IRMP - Public Thesis defense


16 décembre 2019



Chemin du Cyclotron, 2 - Auditoire CYCL 01

Search for an extended scalar sector through the H→ZA→l+l-bb process in pp collisions at √s = 13 TeV by Alessia SAGGIO

Pour l’obtention du grade de Docteur en sciences

The validity of the Standard Model (SM) of particle physics, a theoretical framework that describes the fundamental particles and the interactions between them, has been remarkably proven by a large programme of experimental researches, culminating in the discovery of the Higgs boson in 2012 at CERN. Despite this, the existence of new physics is strongly suggested by the inability of the SM to explain in a satisfactory way some observed phenomena, such as the existence of dark matter and the matter/antimatter asymmetry in the Universe. To overcome these problems, many theoretical models beyond the SM predicting the existence of new particles are postulated. Among these, the two-Higgs-doublet model (2HDM) is one of the simplest extensions of the SM scalar sector, and entails the presence of five new bosons. In this thesis, the search for two of these particles, H and A, is presented.The search is performed through a statistical analysis of the data collected by the CMS experiment at the CERN Large Hadron Collider (LHC) during 2016 at a center-of-mass energy of 13 TeV and integrated luminosity of 35.9/fb. The two particles are searched for via the decay chain H → ZA, with the Z boson decaying to a pair of leptons and the A boson decaying to a pair of b quarks. The analysis strategy exploits the main features of the signal process in order to achieve an improvement in the sensitivity and obtain competitive results. Upper limits are set on the product of the signal production cross section and branching ratio, and the results are also interpreted in the context of the 2HDM, allowing for exclusion of the model's parameters in a certain range. Another viable way of looking for new physics is represented by searches for small deviations from the the SM predictions, for which the Standard Model Effective Field Theory (SMEFT) represents the most natural way of interpretation. In this thesis, a case-study is also presented that infers constraints on a Wilson coefficient of the SMEFT with the Matrix Element Method, using the MoMEMta package. The results look promising for future applications to data analyses. It is an exciting moment for particle physics. During Run 2, the LHC has provided an integrated luminosity of about 150/fb. The increase in the statistics of the data to be analyzed, together with improved analysis techniques, are key for an improvement in the sensitivity of future searches, essential to reach the ultimate goal of finding new physics.

Jury members :

  • Prof. Christophe Delaere (UCLouvain), supervisor
  • Prof. Vincent Lemaitre (UCLouvain), chairperson
  • Prof. Fabio Maltoni (UCLouvain), secretary
  • Dr. Tommaso Dorigo (INFN Padova, Italy)
  • Prof. Ben Kilminster (UZH Zurich, Switzerland)

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