DiHiggs In VBF Events : Exploring the Higgs boson properties through vector boson fusion HH production – DIVE

The investigation of the properties of the Higgs boson (H) is a necessary step to understand the breaking of the electroweak symmetry and verify whether the Brout-Englert-Higgs (BEH) mechanism is actually realized in Nature. The production of Higgs boson pairs (HH) in vector boson fusion (VBF) provides a key access to fundamental properties of the Higgs boson. It involves at the same time the HHH, VVH, and VVHH (with V=W,Z) interactions, and features a cancellation between the amplitudes involving VVH and VVHH with longitudinally polarized vector bosons that is a direct consequence of the doublet structure of the Higgs field postulated in the BEH mechanism. The presence of physics that is as yet undiscovered may alter this cancellation, manifesting as enhancements of the longitudinal scattering amplitude with increases by up to two orders of magnitude in the total cross section, driven by the HH invariant mass (mHH) region above 1 TeV.

While first attempts at studying the VBF HH mechanism have been performed with the Run 2 data set at the LHC, these have been limited to studying anomalous VVHH couplings. The Run 2 and Run 3 data sets of the ATLAS experiment, corresponding to about 400 fb-1, will enable a tight constraint of the Higgs boson properties in VBF HH events with unprecedented sensitivity if explored with suitable experimental analysis techniques.

The project targets the study of VBF HH in the high branching fraction decay channels bbbb and bbtautau, aiming at constraining separately the longitudinal and transversal scattering components. This property has never been studied at the LHC and will unlock the full physics potential of this production mode. The polarization of the scattering vector bosons can be extracted by studying the kinematic information of the event and in particular mHH. The bbtautau channel will be studied in both the low and high mHH regime, while the bbbb channel will focus on the high mHH regime only where the sensitivity is maximal. The project will enable the development of dedicated analyses for these topologies, including the study of methods to precisely model and efficiently reject the background processes to optimize the experimental sensitivity.

The development of dedicated methods to identify decays of highly boosted Higgs boson to bb or tautau, that result in final states with overlapping decay products, will be investigated to maximize the sensitivity. We will explore the improvement of the existing method for the H->bb identification and develop new techniques for H->tautau decays that currently are not available in ATLAS at the required level of performance. To this end we will explore graph neural networks techniques, which have already proved to be excellently suited for this task, combining the information from the individual jet constituents. After the development of the algorithms we will work on their characterisation, validation and calibration with ATLAS Run 2 and Run 3 data.

The results will be coherently developed across the full analysis chain of the three channels, from the initial modeling of the signal in its scattering components until the final interpretation of the results. In addition to traditional constraints in terms of anomalous couplings, the project will explore the interpretation in terms of effective field theories.

The project is presented by a consortium of researchers at the IJCLab, CPPM, and LPNHE laboratories. Deep expertise on the proposed research topics is present in the three groups, and their synergy will enable the achievement of the scientific goals. The funding requested will support three postdoctoral positions of two years each, one for each laboratory. The results will be published as peer-reviewed papers and presented in international conferences.