Search for a high mass Higgs-like boson produced via Vector Boson Fusion, in the decay channel $H \to ZZ \to l^+ l^- q \bar q (l = e, \mu)$ with the CMS detector

The Higgs mechanism is an essential element of the Standard Model (SM) of particles and their interactions, explaining the origin of the mass of elementary particles and playing a key role in the physics of electroweak symmetry breaking. A suitable Higgs boson candidate, compatible with the one predicted by the Higgs mechanism, has recently been found with a mass around 125 GeV. However, many models predict more than a single boson, so further searches for Higgs-like particles using the SM Higgs as a benchmark model are being pursued. The Higgs boson can be produced via four production mechanisms at the LHC: gluon- fusion (gg, the main mode at the LHC, in the whole mass range between 100 and 1000 GeV), Vector Boson Fusion (VBF, where the Higgs is produced via the fusion of two vector bosons which are emitted by two quarks) and associate V H (with a vector boson) and tt ̄H (with a top-antitop couple) production. All these production modes, together with the resonance properties, are currently under investigation. The VBF topology is characterized by the presence of two additional jets in the final state, with high η separation and high invariant mass. In the Standard Model this production mechanism has a lower cross section than the gg one (about 10 times smaller) and thus its observation is more challenging. The H → ZZ → 2l2q channel is one of the best suited to observe such a topology, because of the large branching ratio of the decay Z → 2q. The main background affecting this decay channel is given by Drell-Yan plus jets events; the cross section of this process decreases by a factor of 100 when requesting the presence of four jets instead of two. Thus, the VBF topology benefits from a highly reduced background with respect to the gg production mode, such that even if the VBF production mechanism has a cross section smaller than the gluon-fusion, a ten times higher signal-to-noise ratio is expected. Furthermore, in beyond-SM models, VBF production could be enhanced with respect to gg. If the considered Higgs mass is more than twice the Z mass, both Z bosons are on shell, and the couple of jets from the hadronic Z can be easily identified among the jets in the event by requiring their invariant mass to be close to the Z nominal mass. Moreover, the VBF jets can be easily identified thanks to their peculiar topology. In 2013 results have been published [1] about the search for a high mass Higgs-like boson in this decay channel, for the gluon fusion production mechanism, in the mass range between 230 and 600 GeV. The VBF category was not considered, not even the possibility to reconstruct the couple of jets in the final state as a single merged jet in case of boosted topology. The analysis, investigating a mass range between 230 and 600 GeV, showed no evidence for such a SM-like Higgs boson and had the sensitivity to set upper limits on its production cross section and to exclude this resonance at 95% confidence level in the mass range between 275 and 600 GeV. The analysis which is presented here has been performed to study this decay channel in the VBF production mode. Adding the VBF contribution to the main gluon-fusion analysis helps improving the overall sensitivity in the search for a high mass SM-like Higgs boson, especially at very high masses, allowing to expand the mass range where an exclusion is feasible. In fact, the new analysis, accounting for the VBF production also with the merged-jet topology, allows to set upper limits on the production cross section of a Higgs-like particle in the mass range between 230 and 1000 GeV and has enough sensitivity to exclude a SM-like Higgs boson at 95% confidence level in the mass range between 305 and 744 GeV. In addition, the results have been re-interpreted in terms of an electroweak singlet scalar mixing with the 125 GeV Higgs boson, and exclusion limits have been set.