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Electro-Weak corrections at the LHC

Periodic Reporting for period 1 - EWatLHC (Electro-Weak corrections at the LHC)

Reporting period: 2015-10-01 to 2017-09-30

The Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) is an extraordinary international project. It represents humanity’s latest effort towards greater understanding of the basic composition of the reality that we observe. At the LHC, protons travelling extremely close to the speed of light, at energies never reached before, collide head-on, producing spectacular events which are recorded by sophisticated, state-of-the-art detectors, such as those of the ATLAS and CMS experiments. The study of the recorded events allows scientists to test predictions and theories in order to understand the most intime questions about reality and our universe, in a joint effort of thousands of theoretical and experimental physicists from all over the world.
The LHC experiments have already provided spectacular results which have improved, or even changed, the way we understand the reality: the discovery of a new particle, announced on July 4th, 2012, which can now be identified with the remainder scalar boson of the spontaneous symmetry breaking mechanism predicted by Higgs, Brout, Englert, Guralnik, Hagen and Kibble, and for which Higgs and Englert have been awarded the Nobel Prize of Physics in 2013, is the most shining example. Indeed, the discovery of this new boson renders the Standard Model (SM) of fundamental interactions a consistent theory, and provides a proof for the existence of a new fundamental force (besides electromagnetism, strong and weak interactions and gravity), of which the new boson is the mediator. If on the one hand the SM is now a complete and consistent theory, on the other hand it may not be the final explanation of fundamental interactions: there are several facts which cannot be accommodated within the SM as it is now, among which the most famous ones are the need of a particle which could explain the existence of dark- matter and the mechanism which gives mass to neutrinos. These facts have triggered theorists to build broader theories than the SM, which typically predict the existence of new particles or the deviation of fundamental quantities from their SM values. After the end of the LHC Run I, no such new particle has been discovered, nor any deviation of fundamental parameters from the value predicted by the SM has been measured. Such a non-discovery can be certainly seen as an important confirmation of the SM, but the hope is that with that something will be found which points to some physics beyond the SM. In order to match the excellent quality of the experimental data and the improving precision on particle masses and couplings measurements, accurate and reliable predictions from the theory side are strongly needed.

In fact, my research activity is focused on the development of methods and tools capable of providing accurate predictions for collider physics, which can be used to study the phenomenology of fundamental interactions with greater precision. In particular, the research activity supported by this action regards the study of a class of corrections that affect the production rates (also known as cross sections) of the particles produced at the LHC, the so-called ElectroWeak corrections. The phenomenology of this class of corrections will be studied for different scattering reactions, and eventually a computer code capable of computing these corrections in an automatic manner will be developed and made public. Such a public and automatic code will be of great usefulness for the community, in particular for those scientists who are not familiar with the details of these computations (in particular students and experimental collaborations), as such details will be completely hidden to the final user.
Already before the beginning of the action, the researcher has published a work on ElectroWeak corrections to the production of a top pair in association with a Higgs boson or a heavy vector boson (W and Z). The results of this publication have had a large impact on the scientific community, and the researcher has often been invited as a relator on top and Higgs associated production. During the course of the action the results in the above publication have been employed in three subsequent reports authored by the researcher: the first is the proceedings of the “Les Houches 2015: Physics at TeV colliders” conference, for the Standard Model group report, where electro-weak corrections computed by different groups were compared; the second report is the CERN Yellow Report on future Hadron-Hadron colliders, where ElectroWeak corrections for a variety of processes (top-pair, vector-boson pair, Higgs and vector boson, top-pair and Higgs or vector-boson) have been computed for a future 100 TeV hadron collider (see chapter 17). The third report is the Fourth Yellow Report of the LHC Higgs Cross-Section Working group, where the new recommended cross sections for top pair and Higgs or vector-boson associated production (the latter is treated as a background process for the former), now include electro-weak corrections.
On top of these three reports, the researcher has produced five scientific publications (three have been accepted by a journal, while two are in course of publication) about the computation of electroweak corrections for several processes.

The automation of the computation of ElectroWeak corrections can thus be considered completed, as it is shown by the variety of publications mentioned above. The proof-of-concept paper on the automation of ElectroWeak corrections is in advanced progress, together with the final steps for the release of the corresponding public code.
In many cases, ElectroWeak corrections have been computed for the first time, and have entered the reference predictions used by the experiments at CERN. The publication of a code capable of performing EW corrections in an automatic manner is at a very advanced stage. Such a code will help scientists to fully profit of the excellent data recorded by the CERN experiments.