Performance of $b$-Jet Identification in the ATLAS Experiment

TL;DR

The paper documents ATLAS's comprehensive program for identifying b-jets using lifetime-based and muon-based tagging, along with their online trigger implementations. It develops and validates multiple algorithms (JetProb/IP3D/SV1/JetFitter/MV1 and SMT) and calibrates their data-to-simulation efficiencies for b, c, and mistag rates across jet pT and η. Key contributions include muon-based pT_rel and System8 methods, tt̄-based calibrations, SMT calibrations, and D* and W+c strategies for c-jet tagging, all combined to yield precise scale factors with quantified systematics. The results enable robust use of b-tagging in ATLAS analyses, including high-purity hadronic channels and top/Higgs/neutrino studies, while demonstrating controlled pile-up performance and improved trigger-level tagging capabilities.

Abstract

The identification of jets containing $b$ hadrons is important for the physics programme of the ATLAS experiment at the Large Hadron Collider. Several algorithms to identify jets containing $b$ hadrons are described, ranging from those based on the reconstruction of an inclusive secondary vertex or the presence of tracks with large impact parameters to combined tagging algorithms making use of multi-variate discriminants. An independent $b$-tagging algorithm based on the reconstruction of muons inside jets as well as the $b$-tagging algorithm used in the online trigger are also presented.The $b$-jet tagging efficiency, the $c$-jet tagging efficiency and the mistag rate for light flavour jets in data have been measured with a number of complementary methods. The calibration results are presented as scale factors defined as the ratio of the efficiency (or mistag rate) in data to that in simulation. In the case of $b$ jets, where more than one calibration method exists, the results from the various analyses have been combined taking into account the statistical correlation as well as the correlation of the sources of systematic uncertainty.

Figures (55)

• Figure 1: The vertex mass (a), energy fraction (b) and vertex finding efficiency (c) of the inclusive secondary vertices found by the SV1 algorithm, for three different flavours of jets.
• Figure 2: The vertex mass (top), energy fraction (middle) and flight length significance (bottom) for $b$ jets (left), $c$ jets (middle) and light-flavour jets (right), split according to the decay chain topology found by JetFitter. In the case that no vertex with at least two outgoing tracks has been reconstructed, these quantities are computed from reconstructed single track vertices as explained in the text. The distributions are obtained from a simulated sample of $t\bar{t}$ events generated with POWHEGbib:powhegbib:powheg2 interfaced to PYTHIA.
• Figure 3: Distribution of the IP3D (a), SV1 (b) and IP3D+JetFitter (c) weights, for $b$, $c$ and light-flavour jets. These three weights are used as inputs for the MV1 algorithm. The spikes at $w_{\rm IP3D}\approx -20$ and $\approx -30$ correspond to pathological cases where the IP3D weight could not be computed, due to the absence of good-quality tracks. The spike at $w_{\rm SV1}\approx -1$ corresponds to jets in which no secondary vertex could be reconstructed by the SV1 algorithm, and where discrete probabilities for a $b$ and light-flavour jet not to have a vertex are assigned. The irregular behaviour in $w_{\rm IP3D+JetFitter}$ arises because both the $w_{\rm IP3D}$ and the $w_{\rm JetFitter}$ distribution (not shown) exhibit several spikes.
• Figure 4: Distributions of the correlations between the IP3D, SV1 and IP3D+JetFitter weights, for $b$ jets (top), $c$ jets (middle) and light-flavour jets (bottom). The spikes at $w_{\rm IP3D}\approx -20$ and $\approx -30$ correspond to pathological cases where the IP3D weight could not be computed, due to the absence of good-quality tracks. The spike at $w_{\rm SV1}\approx -1$ corresponds to jets in which no secondary vertex could be reconstructed by the SV1 algorithm, and where discrete probabilities for a $b$ and light-flavour jet not to have a vertex are assigned.
• Figure 5: Distribution of the tagging weight obtained with the MV1 algorithm, for three different flavours of jets.