Beyond the Standard Higgs at the LHC: present constraints on Little Higgs models and future prospects
Marco Tonini · DESY (CERN, DESY, Fermilab, IHEP, and SLAC) · 2022
This thesis discusses the consistency of different Little Higgs models with the collected collider data as of the summer of 2013. Moreover, future prospects for possible discoveries and mass measurement methods of new physics signals at the foreseen LHC run II with increased center--of--mass energy are presented. Little Higgs models belong to a class of extensions of the Standard Higgs model, predicting a strong interaction regime at a compositeness scale $\Lambda = 4 \pi f$ above the electroweak scale $v$. The Higgs boson arises as a pseudo--Goldstone boson of an approximate global symmetry spontaneously broken at the scale $f$. A natural hierarchy between the compositeness and the electroweak scale is introduced by the Collective Symmetry Breaking mechanism: one--loop diagrams generating the Higgs mass term are forced to be at most logarithmically sensitive to $\Lambda$. A naturally light Higgs boson can thus be accommodated, consistently with a perturbative theory until a scale of order $10 \, \textrm{TeV}$. We have probed the parameter space of three prominent examples of Little Higgs models, namely the Simplest Little Higgs model, the Littlest Higgs model, and the Littlest Higgs model with T--parity, against electroweak precision observables and the collected LHC data concerning both Higgs properties and direct searches for new particles, with $\sqrt{s} = 7, \, 8 \, \textrm{TeV}$ and up to $25 \, \textrm{fb}^{-1}$ of integrated luminosity. Lower bounds on the scale $f$ are set, within a certain degree of confidence level, which allow to draw conclusions on the ``naturalness'' of the different models. Optimisations of the existing direct searches setups, assuming a Little Higgs signal, as well as dedicated mass measurement methods designed for the foreseen LHC runs with $\sqrt{s} = 13, \, 14 \, \textrm{TeV}$ are thoroughly discussed and proposed in this thesis. Special attention will be dedicated to final states including either a large or negligible fraction of missing transverse momentum. In particular, we will propose a dedicated collider search tailored for the discovery and mass measurement of a top partner, exploiting jet--substructure techniques and optimised kinematical selection cuts, as well as a mass measurement method for the topology of semi--invisibly decaying particles, pair produced from the decay of a resonance.