LHC passes a new milestone as a precision machine
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.
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.
- Inclusive and differential cross-sections for dilepton tt¯ production measured in 13 TeV proton–proton collisions with the ATLAS detector (arXiv:2303.15340, see figures)
- Measurement of the tt¯ production cross-section and lepton differential distributions in eμ dilepton events from pp collisions at 13 TeV with the ATLAS detector (Eur. Phys. J. C 80 (2020) 528, arXiv:1910.08819, see figures)
- Measurement of the tt¯ cross-section and tt¯/Z cross-section ratio using LHC Run 3 proton-proton collision data at a centre-of-mass energy of 13.6 TeV (ATLAS-CONF-2022-070)
- Moriond EW 2023 presentation by Lisa Shabalina: Top highlights from ATLAS
- Moriond EW 2023 presentation by Evgeniya Cheremushkina: New measurements at 13.6 TeV in ATLAS
- Moriond EW 2023 presentation by Soureek Mitra: ttbar and t(t)+X measurements in ATLAS + CMS
- CMS Collaboration: First measurement of the top quark pair production cross section in proton-proton collisions at 13.6 TeV (CMS-PAS-TOP-22-012)
- CMS Collaboration: Measurement of differential tt¯ production cross sections in the full kinematic range using lepton + jets events from proton-proton collisions at 13 TeV (Phys. Rev. D 104 (2021) 092013)
- CMS Collaboration: Measurement of the tt¯ production cross section, the top quark mass, and the strong coupling constant using dilepton events in pp collisions at 13 TeV (EPJC 79 (2019) 368)
- ATLAS moves into top gear for Run 3, Physics Briefing, November 2022