Periodic Reporting for period 3 - LHCDMTOP (Novel Dark Matter Searches with Top Quarks at the Large Hadron Collider)
Reporting period: 2019-05-01 to 2020-04-30
This project addresses directly the two most important unanswered questions in particle physics: the Standard Model (SM) hierarchy problem and the nature of dark matter (DM). The SM was recently completed with the discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012. We know, however, that the SM cannot be the end of the story for fundamental physics, because it suffers from two major flaws: a lack of stability for the mass of the Higgs boson (the hierarchy problem), and a lack of a candidate for the invisible DM particles known to make up most of the matter in the universe. The project addresses both of these key problems of modern physics by searching at the LHC for new beyond the SM (BSM) partner states for the SM top quark decaying to new DM particles. The greatly increased quantities of data and world-record collision energies generated by the LHC in 2015-18 provide an unprecedented opportunity to find such top partners. Confirmation of their existence would solve the hierarchy problem by providing a mechanism for stabilising the mass of the Higgs boson, while first observation of DM at the LHC would revolutionise our understanding of cosmology and provide a key pointer to the physics of the very early universe. Many leading BSM physics models predict the existence of both top partners and DM, and so this interdisciplinary project provides a unique opportunity to take the next major step forward in developing a unified theory of nature. The project focuses on top partners which decay to a top quark and a DM particle, with the former decaying purely to jets and the latter escaping the detector unseen. The project uses novel kinematic techniques to identify and characterise this signal in LHC data, and also accurately measure for the first time the dominant SM background process of associated production of top quarks and a Z boson, which is of great theoretical interest in its own right.
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.
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.
The project has significantly advanced the field of particle physics beyond the state-of-the-art in 2015 when the project was first proposed. At that time, a large number of theories of new physics predicting the existence of top partner particles and dark matter particles were viable, including those with a relatively high level of ‘naturalness’ (a property related to their ability to solve the hierarchy problem of the Standard Model of particle physics). As a result of the results produced by this project many of these models have now been excluded. Those which remain are typically much less natural and hence less well-motivated theoretically. This has changed the focus of the theoretical particle physics and cosmology communities towards a much broader range of alternative models, which often lack the strong motivation of naturally solving the hierarchy problem as well as providing a good dark matter candidate. This has also led to a widespread fundamental re-evaluation of the likely origin of dark matter in the universe.