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Dec 18, 2022
In their first Run 3 results, the ATLAS collaboration measured two Standard Model processes: the production of Z bosons and top-quark pairs
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Image above: Event display of a pair of top quarks decaying, recorded in the ATLAS detector on 18 July 2022. (Image: CERN).
After over three years of upgrade and maintenance work, the Large Hadron Collider began its third period of operation (Run 3) in July 2022. Since then, the world’s most powerful particle accelerator has been colliding protons at a record-breaking energy of 13.6 TeV. The ATLAS collaboration has just released its first measurements of these record collisions, studying data collected in the first half of August 2022.
The researchers measured the rates of two well-known processes: the production of top-quark pairs and the production of a Z boson, which proceed through strong and electroweak interactions, respectively. The ratio of their cross sections is sensitive to the inner structure of the proton, and their measurement sets constraints on the relative probabilities that reactions are initiated by quarks and gluons.
These early measurements also validate the functionality of the ATLAS detector and its reconstruction software, which underwent many improvements in preparation for Run 3.
Physicists focused on Z-boson decays to electron and muon pairs, and on top-quark decays to a W boson and a jet – collimated sprays of particles – originating from a bottom quark. The W boson subsequently decays into one electron or muon and an invisible neutrino. As the analysis uses very early Run 3 data, physicists relied on preliminary calibrations of the leptons, jets and luminosity. These were derived promptly after the first data became available.
ATLAS measured a top-quark pair to Z boson production ratio that is consistent with the Standard Model prediction within the current experimental uncertainty of 4.7%.
The calibration and corresponding uncertainties will be improved as more data is processed. Future updates of the calibration will allow researchers to measure the cross sections with greater precision.
To validate their results, physicists performed a series of cross-checks. These included measuring the ratio of the cross section each time the LHC was injected with a new fill of protons for a data-taking run.
More analyses using the Run 3 data will follow, exploiting the unprecedented energies and the increased LHC data set.
Read more on the ATLAS website: https://atlas.cern/Updates/Briefing/First-Run3-Measurements
Note:
CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.
The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.
Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 23 Member States.
Related links:
Large Hadron Collider (LHC): https://home.cern/about/accelerators/large-hadron-collider
ATLAS experiment: https://home.cern/science/experiments/atlas
Z boson: https://home.cern/science/physics/z-boson
W boson: https://home.cern/science/physics/w-boson-sunshine-and-stardust
For more information about European Organization for Nuclear Research (CERN), Visit: https://home.cern/
Image (mentioned), Text, Credits: CERN/By ATLAS collaboration.
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