CMS measures the mass of the top quark with unparalleled accuracy
The classical signature of a top-quark pair produced in LHC collisions is four jets (yellow cones), one muon (red line, also detected by the CMS muon detectors as red boxes), and missing energy from a neutrino (pink arrow). Credit: CERN

The mass of the top quark is the heaviest known elementary particle and has been measured by the CMS collaboration. The value of the top-quark mass is estimated by the latest result. New analysis methods and improved procedures help to improve the accuracy of the measurement.

Understanding our world at the smallest scale is dependent on knowing the top-quark mass. The internal consistency of the mathematical description of all elementary particles, called the Standard Model, can be tested by knowing this heaviest elementary particle as closely as possible.

The Standard Model can be used to predict the top-quark mass. The W-boson mass can be predicted using the top-quark and Higgs-boson mass. The definition of mass is still hard to pin down because of the effect of quantum-physics corrections.

Our knowledge of the stability of our universe depends on our knowledge of the top-quark mass. The universe is very close to a metastable state with the current measurement of the top-quark mass. The universe would be less stable in the long term if the top-quark mass was slightly different.

The latest measurement of the top-quark mass was made using data from the CMS detector, which measured five different properties of collision events in which a pair of top quarks is produced. The properties are dependent on the top-quark mass.

The team gained an in-depth understanding of the remaining experimental and theoretical uncertainties after performing an extremely precise calibration of the CMS data. The method that was used to derive the final value of the top-quark mass meant that some of the uncertainties could be estimated more accurately. The result is in line with the predictions from the Standard Model.

The new method to measure the top-quark mass has made a significant leap forward. The treatment of uncertainties and the use of more properties has vastly improved the measurement. When the new approach is applied to the more extensive dataset recorded by theCMS detector, there will be another big step.

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