Collision event displays of top-quark production from ATLAS (left) and CMS (right). (Image: ATLAS/CMS/CERN)
Among the known fundamental particles of the Universe, the top quark claims the heavyweight title, with a mass 184 times that of the proton. Measuring its precise value - along with that of the Higgs boson - provides crucial information about the theoretical underpinnings of the Standard Model of particle physics. It also allows researchers to improve the precision of theoretical calculations with which to compare experimental data, better enabling them to search for new physics phenomena.
For the first time, the ATLAS and CMS collaborations have joined forces to measure the mass of this fundamental particle. Their new result, presented at the 16th International Workshop on Top Quark Physics, takes a weighted average of 15 previous individual measurements from ATLAS and CMS to give a precise new determination of the top-quark mass.
The 15 previous measurements, six from ATLAS and nine from CMS (see figure 2), were based on data samples of proton-proton collisions collected by ATLAS and CMS in 2011 and 2012, during the first run (Run 1) of the Large Hadron Collider (LHC).
Though ATLAS and CMS have independent data samples, their measurements do have some shared sources of (systematic) uncertainty. These sources can include shared theoretical modelling of the top-quark production and decay, as well as of the background processes that mimic them. They can also include the presence of multiple, simultaneous collisions affecting measurements in both experiments in similar ways, and a common understanding of the internal structure of the colliding protons.
Therefore, when combining their measurements, the ATLAS and CMS collaborations have to be careful not to double count any of these shared uncertainties. For example, the top quark almost always decays to a W boson and a bottom quark. In these decays, the bottom quark produces a unique spray, or "jet", of particles, called a b-jet. Determining the energy of these b-jets relies on simulations of jet formation that are common to both experiments. This leads to shared systematic uncertainties in the top-quark mass measurements of ATLAS and CMS that have to be accounted for.
After conducting a detailed study of these shared uncertainties, the ATLAS and CMS researchers combined their 15 previous measurements to obtain the most precise determination of the top-quark mass to date: 172.52 billion electronvolts (GeV) with a total uncertainty of 0.33 GeV (see figure 2). The researchers also examined the ATLAS- and CMS-only combinations of the top-quark mass measurements and found them to be consistent with the joint-experiment combination, giving further confidence in the new result.
The new result, based on Run 1 data, is a good example of the meticulous work that is required to understand LHC data. Such work can go on for many years after the data are collected. The top-quark mass has also been measured with high precision using data from Run 2, and the unprecedented number of top quarks produced in this run provides further opportunities for ATLAS and CMS to innovate and improve on this important measurement. Looking forward, the ongoing Run 3 will allow the collaborations to continue their investigation of this fascinating particle.
