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Final Report Summary - EWBGANDLHC (Electroweak Baryogenesis in the Era of the LHC)

The discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) in 2012 has been a key milestone in our understanding of Nature’s fundamental matter constituents and the interactions among them, confirming the predictions of the Standard Model (SM) of particle physics and establishing the Brout-Englert-Higgs mechanism as responsible for the breaking of the electroweak (EW) symmetry. The energy scale given by the Higgs vacuum value (vev) – causing EW symmetry breaking – is known as the EW scale. Studying the properties of the Higgs boson at the LHC is a major avenue to probe the existence of new physics beyond the SM, and shed light on the origin of the EW scale.

At the same time, gravitational wave (GW) astronomy is experiencing a revolution, and during the coming decade it will lead us to directly access via GW the high energies attained in the early Universe. The future space-based eLISA GW observatory, planned as next L-Class Mission by the European Space Agency (ESA), will directly probe the EW epoch of the early Universe, providing a new avenue to study Higgs physics.

The research project EWBGandLHC investigates the origin of the observed matter-antimatter asymmetry of the Universe, the so-called baryon asymmetry. This is a major open puzzle at the interface of particle physics and early Universe cosmology, in need for new physics beyond the SM. Remarkably, the cosmological history of the Higgs field provides various key ingredients for a dynamical generation of the baryon asymmetry in the early Universe. The research project explores the links between Higgs physics and the early Universe to test the generation of the cosmic baryon asymmetry at the EW scale through LHC data and the future LISA GW observatory.

The project articulates around four key objectives:

A) Characterizing the nature of the EW phase transition – the process of EW symmetry breaking in the early Universe – in theories beyond the SM. The properties of the EW phase transition are of utmost importance for the generation of baryon asymmetry at the EW scale.

B) Assessment of the parameter space in theories beyond the SM where baryogenesis at the EW scale is viable.

C) Design of a search programme at the LHC for EW baryogenesis scenarios, to be incorporated into the main LHC physics programme.

D) Characterization of the potential GW signatures from the EW phase transition in connection with EW baryogenesis. Assessing the LISA Mission sensitivity requirements to probe such signatures.

In order to maximize the impact of the research project, I have become a member of the eLISA Cosmology Working Group (2014) and of the CERN LHC Higgs Cross Section Working Group (2014). The former ensures a decisive impact of the research on the eLISA Science programme in connection with cosmological phase transitions. Membership of the latter provides an important link to ATLAS and CMS Experimental Collaborations, thus aiding to the successful completion of Objective C), which constitutes the main goal of the research project.

In the course of pursuing the envisaged research programme, I have achieved three key milestones:

1) The development of an Effective Field Theory framework for non-minimal Higgs sectors, allowing to study in a systematic way the potential deviations from the Standard Model (SM) predictions in LHC measurements due to the presence of new scalars beyond the Higgs boson. Our analysis also allows to assess the validity of the Higgs Effective Field Theory on general grounds. This work has been published in JHEP 1510 (2015) 036, has been cited 46 times in around a year and is an important reference for the "CERN Yellow Report 4 (2016): Deciphering the Nature of the Higgs Sector" (of which I am a co-Author) which contains the present knowledge of the Higgs boson and its properties.

2) The discovery and analysis of a new LHC signature which provides a direct link to the strength of the electroweak phase transition (as needed for electroweak baryogenesis) in scenarios with more than one Higgs doublet. This work has been published in Physical Review Letters (Phys. Rev. Lett. 113 (2014) 211802), and its signature has merited a dedicated search by the CMS Collaboration (CMS-PAS-HIG-15-001, Phys. Lett. B759 (2016) 369). This search has subsequently been established as a key part of the Beyond the SM Higgs search programme of CMS (summarized in CMS-PAS-HIG-16-007) during Run 1 at 8 TeV and Run 2 at 13 TeV.

3) A precise and complete treatment of GW signatures from the EW phase transition, published in JCAP 1604 (2016) 001 and being part of the eLISA Cosmology Working Group effort to provide a state-of-the-art assessment of the eLISA physics case regarding cosmological GW sources. This work has been cited 45 times in less than a year and more importantly, its results have been key in the eLISA Science Advisory Commitee (so-called GOAT Commitee) decision to move from an initial eLISA mission design with 2 interferometer arms – 4 links to a prospective design with 3 interferometer arms – 6 links (as of September 2016). This highlights the impact of this research project on the eLISA science objectives.

These three milestones are in direct correspondence with Objectives A), C) and D) of the Research
Project. In addition, I have completed important work on LHC probes of extended Higgs sectors, both for extra scalar doublets (published in Phys. Rev. D93 (2016) 115033 and JHEP 1609 (2016) 093) and extra scalar singlets (published in Phys. Rev. D94 (2016) 035022), which have further strengthened the results from Objectives A) and D). Finally, I have completed a study of the viability of baryogenesis in nonminimal Higgs sectors (ArXiv:1611.05874, to be published in JHEP), leading to a successful connection among Objectives A), B) and C) of the Research Project.

Overall, this Research Project has been key in establishing the study of the electroweak phase transition and baryogenesis as part of the LHC Higgs physics programme, as well as in shaping the science objectives of the future eLISA Mission regarding cosmological GW sources. Both outcomes are bound to have a strong long-term scientific impact.

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United Kingdom
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