Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - LHCDMTOP (Novel Dark Matter Searches with Top Quarks at the Large Hadron Collider)

Teaser

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)...

Summary

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.

Work performed

The project has two main objectives – searching for top+DM new physics (work-package 1) and seeking and measuring the rare ttZ standard model process (work-package 2). Work-package 1 has been focused on analysing the latest data acquired by the ATLAS experiment as the LHC has dramatically increased its particle collision rate over the past three years. Results produced so far show no evidence for new physics but have allowed world-leading limits to be set on top+DM properties. Preliminary results obtained with the data acquired by ATLAS in 2015 and early 2016 were presented at the major ICHEP conference in 2016 while definitive results with a factor-of-three larger dataset obtained in 2015 and 2016 were published in December 2017. In work-package 2 the team is working with ATLAS collaborators towards a major journal publication focused on the ttZ channel based upon the 2015 and 2016 ATLAS dataset.

Final results

The most commonly studied model for new physics predicting the production of top quarks and dark matter at the LHC is supersymmetry (SUSY). In SUSY models the partner of the top quark, known as the top squark or ‘stop’ can be produced in pairs in LHC collisions, with each decaying to a top quark and a dark matter particle. A ‘natural’ mass for the stop, predicted by SUSY theory, is expected to be around 1000 GeV. At the start of this project the world’s best limits on the mass of the stop, obtained with LHC Run-1 data from 2012, required it to have a mass greater than 750 GeV for massless DM particles. The latest results from ATLAS and this project, obtained with the much larger 2015+2016 LHC Run-2 dataset and a much higher collision energy, require the stop mass to be greater than 1000 GeV, thereby greatly constraining the possibilities for SUSY top+DM models to solve the dark matter problem.

Before the end of the project, using the complete ATLAS 2015-2018 Run-2 dataset, we expect either to find first evidence for the production of top+DM or to set still more stringent constraints on the properties of top+DM models such as the SUSY top squark. The mass limits for the stop should extend to 1250 GeV or higher. In addition, clear evidence of the associated production of top quarks and a Z boson (ttZ), a major background process for top+DM searches, will be obtained and the first detailed measurements of ttZ production carried out. These measurements could reveal evidence for new physics affecting the production rate and properties of this process.

Website & more info

More info: https://atlas.cern.