Measurement of Higgs boson production via gluon fusion in the HWW decay channel with the CMS experiment

This thesis reports measurements of the Higgs boson production in proton-proton collisions at a centre-of-mass energy of 13 TeV with data collected by the CMS experiment during the Run 2 operation period (2016-2018) of the Large Hadron Collider at CERN. The measurement is per-formed using a sample of event candidates to originate from the decay of a Higgs boson into apair of W bosons, which then decay into a charged lepton and a neutrino resulting in a final state with two charged leptons of different flavour. The thesis focuses on the study of the Higgs boson production through the gluon fusion process, which is the most favoured one at a centre-of-mass energy of 13 TeV. The optimisation of the physics object selection in the CMS configuration during the Run 2 is described first. Then, the evaluation of the main backgrounds are carefully scrutinised and the signal selection is decided. The main physics result is the Higgs boson production cross section, quantified in terms of a signal strength modifier, which is defined as the ratio of the observed cross section and the prediction from the Standard Model. The analysis is carried out in the inclusive phase space and in fine-grained exclusive phase space bins in the so-called Simplified Template Cross-Section (STXS) framework. The STXS framework has been developed with the aim to reduce the dependence on the underlying physics model and its theoretical uncertainties, while keeping a high sensitivity of the measurements. The signal strength modifiers in both the inclusive and STXS analysis are obtained from a maximum likelihood fit, using as discriminant variables the dilepton invariant mass and the transverse mass defined with the transverse momentum of the dilepton system and the missing transverse momentum. The value of the systematic uncertainties in the analysis is also adjusted in the fit,and dedicated control regions are used to constrain the contribution from some background processes. A multiple event categorisation is performed in the signal region for the inclusive analysis depending on the number of reconstructed jets with high transverse momentum, the flavour of the leptons, their charge and the transverse momentum of the subleading lepton, resulting in an optimised overall sensitivity to the Higgs boson signal. Signal strength modifiers are obtained for the 0- and 1-jet event categories, independently and combined, for the data collected in the three years. All signal strength modifiers are found to be consistent with 1, indicating that the Standard Model properly reproduces the experimental data. The combined result for the global signal strength using the full dataset is μ_$_{global}$= 0.90$8_{-0.09}^{+0.10}$. The precision of the measurement is dominated by systematic effects coming from the theoretical model and instrumental sources. Signal strength modifiers are determined for one STXS bin with 0-jets, 3 bins with 1-jet of different regions in the Higgs boson transverse momentum ($p$$_{T}^{H}$ < 60 GeV, 60< $p$$_{T}^{H}$ < 120 GeV and 120< $p$$_{T}^{H}$ <200 GeV) and a high Higgs boson transverse momentum bin ($p$$_{T}^{H}$ >200 GeV), which may be especially sensitive to new physics effects. These bins are specific to the gluon fusion production mode and are defined in a fully orthogonal way with respect to STXS bins characteristic of other production modes. The measured signal strength modifiers for the studied STXS bins are in all cases consistent with the Standard Model within their uncertainties. Finally, production cross sections in the STXS bins are derived from the measured signal strength modifiers. The production cross section of the most sensitive 0-jet bin is measured to be σ$_{_gg}$$_{H0J}$ = 606±76 fb, and the combined result of the STXS bins for the gg→H × B(H→WW) cross section is σ_$_{ggH}^{STXS}$ = 1.08$_{−0.11}^{+0.12}$ pb. These measurements are some of the main results of the CMS analysis of the Higgs boson production studies exploiting the decay channel into a pair of W bosons, with a leptonic decay of the W bosons.