LHC passes a new milestone as a precision machine

New ATLAS measurement of top-quark-pair cross section achieves 1.8% precision

24 March 2023 | By

The top quark is the heaviest known elementary particle and plays a significant role in our understanding of the fundamental building blocks of nature. At the peak of LHC Run 2 (2015-2018), the ATLAS experiment recorded up to 30 top quarks every second. This incredible rate is allowing researchers to open new windows into the subatomic world, revealing the properties and interactions of this unique particle with incredible precision. In particular, measurements of the production rate (or cross section) for top-quark pairs provide a crucial test of the Standard Model of particle physics. If measurements differ from predicted values, this could either indicate limitations in the theoretical framework, or be a potential first indicator of new phenomena.

The ATLAS Collaboration has just published the most precise measurement to date of the production cross section of top-quark pairs. The measured value is 829 ± 15 picobarns and has a relative uncertainty of just 1.8%. It is in agreement with the most advanced theoretical predictions, and with the earlier precise measurements conducted by both the ATLAS and CMS Collaborations.

The ATLAS Collaboration has just published the most precise measurement to date of the production cross section of top-quark pairs.

The incredible precision of this new measurement is due, in large part, to its use of ATLAS’ latest LHC luminosity measurement of the full Run-2 dataset. Luminosity quantifies the total number of proton interactions in a given dataset, allowing physicists to more accurately evaluate the probabilities of interesting processes occurring – in this case, the production of top-quark pairs.

Plots or Distributions,Physics,ATLAS
Top-quark-pair differential cross-section as a function of the sum of the leptons energies, as measured by ATLAS at 13 TeV using events in the di-lepton channel. It is compared to different theoretical predictions (MC). (Image: ATLAS Collaboration/CERN)

As in previous precision results, researchers examined collision events (see display above) where one top quark decays into an electron, a neutrino and a b-quark, while the other decays into a muon, a neutrino and a b-quark. These events leave distinctive signals in the ATLAS detector, enabling physicists to collect a very pure sample of top-pair events with a minimal level of background events. Further, they developed novel techniques for reconstructing the properties of the electrons and muons involved in the decays, and used data to determine how efficiently the b-quarks were being tagged and reconstructed.

In addition to the cross-section measurement, researchers determined ​​the production rate as a function of the kinematics of the produced leptons (see figure). These measurements were then compared to predictions from several event-generator programmes, which are used to simulate top-quark events at the LHC. The results highlighted some discrepancies with respect to the predictions (all generators predict a harder spectrum), indicating the need for more refined theoretical modelling.

This latest result shows the strength of the LHC as a precision machine – achieving results well beyond what was deemed possible at a hadron collider. The collaboration is looking forward to more proton–proton collision data in Run 3 with higher collision energy, which will allow researchers to further reduce statistical uncertainties and match this level of precision in other results.

About the event display: Display of a top-quark pair candidate event from proton-proton collisions recorded by ATLAS. This event was recorded on 13 June 2015 when protons with an energy of 6.5 TeV per beam were delivered by the LHC. The red line shows the path of a muon through the detector. The green line shows the path of an electron through the detector (yellow/orange blocks). The event contains two jets that have passed b-tagging requirements and these are indicated with yellow cones. The upper-left view shows the same event in the transverse plane, highlighting the direction of the missing transverse momentum (dashed white line).

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