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Uncovering New Phenomena at the TeV Scale With Top Quarks

Periodic Reporting for period 4 - NPTEV-TQP2020 (Uncovering New Phenomena at the TeV Scale With Top Quarks)

Reporting period: 2019-03-01 to 2020-02-29

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 being 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.
"The group has been focussing on five measurements: 1) the mass of the top quark; 2) the test for the behaviour of matter versus anti-matter in top quark decays; 3) the search for new particles decaying to a pair of top quarks; 4) the search for rare top quark decays; 5) the measurement of the interaction between top quarks and Z bosons. 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 completed a precision measurement of the top quark mass with an uncertainty of 0.45%, using a technique with uncertainties largely independent on those from previous studies, which opens new possibilities for additional precision and to understand the mechanisms involved in the top decay. The result has been presented at the 2019 International Workshop on Top Quark Physics in Beijing and is being submitted for publication. The second 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. This work has been presented at multiple international conferences and workshops. Furthermore, we are conducting a search for heavy new particles decaying to top quarks, and for extremely rare decays of the top quark which, if observed would indicate the presence of a new dynamics in the top quark decay. The last of the topics is a measurement of the interaction of top quarks with a type of weak interactions mediated by Z bosons, and we have concluded a test of how single top quarks couple with Z bosons. 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 over 400 articles since the beginning of the project."
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. We have completed a measurement with a precision of 0.45%, which opens new possibilities to probe the top quark mass at the level of the non-perturbative quantum chromodynamics scale, and to have further insights into the mechanism, such as colour flow, that enter the decay of the top quark in top pair events. 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%. This is the first measurement of its kind and promises to give much increased precision results in the future. 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. We are conducting a search for neutral broad resonances with mass up to 3-3.5 TeV decaying to top quark pairs and a search for the exotic process t->Zc, and will perform tests for anomalous couplings between top quark and Z bosons. On the last aspect, we have published a measurement of the coupling between single top quarks and Z bosons. As part of the ATLAS experiment, 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, precision tests of the recently discovered Higgs boson. These analyses will become increasingly more precise using the over 100 fb-1 of collision data already provided by the LHC.
Display of candidate top quark and anti-top quark particles decaying in the ATLAS detector