Higgs to diphotons: Shedding light on HIggs boson precision measurements and searches for new resonances – HIGGSENLIGHTENED

The HIGGSENLIGHTENED project proposes to deepen understanding of electroweak symmetry breaking by studying the 125 GeV Higgs boson in the high-precision diphoton decay channel, in the areas of the Higgs boson self-coupling, its CP nature and natural width, studies which could also serve to search for indirect evidence of BSM physics. A fourth area of work is a direct search for additional low-mass Higgs bosons in the diphoton channel. Underlying these objectives are proposed novel reconstruction techniques for electromagnetically interacting particles and novel background estimation techniques.
The measurement of the Brout-Englert-Higgs boson potential form has not yet been performed directly. It can be accessed via the Higgs boson self-couplings, and measured through the pair production of Higgs bosons (HH), requiring at least the Run 3 data. The most recent CMS paper, in the non-resonant case using the process HH? bb??, when combined with other channels, reaches a sensitivity of ~2.5 times the SM HH production cross section. We plan to improve the bb?? sensitivity by at least an additional ~20%, using ‘deep’ machine learning technologies. In combination with other channels, we hope to exclude a null value of the Higgs boson trilinear self-coupling.
Higgs boson width constraint: The combined data of Runs 2 and 3 may permit an indirect constraint of the 125 GeV Higgs boson width through the diphoton decay channel, via a method independent from the ‘off-shell’ method using the H?ZZ*?4 lepton channel. The interference between the Higgs boson production process gg?H??? and the direct diphoton process gg????introduces a downward shift in the invariant diphoton mass and a suppression of the production cross section. Both observables depend on the width of the H, but also on the transverse momentum of the diphoton system, making possible a width constraint via a differential measurement in diphoton transverse momentum. An upward mass shift, corresponding to constructive interference, would be evidence of BSM physics.
Higgs boson CP studies in the diphoton channel: After having demonstrated that constraints from the study of the CP structure in the VBF production mode could be achieved in the diphoton decay channel, we are now pursuing a publication planned for the full Run 2 data set. In the Hff sector, CP-odd couplings have have been studied in Run 2 in the ttH production mode by ATLAS and CMS with 3s level exclusions of the pure CP-odd coupling. Obtaining stronger exclusions will require the data of Run 3. The ttH CP analysis in the diphoton channel could be improved in several ways: with the addition of categories targeting specifically the tH production mode and with global fits within EFT or the anomalous couplings framework including other production modes. Pursuing these studies may allow to pinpoint small deviations not yet within reach.
Direct search for a second low-mass Higgs boson: An excess of 2.8? at an invariant mass of 95.3 GeV has been observed in the CMS data of 2012 and 2016 in the diphoton channel. The analysis of the remaining Run 2 data should give a strong indication, but Run 3 will allow a final response, also permitting an increase in search sensitivity by a factor ~3. Novel ECAL reconstruction techniques could make possible increased rejection of electrons from the pp?e+e- Drell-Yan background process. We plan a purely data-driven modeling of this process, and to extend the search zone’s lower mass boundary to ~20 GeV via the use of alternative triggering methods or subpopulations of boosted events.
Novel ECAL reconstruction techniques, under development in the CMS ECAL using advanced machine learning methods, have shown possible improvements regarding both energy resolution and particle identification in particular, discrimination between photons/electrons/jets. These algorithms could have widespread impact, notably for our four areas of work.