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Malgorzata Worek - Theoretical Aspects of Top-Quark Physics at the LHC

RTG Seminar on May 4th, 2022

The top quark is the heaviest known elementary particle. Within the framework of the Standard Model, its mass is proportional to its coupling to the Higgs boson, which implies a value close to unity for the latter quantity. This fact already makes the top quark unique among the fermions, and one may suspect a deep relation of top quarks to New Physics.

A very different feature which makes the heaviest of the quarks so special is the way it decays, i.e. almost exclusively into the W-boson and the b-quark. Assuming a leptonic decay of the W-boson, the final state contains b-quark jets, leptons and neutrinos. At the detector level, neutrinos are only identified as missing transverse energy. On the other hand, jets, missing energy and charged leptons are a typical signal of New Physics, making top-quark production one of the most important backgrounds for searches. Interestingly, the top-quark life time is shorter than the typical hadronisation time scale. This allows to view the top quark as an unconfined quark. Moreover, a non-vanishing polarisation induced by the production process is transferred with high efficiency to the top quark decay products and may be analysed by means of angular distributions. Very important applications in top-quark physics are a high-precision determination of the top-quark properties (among others: the top-quark mass, the coupling of the top quark to the W and Z gauge bosons and the top-Higgs Yukawa coupling). Top-quark physics is well anchored in the research programme of the CERN Large Hadron Collider (LHC). The collider is, in fact, dubbed a top-factory due to the very high production rate of top-anti-top pairs. Not only tt production but also more complicated final states are measured at the LHC.

Given such rich phenomenological applications it is essential to describe all features of tt(+X) process as accurately as possible on the theory side in order to either deepen our understanding of the Standard Model or maximise the sensitivity to deviations from it. In this lecture, I will summarise the current state of theoretical predictions for top-quark physics at the LHC.

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