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Decoding top quarks: Precision in heavy-flavour partner production

26 September 2024 | By

Two new studies from the ATLAS Collaboration have revealed how top-quark pairs are produced alongside heavy-flavour quarks like bottom and charm. These studies not only deepen the understanding of the top quark’s relationship with bottom and charm quarks within the framework of Quantum Chromodynamics (QCD), but also pave the way for new explorations of rare processes involving top quarks – such as the top quark’s interaction with the Higgs boson and the simultaneous production of four top quarks.

In proton-proton collisions at the LHC, top-quark pairs are frequently produced along with additional heavy-flavour quarks, including bottom and charm quarks (“b” and “c”, see Figure 1). These events can provide physicists with valuable insights into QCD, the theory that describes the strong force. Precisely determining the cross sections of these processes also enables researchers to more effectively distinguish them from rarer phenomena such as tt̅H production, where a top-quark pair is produced with a Higgs boson, and 4-top production, where two top-quark pairs are created in a single event. In their latest studies, ATLAS researchers analysed collision data from LHC Run 2 (2015–2018) to measure how often top-quark pairs are produced alongside b-quarks and c-quarks and to explore the detailed dynamics of these interactions.

Physics,ATLAS — Figure 1: Feynman diagrams for top-quark pair production with additional heavy-flavour jets, from left to right: with an additional b/c-quark pair via initial-state gluon radiation where both b/c-quarks form a jet each; with an additional b/c-quark pair via initial-state gluon radiation where the two b/c-quarks are in the same jet; and, with an additional single b/c-quark originating from the initial state. (Image: ATLAS Collaboration/CERN)

The beauty of precision

The first ATLAS study focuses on how often top-quark pairs are produced alongside additional “jets” of particles produced by b-quarks (b-jets). To identify b-jets, researchers use tools called flavour-tagging algorithms that look for specific patterns, such as how far particles traverse through the detector before they decay, to tag jets from b-quarks with high accuracy. By analysing events with opposite-charge electron-muon pairs and at least three or four b-jets in the final state, ATLAS physicists made the most precise measurements of the inclusive cross-sections in these categories to date, with uncertainties as low as 8.5%. These measurements surpass the accuracy of current theoretical predictions, especially in collision events with higher numbers of b-jets. The results were compared to multiple theoretical predictions to assess the accuracy of the modelling of this process.

Researchers also examined several kinematic properties, such as the momenta of the b-jets and total jet energy (see Figure 2), to test how well simulations of top-quark-pair production match the data. While the tested models performed well in events with two b-jets, these simulations struggled to accurately predict events with three or more b-jets. The predictions from the Sherpa model showed the best consistency across different numbers of b-jet, while the Powheg+Herwig7 predictions excelled in modelling certain variables, like b-jet momentum and total hadronic energy. These findings point to areas where theoretical models need improvement to better capture the complexities of additional b-jet production.

Charmed precision

The second ATLAS study, presented today at the TOP 2024 conference, breaks new ground by providing the first distinct ATLAS measurement of top-quark-pair production with additional jets originating from c-quarks (c-jets). Physicists analysed collision events with one or two leptons, using a custom flavour-tagging algorithm developed specifically for this study to distinguish c-jets from b-jets and other jets. This advanced tagging technique was essential because c-jets are even more challenging to identify than b-jets, as they have shorter lifetimes and produce less distinct signatures in the detector.

The study found that most theoretical models provided reasonable agreement with the data, though they generally under-predicted the production rates of c-jets (see Figure 3). Among the tested models, Powheg+Pythia8 showed the best agreement, while others under-predicted the cross sections by up to 40%, leading to deviations from data of up to two standard deviations. Accurate simulations of Standard Model predictions are critical for precision measurements and rare-process searches. These results highlight the need for refined simulations of c-quark production in top-quark-pair events to improve future measurements.

Laying the foundation for future discoveries

Both studies provide crucial insights into top-quark pair production with heavy-flavour quarks: the first detailed exploration of kinematic properties for events with bottom quarks and the first dedicated measurement of top-quark pairs with charm quarks by ATLAS. However, they also reveal challenges in current theoretical models, particularly when predicting events with multiple bottom or charm quarks. Improving these models will be essential for enhancing background estimations in the search for even rarer processes. These advancements not only improve our understanding of fundamental interactions within the Standard Model but also pave the way for discoveries that could push the boundaries of known physics.