over earlier methods using prescaled single-jet triggers.
• The scale factors for both $b$-jet and light-flavour mistagging efficiencies are found to be stable across different jet $p_T$ and $\eta$ regions, indicating robustness of the method.
• The data-driven validation method confirms that the extrapolation between nominal and flip tagger responses is reliable, supporting the consistency of the calibration.