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The Higgs quest - exploring electroweak symmetry breaking at the LHC

Final Report Summary - HIGGSTOOLS (The Higgs quest - exploring electroweak symmetry breaking at the LHC)

The research goal of HiggsTools (ITN 2012 HiggsTools 316704) is the investigation of electroweak symmetry breaking. This question lies at the very frontier of knowledge of theoretical particle physics and phenomenology and, in fact, the primary goal of the Large Hadron Collider (LHC) at CERN is to unveil the mechanism of electroweak symmetry breaking. HiggsTools consists of 10 Full Partners and 10 Associated Partners from European Universities and Research Institutes, one International Organisation (CERN), and 6 Associated Partners from the private sector. The project encompassed both theoretical and experimental particle physics and 35 scientists are actively involved in the supervision and training of the ESRs recruited by the project. The overall progress was in line with the envisaged timescales and deliverables, and within budget.

Throughout the project the Supervisory Board steered intensive coordination efforts in order to make the network successful. The web page was actively developed to serve as a recruitment tool, a management tool and as a tool for increasing the visibility of the network.

The web interface for the selection of ESR fellows facilitated the coordination between the Recruitment Team and the partners and led to a very successful recruitment of the fellows. All of the 500 person/months were filled with 23 recruitments to the 21 ESR positions (two resigned their position and left for posts in the private sector). The gender split of those appointed is 74\% male (17/23) and 26\% female (6/23).

Major research achievements were reached in all the Work Packages. Our ESRs made a very significant research contribution with 63 publications plus many contributions to ATLAS and CMS papers - several of the ESRs are now signatories on all CMS (Haddad, Pata) or ATLAS (Gutierrez, Gonella, Megy, Wolf, Bahmani, Melini) papers -totalling more than 970.

In total, 285 papers were published by members of the network, and more than 34\% of those papers were the outcome of networked research, involving researchers from at least two different partners. Our participation in both the ATLAS and CMS Collaborations and through the Higgs Cross Section Working Group guaranteed an underlying level of inter-node collaboration which we sustained through an active programme of exchange visits. To reinforce the close collaboration between the experimental and theoretical ESRs we collaborated on a HiggsTools Handbook, "The HiggsTools Handbook: Concepts and observables for deciphering the Nature of the Higgs Sector", which summarised the work done in the working groups established in the Young Researchers Meetings.

The training was as anticipated in the DoW. The ESRs profited from high-level training, tutored directly by world leaders in the field. So far, twelve of the ESRs have completed their PhD, two of them are now working in the private sector, one is taking a well earned vacation, while nine are continuing their research in particle physics. The remaining ESRs are still working in the network and will complete their PhDs in 2018. Secondments played an outstanding and very fruitful training and networking role. Outreach activities included public lectures according to the DoW. The website and Facebook account further adds to outreach.

Over the course of the network, the LHC started operations at the unprecedented energy of 13 TeV. The first 13 TeV data in 2015 provided some intriguing glimpses of a possible new resonance with an invariant mass of 750 GeV, leading to intense theoretical speculation. However, with more data it turned out to be a statistical fluctuation. Nevertheless, this was a very exciting time. The LHC continued its 13 TeV operation in 2016 and 2017 breaking the luminosity records, and will complete Run 2 in 2018 before a longer shutdown. The first tranche of 13 TeV data has been analysed. The ESRs of our network have played a key role in developing the theoretical tools that enable the properties of the Higgs boson to be studied precisely. As the full data set is accumulated, many of our ESRs will continue to study the rarer decays of the Higgs boson and possibly detect small, subtle differences between what the boson looks like in experiments, and what the Standard Model predicts.

The network contributed to several experimental studies of Run 1 data including the precise determination of the Higgs boson mass and more precise determinations of the couplings to standard model particles as well as searches for the H to bB decay in ZH associated production, and constraints on the Higgs boson width, the spin-parity of the Higgs boson and anomalous HVV couplings and the first searches for Higgs boson pair production. In addition, much work was done to prepare for the final combination of ATLAS and CMS Run 1 results for the measurement of the Higgs boson mass. A number of studies were prepared for the very complicated ttH final states, including H decay to bb. A new observable for studying the digamma decay of the Higgs was proposed and studied. On the theory side, Higgs couplings and Higgs properties (consistency with the SM hypothesis) were discussed in several papers including the theoretical treatment used in setting an experimental bound for the intrinsic width of the Higgs boson, probing the SM with Higgs signal rates, interference effects in Beyond-the-SM (BSM) processes and Higgs self-coupling measurements as well as the interpretation of the observed signal in a variety of BSM models. A lot of work was done on extending the effective field theory description for deviations from the SM.

The network developed specialized codes for the SM-like Higgs boson scenario. Highlights include the approximate results for the Higgs production cross section via gluon fusion at N3LO, high energy resummation of the Higgs Transverse momentum distribution, fully differential NNLO predictions for associated ZH production, vctor boson fusion and Higgs+jet production and the NLO corrections to the H decay width and Higgs pair production. An extensive study of the Higgs signal in the two-photon decay mode was made for the phistar variable proposed in WP1. Non-standard Higgs sectors have also been studied leading to data driven constraints on the parameter space of a variety of BSM models. The teams are also actively providing more precise theoretical predictions for background processes including single and pair production of W and Z bosons and top-quark production. Highlights include top-quark pair production cross sections at approximate N3LO, and NNLO results for tt, ZZ, W pair, Z+jet and gluonic jet production together with NLO predictions merged with a parton shower for tt + multijet and triple vector boson production. In addition, the first steps towards the automation of the NLO electroweak corrections were made with the study of W + multijet events.

The network produced several important results for PDFs, in particular improved PDFs using LHC data including a global analysis to NNLO accuracy and provided an updated recommendation for the usage of sets of parton distribution functions (PDFs) and the assessment of PDF and PDFs uncertainties suitable for applications at the LHC Run II. From a more formal point of view, major theoretical steps towards establishing a consistent set of PDFs at NNLO taking into account the masses of the charm and bottom quarks were made including innovative results in the calculation of multi-loop diagrams, including massive 3-loop graphs, iterated binomial sums and their associated iterated integrals, new ways for regularizing gauge theories, symbolic summation methods, three-loop heavy flavour Wilson coefficients and non-planar Feyman integrals.

The website for the project is