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Boosting precision of top-quark mass measurement with ATLAS

25 March 2025 | By

The ATLAS Collaboration at CERN performed its most precise single measurement of the mass of the top quark, using high-transverse-momentum (“boosted”) top quarks.

Physics,ATLAS — Figure 1: The invariant mass distribution of the top jet, which is used to reconstruct the boosted top quark, predicted from simulated signal events with three different top quark mass values: 171, 172.5, and 174 GeV. (Image: ATLAS Collaboration/CERN)

The top quark is no ordinary particle. Weighing approximately 173 GeV – as much as a gold atom – it is the heaviest known fundamental particle in the Standard Model. A precise measurement of its mass is essential for testing the consistency of the Standard Model and probing for new physics phenomena, which could appear as subtle deviations from predictions.

In a new study submitted to Physics Letters B, the ATLAS Collaboration analysed the full LHC Run 2 dataset (2015–2018) to study top-antitop-quark pairs produced in proton-proton collisions. However, there’s a challenge: the top quark decays almost instantly into a W boson and a bottom (b) quark, never reaching the detector. Physicists can only study it by piecing together its decay products.

For this search, researchers focused on collision events where one high-momentum top quark decays hadronically, creating tightly collimated “jets” of particles in the ATLAS detector (see event display). Together, these jets form a single “top jet” with properties reflecting those of the originating top quark. This allowed the ATLAS team to establish a clear link between the top jet’s mass and that of the top quark as shown in Figure 1.

This new result is the ATLAS Collaboration’s most precise top-quark mass measurement using a single decay channel: 172.95 ± 0.53 GeV.

Achieving a precise measurement first required addressing several key systematic uncertainties. For instance, physicists needed to carefully calibrate their measurements of the jet energies. This was achieved by reconstructing the W boson inside the top-jet and measuring its mass distribution. Since this distribution is sensitive to the jet energy scale – but not, crucially, to the top quark mass – researchers could use it to disentangle jet energy scale uncertainties from their mass measurement. This reduced jet energy scale uncertainties in the top-quark mass measurement by an impressive 80%.

Another major source of uncertainty arose from the theoretical modeling of how the b-quark radiates energy during the top quark decay. The ATLAS team tackled this by identifying a new observable that differentiates between various models and used it to determine which model best fits the data, ultimately reducing the uncertainty by 80%.

This new result is the ATLAS Collaboration’s most precise top-quark mass measurement using a single decay channel: 172.95 ± 0.53 GeV. It is in excellent agreement with the LHC Run 1 combination (see Figure 2), which remains the most precise top quark mass determination to date.

As LHC Run 3 continues and the High-Luminosity LHC upgrade appears on the horizon, the sheer volume of top quark events is set to skyrocket. This surge in data will lead to even more precise measurements, enabling increasingly stringent tests of the Standard Model.

About the event display: Candidate event for the top-antitop-quark pair process with a high-transverse-momentum, hadronically-decaying top quark. The hadronically-decaying top quark is reconstructed from the three particle jets in the upper half of the detector. One of the jets originates from a bottom quark (“b-tagged” in blue cone) and the other two jets (yellow cones) are consistent with being produced from a W-boson decay. The second top quark is reconstructed from the signals in the lower half of the detector: a second b-tagged jet (blue), an additional jet (yellow), a muon (red line) and the missing transverse momentum (not shown). Significant energy deposits are seen in the electromagnetic (green) and hadronic (yellow) calorimeters. Tracks of charged particles in the inner detector are visualised as orange lines. (Image: ATLAS Collaboration/CERN)