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Uncovering New Phenomena at the TeV Scale With Top Quarks

Periodic Reporting for period 5 - NPTEV-TQP2020 (Uncovering New Phenomena at the TeV Scale With Top Quarks)

Reporting period: 2020-03-01 to 2021-02-28

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. The measurements performed have opened new directions in the precision tests of the dynamics of the top quark, and pushed the search for top quark resonances and new couplings to unprecedented accuracy.
The NPTEV-TQP2020 project involved 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 towards better modelling of 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 comparing 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 originated 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. We have finalised the strategy of the search for heavy new particles decaying to top quarks using the dilepton final state of top pair events. This technique has the sensitivity enhanced by probing the effects of spin, a relativistic property of particles. On simulated data, the technique is sensitive to new heavy particles decaying to quarks with a mass of up to 3-3.5 TeV, and the technique will soon be applied to collision data of the ATLAS experiment.
Extremely rare decays of the top quark to a Z boson have been searched for in a sample of 139 inverse femtobarns of proton-proton collision data, at a centre of mass energy of 13 TeV, collected by the ATLAS experiment. No evidence of such events is found and an upper limit (95% C.L.) on the top to Z+charm branching ratio of 1.2 10^-4 has been placed. The last of the topics is a measurement of the interaction of top quarks with a type of weak interactions mediated by Z bosons, where we have concluded the observation of associated production of a top quark and Z boson, found to be consistent with the Standard Model prediction. 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. Results of this programme have been presented at over 35 venues, including seminars and conferences, and at 11 outreach events.
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 gives further insights into the mechanisms that characterise the decay of the top quark. 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 decaying to top quarks, and how to optimise the event reconstruction to search for such new particles using a novel event final state. In particular, we have developed and optimised in simulations the technique to search for neutral broad resonances with mass up to 3-3.5 TeV decaying to top quark pairs. The programme also included the search for the exotic process t->Zc, which led to an improvement in sensitivity with respect to the previous ATLAS result of a factor of 2. We have published the first observation with the ATLAS experiment of associated production of a top quark and a Z boson. 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.
Display of a ttbar candidate event from proton-proton collisions recorded by ATLAS
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Display of candidate top quark and anti-top quark particles decaying in the ATLAS detector
Top quark mass likelihood: the best-fit value and the statistical and total uncertainty profiles.