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Precision Physics and Discovery at Hadron Colliders with Heavy Quarks

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Another glimpse of new physics

The production of pairs of top-antitop quarks is one of the phenomena being observed with high precision at the Large Hadron Collider (LHC) of CERN. On the theoretical side, EU-funded scientists sought predictions at the same level of accuracy to test the Standard Model of fundamental interactions and find possible small deviations from it as signals of new physics.

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Due to its uniquely large mass, the top quark is an excellent probe of the mechanism of mass generation and it plays special roles in many new physics scenarios. At the LHC, top quarks are predominantly produced in pairs of top-antitop quarks at rates that reach millions per year. The abundant statistics allow both production and decay rates to be measured with high accuracy. To detect possible signals of new physics, these very precise measurements need to be matched with similarly accurate theoretical predictions. The EU-funded project TOPPHYSICS (Precision physics and discovery at hadron colliders with heavy quarks) focused on the techniques to achieve the required theoretical accuracy with quantum chromodynamics corrections up to next-to-next-to-leading order (NNLO). In perturbation theory, measurable observables are expressed in power series of the coupling constants. If the leading order term of this series is kept, the corresponding theoretical prediction is highly inaccurate and it gives a mere qualitative description of the process. When the next-to-leading order (NLO) term is considered, qualitatively reliable predictions can be obtained. Since the LHC has raised the level of precision, physicists have into take into account NNLO contributions. Calculations at NNLO pose several technical challenges. They involve the evaluation of complicated loop integrals. These integrals exhibit a non-trivial analytical structure that was handled by TOPPHYSICS within a new mathematical framework. Part of TOPPHYSICS was devoted to NLO analysis of new physics effects in top-antitop quark pair and single-top quark production at the LHC. A Monte Carlo event generator was developed for use in the search of new physics at the LHC. After 27 months of shutdown and re-commissioning, the LHC began its much-anticipated second run in 2015. Project work has contributed to provide the physics community with the tools to look for signs of physics beyond the Standard Model.

Keywords

Physics, Large Hadron Collider, Standard Model, heavy quarks, quantum chromodynamics

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