The project had two main objectives: (1) seeking new top quark partners decaying to dark matter particles, and (2) seeking and measuring the rare Standard Model process of top quark pair production in association with a Z boson.
In the context of objective (1), the project completed the world’s most sensitive search for the production of top quark partners decaying to dark matter particles. Unfortunately, this search did not find evidence for new physics beyond the Standard Model of particle physics, but it set the world’s best limits on the properties of these particles, excluding top partners with masses up to 1250 GeV. This represents a 90% improvement in mass limit with respect to the state-of-the-art when the project was proposed in 2015. The results rule out most ‘natural’ dark matter models solving the hierarchy problem of the Standard Model with top partners, including many predicted by supersymmetry (SUSY) theory, with profound implications for our understanding of nature at the smallest scales. The final results of this search, carried out with the full LHC Run-2 dataset acquired in 2015-2018, are described in our paper recently published in European Physical Journal C (Eur. Phys. J C 80 (2020) 737). Prior to the production of these final results the project also generated results with an intermediate dataset published in the Journal of High Energy Physics (JHEP 12 (2017) 085).
In the context of objective (2), the project observed the production of top quark pairs in association with a Z boson (ttZ) at greater than 5 sigma statistical significance, which is the criterion required in particle physics for a definitive discovery. The project then went on to achieve the world’s most sensitive measurements of ttZ production. The measurements are in excellent agreement with the latest theoretical predictions, providing stringent constraints on models of new physics predicting modified interactions between top quarks and Z bosons. Our initial observation of ttZ production was published in Phys. Rev. D 99 (2019) 072009. The results of our detailed measurements of ttZ production are published in European Physical Journal C (Eur. Phys. J C 81 (2021) 737).
The project has also contributed to a number of other important searches for possible signatures of dark matter production at the LHC, using related techniques. Unfortunately, these searches, too, did not find evidence of new physics, however they also have contributed profound insights into the structure of the universe at the smallest scales.
