polarized on higgs? - measurement of the higgs couplings to polarized vector bosons
Lucrezia Bruni is a PhD student in the research group Energy, Materials & Systems (EMS). Her supervisor is prof.dr.ing. B. van Eijk from the Faculty of Science and Technology.
The Standard Model (SM) is the theoretical framework at the basis of particle physics. It gives a description of the fundamental particles and their interactions. Elementary particles acquire their mass thanks to the spontaneous breaking of the electroweak symmetry and thanks to the introduction of a complex Higgs field. The Brout-Englert-Higgs mechanism adds to the theory a new particle, the Higgs boson, which has been discovered in 2012 by the ATLAS and CMS collaborations at the LHC. The study of its properties can either confirm its SM-like nature or may open doors to new physics. For example, there are Beyond Standard Model theories that foresee the existence of more than one Higgs boson. There are some features of this intriguing particle that have not been investigated yet. One of those, discussed in this thesis, is how it couples to longitudinally and transversally polarized vector bosons. The SM predicts the longitudinal coupling parameter and the transversal one to be equal and finely tuned in order to restore unitarity in the scattering of electroweak W bosons. If an anomalous coupling were observed, this would hint at new physics in the interactions between the Higgs and the vector bosons. In the present thesis, these couplings are studied in the case in which the Higgs is produced via vector boson fusion (VBF) and decays into the final state.
The data analyzed has been collected by the ATLAS experiment at LHC during 2015 and 2016, corresponding to an integrated luminosity of 36.1 .
This channel has a very distinctive signature but suffers from the presence of large backgrounds, such as, for example, the production of top-quark pairs.
To disentangle the signal from the backgrounds, multivariate classifiers are built. Anomalies in the longitudinal or transversal coupling directly affect the production cross-section, which presents a strong dependence on the longitudinal coupling and a mild one on the transversal one. The kinematic distributions of the two leading jets, related to the intrinsic structure of the VBF production vertex, carry information about the polarization. In particular, the shape of the distribution of the angle between the two leading jets in the plane perpendicular to the beam is sensitive to BSM effects. Exploiting these kinematic features, maximum likelihood fits are performed on the coupling parameters. The observed best values of the longitudinal and transversal coupling parameters are found to be in agreement with the SM predictions within the errors.
