Since the scrutiny of the Run-II sample did not yield any signs of new physics, my interest turned to the highest priority in the field, as stated clearly in the 2020 update of the European Strategy for Particle Physics: the most in-depth possible understanding of the Higgs boson. Two Higgs couplings are currently the biggest open questions in the SM scalar sector: its couplings to fermions and its self-coupling. The coupling to fermions has no dynamic mechanism behind it and a seemingly arbitrary strength, expressed through a Yukawa coupling which is different for each fermion. This is the only part of the SM that distinguishes between the different generations of fermions in the SM. The LHC experimental program aims at a higher precision for the currently measured couplings, an approximate measurement of the Higgs self-coupling, and the observation of the decay of the Higgs boson to 2^nd-generation quarks, in particular the charm quark. The systematic study of the Higgs couplings requires very high numbers of Higgs bosons, and this is the centerpiece of the current and future LHC physics program: a major increase in luminosity, to yield a factor ten more proton-proton collisions than to this day. Of note is that the leptonic decay H→μμ is within reach in the current LHC Run III, and this might be argued as establishing the coupling to the 2^nd-generation fermions. However, the coupling of the Higgs to 2^nd-generation quarks is independent – and most likely more important: thus far the quark sector has exhibited a rich flavor structure without yet an equivalent observed counterpart in the lepton sector. The primary example is the existence of CP violation in the quark sector – something that remains elusive in the lepton sector to date and will perhaps be observed at HyperK (Japan) and DUNE (US).

Motivated by the LHC Higgs physics program, in 2022 I launched a new research project at NKUA aiming at the observation of the Higgs boson (Yukawa) coupling to the charm quark, y_c. The project engages scientists from CERN, Brown University and University of California at Santa Barbara (US), Ghent University (Belgium), and Peking University (China). The direct measurement of y_c is very challenging because of immense backgrounds from hadronic interactions producing c-quark pairs and the very significant difficulties associated with identifying (“tagging”) jets produced by c quarks. These issues are analogous to those encountered in the measurement of the b-quark Yukawa coupling, y_b. The case of the c quark is much more difficult as its mass is 4 times smaller than the b-quark mass and thus the H→cc rate is 16 times smaller than the H→bb rate. The strategy followed for the measurement of y_b was to exploit all Higgs boson production modes based on their signal-to-background fractions. Most sensitive are the associated production modes, vector boson plus Higgs boson (VH) and top-quark pair plus Higgs boson (ttH), because the presence of the associated heavy particles makes the final state more distinct from states produced via background hadronic processes. The gluon fusion (ggH) and vector boson fusion (VBF) modes, although having larger cross sections than the associated production, are less sensitive but still contribute. The observation of y_b came from the combination of the measurements in all modes. Following this strategy, both ATLAS and CMS are searching for y_c, starting with the VH production mode, with the results being yet far from conclusive. Its observation requires effort in all production modes, large datasets and advanced analysis techniques for flavor discrimination and background reduction. This translates to a systematic program of measurements to lay the grounds for the eventual observation of the H→cc decay at the HL-LHC and contrast it with the H→bb decay. The program makes extensive use of the most recent state-of-the-art ML algorithms based on the most advanced present architectures, such as graph networks and transformers.

Another process which is recently receiving attention as an additional constraint of y_c is the Higgs boson production in association with a jet originating from a c quark, where the c quark is produced via gluon interactions within the colliding protons. This mode has a relatively low sensitivity to y_c, but it has the advantage that the Higgs boson can be reconstructed in any decay mode. By selecting the "cleanest" decays, for example into a photon pair or a vector boson pair subsequently decaying into four leptons, both of which were used for the discovery of the Higgs boson, a signal-to-background fraction that allows a meaningful search for the Yukawa coupling can be achieved. My program in Higgs boson physics includes a search for y_c in the H+c production mode, with the Higgs boson decaying into four leptons, using data from LHC Run III. To exploit the full production rate, we are planning to use novel algorithms for the decay vertex reconstruction of heavy-flavored hadrons, not clustered in jets, which are currently being developed by CMS.