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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - NPTEV-TQP2020 (Uncovering New Phenomena at the TeV Scale With Top Quarks)

Teaser

Summary

Our understanding of the subatomic world and of the very fabric of the space-time is encompassed in a theory which is the result of all past experimental observations and theoretical developments: the Standard Model of Particle Physics. Yet cosmological observations and theoretical arguments lead us to conclude that new phenomenology, new particles, forces, or a new space-time structure is waiting to be uncovered. In particular, theoretical arguments suggest that new phenomena should appear in the laboratory at high energies, of the order of tera-electronvolts, and will be accompanied by modifications to the dynamics of the heaviest elementary particle known: the top quark. The aim of this programme is to perform five measurements involving top quarks with the data that is being collected by the ATLAS experiment at the European Centre for Nuclear Research\'s (CERN) Large Hadron Collider (LHC). The measurements to be performed are sensitive to a broad spectrum of new particles decaying to top quarks or produced with top quarks in proton-proton collisions, as well as to the presence of new couplings or extra dimensions. This programme addresses key questions such as: What are the fundamental particles? What is the nature of space-time? Is there a unified theoretical framework? What is the origin of the matter-antimatter asymmetry in our Universe? Discoveries of new forces or space-time structure have the potential to revolutionise our view of the building blocks and of the fabric of our Universe and ourselves.

Work performed

\"The group has been focussing mainly on two measurements: the mass of the top quark and the test for the behaviour of matter versus anti-matter in top quark decays. The top quark is the most massive elementary particle ever discovered and because of this it affects the behaviour of other particles, such as the Higgs boson. A difference of only 1% in the value of the top quark mass appears to make huge differences in the expected dynamics of the Higgs field, that is the energy field that permeates the universe. We have been working on the reconstruction of collision events and backgrounds recorded with the ATLAS detector in 2015 and 2016, towards isolating a sample of top quarks in which to make a precise measurement of the top quark mass. The most challenging part, currently ongoing, is the estimate of the systematic uncertainties arising from our modelling of the events. The second main activity has been the search for matter versus antimatter differences in the particles from the top quark decay. Our Universe is made of matter-particles, yet matter is always produced from pure energy together with anti-matter. If the anti-matter disappeared over time perhaps this is due to different decay characteristics of the matter and anti-matter particles. The top quark decays almost always into a b-quark and a W-boson, and from a top and anti-top pair it is possible to analyse the decay chain production matter and anti-matter particles. From a sample of data collected with the ATLAS detector in 2012 we have investigated for the first time possible differences in the decay chain of the b-quarks from top decay. Differences attributed to \"\"mixing\"\" (the transformation of a b-quark into an anti-b quark) have been probed at the level of 2.8% and differences in the behaviour of decays to matter and anti-matter have been probed at the level of 0.5% to 1%. We have observed no detectable differences and published the results in a journal paper (Journal of High Energy Physics 02 (2017) 071). This work has been presented at multiple international conferences and workshops. Ultimately we will repeat similar tests increasing the precision by up to a factor of three by the end of this ERC programme. Furthermore, we have laid down the foundation for the upcoming search for heavy new particles decaying to top quarks. This involved the definition of the primary observables to use, modelling the decays of hypothetical benchmark particles, and how to optimise the event reconstruction. Finally we have contributed to the ATLAS experiment\'s operations, calibrations and development, which are key to wide-ranging investigations of the dynamics of subatomic particles in proton-proton collisions at multi-TeV energy. These have been co-authored in 213 articles since the beginning of the project.\"

Final results

Significant progress has been made by our group to probe the top quark mass using only well measured electrons and muons in the final state from the decay of W bosons and b-quarks, using a new technique at the Large Hadron Collider. Once the data analysis is complete we expect a very competitive precision in the measured value of the top mass with minimal dependence on the calibration of the particle jets. We have published the first test of CP-symmetry violation in the decay chain of b form top quarks. We have observed no detectable effects within a range of sensitivities between 0.5% and 2.8%. By the end of the project the larger collision datasets will allow a test with precision increased by up to a factor of three. We have studied the simulated behaviour of hypothetical benchmark particles, and how to optimise the event reconstruction to search for heavy new particles decaying to top quarks. Future progress will involve performing the search for neutral broad resonances with mass up to 3-3.5 TeV decaying to top quark pairs, a search for the exotic process t->Zc and tests for anomalous couplings between top quark and Z bosons. As part of the ATLAS experiment\'s programme, we have contributed through detector operations, calibration and development to a wide raging programme including stringent tests of the Standard Model of particle physics, search for new particles, and probed with increasing precision the properties of the recently discovered Higgs boson.
These will become increasingly more precise as the LHC continues to provide collision data up to 100 fb-1 by the end of 2018.